Author name code: klimchuk
ADS astronomy entries on 2022-09-14
author:"Klimchuk, James A." or author:"Klimchuk, Jim" 
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Title: Advancing Theory and Modeling Efforts in Heliophysics
Authors: Guo, Fan; Antiochos, Spiro; Cassak, Paul; Chen, Bin; Chen,
   Xiaohang; Dong, Chuanfei; Downs, Cooper; Giacalone, Joe; Haggerty,
   Colby C.; Ji, Hantao; Karpen, Judith; Klimchuk, James; Li, Wen; Li,
   Xiaocan; Oka, Mitsuo; Reeves, Katharine K.; Swisdak, Marc; Tu, Weichao
Bibcode: 2022arXiv220903611G
Altcode:
  Heliophysics theory and modeling build understanding from fundamental
  principles to motivate, interpret, and predict observations. Together
  with observational analysis, they constitute a comprehensive scientific
  program in heliophysics. As observations and data analysis become
  increasingly detailed, it is critical that theory and modeling develop
  more quantitative predictions and iterate with observations. Advanced
  theory and modeling can inspire and greatly improve the design of
  new instruments and increase their chance of success. In addition,
  in order to build physics-based space weather forecast models, it is
  important to keep developing and testing new theories, and maintaining
  constant communications with theory and modeling. Maintaining a
  sustainable effort in theory and modeling is critically important
  to heliophysics. We recommend that all funding agencies join forces
  and consider expanding current and creating new theory and modeling
  programs--especially, 1. NASA should restore the HTMS program to its
  original support level to meet the critical needs of heliophysics
  science; 2. a Strategic Research Model program needs to be created to
  support model development for next-generation basic research codes;
  3. new programs must be created for addressing mission-critical theory
  and modeling needs; and 4. enhanced programs are urgently required
  for training the next generation of theorists and modelers.

Title: Contribution of spicules to solar coronal emission
Authors: Sow Mondal, Shanwlee; Klimchuk, James A.; Sarkar, Aveek
Bibcode: 2022arXiv220805240S
Altcode:
  Recent high-resolution imaging and spectroscopic observations have
  generated renewed interest in spicules' role in explaining the hot
  corona. Some studies suggest that some spicules, often classified as
  type II, may provide significant mass and energy to the corona. Here
  we use numerical simulations to investigate whether such spicules can
  produce the observed coronal emission without any additional coronal
  heating agent. Model spicules consisting of a cold body and hot tip
  are injected into the base of a warm ($0.5$ MK) equilibrium loop with
  different tip temperatures and injection velocities. Both piston- and
  pressure-driven shocks are produced. We find that the hot tip cools
  rapidly and disappears from coronal emission lines such as Fe XII $195$
  and Fe XIV $274$. Prolonged hot emission is produced by pre-existing
  loop material heated by the shock and by thermal conduction from
  the shock. However, the shapes and Doppler shifts of synthetic line
  profiles show significant discrepancies with observations. Furthermore,
  spatially and temporally averaged intensities are extremely low,
  suggesting that if the observed intensities from the quiet Sun and
  active regions were solely due to type II spicules, one to several
  orders of magnitude more spicules would be required than have been
  reported in the literature. This conclusion applies strictly to the
  ejected spicular material. We make no claims about emissions connected
  with waves or coronal currents that may be generated during the ejection
  process and heat the surrounding area.

Title: The Coronal Veil
Authors: Malanushenko, A.; Cheung, M. C. M.; DeForest, C. E.; Klimchuk,
   J. A.; Rempel, M.
Bibcode: 2022ApJ...927....1M
Altcode: 2021arXiv210614877M
  Coronal loops, seen in solar coronal images, are believed to
  represent emission from magnetic flux tubes with compact cross
  sections. We examine the 3D structure of plasma above an active
  region in a radiative magnetohydrodynamic simulation to locate volume
  counterparts for coronal loops. In many cases, a loop cannot be linked
  to an individual thin strand in the volume. While many thin loops are
  present in the synthetic images, the bright structures in the volume
  are fewer and of complex shape. We demonstrate that this complexity
  can form impressions of thin bright loops, even in the absence of thin
  bright plasma strands. We demonstrate the difficulty of discerning
  from observations whether a particular loop corresponds to a strand in
  the volume, or a projection artifact. We demonstrate how apparently
  isolated loops could deceive observers, even when observations from
  multiple viewing angles are available. While we base our analysis
  on a simulation, the main findings are independent from a particular
  simulation setup and illustrate the intrinsic complexity involved in
  interpreting observations resulting from line-of-sight integration
  in an optically thin plasma. We propose alternative interpretation
  for strands seen in Extreme Ultraviolet images of the corona. The
  "coronal veil" hypothesis is mathematically more generic, and
  naturally explains properties of loops that are difficult to address
  otherwise-such as their constant cross section and anomalously high
  density scale height. We challenge the paradigm of coronal loops as
  thin magnetic flux tubes, offering new understanding of solar corona,
  and by extension, of other magnetically confined bright hot plasmas.

Title: Impact of 3D Structure on Magnetic Reconnection
Authors: Daldorff, Lars K. S.; Leake, James E.; Klimchuk, James A.
Bibcode: 2022ApJ...927..196D
Altcode: 2022arXiv220204761D
  Results from 2.5D and 3D studies of the onset and development of the
  tearing instability are presented, using high-fidelity resistive
  MHD simulations. A limited parameter study of the strength of the
  reconnecting field (or shear angle) was performed. An initially simple
  1D equilibrium was used, consisting of a modified force-free current
  sheet, with periodic boundary conditions in all directions. In all
  cases, the linear and nonlinear evolution led to a primary current
  sheet between two large flux ropes. The global reconnection rate during
  this later stage was analyzed in all simulations. It was found that
  in 2.5D the primary current sheet fragmented owing to plasmoids, and
  as expected, the global reconnection rate, calculated using multiple
  methods, increases with the strength of the reconnecting field owing to
  a stronger Alfvén speed. In 3D, the presence of interacting oblique
  modes of the tearing instability complicates the simple 2.5D picture,
  entangling the magnetic field of the inflow and introducing a negative
  effect on the reconnection rate. The two competing effects of stronger
  Alfvén speed and entangling, which both increase with the strength of
  the reconnecting field, resulted in a decrease in the reconnection rate
  with increasing reconnecting field. For all simulations, the 3D rates
  were less than in 2.5D but suggest that as one goes to weak reconnecting
  field (or strong guide field) the system becomes more 2.5D-like and the
  2.5D and 3D rates converge. These results have relevance to situations
  like nanoflare heating and flare current sheets in the corona.

Title: Static and dynamic solar coronal loops with cross-sectional
    area variations
Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A.; Barnes,
   W. T.
Bibcode: 2022MNRAS.509.4420C
Altcode: 2021arXiv211109339C; 2021MNRAS.tmp.2869C
  The Enthalpy Based Thermal Evolution of Loops approximate model for
  static and dynamic coronal loops is developed to include the effect
  of a loop cross-sectional area which increases from the base of the
  transition region (TR) to the corona. The TR is defined as the part of a
  loop between the top of the chromosphere and the location where thermal
  conduction changes from an energy loss to an energy gain. There are
  significant differences from constant area loops due to the manner in
  which the reduced volume of the TR responds to conductive and enthalpy
  fluxes from the corona. For static loops with modest area variation the
  standard picture of loop energy balance is retained, with the corona
  and TR being primarily a balance between heating and conductive losses
  in the corona, and downward conduction and radiation to space in the
  TR. As the area at the loop apex increases, the TR becomes thicker
  and the density in TR and corona larger. For large apex areas, the
  coronal energy balance changes to one primarily between heating and
  radiation, with conduction playing an increasingly unimportant role,
  and the TR thickness becoming a significant fraction of the loop
  length. Approximate scaling laws are derived that give agreement with
  full numerical solutions for the density, but not the temperature. For
  non-uniform areas, dynamic loops have a higher peak temperature and
  are denser in the radiative cooling phase by of order 50 per cent than
  the constant area case for the examples considered. They also show
  a final rapid cooling and draining once the temperature approaches 1
  MK. Although the magnitude of the emission measure will be enhanced
  in the radiative phase, there is little change in the important
  observational diagnostic of its temperature dependence.

Title: Flows in Enthalpy-based Thermal Evolution of Loops
Authors: Rajhans, Abhishek; Tripathi, Durgesh; Bradshaw, Stephen J.;
   Kashyap, Vinay L.; Klimchuk, James A.
Bibcode: 2022ApJ...924...13R
Altcode: 2021arXiv211003204R
  Plasma-filled loop structures are common in the solar corona. Because
  detailed modeling of the dynamical evolution of these structures is
  computationally costly, an efficient method for computing approximate
  but quick physics-based solutions is to rely on space-integrated 0D
  simulations. The enthalpy-based thermal evolution of loops (EBTEL)
  framework is a commonly used method to study the exchange of mass
  and energy between the corona and transition region. EBTEL solves for
  density, temperature, and pressure, averaged over the coronal part of
  the loop, velocity at coronal base, and the instantaneous differential
  emission measure distribution in the transition region. The current
  single-fluid version of the code, EBTEL2, assumes that at all stages the
  flows are subsonic. However, sometimes the solutions show the presence
  of supersonic flows during the impulsive phase of heat input. It is thus
  necessary to account for this effect. Here, we upgrade EBTEL2 to EBTEL3
  by including the kinetic energy term in the Navier-Stokes equation. We
  compare the solutions from EBTEL3 with those obtained using EBTEL2, as
  well as the state-of-the-art field-aligned hydrodynamics code HYDRAD. We
  find that the match in pressure between EBTEL3 and HYDRAD is better than
  that between EBTEL2 and HYDRAD. Additionally, the velocities predicted
  by EBTEL3 are in close agreement with those obtained with HYDRAD when
  the flows are subsonic. However, EBTEL3 solutions deviate substantially
  from HYDRAD's when the latter predicts supersonic flows. Using the
  mismatches in the solution, we propose a criterion to determine the
  conditions under which EBTEL can be used to study flows in the system.

Title: Magnetic Reconnection in 3D vs. 2D and Dependence on Magnetic
    Shear
Authors: Daldorff, Lars; Leake, James; Klimchuk, James
Bibcode: 2021AGUFMSH25E2125D
Altcode:
  Magnetic reconnection plays a central role for heating the solar
  coronal plasma including during flares as well as other places in
  the heliosphere and beyond, where each event can be characterized by
  its geometry and magnetic configurations, forming structures that we
  can categorize to be more 2D or 3D. The difference in reconnection
  rate between 2D and 3D simulations have been reported in multiple
  studies over the years. We report on a study of the reconnection rate
  using the resistive MHD code LaRe3D. We show that the rate depends
  strongly on the existence and interaction of different tearing layers
  (oblique tearing modes) within the current sheet. Such modes are only
  present with a finite guide field and a spatial dependence in this
  third direction. We find, as have others, that reconnection rates are
  artificially high in 2D simulations, and we offer an explanation.

Title: Distributed Space Telescopes Enabled by Constellation of
    Small Satellites
Authors: Kamalabadi, Farzad; Lightsey, E.; Rabin, Douglas; Daw,
   Adrian; D'Amico, Simone; Koenig, Adam; Chamberlin, Philip; Woods,
   Thomas; Gupta, Subhanshu; Ekici, Eylem; Sample, John; Park, Hyeongjun;
   Alexeenko, Alina; Hwang, John; Denis, Kevin; Klimchuk, James
Bibcode: 2021AGUFM.A33C..03K
Altcode:
  New pathways to high-resolution sensing and imaging for a multitude
  of high-priority scientific investigations are being realized by small
  multi-spacecraft systems. Such pathfinder mission concepts circumvent
  the limitations of conventional remote sensing/imaging systems by
  utilizing multiple baselines, synthesized apertures, diffractive
  optics, combined with computational imaging via interferometry,
  tomography, or super-resolution. Regardless of the specific scientific
  questions targeted, such small satellite constellation pathfinders
  require technological breakthroughs in precision formation flying
  and associated advances in guidance, navigation, and control;
  proximity operations and associated autonomy and robust orbit
  design with passive safety; innovations in sensor miniaturization;
  inter-satellite communication; and sophisticated computational sensing
  and reconstruction algorithms. We describe recent advances in such
  enabling technologies in the context of a scalable ultra-high-resolution
  spectral imaging mission for investigating the solar corona currently
  under development by a multi-university consortium in collaboration
  with NASA and under sponsorship by NSF.

Title: Study of Type III Radio bursts in the Closed Corona and the
Solar Wind from Small-scale Reconnection: Observations
Authors: Chhabra, Sherry; Klimchuk, James; Gary, Dale
Bibcode: 2021AGUFMSH24B..06C
Altcode:
  It is widely agreed that the ubiquitous presence of reconnection
  events and the associated impulsive heating (nanoflares) are a strong
  candidate for heating the magnetically closed corona. Whether nanoflares
  accelerate energetic particles like fullsized flares is unknown. The
  lack of strong emission in hard Xrays suggests that the quantity of
  highly energetic particles is small. There could, however, be large
  numbers of mildly energetic particles (~ 10 keV). Similarly, in the
  context of the solar wind, these energetic particles can originate
  from interchange reconnection, streamer tip reconnection, or turbulence
  reconnection in the solar wind itself, in which case they stream away
  from the Sun along the open field lines. To understand whether these
  processes are efficient at accelerating particles, we search for the
  type III radio bursts that they may produce. The timelag technique that
  was developed to study subtle delays in light curves from different
  EUV channels [Viall &amp; Klimchuk 2012] can also be used to detect
  subtle delays at different radio frequencies. We have modeled the
  expected radio emission from mildly energetic particles propagating
  in the closed corona and open corona/solar wind. The models were used
  to test and calibrate the technique. We are currently applying the
  technique to radio observations from VLA (Very Large Array), LOFAR
  (LowFrequency Array), LWA, NM (Long Wavelength Array, New Mexico), and
  the FIELDS experiment (encounters 1-6) to search for such signatures
  of type IIIs. We also plan to investigate the relationship between
  the bursts and activity on the Sun, such as the presence/absence of
  active regions, relationship with their age etc. We will report the
  results from our analysis.

Title: Coronal Heating: A Coupled Multi-Scale Problem
Authors: Klimchuk, James
Bibcode: 2021AGUFMSH13A..01K
Altcode:
  Despite substantial observational and theoretical progress, the heating
  of the magnetically closed corona to temperatures 1000 times hotter than
  the solar surface has yet to be fully explained. Like many problems in
  Heliophysics, it involves an enormous range of spatial scales that are
  coupled in fundamental ways. In this talk, I will review our current
  understanding of how the coronal plasma is heated and how it responds
  to produce the time variable spectrum of radiation that is an important
  driver of space weather. I will emphasize that future progress requires
  a more integrated approach than has typically been used.

Title: Onset of Magnetic Reconnection in the Solar Corona
Authors: Leake, James; Klimchuk, James; Daldorff, Lars
Bibcode: 2021AGUFMSH34C..01L
Altcode:
  Magnetic reconnection plays a vital role in the dynamics, heating,
  and emission in the solar corona. It also plays a role in the onset
  of transient events such as jets, spicules, filament eruptions, and
  coronal mass ejections. One of the major questions still unanswered
  is the nature of the onset of magnetic reconnection. How is it that
  the solar corona can slowly build up magnetic energy and then rapidly
  release it to manifest in the above phenomena? Recent theoretical
  developments have considered the nature of the tearing instability in
  dynamically thinning current sheets in terms of this rapid switch-on of
  reconnection. We present a study of the onset of magnetic reconnection
  in 3D dynamically evolving current sheets unstable to both oblique and
  parallel modes of the tearing instability. While it is not currently
  possible to realize the actual parameters in the solar corona such as
  Lundquist number and current sheet thickness, we are able to perform
  our study in a regime which has relevant ratios of tearing timescale
  to dynamic timescale that allow us to make relevant conclusions about
  the solar corona.

Title: Signatures of Type III Solar Radio Bursts from Nanoflares:
    Modeling
Authors: Chhabra, Sherry; Klimchuk, James A.; Gary, Dale E.
Bibcode: 2021ApJ...922..128C
Altcode: 2021arXiv210903355C
  There is a wide consensus that the ubiquitous presence of magnetic
  reconnection events and the associated impulsive heating (nanoflares)
  are strong candidates for solving the solar coronal heating
  problem. Whether nanoflares accelerate particles to high energies
  like full-sized flares is unknown. We investigate this question by
  studying the type III radio bursts that the nanoflares may produce on
  closed loops. The characteristic frequency drifts that type III bursts
  exhibit can be detected using a novel application of the time-lag
  technique developed by Viall &amp; Klimchuk (2012) even when there
  are multiple overlapping events. We present a simple numerical model
  that simulates the expected radio emission from nanoflares in an active
  region, which we use to test and calibrate the technique. We find that
  in the case of closed loops the frequency spectrum of type III bursts
  is expected to be extremely steep such that significant emission is
  produced at a given frequency only for a rather narrow range of loop
  lengths. We also find that the signature of bursts in the time-lag
  signal diminishes as: (1) the variety of participating loops within
  that range increases; (2) the occurrence rate of bursts increases;
  (3) the duration of bursts increases; and (4) the brightness of bursts
  decreases relative to noise. In addition, our model suggests a possible
  origin of type I bursts as a natural consequence of type III emission
  in a closed-loop geometry.

Title: Computing Emission Signatures from Coronal MHD Models Without
    a Realistic Lower Atmosphere
Authors: Klimchuk, James; Knizhnik, Kalman; Uritsky, Vadim
Bibcode: 2021AGUFMSH43A..08K
Altcode:
  The emission properties of the corona depend crucially on its energetic
  and dynamic coupling with the lower atmosphere, involving processes like
  thermal conduction and radiation cooling, chromospheric evaporation, and
  draining/condensation. Incorporating these processes into an MHD model
  is extremely challenging, because of the need to spatially resolve both
  the transition region and current sheets involved in the heating, both
  of which are thin and non-stationary. (New methods lesson the demands
  on resolving the transition region, especially when the heating is
  quasi steady.) We have developed a simple technique that can be applied
  post facto to MHD simulations without a realistic lower atmosphere to
  obtain useful approximations of the nonuniform and evolving radiation
  spectrum. We demonstrate the technique with a simulation of an initially
  uniform plasma/magnetic field system that is subjected to small-scale
  driving at the photospheric boundary. Important results are obtained
  concerning the nature of both the diffuse component of the corona and
  bright coronal loops.

Title: Signatures of Type III Radio Bursts from Small-scale
    Reconnection Events in the Solar Wind
Authors: Chhabra, S.; Klimchuk, J.; Gary, D.; Psp/Fields Team
Bibcode: 2021AAS...23812307C
Altcode:
  We look for evidence of energetic particles in the solar wind,
  that could be produced by reconnection in the solar wind itself or
  reconnection in the lower corona, where particles escape on open field
  lines, from e.g. interchange reconnection.We expect reconnection to be
  common at the current sheets that separate the thin magnetic strands
  that make up the corona and solar wind, but whether it is efficient
  at accelerating particles is an open question. Type III radio bursts
  have been very frequently observed in the solar wind over the past
  few decades. Energetic electron beams propagating along magnetic
  field lines cause a bump-on-tail instability generating Langmuir
  waves. Produced by their interaction with other particles and waves,
  type III bursts exhibit a characteristic drift in frequency as they
  propagate through the density gradient in the field. An interesting
  question is whether there is a ubiquitous presence of type IIIs in the
  radio 'background' observed. The radio background outside of clearly
  identified bursts may actually be comprised of multiple overlapping
  events. The time-lag technique that was developed to study subtle delays
  in light curves from different EUV channels [Viall &amp; Klimchuk 2012]
  can also be used to detect subtle delays at different frequencies in
  the radio background even when there is no hint in the individual light
  curves. The FIELDS instrument onboard the Parker Solar Probe (PSP) is
  utilized to investigate the solar wind for these signatures. We perform
  a systematic study of the observed type III storms in Encounters 1-5,
  to understand the signatures that can identify their presence and
  the different features observed using the technique. Our findings are
  then employed to study the times where no activity is visibly detected
  by the instrument. We are currently analyzing multiple periods of no
  visible activity and will report our findings.

Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS)
Authors: Caspi, A.; Shih, A. Y.; Panchapakesan, S.; Warren, H. P.;
   Woods, T. N.; Cheung, M.; DeForest, C. E.; Klimchuk, J. A.; Laurent,
   G. T.; Mason, J. P.; Palo, S. E.; Seaton, D. B.; Steslicki, M.;
   Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński, M.; Schattenburg,
   M.; The CubIXSS Team
Bibcode: 2021AAS...23821609C
Altcode:
  The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U
  CubeSat proposed to NASA H-FORT. CubIXSS is motivated by a compelling
  overarching science question: what are the origins of hot plasma in
  solar flares and active regions? Elemental abundances are a unique
  diagnostic of how mass and energy flow into and within the corona,
  and CubIXSS addresses its science question through sensitive, precise
  measurements of abundances of key trace ion species, whose spectral
  signatures reveal the chromospheric or coronal origins of heated plasma
  across the entire temperature range from ~1 to &gt;30 MK. CubIXSS
  measurements of the coronal temperature distribution and elemental
  abundances directly address longstanding inconsistencies from prior
  studies using instruments with limited, differing temperature and
  composition sensitivities. <P />CubIXSS comprises two co-optimized
  and cross-calibrated instruments that fill a critical observational
  gap: <P />MOXSI, a novel diffractive spectral imager using a pinhole
  camera and X-ray transmission diffraction grating for spectroscopy of
  flares and active regions from 1 to 55 Å, with spectral and spatial
  resolutions of 0.28-0.37 Å and 29-39 arcsec FWHM, respectively;
  and <P />SASS, a suite of four spatially-integrated off-the-shelf
  spectrometers for high-cadence, high-sensitivity X-ray spectra from
  0.5 to 50 keV, with spectral resolution of 0.06-0.5 keV FWHM across
  that range. <P />If selected for implementation, CubIXSS will launch
  in late 2023 to mid-2024 to observe intense solar flares and active
  regions during the rising phase and peak of the solar cycle. Its 1-year
  prime mission is well timed with perihelia of Parker Solar Probe and
  Solar Orbiter, and with the launches of complementary missions such
  as the PUNCH Small Explorer. CubIXSS is a pathfinder for the next
  generation of Explorer-class missions with improved capabilities for
  SXR imaging spectroscopy. We present the CubIXSS motivating science
  background, its suite of instruments and expected performances, and
  other highlights from the completed Concept Study Report, including
  novel analysis techniques to fully exploit the rich data set of CubIXSS
  spectral observations.

Title: Cross Sections of Coronal Loop Flux Tubes
Authors: Klimchuk, J. A.; DeForest, C.
Bibcode: 2021AAS...23832808K
Altcode:
  Coronal loops reveal crucial information about the nature of
  both coronal magnetic fields and coronal heating. The shape of the
  corresponding flux tube cross section and how it varies with position
  are especially important properties. They are a direct indication of
  the expansion of the field and of the cross-field spatial distribution
  of the heating. We have studied 20 loops using high spatial resolution
  observations from the first flight of the Hi-C rocket experiment,
  measuring the intensity and width as a function of position along
  the loop axis. We find that intensity and width tend to either be
  uncorrelated or to have a direct dependence, such that they increase
  or decrease together. This implies that the flux tube cross sections
  are approximately circular under the assumptions that the tubes have
  non-negligible twist and that the plasma emissivity is approximately
  uniform along the magnetic field. The shape need not be a perfect circle
  and the emissivity need not be uniform within the cross section, but
  sub-resolution patches of emission must be distributed quasi-uniformly
  within an envelope that has an aspect ratio of order unity. This raises
  questions about the suggestion that flux tubes expand with height,
  but primarily in the line-of-sight direction so that the corresponding
  (relatively noticeable) loops appear to have roughly uniform width,
  a long-standing puzzle. It also casts doubt on the idea that most
  loops correspond to simple warped sheets, although we leave open the
  possibility of more complex manifold structures.

Title: Nonthermal Velocity in the Transition Region of Active Regions
    and Its Center-to-limb Variation
Authors: Ghosh, Avyarthana; Tripathi, Durgesh; Klimchuk, James A.
Bibcode: 2021ApJ...913..151G
Altcode: 2021arXiv210315081G
  We derive the nonthermal velocities (NTVs) in the transition region
  of an active region using the Si IV 1393.78 Å line observed by
  the Interface Region Imaging Spectrograph and compare them with the
  line-of-sight photospheric magnetic fields obtained by the Helioseismic
  and Magnetic Imager on board the Solar Dynamics Observatory. The active
  region consists of two strong field regions with opposite polarity,
  separated by a weak field corridor that widened as the active region
  evolved. The means of the NTV distributions in strong field regions
  (weak field corridors) range between ∼18-20 (16-18) km s<SUP>-1</SUP>,
  albeit the NTV maps show a much larger range. In addition, we identify
  a narrow lane in the middle of the corridor with significantly reduced
  NTV. The NTVs do not show a strong center-to-limb variation, albeit
  they show somewhat larger values near the disk center. The NTVs are
  well correlated with redshifts as well as line intensities. The results
  obtained here and those presented in our companion paper on Doppler
  shifts suggest two populations of plasma in the active region emitting
  in Si IV. The first population exists in the strong field regions and
  extends partway into the weak field corridor between them. We attribute
  this plasma to spicules heated to ∼0.1 MK (often called type II
  spicules). They have a range of inclinations relative to vertical. The
  second population exists in the center of the corridor, is relatively
  faint, and has smaller velocities, likely horizontal. These results
  provide further insights into the heating of the transition region.

Title: How Turbulent is the Magnetically Closed Corona?
Authors: Klimchuk, James A.; Antiochos, Spiro K.
Bibcode: 2021FrASS...8...83K
Altcode: 2021arXiv210512212K
  We argue that the magnetically closed corona evolves primarily
  quasi-statically, punctuated by many localized bursts of activity
  associated with magnetic reconnection at a myriad of small current
  sheets. The sheets form by various processes that do not involve
  a traditional turbulent cascade whereby energy flows losslessly
  through a continuum of spatial scales starting from the large scale
  of the photospheric driving. If such an inertial range is a defining
  characteristic of turbulence, then the magnetically closed corona is
  not a turbulent system. It nonetheless has a complex structure that
  bears no direct relationship to the pattern of driving.

Title: The Heating of the Solar Corona
Authors: Viall, Nicholeen M.; De Moortel, Ineke; Downs, Cooper;
   Klimchuk, James A.; Parenti, Susanna; Reale, Fabio
Bibcode: 2021GMS...258...35V
Altcode:
  No abstract at ADS

Title: High resolution soft X-ray spectroscopy and the quest for
    the hot (5-10 MK) plasma in solar active regions
Authors: Del Zanna, Giulio; Andretta, Vincenzo; Cargill, Peter J.;
   Corso, Alain J.; Daw, Adrian N.; Golub, Leon; Klimchuk, James A.;
   Mason, Helen E.
Bibcode: 2021FrASS...8...33D
Altcode: 2021arXiv210306156D
  We discuss the diagnostics available to study the 5--10 MK plasma in
  the solar corona, which is key to understanding the heating in the
  cores of solar active regions. We present several simulated spectra,
  and show that excellent diagnostics are available in the soft X-rays,
  around 100 Angstroms, as six ionisation stages of Fe can simultaneously
  be observed, and electron densities derived, within a narrow spectral
  region. As this spectral range is almost unexplored, we present an
  analysis of available and simulated spectra, to compare the hot emission
  with the cooler component. We adopt recently designed multilayers to
  present estimates of count rates in the hot lines, with a baseline
  spectrometer design. Excellent count rates are found, opening up
  the exciting opportunity to obtain high-resolution spectroscopy of
  hot plasma.

Title: The Fascinating Phenomenon of Thermal Nonequilibrium
Authors: Klimchuk, James
Bibcode: 2021cosp...43E.960K
Altcode:
  Thermal nonequilibrium (TNE) is an intriguing, counter-intuitive
  situation in which the plasma contained in a coronal magnetic loop is
  constantly evolving even though the heating is perfectly steady. It
  is believed to be the explanation for prominences, coronal rain, and
  long-period pulsating loops on the Sun, and there may be additional
  applications in other astrophysical settings. In this talk, I will
  attempt to peal away the mystery of TNE and provide some basic physical
  insights. I will present two analytical formulae for predicting
  whether TNE will occur, including the important role of asymmetries
  in preventing TNE. Finally, I will explain how TNE is related to,
  but different from, thermal instability.

Title: Why do different current sheets reconnect differently?
Authors: Daldorff, L. K. S.; Leake, J. E.; Klimchuk, J. A.
Bibcode: 2020AGUFMSH034..03D
Altcode:
  What variability can we expect for heating events from magnetic
  reconnection in the solar corona is an important research
  question. Complex flows in the photosphere entangle, twist, and compress
  the magnetic field in the chromosphere and corona, resulting in a
  complex forest of evolving current sheets. These sheets undergo tearing
  and magnetic reconnection, the details of which depend on the properties
  of the sheet, including its dimensions in all three directions, as
  well as the shear angle of the field. The associated energy release
  and heating can be very different for different sheets. We discuss
  the possibilities with emphasis on the role played by the interaction
  among the different tearing modes, parallel and oblique, that are
  present in the system.

Title: Transition Region Contribution to AIA Observations in the
    Context of Coronal Heating
Authors: Schonfeld, S. J.; Klimchuk, J. A.
Bibcode: 2020ApJ...905..115S
Altcode: 2020arXiv200906759S
  We investigate the ratio of coronal and transition region intensity
  in coronal loops observed by the Atmospheric Imaging Assembly (AIA)
  on the Solar Dynamics Observatory (SDO). Using Enthalpy-based Thermal
  Evolution of Loops (EBTEL) hydrodynamic simulations, we model loops
  with multiple lengths and energy fluxes heated randomly by events drawn
  from power-law distributions with different slopes and minimum delays
  between events to investigate how each of these parameters influences
  observable loop properties. We generate AIA intensities from the corona
  and transition region for each realization. The variations within and
  between models generated with these different parameters illustrate
  the sensitivity of narrowband imaging to the details of coronal
  heating. We then analyze the transition region and coronal emission
  from a number of observed active regions and find broad agreement
  with the trends in the models. In both models and observations, the
  transition region brightness is significant, often greater than the
  coronal brightness in all six "coronal" AIA channels. We also identify
  an inverse relationship, consistent with heating theories, between the
  slope of the differential emission measure (DEM) coolward of the peak
  temperature and the observed ratio of coronal to transition region
  intensity. These results highlight the use of narrowband observations
  and the importance of properly considering the transition region in
  investigations of coronal heating.

Title: Updates on the Fundamentals of Impulsive Energy Release in
    the Corona Explorer (FIERCE) mission concept
Authors: Shih, A. Y.; Glesener, L.; Krucker, S.; Guidoni, S. E.;
   Christe, S.; Reeves, K.; Gburek, S.; Caspi, A.; Alaoui, M.; Allred,
   J. C.; Battaglia, M.; Baumgartner, W.; Dennis, B. R.; Drake, J. F.;
   Goetz, K.; Golub, L.; Hannah, I. G.; Hayes, L.; Holman, G.; Inglis,
   A.; Ireland, J.; Kerr, G. S.; Klimchuk, J. A.; McKenzie, D. E.; Moore,
   C. S.; Musset, S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage,
   S. L.; Schwartz, R.; Seaton, D. B.; Steslicki, M.; Woods, T. N.
Bibcode: 2020AGUFMSH0480012S
Altcode:
  The Fundamentals of Impulsive Energy Release in the Corona Explorer
  ( FIERCE ) Medium-Class Explorer (MIDEX) mission concept addresses
  the following science questions: <P />What are the physical origins
  of space-weather events? <P />How are particles accelerated at the
  Sun? <P />How is impulsively released energy transported throughout
  the solar atmosphere? <P />How is the solar corona heated? <P />FIERCE
  achieves its science objectives through co-optimized X-ray and extreme
  ultraviolet (EUV) observations by the following instruments: <P />FOXSI,
  a focusing hard X-ray spectroscopic imager that is able to capture the
  full range of emission in flares and CMEs (e.g., faint coronal sources
  near bright chromospheric sources) <P />THADIS, a high-resolution,
  fast-cadence EUV imager that will not saturate for even intense flares
  to follow dynamic changes in the configuration of plasma structures
  <P />STC, a soft X-ray spectrometer that provides detailed thermal and
  elemental composition diagnostics <P />If selected, FIERCE will launch
  in 2025, near the peak of the next solar cycle, which is also well timed
  with perihelia of Parker Solar Probe and Solar Orbiter . We describe the
  status and latest updates of the mission concept since it was proposed
  to NASA last year. We also highlight the anticipated science return
  from co-observations with other observatories/instruments such as the
  Expanded Owens Valley Solar Array (EOVSA) or the STIX instrument on
  Solar Orbiter .

Title: The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS)
Authors: Caspi, A.; Shih, A. Y.; Warren, H.; Winebarger, A. R.; Woods,
   T. N.; Cheung, C. M. M.; DeForest, C.; Klimchuk, J. A.; Laurent,
   G. T.; Mason, J. P.; Palo, S. E.; Schwartz, R.; Seaton, D. B.;
   Steslicki, M.; Gburek, S.; Sylwester, J.; Mrozek, T.; Kowaliński,
   M.; Schattenburg, M.
Bibcode: 2020AGUFMSH0480007C
Altcode:
  The CubeSat Imaging X-ray Solar Spectrometer (CubIXSS) is a 6U
  CubeSat currently in a formulation phase under the 2019 NASA H-FORT
  program. CubIXSS is motivated by a compelling overarching science
  question: what are the origins of hot plasma in solar flares and active
  regions? Elemental abundances are a unique diagnostic of how mass
  and energy flow into and within the corona, and CubIXSS addresses
  its science question through sensitive, precise measurements of
  abundances of key trace ion species, whose spectral signatures reveal
  the chromospheric or coronal origins of heated plasma across the
  entire range of coronal temperatures, from ~1 to &gt;30 MK. CubIXSS
  measurements of the coronal temperature distribution and elemental
  abundances directly address longstanding inconsistencies from prior
  studies using instruments with limited, differing temperature and
  composition sensitivities. <P />CubIXSS comprises two co-optimized and
  cross-calibrated instruments that fill a critical observational gap:
  <P />MOXSI, a novel diffractive spectral imager using a pinhole camera
  and X-ray transmission diffraction grating to achieve spectroscopy of
  flares and active regions from 1 to 55 Å, with spectral resolution of
  0.24 Å FWHM and a spatial resolution of 25 arcsec FWHM; and <P />SASS,
  a suite of four spatially-integrated off-the-shelf spectrometers for
  high-cadence, high-sensitivity measurements of soft and hard X-rays,
  from 0.5 to 50 keV, with spectral resolution from 0.06 to 0.5 keV
  FWHM. <P />If selected for implementation, CubIXSS will launch in
  mid-2023 to observe intense solar flares and active regions during
  the rising phase of the solar cycle. Its nominal 1-year mission is
  well timed with perihelia of Parker Solar Probe and Solar Orbiter,
  and with the launches of complementary missions such as the PUNCH
  Small Explorer. CubIXSS is also a pathfinder for the next generation
  of Explorer-class missions with improved capabilities for SXR imaging
  spectroscopy. We present the CubIXSS motivating science background, its
  suite of instruments and expected performances, and other highlights
  from the completed Concept Study Report, including novel analysis
  techniques to fully exploit the rich data set of CubIXSS spectral
  observations.

Title: Signatures of Type III Solar Radio Bursts from Nanoflares:
    Final Results
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
Bibcode: 2020AGUFMSH0430016C
Altcode:
  The heating mechanisms responsible for the million degree solar corona
  remain one of the most intriguing problems in space science. It is
  widely agreed, that the ubiquitous presence o f reconnection events and
  the associated impulsive heating (nanoflares) are a strong candidate in
  solving this problem [Klimchuk J.A., 2015 and references therein]. <P
  />Whether nanoflares accelerate energetic particles like full sized
  flares is unknown. The lack of strong emission in hard X rays suggests
  that the quantity of highly energetic particles is small. There could,
  however, be large numbers of mildly energetic particles (~ 10 keV). We
  investigate such particles by searching for the type III radio bursts
  that they may produce. If energetic electron beams propagating along
  magnetic field lines generate a bump on tail instability, they will
  produce Langmuir waves, which can then interact with other particles
  and waves to give rise to emission at the local plasma frequency and
  its first harmonic. Type III radio bursts bursts are characteristically
  known to exhibit high frequency drifts as the beam propagates through
  a density gradient. The time lag technique that was developed to study
  subtle delays in light curves from different EUV channels [Viall &amp;
  Klimchuk 2012] can also be used to detect subtle delays at different
  radio frequencies. We have modeled the expected radio emission from
  nanoflares, which we used to test and calibrate the technique. We will
  present the final results of our modeling efforts along with results
  from application of the technique to actual radio observations from VLA
  (Very Large Array), MWA (Murchison Widefield Array) and seeking data
  from LOFAR (Low Frequency Array) as well.We are also using data from the
  PSP (Parker Solar Probe) to look for similar reconnection signatures in
  the Solar Wind. Our goal is to determine whether nanoflares accelerate
  energetic particles and to determine their properties. The results
  will have important implications for both the particle acceleration
  and reconnection physics.

Title: Cross Sections of Coronal Loop Flux Tubes
Authors: Klimchuk, J. A.; DeForest, C.
Bibcode: 2020AGUFMSH0370001K
Altcode:
  Coronal loops reveal crucial information about the nature of
  both coronal magnetic fields and coronal heating. The shape of the
  corresponding flux tube cross section and how it varies with position
  are especially important properties. They are a direct indication of
  the expansion of the field and of the cross-field spatial distribution
  of the heating. We have studied 20 loops using high spatial resolution
  observations from the first flight of the Hi-C rocket experiment,
  measuring the intensity and width as a function of position along
  the loop axis. We find that intensity and width tend to either be
  uncorrelated or to have a direct dependence, such that they increase
  or decrease together. This implies that the flux tube cross sections
  are approximately circular under the assumptions that the tubes have
  non-negligible twist and that the plasma emissivity is approximately
  uniform along the magnetic field. The shape need not be a perfect circle
  and the emissivity need not be uniform within the cross section, but
  sub-resolution patches of emission must be distributed quasi-uniformly
  within an envelope that has an aspect ratio of order unity. This raises
  questions about the suggestion that flux tubes expand with height,
  but primarily in the line-of-sight direction so that the corresponding
  (relatively noticeable) loops appear to have roughly uniform width,
  a long-standing puzzle. It also casts doubt on the idea that most
  loops correspond to simple warped sheets, although we leave open the
  possibility of more complex manifold structures.

Title: Can nanoflare heating define the coronal loop size?
Authors: Uritsky, V. M.; Knizhnik, K.; Klimchuk, J. A.
Bibcode: 2020AGUFMSH0370002U
Altcode:
  The idea of nanoflare heating first coined by T. Gold (1964) and further
  developed by E. Parker (1972) has resulted in a dramatic paradigm shift
  in our understanding of the Sun, by emphasizing the critical role of
  the microscale physics in the formation of huge coronal structures such
  as coronal loops. Nanoflares are small, short-living energy conversion
  events in which the magnetic free energy is efficiently converted into
  heat. While the outcome of a single nanoflare is negligibly small, their
  cooperative contribution to the bulk coronal heating can be enormous. In
  this talk, we present recent advances in the numerical modeling of the
  nanoflare heating process. Our simulations demonstrate that densely
  distributed nanoflare events occur naturally inside a topologically
  complex coronal loop energized by photospheric vortices. These events
  lead to a formation of visible loops of various sizes, with the largest
  loop size imposed by the photospheric convection pattern and the
  smaller-scale loops reflecting clusters of nanoflares. Observational
  implications of our findings will be discussed.

Title: Spectroscopic Constraints on the Dimension of Active Region
    Loops Along the Line of Sight
Authors: Kucera, T. A.; Young, P. R.; Klimchuk, J. A.; DeForest, C.
Bibcode: 2020AGUFMSH041..05K
Altcode:
  Understanding the cross sections of coronal loops and how they vary
  along the loop is important both for understanding coronal heating
  and how the loops are shaped by the coronal magnetic field. To better
  address this question we have developed a new method to constrain the
  dimension of loops along the line of sight by utilizing spectroscopic
  observations. We apply this method to a cool (5.5&lt;logT&lt;6.2)
  loop using data from the Hinode/EUV Imaging Spectrometer (EIS) with
  supporting data from Solar Dynamic Observatory (SDO) and the Solar
  TErrestrial RElations Observatory (STEREO), and discuss the results and
  their limitations. Our results are consistent with circular loop cross
  sections, but could also be consistent with aspect ratios of 2 or 3.

Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena in Solar and
    Heliospheric Plasmas
Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.;
   Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee,
   A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.;
   Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso,
   L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.;
   Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink,
   G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto,
   K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare,
   J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le,
   A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.;
   Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge,
   J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.;
   Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.;
   Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200908779J
Altcode:
  Magnetic reconnection underlies many explosive phenomena in the
  heliosphere and in laboratory plasmas. The new research capabilities in
  theory/simulations, observations, and laboratory experiments provide the
  opportunity to solve the grand scientific challenges summarized in this
  whitepaper. Success will require enhanced and sustained investments
  from relevant funding agencies, increased interagency/international
  partnerships, and close collaborations of the solar, heliospheric,
  and laboratory plasma communities. These investments will deliver
  transformative progress in understanding magnetic reconnection and
  related explosive phenomena including space weather events.

Title: Cross Sections of Coronal Loop Flux Tubes
Authors: Klimchuk, James A.; DeForest, Craig E.
Bibcode: 2020ApJ...900..167K
Altcode: 2020arXiv200715085K
  Coronal loops reveal crucial information about the nature of
  both coronal magnetic fields and coronal heating. The shape of the
  corresponding flux tube cross section and how it varies with position
  are especially important properties. They are a direct indication of
  the expansion of the field and of the cross-field spatial distribution
  of the heating. We have studied 20 loops using high spatial resolution
  observations from the first flight of the Hi-C rocket experiment,
  measuring the intensity and width as a function of position along
  the loop axis. We find that intensity and width tend to either be
  uncorrelated or to have a direct dependence, such that they increase
  or decrease together. This implies that the flux tube cross sections
  are approximately circular under the assumptions that the tubes have
  nonnegligible twist and that the plasma emissivity is approximately
  uniform along the magnetic field. The shape need not be a perfect circle
  and the emissivity need not be uniform within the cross section, but
  subresolution patches of emission must be distributed quasi-uniformly
  within an envelope that has an aspect ratio of order unity. This raises
  questions about the suggestion that flux tubes expand with height,
  but primarily in the line-of-sight direction so that the corresponding
  (relatively noticeable) loops appear to have roughly uniform width,
  a long-standing puzzle. It also casts doubt on the idea that most
  loops correspond to simple warped sheets, although we leave open the
  possibility of more complex manifold structures.

Title: The Significance of the Transition Region in AIA Channels:
    Modeling and Observations
Authors: Schonfeld, S. J.; Klimchuk, J.
Bibcode: 2020SPD....5121014S
Altcode:
  We investigate the relative contributions from the transition region
  and corona of coronal loops observed by the Atmospheric Imaging
  Assembly (AIA) on the Solar Dynamics Observatory (SDO). Using EBTEL
  (Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations,
  we model loops with multiple lengths and energy fluxes heated randomly
  by events drawn from power-law distributions with different slopes
  and minimum event sizes to investigate how each of these parameters
  influences observable loop properties. We generate AIA intensities from
  the corona and transition region for each realization. The variations
  within and between models generated with these different parameters
  illustrate the sensitivity of narrowband imaging to the details of
  coronal heating. We then analyze the transition region and coronal
  emission from a number of observed active regions and find broad
  agreement with the trends in the models. We find that in both models
  and observations, the transition region brightness is significant,
  often greater than the coronal brightness in all six "coronal" AIA
  channels. These results highlight the use of narrowband observations
  and the importance of properly considering the transition region in
  investigations of coronal heating.

Title: The Onset of 3D Magnetic Reconnection and Heating in the
    Solar Corona
Authors: Leake, James E.; Daldorff, Lars K. S.; Klimchuk, James A.
Bibcode: 2020ApJ...891...62L
Altcode: 2020arXiv200102971L
  Magnetic reconnection, a fundamentally important process in
  astrophysics, is believed to be initiated by the tearing instability
  of an electric current sheet, a region where magnetic field abruptly
  changes direction. Recent studies have suggested that the amount of
  magnetic shear in these structures is a critical parameter for the
  switch-on nature of magnetic reconnection in the solar atmosphere,
  at large spatial scales. We present results of visco-resistive
  magnetohydrodynamic simulations of magnetic reconnection in 3D current
  sheets with conditions appropriate to the solar corona. We follow the
  evolution of the linear and nonlinear 3D tearing instability. We find
  that, depending on the parameter space, magnetic shear can play a vital
  role in the onset of significant energy release and plasma heating. Two
  regimes in our study exist, dependent on whether the current sheet is
  longer or shorter than the wavelength of the fastest growing mode, thus
  determining whether subharmonics are present in the actual system. In
  one parameter regime, where the fastest growing parallel mode has
  subharmonics, the subsequent coalescence of 3D plasmoids dominates
  the nonlinear evolution, with magnetic shear playing only a weak role
  in the amount of energy released. In the second parameter regime,
  where the fastest growing parallel mode has no subharmonics, only
  strongly sheared current sheets, where 3D effects are strong enough,
  show any significant energy release. We expect both regimes to exist
  on the Sun, and so our results have important consequences for the
  question of reconnection onset in various solar physics applications.

Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
   F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
   Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
   Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
   Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
   Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
   Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200400079J
Altcode:
  This white paper summarizes major scientific challenges and
  opportunities in understanding magnetic reconnection and related
  explosive phenomena as a fundamental plasma process.

Title: Volume-filling Simulations of Coronal Loops Heated by
    Nanoflares
Authors: Plowman, J.; Barnes, W.; Bradshaw, S. J.; Caspi, A.; DeForest,
   C.; Klimchuk, J. A.
Bibcode: 2019AGUFMSH53B3380P
Altcode:
  We present results of a coronal simulation consisting of loop strands
  that fill the coronal volume in a self-consistent fashion. The
  simulation is heated by a fully controllable 3D distribution, which
  can be specified independent of the loop geometry and can include
  nanoflares and continuous heating. The heating is then mapped to the
  loop strands, and the physics of each strand are simulated using the
  HYDRAD field-aligned hydrodynamics code. The simulation is applied
  to a small example active region and used to produce synthetic AIA
  data, which are then processed to produce a distribution of coronal
  EUV brightening events. This distribution is then compared with that
  found in the real AIA data for the same region, and we use the results
  to determine if the observations are consistent with our prescribed
  heating distribution.

Title: Erratum: “The Role of Magnetic Helicity in Coronal Heating”
(<A href="https://doi.org/10.3847/1538-4357/ab3afd">2019, ApJ,
    883, 26</A>)
Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore,
   C. R.
Bibcode: 2019ApJ...887..270K
Altcode:
  No abstract at ADS

Title: Combined Next-Generation X-ray and EUV Observations with the
    FIERCE Mission Concept
Authors: Shih, A. Y.; Glesener, L.; Christe, S.; Reeves, K.; Gburek,
   S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis,
   B. R.; Drake, J. F.; Goetz, K.; Golub, L.; Guidoni, S. E.; Inglis,
   A.; Hannah, I. G.; Holman, G.; Hayes, L.; Ireland, J.; Kerr, G. S.;
   Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset,
   S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton,
   D. B.; Steslicki, M.; Woods, T. N.
Bibcode: 2019AGUFMSH33A..08S
Altcode:
  While there have been significant advances in our understanding
  of impulsive energy release at the Sun through the combination
  of RHESSI X-ray observations and SDO/AIA EUV observations, there
  is a clear science need for significantly improved X-ray and EUV
  observations. These new observations must capture the full range
  of emission in flares and CMEs (e.g., faint coronal sources near
  bright chromospheric sources), connect the intricate evolution of
  energy release with dynamic changes in the configuration of plasma
  structures, and identify the signatures of impulsive energy release in
  even the quiescent Sun. The Fundamentals of Impulsive Energy Release
  in the Corona Explorer ( FIERCE ) MIDEX mission concept makes these
  observations by combining the two instruments previously proposed on the
  FOXSI SMEX mission concept - a focusing hard X-ray spectroscopic imager
  and a soft X-ray spectrometer - with a high-resolution EUV imager that
  will not saturate for even intense flares. All instruments observe at
  high cadence to capture the initiation of solar transient events and
  the fine time structure within events. FIERCE would launch in mid-2025,
  near the peak of the next solar cycle, which is also well timed with
  perihelions of Parker Solar Probe and Solar Orbiter.

Title: The Sensitivity of AIA Observations to Coronal Heating
    Parameters
Authors: Schonfeld, S.; Klimchuk, J. A.
Bibcode: 2019AGUFMSH41F3323S
Altcode:
  We explore the effects of changing heating parameters in closed
  coronal loops on the intensity of Atmospheric Imaging Assembly (AIA)
  observations. Using EBTEL (Enthalpy-Based Thermal Evolution of Loops)
  hydrodynamic simulations, we produce realizations of coronal loops
  with a series of heating events randomly drawn from a power law
  distribution. We repeat this procedure for multiple loop lengths,
  heating intensities, and characteristic heating frequencies to
  investigate how each of these parameters individually and in combination
  influences observable loop properties. We generate AIA intensities from
  the corona and transition region for each realization. The variations
  within and between models generated with these different parameters
  illustrate the sensitivity of narrowband imaging to the details of
  coronal heating. Using insights from this analysis, we generate images
  of observed coronal active regions using the GX Simulator SSW IDL
  package and interpret the causes of discrepancies between the models
  and observations.

Title: Study of Type III Solar Radio Bursts in Nanoflares
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
Bibcode: 2019AGUFMSH23C3337C
Altcode:
  The heating mechanisms responsible for the million-degree solar corona
  remain one of the most intriguing problems in space science. It is
  widely agreed, that the ubiquitous presence of reconnection events and
  the associated impulsive heating (nanoflares) are a strong candidate in
  solving this problem [Klimchuk J.A., 2015 and references therein]. <P
  />Whether nanoflares accelerate energetic particles like full-sized
  flares is unknown. The lack of strong emission in hard X-rays suggests
  that the quantity of highly energetic particles is small. There could,
  however, be large numbers of mildly energetic particles (~ 10 keV). We
  investigate such particles by searching for the type III radio bursts
  that they may produce. If energetic electron beams propagating along
  magnetic field lines generate a bump-on-tail instability, they will
  produce Langmuir waves, which can then interact with other particles
  and waves to give rise to emission at the local plasma frequency and
  its first harmonic. Type III bursts are characteristically known
  to exhibit high frequency drifts as the beam propagates through a
  density gradient. The time-lag technique that was developed to study
  subtle delays in light curves from different EUV channels [Viall &amp;
  Klimchuk 2012] can also be used to detect subtle delays at different
  radio frequencies. We have modeled the expected radio emission from
  nanoflares, which we used to test and calibrate the technique. We are
  applying the technique to actual radio observations from VLA (Very Large
  Array), MWA (Murchison Widefield Array) and seeking data from LOFAR
  (Low-Frequency Array) as well. We also plan to use data from the PSP
  (Parker Solar Probe) to look for similar reconnection signatures in
  the Solar Wind. Our goal is to determine whether nanoflares accelerate
  energetic particles and to determine their properties. The results
  will have important implications for both the particle acceleration
  and reconnection physics.

Title: The magnetic structure and electrodynamics of the emerging
    solar wind
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
   Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
   Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; Dudok de Wit, T.;
   Eastwood, J. P.; Webster, J.; Farrell, W. M.; Fong, C.; Goetz, K.;
   Goldstein, M. L.; Goodrich, K.; Harvey, P.; Horbury, T. S.; Howes,
   G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
   K. E.; Krasnoselskikh, V.; Krucker, S.; Laker, R.; Larson, D. E.;
   MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Martinez Oliveros,
   J. C.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F.;
   Phan, T.; Pulupa, M.; Raouafi, N. E.; Salem, C. S.; Stansby, D.;
   Stevens, M. L.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
Bibcode: 2019AGUFMSH11A..05B
Altcode:
  Convection and rotation drive the solar dynamo and, ultimately,
  provide the mechanical energy flux required to heat the solar corona
  and accelerate the solar wind. However, the way in which energy is then
  dissipated to heat the corona and wind are not well understood. Some
  energization models invoke non-thermal energy flux imparted by plasma
  Alfvén waves, while others rely on a carpet of small nano-flares as
  energy input, however these models have been unconstrained by direct
  measurements of the solar wind near its origin. Here we use in situ
  measurements from the FIELDS instrument suite during the first solar
  encounter (E1) at 35.7 solar radii (Rs) of the NASA Parker Solar
  Probe (PSP) mission to reveal the magnetic structure and kinetics of
  slow Alfvénic solar wind emerging from a small, equatorial coronal
  hole. Our measurements show that, at solar minimum, the slow wind can
  escape from above the low-lying, complex magnetic structures of the
  equatorial streamer belt, carrying a magnetic field that is highly
  dynamic, exhibiting polarity reversals on timescales from seconds
  to hours. These rapidly oscillating field structures are associated
  with clustered radial jets of plasma in which the energy flux is
  dramatically enhanced and turbulence levels are higher. Time intervals
  between groups of jets indicate a solar wind that is steady with a
  mostly radial magnetic field and relatively low levels of Alfvénic
  turbulent fluctuations. This 'quiet' wind however shows clear signatures
  of plasma micro-instabilities associated with ion and electron beams
  and velocity-space structure.

Title: The Onset and Development of 3D Magnetic Reconnection in the
Solar Corona: New Insights
Authors: Klimchuk, J. A.; Daldorff, L. K. S.; Leake, J. E.; Knizhnik,
   K. J.
Bibcode: 2019AGUFMSH52A..08K
Altcode:
  Magnetic reconnection, one of the most important processes in the
  universe, is believed to be initiated in most cases by the tearing
  instability of an electric current sheet. Whether and how the tearing
  develops nonlinearly and releases substantial amounts of magnetic
  free energy depends on the relative growth rates of the different
  normal modes that are allowed in the system. In the general case
  where there is a guide field—in which the field rotates across the
  sheet by a shear angle—there exist both parallel modes centered
  on the sheet and oblique modes offset from it. We have performed a
  series of resistive magnetohydrodynamic (MHD) simulations of weakly
  perturbed, triply periodic current sheets with conditions appropriate
  to the solar corona to study how the different modes evolve and
  interact. We find three primary evolutionary paths: (1) a parallel
  mode dominates and saturates at a level that releases only a small
  amount of energy, but is large enough to choke off the growth of other
  modes; (2) parallel and oblique modes grow at nearly the same rate
  and interact violently as they become nonlinear, releasing a large
  amount of energy; (3) subharmonics of the fastest growing parallel
  mode induce a coalescence of islands, also releasing a large amount
  of energy. Which path will be taken depends in a predictable way on
  the length, thickness, and shear angle of the current sheet, as well
  as the resistivity. A critical issue is whether the wavelength of the
  fastest growing mode is longer or shorter than the current sheet,
  i.e., whether subharmonics exist. We expect all three behaviors on
  the Sun. These results have important implications for the question
  of reconnection onset. Observed phenomena require the buildup of high
  levels of magnetic stress before reconnection switches on to release
  the stored magnetic energy. Physical details of these simulations will
  be discussed in a companion presentation by Daldorff et al.

Title: The Onset and Development of 3D Magnetic Reconnection in the
Solar Corona: Important Physical Details
Authors: Daldorff, L. K. S.; Leake, J. E.; Klimchuk, J. A.; Knizhnik,
   K. J.
Bibcode: 2019AGUFMSH53B3366D
Altcode:
  Magnetic reconnection, one of the most important processes in the
  universe, is believed to be initiated in most cases by the tearing
  instability of an electric current sheet. Whether and how the tearing
  develops nonlinearly and releases substantial amounts of magnetic
  free energy is a crucial unanswered question. Using a series of
  resistive magnetohydrodynamic (MHD) simulations of weakly perturbed,
  triply periodic current sheets with conditions appropriate to the
  solar corona, we have found that the answer depends on the relative
  growth rates of the different normal modes that are allowed in
  the system. Dramatically different evolutionary paths are possible
  depending on the length, thickness, and shear angle of the current
  sheet, as well as the resistivity. The basic results are discussed in
  the companion presentation by Klimchuk et al. Here, we delve into the
  important physical details underpinning the complex behavior.

Title: FIERCE Science: Expected Results From a High-Energy
    Medium-Class Explorer
Authors: Glesener, L.; Shih, A. Y.; Christe, S.; Reeves, K.; Gburek,
   S.; Alaoui, M.; Allred, J. C.; Baumgartner, W.; Caspi, A.; Dennis,
   B. R.; Drake, J. F.; Golub, L.; Goetz, K.; Guidoni, S. E.; Hannah,
   I. G.; Hayes, L.; Holman, G.; Inglis, A.; Ireland, J.; Kerr, G. S.;
   Klimchuk, J. A.; Krucker, S.; McKenzie, D. E.; Moore, C. S.; Musset,
   S.; Reep, J. W.; Ryan, D.; Saint-Hilaire, P.; Savage, S. L.; Seaton,
   D. B.; Steslicki, M.; Woods, T. N.
Bibcode: 2019AGUFMSH31C3313G
Altcode:
  A variety of individual X-ray and EUV instruments have probed
  high-energy aspects of the Sun over the decades, each contributing
  pieces to the puzzles of the energization, heating, and acceleration of
  solar plasma and particles. But fundamental difficulties in sensitivity
  and dynamic range impart big challenges in probing the details of
  particle acceleration sites, understanding how eruptions and flares are
  initiated, and tracking the intricacies of energy transfer as flares
  evolve. The Fundamentals of Impulsive Energy Release in the Corona
  Explorer ( FIERCE ) mission will make substantial leaps forward in
  these scientific ventures by combining a variety of instruments into
  one platform, each optimized to have high sensitivity and dynamic
  range. FIERCE is a proposed NASA Heliophysics Medium-Class Explorer
  that will investigate high-energy solar phenomena across a variety
  of spectral and spatial dimensions. It combines hard X-ray imaging
  spectroscopy (via focusing, for the first time for a solar-dedicated
  spacecraft), spatially integrated soft X-ray spectroscopy, and fast,
  high-resolution extreme ultraviolet imaging at coronal and flare
  temperatures. FIERCE uses this array of instruments to make important
  contributions toward probing the genesis of space weather events,
  the acceleration of particles, the transport of flare energy, and the
  heating of the corona. Here, we present some of the expected science
  outcomes for the FIERCE observatory, concentrating on the ways in which
  FIERCE can probe confined and eruptive events, particle acceleration
  everywhere it may occur on the Sun, and the connections of solar
  high-energy phenomena to the heliosphere.

Title: The Distinction Between Thermal Nonequilibrium and Thermal
    Instability
Authors: Klimchuk, James A.
Bibcode: 2019SoPh..294..173K
Altcode: 2019arXiv191111849K
  For some forms of steady heating, coronal loops are in a state of
  thermal nonequilibrium and evolve in a manner that includes accelerated
  cooling, often resulting in the formation of a cold condensation. This
  is frequently confused with thermal instability, but the two are
  in fact fundamentally different. We explain the distinction and
  discuss situations where they may be interconnected. Large-amplitude
  perturbations, perhaps associated with MHD waves, likely play a role in
  explaining phenomena that have been attributed to thermal nonequilibrium
  but also seem to require cross-field communication.

Title: Highly structured slow solar wind emerging from an equatorial
    coronal hole
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
   Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
   Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; de Wit, T. Dudok;
   Eastwood, J. P.; Ergun, R. E.; Farrell, W. M.; Fong, C.; Goetz,
   K.; Goldstein, M.; Goodrich, K. A.; Harvey, P. R.; Horbury, T. S.;
   Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
   K. E.; Krasnoselskikh, V. V.; Krucker, S.; Laker, R.; Larson, D. E.;
   MacDowall, R. J.; Maksimovic, M.; Malaspina, D. M.; Martinez-Oliveros,
   J.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F. S.;
   Phan, T. D.; Pulupa, M.; Raouafi, N. E.; Salem, C.; Stansby, D.;
   Stevens, M.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
Bibcode: 2019Natur.576..237B
Altcode:
  During the solar minimum, when the Sun is at its least active, the solar
  wind<SUP>1,2</SUP> is observed at high latitudes as a predominantly fast
  (more than 500 kilometres per second), highly Alfvénic rarefied stream
  of plasma originating from deep within coronal holes. Closer to the
  ecliptic plane, the solar wind is interspersed with a more variable slow
  wind<SUP>3</SUP> of less than 500 kilometres per second. The precise
  origins of the slow wind streams are less certain<SUP>4</SUP>; theories
  and observations suggest that they may originate at the tips of helmet
  streamers<SUP>5,6</SUP>, from interchange reconnection near coronal hole
  boundaries<SUP>7,8</SUP>, or within coronal holes with highly diverging
  magnetic fields<SUP>9,10</SUP>. The heating mechanism required to
  drive the solar wind is also unresolved, although candidate mechanisms
  include Alfvén-wave turbulence<SUP>11,12</SUP>, heating by reconnection
  in nanoflares<SUP>13</SUP>, ion cyclotron wave heating<SUP>14</SUP>
  and acceleration by thermal gradients<SUP>1</SUP>. At a distance of
  one astronomical unit, the wind is mixed and evolved, and therefore
  much of the diagnostic structure of these sources and processes
  has been lost. Here we present observations from the Parker Solar
  Probe<SUP>15</SUP> at 36 to 54 solar radii that show evidence of
  slow Alfvénic solar wind emerging from a small equatorial coronal
  hole. The measured magnetic field exhibits patches of large,
  intermittent reversals that are associated with jets of plasma and
  enhanced Poynting flux and that are interspersed in a smoother and
  less turbulent flow with a near-radial magnetic field. Furthermore,
  plasma-wave measurements suggest the existence of electron and ion
  velocity-space micro-instabilities<SUP>10,16</SUP> that are associated
  with plasma heating and thermalization processes. Our measurements
  suggest that there is an impulsive mechanism associated with solar-wind
  energization and that micro-instabilities play a part in heating, and
  we provide evidence that low-latitude coronal holes are a key source
  of the slow solar wind.

Title: Spectroscopic Constraints on the Cross-sectional Asymmetry
    and Expansion of Active Region Loops
Authors: Kucera, T. A.; Young, P. R.; Klimchuk, J. A.; DeForest, C. E.
Bibcode: 2019ApJ...885....7K
Altcode:
  We explore the constraints that can be placed on the dimensions of
  coronal loops out of the plane of the sky by utilizing spectroscopic
  observations from the Hinode/EUV Imaging Spectrometer (EIS). The
  usual assumption is that loop cross sections are circular. Changes in
  intensity are assumed to be the result of changing density, filling
  factor, and/or point of view. In this work we instead focus on the
  possibility that the loop dimensions may be changing along the line of
  sight while the filling factor remains constant. We apply these ideas
  to two warm (5.5≲ {log}T({{K}})&lt; 6.2) loops observed by EIS in
  Active Region 11150 on 2011 February 6 with supporting observations
  from Solar Dynamics Observatory's Atmospheric Imaging Assembly and
  the Solar TErrestrial RElations Observatory-A's Extreme Ultraviolet
  Imager. Our results are generally consistent with nonexpanding loops
  but could also allow linear expansions of up to a factor of 2.5 along
  a 40 Mm section of one loop and up to a factor of 3.9 in another loop,
  both under the assumption that the filling factor is constant along
  the loop. Expansions in the plane of the sky over the same sections of
  the loops are 1.5 or less. For a filling factor of 1, the results of
  the analysis are consistent with circular cross sections but also with
  aspect ratios of 2 or greater. Count rate statistics are an important
  part of the uncertainties, but the results are also significantly
  dependent on radiometric calibration of EIS and the selection of the
  loop backgrounds.

Title: On Doppler Shift and Its Center-to-limb Variation in Active
    Regions in the Transition Region
Authors: Ghosh, Avyarthana; Klimchuk, James A.; Tripathi, Durgesh
Bibcode: 2019ApJ...886...46G
Altcode: 2019arXiv191012033G
  A comprehensive understanding of the structure of Doppler motions
  in the transition region including the center-to-limb variation and
  its relationship with the magnetic field structure is vital for the
  understanding of mass and energy transfer in the solar atmosphere. In
  this paper, we have performed such a study in an active region using
  the Si IV 1394 Å emission line recorded by the Interface Region Imaging
  Spectrograph and the line-of-sight photospheric magnetic field obtained
  by the Helioseismic and Magnetic Imager onboard the Solar Dynamics
  Observatory. The active region has two opposite polarity strong field
  regions separated by a weak field corridor, which widened as the active
  region evolved. On average, the strong field regions (corridor) show(s)
  redshifts of 5-10 (3-9) km s<SUP>-1</SUP> (depending on the date of
  observation). There is, however, a narrow lane in the middle of the
  corridor with near-zero Doppler shifts at all disk positions, suggesting
  that any flows there are very slow. The Doppler velocity distributions
  in the corridor seem to have two components—a low velocity component
  centered near 0 km s<SUP>-1</SUP> and a high-velocity component centered
  near 10 km s<SUP>-1</SUP>. The high-velocity component is similar to
  the velocity distributions in the strong field regions, which have just
  one component. Both exhibit a small center-to-limb variation and seem
  to come from the same population of flows. To explain these results,
  we suggest that the emission from the lower transition region comes
  primarily from warm type II spicules, and we introduce the idea of a
  “chromospheric wall”—associated with classical cold spicules—to
  account for a diminished center-to-limb variation.

Title: The Role of Asymmetries in Thermal Nonequilibrium
Authors: Klimchuk, James A.; Luna, Manuel
Bibcode: 2019ApJ...884...68K
Altcode: 2019arXiv190509767K
  Thermal nonequilibrium (TNE) is a fascinating situation that occurs
  in coronal magnetic flux tubes (loops) for which no solution to
  the steady-state fluid equations exists. The plasma is constantly
  evolving even though the heating that produces the hot temperatures
  does not. This is a promising explanation for isolated phenomena
  such as prominences, coronal rain, and long-period pulsating loops,
  but it may also have much broader relevance. As known for some time,
  TNE requires that the heating be both (quasi-)steady and concentrated
  at low coronal altitudes. Recent studies indicate that asymmetries are
  also important, with large enough asymmetries in the heating and/or
  cross-sectional area resulting in steady flow rather than TNE. Using
  reasonable approximations, we have derived two formulae for quantifying
  the conditions necessary for TNE. As a rough rule of thumb, the ratio
  of apex to footpoint heating rates must be less than about 0.1, and
  asymmetries must be less than about a factor of 3. The precise values
  are case-dependent. We have tested our formulae with 1D hydrodynamic
  loop simulations and find a very acceptable agreement. These results
  are important for developing physical insight about TNE and assessing
  how widespread it may be on the Sun.

Title: Achievements of Hinode in the first eleven years
Authors: Hinode Review Team; Al-Janabi, Khalid; Antolin, Patrick;
   Baker, Deborah; Bellot Rubio, Luis R.; Bradley, Louisa; Brooks,
   David H.; Centeno, Rebecca; Culhane, J. Leonard; Del Zanna, Giulio;
   Doschek, George A.; Fletcher, Lyndsay; Hara, Hirohisa; Harra,
   Louise K.; Hillier, Andrew S.; Imada, Shinsuke; Klimchuk, James A.;
   Mariska, John T.; Pereira, Tiago M. D.; Reeves, Katharine K.; Sakao,
   Taro; Sakurai, Takashi; Shimizu, Toshifumi; Shimojo, Masumi; Shiota,
   Daikou; Solanki, Sami K.; Sterling, Alphonse C.; Su, Yingna; Suematsu,
   Yoshinori; Tarbell, Theodore D.; Tiwari, Sanjiv K.; Toriumi, Shin;
   Ugarte-Urra, Ignacio; Warren, Harry P.; Watanabe, Tetsuya; Young,
   Peter R.
Bibcode: 2019PASJ...71R...1H
Altcode:
  Hinode is Japan's third solar mission following Hinotori (1981-1982)
  and Yohkoh (1991-2001): it was launched on 2006 September 22 and is in
  operation currently. Hinode carries three instruments: the Solar Optical
  Telescope, the X-Ray Telescope, and the EUV Imaging Spectrometer. These
  instruments were built under international collaboration with the
  National Aeronautics and Space Administration and the UK Science and
  Technology Facilities Council, and its operation has been contributed
  to by the European Space Agency and the Norwegian Space Center. After
  describing the satellite operations and giving a performance evaluation
  of the three instruments, reviews are presented on major scientific
  discoveries by Hinode in the first eleven years (one solar cycle long)
  of its operation. This review article concludes with future prospects
  for solar physics research based on the achievements of Hinode.

Title: The Role of Magnetic Helicity in Coronal Heating
Authors: Knizhnik, K. J.; Antiochos, S. K.; Klimchuk, J. A.; DeVore,
   C. R.
Bibcode: 2019ApJ...883...26K
Altcode: 2019arXiv190903768K
  One of the greatest challenges in solar physics is understanding
  the heating of the Sun’s corona. Most theories for coronal heating
  postulate that free energy in the form of magnetic twist/stress is
  injected by the photosphere into the corona where the free energy is
  converted into heat either through reconnection or wave dissipation. The
  magnetic helicity associated with the twist/stress, however, is expected
  to be conserved and appear in the corona. In previous works, we showed
  that the helicity associated with the small-scale twists undergoes
  an inverse cascade via stochastic reconnection in the corona and
  ends up as the observed large-scale shear of filament channels. Our
  “helicity condensation” model accounts for both the formation
  of filament channels and the observed smooth, laminar structure of
  coronal loops. In this paper, we demonstrate, using helicity- and
  energy-conserving numerical simulations of a coronal system driven
  by photospheric motions, that the model also provides a natural
  mechanism for heating the corona. We show that the heat generated by
  the reconnection responsible for the helicity condensation process is
  sufficient to account for the observed coronal heating. We study the
  role that helicity injection plays in determining coronal heating and
  find that, crucially, the heating rate is only weakly dependent on the
  net helicity preference of the photospheric driving. Our calculations
  demonstrate that motions with 100% helicity preference are least
  efficient at heating the corona; those with 0% preference are most
  efficient. We discuss the physical origins of this result and its
  implications for the observed corona.

Title: Frequency Agile Solar Radiotelescope
Authors: Bastian, Tim; Bain, H.; Bradley, R.; Chen, B.; Dahlin, J.;
   DeLuca, E.; Drake, J.; Fleishman, G.; Gary, D.; Glesener, L.; Guo,
   Fan; Hallinan, G.; Hurford, G.; Kasper, J.; Ji, Hantao; Klimchuk,
   J.; Kobelski, A.; Krucker, S.; Kuroda, N.; Loncope, D.; Lonsdale,
   C.; McTiernan, J.; Nita, G.; Qiu, J.; Reeves, K.; Saint-Hilaire, P.;
   Schonfeld, S.; Shen, Chengcai; Tun, S.; Wertheimer, D.; White, S.
Bibcode: 2019astro2020U..56B
Altcode:
  We describe the science objectives and technical requirements for a
  re-scoped Frequency Agile Solar Radiotelescope (FASR). FASR fulfills
  a long term community need for a ground-based, solar-dedicated, radio
  telescope - a next-generation radioheliograph - designed to perform
  ultra-broadband imaging spectropolarimetry.

Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
   Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Gburek, Szymon;
   Steslicki, Marek; Allred, Joel C.; Battaglia, Marina; Baumgartner,
   Wayne H.; Drake, James; Goetz, Keith; Grefenstette, Brian; Hannah,
   Iain; Holman, Gordon D.; Inglis, Andrew; Ireland, Jack; Klimchuk,
   James A.; Ishikawa, Shin-Nosuke; Kontar, Eduard; Massone, Anna-maria;
   Piana, Michele; Ramsey, Brian; Schwartz, Richard A.; Woods, Thomas N.;
   Chen, Bin; Gary, Dale E.; Hudson, Hugh S.; Kowalski, Adam; Warmuth,
   Alexander; White, Stephen M.; Veronig, Astrid; Vilmer, Nicole
Bibcode: 2019AAS...23422501C
Altcode:
  The Focusing Optics X-ray Solar Imager (FOXSI), a SMEX mission concept
  in Phase A, is the first-ever solar-dedicated, direct-imaging, hard
  X-ray telescope. FOXSI provides a revolutionary new approach to
  viewing explosive magnetic-energy release on the Sun by detecting
  signatures of accelerated electrons and hot plasma directly in
  and near the energy-release sites of solar eruptive events (e.g.,
  solar flares). FOXSI's primary science objective is to understand the
  mystery of how impulsive energy release leads to solar eruptions, the
  primary drivers of space weather at Earth, and how those eruptions are
  energized and evolve. FOXSI addresses three important science questions:
  (1) How are particles accelerated at the Sun? (2) How do solar plasmas
  get heated to high temperatures? (3) How does magnetic energy released
  on the Sun lead to flares and eruptions? These fundamental physics
  questions are key to our understanding of phenomena throughout
  the Universe from planetary magnetospheres to black hole accretion
  disks. FOXSI measures the energy distributions and spatial structure of
  accelerated electrons throughout solar eruptive events for the first
  time by directly focusing hard X-rays from the Sun. This naturally
  enables high imaging dynamic range, while previous instruments have
  typically been blinded by bright emission. FOXSI provides 20-100 times
  more sensitivity as well as 20 times faster imaging spectroscopy
  than previously available, probing physically relevant timescales
  (&lt;1 second) never before accessible. FOXSI's launch in July 2022
  is aligned with the peak of the 11-year solar cycle, enabling FOXSI
  to observe the many large solar eruptions that are expected to take
  place throughout its two-year mission.

Title: Constraints from Hinode/EIS on the Expansion of Active Region
    Loops Along the Line of Sight
Authors: Kucera, Therese A.; Young, Peter R.; Klimchuk, James A.;
   DeForest, Craig
Bibcode: 2019AAS...23411706K
Altcode:
  We explore the constraints that can be placed on the dimensions of
  coronal loops out of the plane of the sky by utilizing spectroscopic
  observations from the Hinode/EUV Imaging Spectrometer (EIS). The
  usual assumption is that loop cross sections are circular. Changes
  in intensity not constant with the measured width are assumed to be
  the result of changing density and/or filling factor. Here we instead
  focus on the possibility that the loop dimensions may be changing along
  the line of sight while the filling factor remains constant. We apply
  these ideas to two cool (5.5&lt;logT&lt;6.2) loops observed by EIS with
  supporting observations from Solar Dynamics Observatory's Atmospheric
  Imaging Assembly (SDO/AIA) and the Solar TErrestrial RElations
  Observatory-A's Extreme Ultraviolet Imager (STEREO-A/EUVI). Our
  results are generally consistent with non-expanding loops, but allow
  for line-of-sight expansion factors up to 3-4. The uncertainties are
  sizable and are driven by count rate statistics, radiometric calibration
  of EIS, and the selection of the loop backgrounds.

Title: Ultrahigh-Resolution Imaging of the Solar Corona using a
    Distributed Diffractive Telescope
Authors: Rabin, Douglas M.; Daw, Adrian N.; Denis, Kevin; Kamalabadi,
   Farzad; Klimchuk, James A.
Bibcode: 2019AAS...23410704R
Altcode:
  Several observational and theoretical considerations suggest that
  energy is often released in the solar corona on small spatial scales
  of order 100 km. It has been a longstanding goal of solar physics to
  subject this hypothesis to direct observational test. However, extreme
  ultraviolet (EUV) and soft x-ray (SXR) telescopes rarely approach
  diffraction-limited performance because conventional reflective optics
  of adequate size typically cannot be manufactured to the requisite
  figure accuracy. Diffractive optics can overcome the angular-resolution
  limitations of EUV/SXR mirrors. We describe a mission approach that
  employs diffractive optics and small satellites flying in formation
  to form a distributed solar telescope operating at EUV wavelengths.

Title: Study of Type III Radio Bursts in Nanoflares
Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.;
   Gary, Dale E.
Bibcode: 2019shin.confE..12C
Altcode:
  The heating mechanisms responsible for the million-degree solar corona
  remain one of the most intriguing problems in space science. It is
  widely agreed, that the ubiquitous presence of reconnection events and
  the associated impulsive heating (nanoflares) are a strong candidate in
  solving this problem [Klimchuk J.A., 2015 and references therein]. <P
  />Whether nanoflares accelerate energetic particles like full-sized
  flares is unknown. The lack of strong emission in hard X-rays suggests
  that the quantity of highly energetic particles is small. There could,
  however, be large numbers of mildly energetic particles ( 10 keV). We
  investigate such particles by searching for the type III radio bursts
  that they may produce. If energetic electron beams propagating along
  magnetic field lines generate a bump-on-tail instability, they will
  produce Langmuir waves, which can then interact with other particles
  and waves to give rise to emission at the local plasma frequency and
  its first harmonic. Type III bursts are characteristically known
  to exhibit high frequency drifts as the beam propagates through a
  density gradient. The time-lag technique that was developed to study
  subtle delays in light curves from different EUV channels [Viall &amp;
  Klimchuk 2012] can also be used to detect subtle delays at different
  radio frequencies. We have modeled the expected radio emission from
  nanoflares, which we used to test and calibrate the technique. We have
  begun applying the technique to actual radio observations from VLA
  (Very Large Array) and seeking data from MWA (Murchison Widefield Array)
  as well. We also plan to use data from the PSP(Parker Solar Probe) to
  look for similar reconnection signatures in the Solar Wind. Our goal is
  to determine whether nanoflares accelerate energetic particles and to
  determine their properties. The results will have important implications
  for both the particle acceleration and reconnection physics."

Title: Studying Coronal Heating with Data Driven Active Region
    Modeling
Authors: Schonfeld, Samuel J.; Klimchuk, James
Bibcode: 2019shin.confE.158S
Altcode:
  Successfully forecasting coronal emission requires an accurate
  understanding of the mechanism(s) that heat the corona to temperatures
  well above a million degrees Kelvin. We explore the relevance of a range
  of coronal heating models by varying the physical parameters needed
  to simulate EUV emission from an active region. We use GX Simulator
  to generate synthetic images from non-linear force-free field (NLFFF)
  magnetic field extrapolations that are filled with plasma using EBTEL
  (Enthalpy-Based Thermal Evolution of Loops) hydrodynamic simulations
  and compare these with AIA images. Modifying the heating function,
  heating event recurrence time, and treatment of the transition region
  allows us to quantify the effects of these parameters on the observed
  coronal emission. We discuss the implications of these findings on
  the nature of heating properties in the magnetically closed corona.

Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
   Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
   J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
   A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
   W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2019BAAS...51c...5J
Altcode: 2019astro2020T...5J
  This is a group white paper of 100 authors (each with explicit
  permission via email) from 51 institutions on the topic of magnetic
  reconnection which is relevant to 6 thematic areas. Grand challenges
  and research opportunities are described in observations, numerical
  modeling and laboratory experiments in the upcoming decade.

Title: ICSF: Intensity Conserving Spectral Fitting
Authors: Klimchuk, James A.; Patsourakos, Spiros; Tripathi, Durgesh
Bibcode: 2019ascl.soft03007K
Altcode:
  ICSF (Intensity Conserving Spectral Fitting) "corrects" (x,y) data in
  which the ordinate represents the average of a quantity over a finite
  interval in the abscissa. A typical example is spectral data, where
  the average intensity over a wavelength bin (the measured quantity)
  is assigned to the center of the bin. If the profile is curved, the
  average will be different from the discrete value at the bin center
  location. ICSF, written in IDL and available separately and as part of
  SolarSoft (ascl:1208.013), corrects the intensity using an iterative
  procedure and cubic spline. The corrected intensity equals the "true"
  intensity at bin center, rather than the average over the bin. Unlike
  other methods that are restricted to a single fitting function,
  typically a spline, ICSF can be used with any function, such as a
  cubic spline or a Gaussian, with slight changes to the code.

Title: Hard X-Ray Constraints on Small-scale Coronal Heating Events
Authors: Marsh, Andrew J.; Smith, David M.; Glesener, Lindsay;
   Klimchuk, James A.; Bradshaw, Stephen J.; Vievering, Juliana; Hannah,
   Iain G.; Christe, Steven; Ishikawa, Shin-nosuke; Krucker, Säm
Bibcode: 2018ApJ...864....5M
Altcode: 2018arXiv180802630M
  Much evidence suggests that the solar corona is heated impulsively,
  meaning that nanoflares may be ubiquitous in quiet and active regions
  (ARs). Hard X-ray (HXR) observations with unprecedented sensitivity
  &gt;3 keV are now enabled by focusing instruments. We analyzed data
  from the Focusing Optics X-ray Solar Imager (FOXSI) rocket and the
  Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft to constrain
  properties of AR nanoflares simulated by the EBTEL field-line-averaged
  hydrodynamics code. We generated model X-ray spectra by computing
  differential emission measures for homogeneous nanoflare sequences
  with heating amplitudes H <SUB>0</SUB>, durations τ, delay times
  between events t <SUB> N </SUB>, and filling factors f. The single
  quiescent AR observed by FOXSI-2 on 2014 December 11 is well fit by
  nanoflare sequences with heating amplitudes 0.02 erg cm<SUP>-3</SUP>
  s<SUP>-1</SUP> &lt;H <SUB>0</SUB> &lt; 13 erg cm<SUP>-3</SUP>
  s<SUP>-1</SUP> and a wide range of delay times and durations. We
  exclude delays between events shorter than ∼900 s at the 90%
  confidence level for this region. Three of five regions observed
  by NuSTAR on 2014 November 1 are well fit by homogeneous nanoflare
  models, while two regions with higher fluxes are not. Generally,
  the NuSTAR count spectra are well fit by nanoflare sequences with
  smaller heating amplitudes, shorter delays, and shorter durations than
  the allowed FOXSI-2 models. These apparent discrepancies are likely
  due to differences in spectral coverage between the two instruments
  and intrinsic differences among the regions. Steady heating (t <SUB>
  N </SUB> = τ) was ruled out with &gt;99% confidence for all regions
  observed by either instrument.

Title: Shifting and broadening of coronal spectral lines by nanoflare
    heating
Authors: López Fuentes, M.; Klimchuk, J. A.
Bibcode: 2018BAAA...60..207L
Altcode:
  The heating of the solar corona by nanoflares is one of the most
  succesful theories in recent times to explain the wide variety of
  available observations. In a recent work we developed and analyzed
  a model based on coronal loops formed by elemental magnetic strands
  that interact with each other, reconnecting and consequently heating
  the plasma. Each of these heating events (or cluster of linked events)
  is considered a nanoflare. In a later article we showed that the model
  reproduces the main statistical characteristics of observed coronal
  loop lightcurves and the typical emission measure distributions obtained
  from observations. One of the predictions of the model is the presence
  of intense plasma flows in the heated strands. The summed radiative
  emission of strands at different temperatures and velocities produce
  spectral lines of coronal ions with characteristic broadenings and
  Doppler shifts. In this work, we study the contribution to emission
  of flows with different temperatures and velocities obtained with our
  model and construct synthetic spectral lines that we plan to compare
  in the future with observational results.

Title: Study of Type III Radio Bursts in Nanoflares
Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.
Bibcode: 2018shin.confE..18C
Altcode:
  The heating mechanisms responsible for the million-degree solar corona
  remain one of the most intriguing problems in space science. It is
  widely agreed, that the ubiquitous presence of reconnection events and
  the associated impulsive heating (nanoflares) are a strong candidate
  in solving this problem [Klimchuk J.A., 2015 and references therein].

Title: Power-Law Statistics of Driven Reconnection in the Magnetically
    Closed Corona
Authors: Knizhnik, Kalman Joshua; Uritsky, Vadim M.; Klimchuk, James
   A.; DeVore, C. Richard
Bibcode: 2018tess.conf21164K
Altcode:
  Numerous observations have revealed that power-law distributions
  are ubiquitous in energetic solar processes. Hard X-rays, soft
  X-rays, extreme ultraviolet radiation, and radio waves all display
  power-law frequency distributions. Since magnetic reconnection is
  the driving mechanism for many energetic solar phenomena, it is
  likely that reconnection events themselves display such power-law
  distributions. In this work, we perform numerical simulations of the
  solar corona driven by simple convective motions at the photospheric
  level. Using temperature changes, current distributions, and Poynting
  fluxes as proxies for heating, we demonstrate that energetic events
  occurring in our simulation display power-law frequency distributions,
  with slopes in good agreement with observations. We suggest that
  the braiding-associated reconnection in the corona can be understood
  in terms of a self-organized criticality model driven by convective
  rotational motions similar to those observed at the photosphere.

Title: The Onset of Magnetic Reconnection in the Solar Corona
Authors: Leake, James Edward; Daldorff, Lars K. S.; Klimchuk, James
   A.; Knizhnik, Kalman Joshua
Bibcode: 2018tess.conf10418L
Altcode:
  Understanding why the release of magnetic energy on the Sun only
  occurs after substantial stresses have been built up is a fundamental
  question. We investigate the onset of magnetic reconnection in
  the corona, initially focusing on the linear growth and non-linear
  interaction of both parallel and oblique tearing modes in 3D sheared
  coronal current sheets. We then study the critical parameters of
  current sheet thickness and magnetic field angle. We also investigate
  the release of magnetic energy in the corona by magnetic reconnection
  driven by realistic surface motions.

Title: The Focusing Optics X-ray Solar Imager (FOXSI)
Authors: Christe, Steven; Shih, Albert Y.; Krucker, Sam; Glesener,
   Lindsay; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel C.; Chen,
   Bin; Battaglia, Marina; Drake, James Frederick; Gary, Dale E.; Goetz,
   Keith; Gburek, Szymon; Grefenstette, Brian; Hannah, Iain G.; Holman,
   Gordon; Hudson, Hugh S.; Inglis, Andrew R.; Ireland, Jack; Ishikawa,
   Shin-nosuke; Klimchuk, James A.; Kontar, Eduard; Kowalski, Adam F.;
   Massone, Anna Maria; Piana, Michele; Ramsey, Brian; Schwartz, Richard;
   Steslicki, Marek; Ryan, Daniel; Warmuth, Alexander; Veronig, Astrid;
   Vilmer, Nicole; White, Stephen M.; Woods, Thomas N.
Bibcode: 2018tess.conf40444C
Altcode:
  We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
  (SMEX) Heliophysics mission that is currently undergoing a Phase A
  concept study. FOXSI will provide a revolutionary new perspective on
  energy release and particle acceleration on the Sun. FOXSI's primary
  instrument, the Direct Spectroscopic Imager (DSI), is a direct imaging
  X-ray spectrometer with higher dynamic range and better than 10x the
  sensitivity of previous instruments. Flown on a 3-axis-stabilized
  spacecraft in low-Earth orbit, DSI uses high-angular-resolution
  grazing-incidence focusing optics combined with state-of-the-art
  pixelated solid-state detectors to provide direct imaging of solar hard
  X-rays for the first time. DSI is composed of a pair of X-ray telescopes
  with a 14-meter focal length enabled by a deployable boom. DSI has a
  field of view of 9 arcminutes and an angular resolution of better than 8
  arcsec FWHM; it will cover the energy range from 3 up to 50-70 keV with
  a spectral resolution of better than 1 keV. DSI will measure each photon
  individually and will be able to create useful images at a sub-second
  temporal resolution. FOXSI will also measure soft x-ray emission down
  to 0.8 keV with a 0.25 keV resolution with its secondary instrument,
  the Spectrometer for Temperature and Composition (STC) provided by
  the Polish Academy of Sciences. Making use of an attenuator-wheel and
  high-rate-capable detectors, FOXSI will be able to observe the largest
  flares without saturation while still maintaining the sensitivity to
  detect X-ray emission from weak flares, escaping electrons, and hot
  active regions. This presentation will cover the data products and
  software that can be expected from FOXSI and how they could be used
  by the community.

Title: The Case for Spectroscopic Observations of Very Hot Plasmas
Authors: Klimchuk, James A.; Daldorff, Lars K. S.; Liu, Yi-Hsin;
   Brosius, Jeffrey W.; Daw, Adrian Nigel; Leake, James Edward
Bibcode: 2018tess.conf11003K
Altcode:
  Explosive magnetic energy release is responsible for many solar
  phenomena, ranging from coronal heating, to jets, to CMEs and
  flares. Despite its importance, many of the basic details of how this
  works are still not well established. In order to make significant
  progress, we must observe the actual energy release process. That means
  measuring plasmas at high temperatures (&gt; 5 MK). Most existing
  coronal observations are of plasma that has cooled dramatically,
  after vital information has been lost, or, worse yet, plasma that has
  evaporated from the chromosphere and is only an indirect bi-product of
  the energy release. Spectroscopic observations are especially valuable
  because of their potential for diagnosing flows, temperatures, and
  densities. Furthermore, only with spectroscopy can we disentangle the
  disparate plasmas that are invariably present along optically-thin
  lines-of-sight. I will discuss these points in more detail, review the
  pros and cons of different wavelength regimes, and present preliminary
  results on synthetic line profiles from a particle-in-cell (PIC)
  simulation of magnetic reconnection.

Title: The Role of Asymmetries in Thermal Non-Equilibrium
Authors: Klimchuk, James A.; Luna Bennasar, Manuel
Bibcode: 2018tess.conf22205K
Altcode:
  Thermal non-equilibrium (TNE) is a fascinating situation that applies
  when quasi-steady coronal heating, including impulsive heating with a
  high repetition frequency, is concentrated at sufficiently low altitudes
  in the corona. No equilibrium exists, and as the atmosphere "searches"
  for one, it undergoes convulsions that typically involve the formation
  of a cold condensation. This is the likely explanation of prominences
  and coronal rain. Under the right conditions, a condensation does not
  fully form, and it has been suggested that such conditions may be
  rather common. Asymmetries in heating or flux tube cross-sectional
  area can have a major impact on the behavior. If the asymmetries
  are large enough, a steady flow develops rather than TNE. We here
  present an analytical formula for predicting steady flow vs. TNE,
  and we compare it with 1D hydrodynamic simulations.

Title: Magnetic Braids in Eruptions of a Spiral Structure in the
    Solar Atmosphere
Authors: Huang, Zhenghua; Xia, Lidong; Nelson, Chris J.; Liu, Jiajia;
   Wiegelmann, Thomas; Tian, Hui; Klimchuk, James A.; Chen, Yao; Li, Bo
Bibcode: 2018ApJ...854...80H
Altcode: 2018arXiv180105967H
  We report on high-resolution imaging and spectral observations of
  eruptions of a spiral structure in the transition region, which
  were taken with the Interface Region Imaging Spectrograph, and the
  Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic
  Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The eruption
  coincided with the appearance of two series of jets, with velocities
  comparable to the Alfvén speeds in their footpoints. Several pieces of
  evidence of magnetic braiding in the eruption are revealed, including
  localized bright knots, multiple well-separated jet threads, transition
  region explosive events, and the fact that all three of these are
  falling into the same locations within the eruptive structures. Through
  analysis of the extrapolated 3D magnetic field in the region, we found
  that the eruptive spiral structure corresponded well to locations
  of twisted magnetic flux tubes with varying curl values along their
  lengths. The eruption occurred where strong parallel currents,
  high squashing factors, and large twist numbers were obtained. The
  electron number density of the eruptive structure is found to be ∼3 ×
  10<SUP>12</SUP> cm<SUP>-3</SUP>, indicating that a significant amount
  of mass could be pumped into the corona by the jets. Following the
  eruption, the extrapolations revealed a set of seemingly relaxed loops,
  which were visible in the AIA 94 Å channel, indicating temperatures
  of around 6.3 MK. With these observations, we suggest that magnetic
  braiding could be part of the mechanisms explaining the formation of
  solar eruption and the mass and energy supplement to the corona.

Title: Power-law Statistics of Driven Reconnection in the Magnetically
    Closed Corona
Authors: Knizhnik, K. J.; Uritsky, V. M.; Klimchuk, J. A.; DeVore,
   C. R.
Bibcode: 2018ApJ...853...82K
Altcode: 2018arXiv180105245K
  Numerous observations have revealed that power-law distributions
  are ubiquitous in energetic solar processes. Hard X-rays, soft
  X-rays, extreme ultraviolet radiation, and radio waves all display
  power-law frequency distributions. Since magnetic reconnection is
  the driving mechanism for many energetic solar phenomena, it is
  likely that reconnection events themselves display such power-law
  distributions. In this work, we perform numerical simulations of the
  solar corona driven by simple convective motions at the photospheric
  level. Using temperature changes, current distributions, and Poynting
  fluxes as proxies for heating, we demonstrate that energetic events
  occurring in our simulation display power-law frequency distributions,
  with slopes in good agreement with observations. We suggest that
  the braiding-associated reconnection in the corona can be understood
  in terms of a self-organized criticality model driven by convective
  rotational motions similar to those observed at the photosphere.

Title: Dressing the Coronal Magnetic Extrapolations of Active Regions
    with a Parameterized Thermal Structure
Authors: Nita, Gelu M.; Viall, Nicholeen M.; Klimchuk, James A.;
   Loukitcheva, Maria A.; Gary, Dale E.; Kuznetsov, Alexey A.; Fleishman,
   Gregory D.
Bibcode: 2018ApJ...853...66N
Altcode:
  The study of time-dependent solar active region (AR) morphology and
  its relation to eruptive events requires analysis of imaging data
  obtained in multiple wavelength domains with differing spatial and
  time resolution, ideally in combination with 3D physical models. To
  facilitate this goal, we have undertaken a major enhancement of our
  IDL-based simulation tool, GX_Simulator, previously developed for
  modeling microwave and X-ray emission from flaring loops, to allow it
  to simulate quiescent emission from solar ARs. The framework includes
  new tools for building the atmospheric model and enhanced routines
  for calculating emission that include new wavelengths. In this paper,
  we use our upgraded tool to model and analyze an AR and compare the
  synthetic emission maps with observations. We conclude that the modeled
  magneto-thermal structure is a reasonably good approximation of the
  real one.

Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
   Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
   Drake, J. F.; Gary, D. E.; Goetz, K.; Gburek, S.; Grefenstette, B.;
   Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland,
   J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.;
   Massone, A. M.; Piana, M.; Ramsey, B.; Schwartz, R.; Steslicki, M.;
   Turin, P.; Ryan, D.; Warmuth, A.; Veronig, A.; Vilmer, N.; White,
   S. M.; Woods, T. N.
Bibcode: 2017AGUFMSH44A..07C
Altcode:
  We present FOXSI (Focusing Optics X-ray Solar Imager), a Small Explorer
  (SMEX) Heliophysics mission that is currently undergoing a Phase A
  concept study. FOXSI will provide a revolutionary new perspective
  on energy release and particle acceleration on the Sun. FOXSI is
  a direct imaging X-ray spectrometer with higher dynamic range and
  better than 10x the sensitivity of previous instruments. Flown
  on a 3-axis-stabilized spacecraft in low-Earth orbit, FOXSI uses
  high-angular-resolution grazing-incidence focusing optics combined
  with state-of-the-art pixelated solid-state detectors to provide direct
  imaging of solar hard X-rays for the first time. FOXSI is composed of
  a pair of x-ray telescopes with a 14-meter focal length enabled by a
  deployable boom. Making use of a filter-wheel and high-rate-capable
  solid-state detectors, FOXSI will be able to observe the largest flares
  without saturation while still maintaining the sensitivity to detect
  x-ray emission from weak flares, escaping electrons, and hot active
  regions. This mission concept is made possible by past experience with
  similar instruments on two FOXSI sounding rocket flights, in 2012 and
  2014, and on the HEROES balloon flight in 2013. FOXSI's hard X-ray
  imager has a field of view of 9 arcminutes and an angular resolution
  of better than 8 arcsec; it will cover the energy range from 3 up to
  50-70 keV with a spectral resolution of better than 1 keV; and it will
  have sub-second temporal resolution.

Title: Anticipated Results from the FOXSI SMEX Mission
Authors: Shih, A. Y.; Christe, S.; Krucker, S.; Glesener, L.;
   Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen, B.;
   Drake, J. F.; Gary, D. E.; Gburek, S.; Goetz, K.; Grefenstette, B.;
   Gubarev, M.; Hannah, I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.;
   Ireland, J.; Ishikawa, S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski,
   A. F.; Massone, A. M.; Piana, M.; Ramsey, B.; Ryan, D.; Schwartz,
   R.; Steslicki, M.; Turin, P.; Veronig, A.; Vilmer, N.; Warmuth, A.;
   White, S. M.; Woods, T. N.
Bibcode: 2017AGUFMSH43C..03S
Altcode:
  While there have been significant advances in our understanding
  of impulsive energy release at the Sun since the advent of RHESSI
  observations, there is a clear need for new X-ray observations that
  can capture the full range of emission in flares (e.g., faint coronal
  sources near bright chromospheric sources), follow the intricate
  evolution of energy release and changes in morphology, and search
  for the signatures of impulsive energy release in even the quiescent
  Sun. The FOXSI Small Explorer (SMEX) mission, currently undergoing a
  Phase A concept study, combines state-of-the-art grazing-incidence
  focusing optics with pixelated solid-state detectors to provide
  direct imaging of hard X-rays for the first time on a solar
  observatory. FOXSI's X-ray observations will provide quantitative
  information on (1) the non-thermal populations of accelerated electrons
  and (2) the thermal plasma distributions at the high temperatures
  inaccessible through other wavelengths. FOXSI's major science questions
  include: Where are electrons accelerated and on what time scales? Where
  do escaping flare-accelerated electrons originate? What is the energy
  input of accelerated electrons into the chromosphere and corona? How
  much do flare-like processes heat the corona above active regions? Here
  we present examples with simulated observations to show how FOXSI's
  capabilities will address and resolve these and other questions.

Title: Conditions for Thermal Non-Equilibrium
Authors: Klimchuk, J. A.; Luna Bennasar, M.
Bibcode: 2017AGUFMSH43A2798K
Altcode:
  Thermal non-equilibrium (TNE) is a fascinating phenomenon in which
  perfectly steady heating produces inherently dynamic behavior in coronal
  loops. Sometimes, but not always, a condensation of cold ( 10,000 K)
  material is produced. This is the likely explanation of coronal rain,
  prominences, and "pulsating loops," and has been suggested to have
  additional application in active regions and even helmet streamers. A
  primary requirement of TNE is that steady heating must be concentrated
  at low altitudes in the corona. Another requirement is that asymmetries
  in the heating and/or loop cross sectional area cannot be too great. We
  present theoretical formulas for predicting whether TNE will occur and
  whether the associated condensations will be full or incomplete. The
  predictions are compared against hydrodynamic simulations.

Title: The Onset of Magnetic Reconnection
Authors: Daldorff, L. K. S.; Klimchuk, J. A.; Leake, J. E.; Knizhnik,
   K. J.
Bibcode: 2017AGUFMSH11B2454D
Altcode:
  A fundamental question concerning the release of the magnetic energy
  on the Sun is why it occurs only after substantial stresses have built
  up in the field. If reconnection were to occur readily, the released
  energy would be insufficient to explain coronal heating, CMEs, flares,
  jets, spicules, etc. How can we explain this switch-on property? What
  is the physical nature of the onset conditions? One idea involves the
  ``secondary instability'' of current sheets, which switches on when
  the rotation of the magnetic field across a current sheet reaches
  a critical angle. Such conditions would occur, for example, at the
  boundaries of flux tubes that become tangled and twisted by turbulent
  photospheric convection. Other ideas involve a critical thickness for
  the current sheet. Our ultimate goal is to simulate the evolution of a
  current sheet that is driven by shear flows at the photopheric boundary
  and extends upward through the chromosphere and transition region and
  into the corona. We report here on the status of our investigation
  into this fundamental problem.

Title: Nanoflare Heating: Observations and Theory
Authors: Klimchuk, James A.
Bibcode: 2017arXiv170907320K
Altcode:
  This is a review of the observational and theoretical evidence for
  nanoflare heating of the magnetically-closed corona.

Title: Current Sheet Proliferation, Turbulence, and the Heating of
    the Magnetically-Closed Corona
Authors: Klimchuk, James A.; Antiochos, Spiro K.
Bibcode: 2017SPD....4830302K
Altcode:
  Electric current sheets in the solar corona are essential to many
  theories of coronal heating and activity. They can form by a number
  of mechanisms. The magnetic field is known to be very clumpy in the
  photosphere, with approximately 100,000 elemental flux tubes in a
  single active region. Convection causes the tubes to become twisted and
  tangled, with current sheets forming unavoidably at their boundaries in
  the corona. Partial reconnections of the tubes as well as a patchiness
  of the reconnection process lead to a multiplication of the number of
  distinct sheets. Quasi-ideal instabilities, such as kinking, multiply
  the numbers even more. We conclude, therefore, that there will be a
  proliferation of current sheets in the corona. An important question
  is whether large-scale (active region size) models of the corona
  need to take this complexity into account to successfully predict
  the distribution of plasma and the resulting radiation. We discuss
  the picture of current sheet proliferation and compare and contrast
  it to MHD turbulence. We also discuss the implication of our results
  for coronal observations.

Title: Diagnosing Coronal Heating in a Survey of Active Regions
    using the Time Lag Method
Authors: Viall, Nicholeen; Klimchuk, James A.
Bibcode: 2017SPD....4840202V
Altcode:
  In this paper we examine 15 different active regions observed with the
  Solar Dynamics Observatory and analyze their nanoflare properties using
  the time lag method. The time lag method is a diagnostic of whether
  the plasma is maintained at a steady temperature, or if it is dynamic,
  undergoing heating and cooling cycles. An important aspect of our
  technique is that it analyses both observationally distinct coronal
  loops as well as the much more prevalent diffuse emission surrounding
  them. Warren et al. (2012) first studied these same 15 active regions,
  which are all quiescent and exhibit a broad range of characteristics,
  including age, total unsigned magnetic flux, area, hot emission, and
  emission measure distribution. We find that widespread cooling is a
  generic property of both loop and diffuse emission from all 15 active
  regions. However, the range of temperatures through which the plasma
  cools varies between active regions and within each active region,
  and only occasionally is there full cooling from above 7 MK to well
  below 1 MK. We find that the degree of cooling is not well correlated
  with slopes of the emission measure distribution measured by Warren et
  al. (2012). We show that these apparently contradictory observations
  can be reconciled with the presence of a distribution of nanoflare
  energies and frequencies along the line of sight, with the average
  delay between successive nanoflare events on a single flux tube being
  comparable to the plasma cooling timescale. Warren, H. P., Winebarger,
  A. R., &amp; Brooks, D. H. 2012, ApJ, 759, 141

Title: Constraints on Nonuniform Expansion in Coronal Loops
Authors: Kucera, Therese A.; DeForest, Craig; Klimchuk, James A.;
   Young, Peter R.
Bibcode: 2017SPD....4810608K
Altcode:
  We use measurements of coronal loop properties to constrain the
  hypothesis that coronal loops expand differently in different
  directions. A long standing problem in understanding coronal loops is
  that although the magnetic field is expected to expand with altitude
  and does indeed seem to do so on scales of active regions, individual
  loops seem to have fairly uniform diameters along the length of the
  loop. Malanushenko &amp; Schrijver (2013) have suggested that loops
  may be expanding, but with a non-circular cross section. In this
  scenario a loop might have a constant width in the plane of the sky,
  but expand along the line of sight. Furthermore, such loops might be
  easier to see from the point of view that does not show expansion. We
  use Hinode/EIS and SDO/AIA data to measure loop intensities, electron
  densities, temperatures and dimensions in order to determine the extent
  to which loops may be expanding along the line of sight.

Title: The Onset of Magnetic Reconnection: Tearing Instability in
    Current Sheets with a Guide Field
Authors: Daldorff, Lars K. S.; Klimchuk, James A.; Leake, James E.;
   Knizhnik, Kalman
Bibcode: 2017SPD....4810616D
Altcode:
  Magnetic reconnection is fundamental to many solar phenomena, ranging
  from coronal heating, to jets, to flares and CMEs. A poorly understood
  yet crucial aspect of reconnection is that it does not occur until
  magnetic stresses have built to sufficiently high levels for significant
  energy release. If reconnection were to happen too soon, coronal heating
  would be weak and flares would be small. As part of our program to study
  the onset conditions for magnetic reconnection, we have investigated
  the instability of current sheets to tearing. Surprisingly little work
  has been done on this problem for sheets that include a guide field,
  i.e., for which the field rotates by less than 180 degrees. This is the
  most common situation on the Sun. We present numerical 3D resistive MHD
  simulations of several sheets and show how the behavior depends on the
  shear angle (rotation). We compare our results to the predictions of
  linear theory and discuss the nonlinear evolution in terms of plasmoid
  formation and the interaction of different oblique tearing modes. The
  relevance to the Sun is explained.

Title: Hard X-Ray Constraints on Small-Scale Coronal Heating Events
Authors: Marsh, Andrew; Smith, David M.; Glesener, Lindsay; Klimchuk,
   James A.; Bradshaw, Stephen; Hannah, Iain; Vievering, Juliana;
   Ishikawa, Shin-Nosuke; Krucker, Sam; Christe, Steven
Bibcode: 2017SPD....4810614M
Altcode:
  A large body of evidence suggests that the solar corona is heated
  impulsively. Small-scale heating events known as nanoflares may be
  ubiquitous in quiet and active regions of the Sun. Hard X-ray (HXR)
  observations with unprecedented sensitivity &gt;3 keV have recently been
  enabled through the use of focusing optics. We analyze active region
  spectra from the FOXSI-2 sounding rocket and the NuSTAR satellite to
  constrain the physical properties of nanoflares simulated with the
  EBTEL field-line-averaged hydrodynamics code. We model a wide range
  of X-ray spectra by varying the nanoflare heating amplitude, duration,
  delay time, and filling factor. Additional constraints on the nanoflare
  parameter space are determined from energy constraints and EUV/SXR data.

Title: A Survey of Nanoflare Properties in Active Regions Observed
    with the Solar Dynamics Observatory
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2017ApJ...842..108V
Altcode:
  In this paper, we examine 15 different active regions (ARs) observed
  with the Solar Dynamics Observatory and analyze their nanoflare
  properties. We have recently developed a technique that systematically
  identifies and measures plasma temperature dynamics by computing time
  lags between light curves. The time lag method tests whether the
  plasma is maintained at a steady temperature, or if it is dynamic,
  undergoing heating and cooling cycles. An important aspect of our
  technique is that it analyzes both observationally distinct coronal
  loops as well as the much more prevalent diffuse emission between
  them. We find that the widespread cooling reported previously for NOAA
  AR 11082 is a generic property of all ARs. The results are consistent
  with impulsive nanoflare heating followed by slower cooling. Only
  occasionally, however, is there full cooling from above 7 MK to well
  below 1 MK. More often, the plasma cools to approximately 1-2 MK
  before being reheated by another nanoflare. These same 15 ARs were
  first studied by Warren et al. We find that the degree of cooling is
  not well correlated with the reported slopes of the emission measure
  distribution. We also conclude that the Fe xviii emitting plasma that
  they measured is mostly in a state of cooling. These results support the
  idea that nanoflares have a distribution of energies and frequencies,
  with the average delay between successive events on an individual flux
  tube being comparable to the plasma cooling timescale.

Title: Exploring impulsive solar magnetic energy release and particle
    acceleration with focused hard X-ray imaging spectroscopy
Authors: Christe, Steven; Krucker, Samuel; Glesener, Lindsay; Shih,
   Albert; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel; Battaglia,
   Marina; Chen, Bin; Drake, James; Dennis, Brian; Gary, Dale; Gburek,
   Szymon; Goetz, Keith; Grefenstette, Brian; Gubarev, Mikhail; Hannah,
   Iain; Holman, Gordon; Hudson, Hugh; Inglis, Andrew; Ireland, Jack;
   Ishikawa, Shinosuke; Klimchuk, James; Kontar, Eduard; Kowalski, Adam;
   Longcope, Dana; Massone, Anna-Maria; Musset, Sophie; Piana, Michele;
   Ramsey, Brian; Ryan, Daniel; Schwartz, Richard; Stęślicki, Marek;
   Turin, Paul; Warmuth, Alexander; Wilson-Hodge, Colleen; White, Stephen;
   Veronig, Astrid; Vilmer, Nicole; Woods, Tom
Bibcode: 2017arXiv170100792C
Altcode:
  How impulsive magnetic energy release leads to solar eruptions and how
  those eruptions are energized and evolve are vital unsolved problems
  in Heliophysics. The standard model for solar eruptions summarizes
  our current understanding of these events. Magnetic energy in the
  corona is released through drastic restructuring of the magnetic
  field via reconnection. Electrons and ions are then accelerated by
  poorly understood processes. Theories include contracting loops,
  merging magnetic islands, stochastic acceleration, and turbulence at
  shocks, among others. Although this basic model is well established,
  the fundamental physics is poorly understood. HXR observations
  using grazing-incidence focusing optics can now probe all of the key
  regions of the standard model. These include two above-the-looptop
  (ALT) sources which bookend the reconnection region and are likely
  the sites of particle acceleration and direct heating. The science
  achievable by a direct HXR imaging instrument can be summarized by the
  following science questions and objectives which are some of the most
  outstanding issues in solar physics (1) How are particles accelerated
  at the Sun? (1a) Where are electrons accelerated and on what time
  scales? (1b) What fraction of electrons is accelerated out of the
  ambient medium? (2) How does magnetic energy release on the Sun lead
  to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI)
  instrument, which can be built now using proven technology and at modest
  cost, would enable revolutionary advancements in our understanding of
  impulsive magnetic energy release and particle acceleration, a process
  which is known to occur at the Sun but also throughout the Universe.

Title: Solving the Coronal Heating Problem using X-ray
    Microcalorimeters
Authors: Christe, Steven; Bandler, Simon; DeLuca, Edward; Caspi,
   Amir; Golub, Leon; Smith, Randall; Allred, Joel; Brosius, Jeffrey W.;
   Dennis, Brian; Klimchuk, James
Bibcode: 2017arXiv170100795C
Altcode:
  Even in the absence of resolved flares, the corona is heated to several
  million degrees. However, despite its importance for the structure,
  dynamics, and evolution of the solar atmosphere, the origin of this
  heating remains poorly understood. Several observational and theoretical
  considerations suggest that the heating is driven by small, impulsive
  energy bursts which could be Parker-style "nanoflares" (Parker 1988)
  that arise via reconnection within the tangled and twisted coronal
  magnetic field. The classical "smoking gun" (Klimchuk 2009; Cargill et
  al. 2013) for impulsive heating is the direct detection of widespread
  hot plasma (T &gt; 6 MK) with a low emission measure. In recent years
  there has been great progress in the development of Transition Edge
  Sensor (TES) X-ray microcalorimeters that make them more ideal for
  studying the Sun. When combined with grazing-incidence focusing optics,
  they provide direct spectroscopic imaging over a broad energy band
  (0.5 to 10 keV) combined with extremely impressive energy resolution
  in small pixels, as low as 0.7 eV (FWHM) at 1.5 keV (Lee 2015),
  and 1.56 eV (FWHM) at 6 keV (Smith 2012), two orders of magnitude
  better than the current best traditional solid state photon-counting
  spectrometers. Decisive observations of the hot plasma associated
  with nanoflare models of coronal heating can be provided by new solar
  microcalorimeters. These measurements will cover the most important part
  of the coronal spectrum for searching for the nanoflare-related hot
  plasma and will characterize how much nanoflares can heat the corona
  both in active regions and the quiet Sun. Finally, microcalorimeters
  will enable to study all of this as a function of time and space in
  each pixel simultaneously a capability never before available.

Title: Unravelling the Components of a Multi-thermal Coronal Loop
    using Magnetohydrodynamic Seismology
Authors: Krishna Prasad, S.; Jess, D. B.; Klimchuk, J. A.; Banerjee, D.
Bibcode: 2017ApJ...834..103K
Altcode: 2016arXiv161104011K; 2017ApJ...834..103P
  Coronal loops, constituting the basic building blocks of the active Sun,
  serve as primary targets to help understand the mechanisms responsible
  for maintaining multi-million Kelvin temperatures in the solar and
  stellar coronae. Despite significant advances in observations and
  theory, our knowledge on the fundamental properties of these structures
  is limited. Here, we present unprecedented observations of accelerating
  slow magnetoacoustic waves along a coronal loop that show differential
  propagation speeds in two distinct temperature channels, revealing
  the multi-stranded and multithermal nature of the loop. Utilizing
  the observed speeds and employing nonlinear force-free magnetic field
  extrapolations, we derive the actual temperature variation along the
  loop in both channels, and thus are able to resolve two individual
  components of the multithermal loop for the first time. The obtained
  positive temperature gradients indicate uniform heating along the loop,
  rather than isolated footpoint heating.

Title: Hard X-ray Detectability of Small-Scale Coronal Heating Events
Authors: Marsh, A.; Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.;
   Smith, D. M.; Hannah, I. G.
Bibcode: 2016AGUFMSH11D..06M
Altcode:
  The nanoflare heating theory predicts the ubiquitous presence
  of hot ( &gt;5 MK) plasma in the solar corona, but evidence for
  this high-temperature component has been scarce. Current hard x-ray
  instruments such as RHESSI lack the sensitivity to see the trace amounts
  of this plasma that are predicted by theoretical models. New hard X-ray
  instruments that use focusing optics, such as FOXSI (the Focusing Optics
  X-ray Solar Imager) and NuSTAR (the Nuclear Spectroscopic Telescope
  Array) can extend the visible parameter space of nanoflare "storms"
  that create hot plasma. We compare active-region data from FOXSI and
  NuSTAR with a series of EBTEL hydrodynamic simulations, and constrain
  nanoflare properties to give good agreement with observations.

Title: Turbulence, Current Sheet Proliferation, and the Heating of
    the Magnetically-Closed Corona
Authors: Klimchuk, J. A.; Antiochos, S. K.; Dahlburg, R. B.
Bibcode: 2016AGUFMSH33A..03K
Altcode:
  Turbulence plays an important role in heating and accelerating the
  solar wind, and it has been proposed to also be important in heating
  active regions and the quiet Sun. These regions are fundamentally
  different from the sources of the solar wind, however, in that they are
  magnetically closed and have a small plasma beta. We suggest that the
  strong, line-tied magnetic field resists being distorted and inhibits
  turbulence from developing. To test this idea, we performed a 3D MHD
  simulation representing a solar active region being driven by slow
  photospheric motions. The conditions said to be necessary for turbulence
  are met, yet the system evolves quasi-statically up to the point where
  a kink instability occurs. We conclude that the magnetically-closed
  corona is not turbulent in the classical sense. There is no inertial
  range of spatial scales where energy flows without dissipation through
  a continuum of eddies. Rather, there is a quasi-static evolution that
  is interrupted by localized and temporary bursts of turbulent behavior
  associated with the tearing and reconnection of current sheets. Because
  of a proliferation of current sheets, these episodes are widespread and
  frequent, with many occurring at the same time within a single active
  region. This picture is fundamentally different from MHD turbulence,
  despite some similarities. In addition to the lack of an inertial
  range, the amount of heating is not independent of the details of
  the dissipation. On the contrary, it depends critically on the onset
  conditions for tearing and reconnection.

Title: Focusing Solar Hard X-rays: Expected Results from a FOXSI
    Spacecraft
Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.;
   Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis,
   A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.;
   Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.;
   Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey,
   N. L. S.; Musset, S.; Swisdak, M.
Bibcode: 2016AGUFMSH13A2282G
Altcode:
  Over the course of two solar cycles, RHESSI has examined high-energy
  processes in flares via high-resolution spectroscopy and imaging of
  soft and hard X-rays (HXRs). The detected X-rays are the thermal
  and nonthermal bremsstrahlung from heated coronal plasma and from
  accelerated electrons, respectively, making them uniquely suited to
  explore the highest-energy processes that occur in the corona. RHESSI
  produces images using an indirect, Fourier-based method and has made
  giant strides in our understanding of these processes, but it has also
  uncovered intriguing new mysteries regarding energy release location,
  acceleration mechanisms, and energy propagation in flares. Focusing
  optics are now available for the HXR regime and stand poised to perform
  another revolution in the field of high-energy solar physics. With
  two successful sounding rocket flights completed, the Focusing Optics
  X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and
  power of direct solar HXR imaging with its vastly superior sensitivity
  and dynamic range. Placing this mature technology aboard a spacecraft
  will offer a systematic way to explore high-energy aspects of the
  solar corona and to address scientific questions left unanswered by
  RHESSI. Here we present examples of such questions and show simulations
  of expected results from a FOXSI spaceborne instrument to demonstrate
  how these questions can be addressed with the focusing of hard X-rays.

Title: The Onset of Magnetic Reconnection: Tearing Instability in
    Current Sheets with a Guide Field
Authors: Daldorff, L. K. S.; Klimchuk, J. A.; Knizhnik, K. J.
Bibcode: 2016AGUFMSH51B2590D
Altcode:
  Magnetic reconnection is fundamental to many solar phenomena, ranging
  from coronal heating, to jets, to flares and CMEs. A poorly understood
  yet crucial aspect of reconnection is that it does not occur until
  magnetic stresses have built to sufficiently high levels for significant
  energy release. If reconnection were to happen too soon, coronal heating
  would be weak and flares would be small. As part of our program to study
  the onset conditions for magnetic reconnection, we have investigated
  the instability of current sheets to tearing. Surprisingly little work
  has been done on this problem for sheets that include a guide field,
  i.e., for which the field rotates by less than 180 degrees. This is the
  most common situation on the Sun. We present numerical 3D resistive MHD
  simulations of several sheets and show how the behaviour depends on the
  shear angle (rotation). We compare our results to the predictions of
  linear theory and discuss the nonlinear evolution in terms of plasmoid
  formation and the interaction of different oblique tearing modes. The
  relevance to the Sun is explained.

Title: The Focusing Optics X-ray Solar Imager (FOXSI) SMEX Mission
Authors: Christe, S.; Shih, A. Y.; Krucker, S.; Glesener, L.;
   Saint-Hilaire, P.; Caspi, A.; Allred, J. C.; Battaglia, M.; Chen,
   B.; Drake, J. F.; Gary, D. E.; Goetz, K.; Grefenstette, B.; Hannah,
   I. G.; Holman, G.; Hudson, H. S.; Inglis, A. R.; Ireland, J.; Ishikawa,
   S. N.; Klimchuk, J. A.; Kontar, E.; Kowalski, A. F.; Massone, A. M.;
   Piana, M.; Ramsey, B.; Gubarev, M.; Schwartz, R. A.; Steslicki, M.;
   Ryan, D.; Turin, P.; Warmuth, A.; White, S. M.; Veronig, A.; Vilmer,
   N.; Dennis, B. R.
Bibcode: 2016AGUFMSH13A2281C
Altcode:
  We present FOXSI (Focusing Optics X-ray Solar Imager), a recently
  proposed Small Explorer (SMEX) mission that will provide a revolutionary
  new perspective on energy release and particle acceleration on the
  Sun. FOXSI is a direct imaging X-ray spectrometer with higher dynamic
  range and better than 10x the sensitivity of previous instruments. Flown
  on a 3-axis stabilized spacecraft in low-Earth orbit, FOXSI uses
  high-angular-resolution grazing-incidence focusing optics combined
  with state-of-the-art pixelated solid-state detectors to provide direct
  imaging of solar hard X-rays for the first time. FOXSI is composed of
  two individual x-ray telescopes with a 14-meter focal length enabled by
  a deployable boom. Making use of a filter-wheel and high-rate-capable
  solid-state detectors, FOXSI will be able to observe the largest flares
  without saturation while still maintaining the sensitivity to detect
  x-ray emission from weak flares, escaping electrons, and hot active
  regions. This SMEX mission is made possible by past experience with
  similar instruments on two sounding rocket flights, in 2012 and 2014,
  and on the HEROES balloon flight in 2013. FOXSI will image the Sun
  with a field of view of 9 arcminutes and an angular resolution of
  better than 8 arcsec; it will cover the energy range from 3 to 100
  keV with a spectral resolution of better than 1 keV; and it will have
  sub-second temporal resolution.

Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring
    the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence,
    and Radio Signatures of Solar Transients
Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell,
   J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa,
   M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess,
   D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen,
   C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.;
   Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.;
   Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson,
   E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury,
   T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper,
   J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh,
   V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.;
   Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas,
   D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.;
   Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.;
   Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin,
   S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.;
   Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle,
   A.; Werthimer, D.; Wygant, J. R.
Bibcode: 2016SSRv..204...49B
Altcode: 2016SSRv..tmp...16B
  NASA's Solar Probe Plus (SPP) mission will make the first in situ
  measurements of the solar corona and the birthplace of the solar
  wind. The FIELDS instrument suite on SPP will make direct measurements
  of electric and magnetic fields, the properties of in situ plasma waves,
  electron density and temperature profiles, and interplanetary radio
  emissions, amongst other things. Here, we describe the scientific
  objectives targeted by the SPP/FIELDS instrument, the instrument
  design itself, and the instrument concept of operations and planned
  data products.

Title: Towards a Physics-Based Flare Irradiance Model
Authors: Hock-Mysliwiec, R. A.; Klimchuk, J. A.; Eparvier, F. G.;
   Woods, T. N.; Balasubramaniam, K. S.
Bibcode: 2016usc..confE..46H
Altcode:
  The Extreme UltraViolet (EUV) irradiance from solar flares is a critical
  driver of short term variability in the Earth's upper atmosphere. The
  EUV Variability Experiment (EVE) onboard NASA's Solar Dynamics
  Observatory (SDO) has been making moderate spectral resolution (0.1
  nm), high time cadence (10 s) measurements of the solar EUV irradiance
  (5-105 nm) since 2010. A key observation from EVE is that flares of the
  same magnitude at one wavelength (e.g. GOES XRS) have different peak
  intensities and time profiles in other wavelengths. As it is impractical
  to measure the entire EUV spectrum with sufficient spectral resolution
  and temporal cadence to capture these differences for space weather
  operations, the next generation of flare irradiance models must be
  able to capture these variations. We have developed a framework for
  a physics-based flare irradiance model based on the EBTEL model. At
  present, this Multi-Strand Flare Irradiance Model (MS-FIM) is able to
  predict EUV lightcurves over a range of coronal temperatures given the
  lightcurves from two EVE lines as inputs. In this paper, we present an
  overview of the Multi-Strand Flare Irradiance Model as well as initial
  results showing its ability to predict the irradiances for a diverse
  range of flares, including EUV late phase flares. We also describe
  preliminary efforts to drive the model with parameters derived from
  images of the flaring region instead of EUV lightcurves.

Title: Signatures of Steady Heating in Time Lag Analysis of Coronal
    Emission
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2016ApJ...828...76V
Altcode: 2016arXiv160702008V
  Among the multitude of methods used to investigate coronal heating,
  the time lag method of Viall &amp; Klimchuk is becoming increasingly
  prevalent as an analysis technique that is complementary to those
  that are traditionally used. The time lag method cross correlates
  light curves at a given spatial location obtained in spectral bands
  that sample different temperature plasmas. It has been used most
  extensively with data from the Atmospheric Imaging Assembly on the
  Solar Dynamics Observatory. We have previously applied the time lag
  method to entire active regions and surrounding the quiet Sun and
  created maps of the results. We find that the majority of time lags
  are consistent with the cooling of coronal plasma that has been
  impulsively heated. Additionally, a significant fraction of the
  map area has a time lag of zero. This does not indicate a lack of
  variability. Rather, strong variability must be present, and it must
  occur in phase between the different channels. We have previously
  shown that these zero time lags are consistent with the transition
  region response to coronal nanoflares, although other explanations are
  possible. A common misconception is that the zero time lag indicates
  steady emission resulting from steady heating. Using simulated and
  observed light curves, we demonstrate here that highly correlated
  light curves at zero time lag are not compatible with equilibrium
  solutions. Such light curves can only be created by evolution.

Title: A Nanoflare-based Cellular Automaton Model and the Observed
    Properties of the Coronal Plasma
Authors: López Fuentes, Marcelo; Klimchuk, James A.
Bibcode: 2016ApJ...828...86L
Altcode: 2016arXiv160703917L
  We use the cellular automaton model described in López Fuentes &amp;
  Klimchuk to study the evolution of coronal loop plasmas. The model,
  based on the idea of a critical misalignment angle in tangled magnetic
  fields, produces nanoflares of varying frequency with respect to the
  plasma cooling time. We compare the results of the model with active
  region (AR) observations obtained with the Hinode/XRT and SDO/AIA
  instruments. The comparison is based on the statistical properties
  of synthetic and observed loop light curves. Our results show that
  the model reproduces the main observational characteristics of the
  evolution of the plasma in AR coronal loops. The typical intensity
  fluctuations have amplitudes of 10%-15% both for the model and the
  observations. The sign of the skewness of the intensity distributions
  indicates the presence of cooling plasma in the loops. We also study
  the emission measure (EM) distribution predicted by the model and obtain
  slopes in log(EM) versus log(T) between 2.7 and 4.3, in agreement with
  published observational values.

Title: The Onset of Magnetic Reconnection: Tearing Instability in
    Current Sheets with a Guide Field
Authors: Daldorff, Lars K. S.; Klimchuk, James A.
Bibcode: 2016shin.confE.110D
Altcode:
  Magnetic reconnection is fundamental to many solar phenomena, ranging
  from coronal heating o jets, to flares and CMEs. A poorly understood yet
  crucial aspect of reconnection is that it does not occur until magnetic
  stresses have built to sufficiently high levels for significant energy
  release. If reconnection were to happen too soon, coronal heating would
  be weak and flares would be small. As part of our program to study
  the onset conditions for magnetic reconnection, we have investigated
  the instability of current sheets to tearing. Surprisingly little work
  has been done on this problem for sheets that include a guide field,
  i.e., for which the field rotates by less than 180 degrees. This is the
  most common situation on the Sun. We present numerical 3D resistive MHD
  simulations of several sheets and show how the behaviour depends on the
  shear angle (rotation). We compare our results to the predictions of
  linear theory and discuss the nonlinear evolution in terms of plasmoid
  formation and the interaction of different oblique tearing modes. The
  relevance to the Sun is explained.

Title: Hard X-ray Detectability of Small-Scale Coronal Heating Events
Authors: Marsh, Andrew; Glesener, Lindsay; Klimchuk, James A.;
   Bradshaw, Stephen; Smith, David; Hannah, Iain
Bibcode: 2016SPD....4720204M
Altcode:
  The nanoflare heating theory predicts the ubiquitous presence
  of hot (~&gt;5 MK) plasma in the solar corona, but evidence for
  this high-temperature component has been scarce. Current hard x-ray
  instruments such as RHESSI lack the sensitivity to see the trace amounts
  of this plasma that are predicted by theoretical models. New hard X-ray
  instruments that use focusing optics, such as FOXSI (the Focusing
  Optics X-ray Solar Imager) and NuSTAR (the Nuclear Spectroscopic
  Telescope Array) can extend the visible parameter space of nanoflare
  “storms” that create hot plasma. We compare active-region data from
  FOXSI and NuSTAR with a series of EBTEL hydrodynamic simulations, and
  constrain nanoflare properties to give good agreement with observations.

Title: The Transition Region Response to a Coronal Nanoflare:
    Forward Modeling and Observations in SDO/AIA
Authors: Viall, Nicholeen; Klimchuk, James A.
Bibcode: 2016SPD....4720202V
Altcode:
  The corona and transition region (TR) are fundamentally coupled
  through the processes of thermal conduction and mass exchange. Yet
  the temperature-dependent emissions from the two locations behave
  quite differently in the aftermath of an impulsive heating event such
  as a coronal nanoflare. In this presentation, we use results from the
  EBTEL hydrodynamics code to demonstrate that after a coronal nanoflare,
  the TR is multithermal and the emission at all temperatures responds
  in unison. This is in contrast to the coronal plasma, which cools
  sequentially, emitting first at higher temperatures and then at lower
  temperatures. We apply the time lag technique of Viall &amp; Klimchuk
  (2012) to the simulated Atmospheric Imaging Assembly (AIA) on the
  Solar Dynamics Observatory emission and show that coronal plasma light
  curves exhibit post-nanoflare cooling time lags, while TR light curves
  show time lags of zero, as observed. We further demonstrate that time
  lags of zero, regardless of physical cause, do not indicate a lack of
  variability. Rather, strong variability must be present, and it must
  occur in unison in the different channels. Lastly, we show that the
  'coronal' channels in AIA can be dominated by bright TR emission. When
  defined in a physically meaningful way, the TR reaches a temperature
  of roughly 60% the peak temperature in a flux tube. The TR resulting
  from impulsive heating can extend to 3 MK and higher, well within the
  range of the 'coronal' AIA channels.

Title: Science Objectives of the FOXSI Small Explorer Mission Concept
Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred,
   Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas,
   Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman,
   Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian;
   Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.;
   Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James
   A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita,
   Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz,
   Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen
Bibcode: 2016SPD....47.0814S
Altcode:
  Impulsive particle acceleration and plasma heating at the Sun, from the
  largest solar eruptive events to the smallest flares, are related to
  fundamental processes throughout the Universe. While there have been
  significant advances in our understanding of impulsive energy release
  since the advent of RHESSI observations, there is a clear need for
  new X-ray observations that can capture the full range of emission
  in flares (e.g., faint coronal sources near bright chromospheric
  sources), follow the intricate evolution of energy release and changes
  in morphology, and search for the signatures of impulsive energy
  release in even the quiescent Sun. The FOXSI Small Explorer (SMEX)
  mission concept combines state-of-the-art grazing-incidence focusing
  optics with pixelated solid-state detectors to provide direct imaging
  of hard X-rays for the first time on a solar observatory. We present
  the science objectives of FOXSI and how its capabilities will address
  and resolve open questions regarding impulsive energy release at the
  Sun. These questions include: What are the time scales of the processes
  that accelerate electrons? How do flare-accelerated electrons escape
  into the heliosphere? What is the energy input of accelerated electrons
  into the chromosphere, and how is super-heated coronal plasma produced?

Title: The Onset of Magnetic Reconnection: Tearing Instability in
    Current Sheets with a Guide Field
Authors: Daldorff, Lars K. S.; Klimchuk, James A.
Bibcode: 2016SPD....4740207D
Altcode:
  Magnetic reconnection is fundamental to many solar phenomena, ranging
  from coronal heating, to jets, to flares and CMEs. A poorly understood
  yet crucial aspect of reconnection is that it does not occur until
  magnetic stresses have built to sufficiently high levels for significant
  energy release. If reconnection were to happen too soon, coronal heating
  would be weak and flares would be small. As part of our program to study
  the onset conditions for magnetic reconnection, we have investigated
  the instability of current sheets to tearing. Surprisingly little work
  has been done on this problem for sheets that include a guide field,
  i.e., for which the field rotates by less than 180 degrees. This is the
  most common situation on the Sun. We present numerical 3D resistive MHD
  simulations of several sheets and show how the behaviour depends on the
  shear angle (rotation). We compare our results to the predictions of
  linear theory and discuss the nonlinear evolution in terms of plasmoid
  formation and the interaction of different oblique tearing modes. The
  relevance to the Sun is explained.

Title: Comparing Loop Cross Sections Observed with Hi-C and AIA/SDO
Authors: Klimchuk, James A.; DeForest, Craig
Bibcode: 2016SPD....47.0301K
Altcode:
  Many studies have reported coronal loop widths measured with AIA/SDO,
  TRACE, and other data. For warm loops (T ~ 1 MK), the characteristic
  diameter is about 1500 km. Sub-structure is likely to exist on smaller
  scales, but the envelope of the "strands" has this typical size. Since
  1500 km (2 arcsec) is not large compared to the spatial resolution of
  the observations, there remained a question about whether the loops
  are actually much thinner. To address this concern, we have measured
  the widths of several loops observed at 193 A by both AIA and the Hi-C
  rocket experiment. Hi-C has 3-6 times better spatial resolution, so if
  the loops are substantially unresolved by AIA, it should be readily
  apparent. We find that the measured widths are very similar. Small
  differences (&lt; 25%) are explainable by uncertainties in the point
  spread functions. We conclude that previous measurements of loop
  widths made by AIA and TRACE are essentially correct. We also find
  little evidence for loop sub-structure at the resolution of Hi-C. The
  individual strands that comprise loops are therefore smaller than 200
  km. These results have important implications for coronal heating.

Title: Intensity Conserving Spectral Fitting
Authors: Klimchuk, J. A.; Patsourakos, S.; Tripathi, D.
Bibcode: 2016SoPh..291...55K
Altcode: 2015SoPh..tmp..180K
  The detailed shapes of spectral-line profiles provide valuable
  information about the emitting plasma, especially when the plasma
  contains an unresolved mixture of velocities, temperatures, and
  densities. As a result of finite spectral resolution, the intensity
  measured by a spectrometer is the average intensity across a wavelength
  bin of non-zero size. It is assigned to the wavelength position at
  the center of the bin. However, the actual intensity at that discrete
  position will be different if the profile is curved, as it invariably
  is. Standard fitting routines (spline, Gaussian, etc.) do not account
  for this difference, and this can result in significant errors when
  making sensitive measurements. We have developed an iterative procedure
  that corrects for this effect. It converges rapidly and is very
  flexible in that it can be used with any fitting function. We present
  examples of cubic-spline and Gaussian fits and give special attention
  to measurements of blue-red asymmetries of coronal emission lines.

Title: Reconnection Between Twisted Flux Tubes - Implications for
    Coronal Heating
Authors: Knizhnik, K. J.; Antiochos, S. K.; DeVore, C. R.; Klimchuk,
   J. A.; Wyper, P. F.
Bibcode: 2015AGUFMSH13B2439K
Altcode:
  The nature of the heating of the Sun's corona has been a long-standing
  unanswered problem in solar physics. Beginning with the work of Parker
  (1972), many authors have argued that the corona is continuously heated
  through numerous small-scale reconnection events known as nanoflares. In
  these nanoflare models, stressing of magnetic flux tubes by photospheric
  motions causes the field to become misaligned, producing current sheets
  in the corona. These current sheets then reconnect, converting the
  free energy stored in the magnetic field into heat. In this work,
  we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD
  simulations that dynamically resolve regions of strong current to study
  the reconnection between twisted flux tubes in a plane-parallel Parker
  configuration. We investigate the energetics of the process, and show
  that the flux tubes accumulate stress gradually before undergoing
  impulsive reconnection. We study the motion of the individual field
  lines during reconnection, and demonstrate that the connectivity of the
  configuration becomes extremely complex, with multiple current sheets
  being formed, which could lead to enhanced heating. In addition, we
  show that there is considerable interaction between the twisted flux
  tubes and the surrounding untwisted field, which contributes further
  to the formation of current sheets. The implications for observations
  will be discussed. This work was funded by a NASA Earth and Space
  Science Fellowship, and by the NASA TR&amp;T Program.

Title: Hard X-ray Detectability of Small Impulsive Heating Events
    in the Solar Corona
Authors: Glesener, L.; Klimchuk, J. A.; Bradshaw, S. J.; Marsh, A.;
   Krucker, S.; Christe, S.
Bibcode: 2015AGUFMSH13B2440G
Altcode:
  Impulsive heating events ("nanoflares") are a candidate to supply the
  solar corona with its ~2 MK temperature. These transient events can
  be studied using extreme ultraviolet and soft X-ray observations,
  among others. However, the impulsive events may occur in tenuous
  loops on small enough timescales that the heating is essentially not
  observed due to ionization timescales, and only the cooling phase is
  observed. Bremsstrahlung hard X-rays could serve as a more direct and
  prompt indicator of transient heating events. A hard X-ray spacecraft
  based on the direct-focusing technology pioneered by the Focusing
  Optics X-ray Solar Imager (FOXSI) sounding rocket could search for these
  direct signatures. In this work, we use the hydrodynamical EBTEL code to
  simulate differential emission measures produced by individual heating
  events and by ensembles of such events. We then directly predict hard
  X-ray spectra and consider their observability by a future spaceborne
  FOXSI, and also by the RHESSI and NuSTAR spacecraft.

Title: Nanoflare Heating of the Quiet Sun
Authors: Viall, N. M.; Klimchuk, J. A.
Bibcode: 2015AGUFMSH31D..05V
Altcode:
  How the solar corona is heated to temperatures of over 1 MK, while
  the photosphere below is only ~ 6000 K remains one of the outstanding
  problems in all of space science. Solving this problem is crucial for
  understanding Sun-Earth connections, and will provide new insight into
  universal processes such as magnetic reconnection and wave-particle
  interactions. We use a systematic technique to analyze the properties
  of coronal heating throughout the solar corona using data taken
  with the Atmospheric Imaging Assembly onboard the Solar Dynamics
  Observatory. Our technique computes cooling times of the coronal plasma
  on a pixel-by-pixel basis and has the advantage that it analyzes all
  of the coronal emission, including the diffuse emission surrounding
  distinguishable coronal features. We have already applied this technique
  to 15 different active regions, and find clear evidence for dynamic
  heating and cooling cycles that are consistent with the 'impulsive
  nanoflare' scenario. What about the rest of the Solar corona? Whether
  the quiet Sun is heated in a similar or distinct manner from active
  regions is a matter of great debate. Here we apply our coronal heating
  analysis technique to quiet Sun locations. We find areas of quiet
  Sun locations that also undergo dynamic heating and cooling cycles,
  consistent with impulsive nanoflares. However, there are important
  characteristics that are distinct from those of active regions.

Title: The Onset of Magnetic Reconnection
Authors: Daldorff, L. K. S.; Klimchuk, J. A.
Bibcode: 2015AGUFMSH13B2437D
Altcode:
  A fundamental question concerning magnetic energy release on the Sun is
  why the release occurs only after substantial stresses have been built
  up in the field. If reconnection were to occur readily, the released
  energy would be insufficient to explain coronal heating, CMEs, flares,
  jets, spicules, etc. How can we explain this switch-on property? What
  is the physical nature of the onset conditions? One idea involves the
  "secondary instability" of current sheets, which switches on when
  the rotation of the magnetic field across a current sheet reaches
  a critical angle. Such conditions would occur at the boundaries of
  flux tubes that become tangled and twisted by turbulent photospheric
  convection, for example. Other ideas involve a critical thickness
  for the current sheet. We report here on the preliminary results of
  our investigation of reconnect onset. Unlike our earlier work on the
  secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), we
  treat the coupled chromosphere-corona system. Using the BATS-R-US MHD
  code, we simulate a single current sheet in a sheared magnetic field
  that extends from the chromosphere into the corona. Driver motions are
  applied at the base of the model. The configuration and chromosphere are
  both idealized, but capture the essential physics of the problem. The
  advantage of this unique approach is that it resolves the current sheet
  to the greatest extent possible while maintaining a realistic solar
  atmosphere. It thus bridges the gap between"reconnection in a box"
  studies and studies of large-scale systems such as active regions. One
  question we will address is whether onset conditions are met first
  in the chromosphere or corona. We will report on the work done on
  the project.

Title: The Details of Coronal Heating Matter!
Authors: Klimchuk, J. A.; Daldorff, L. K. S.
Bibcode: 2015AGUFMSH13B2438K
Altcode:
  Understanding how the magnetically-closed corona is heated remains one
  of the most important goals in Heliophysics. It is generally believed
  that much or most of the heating involves the conversion of energy
  that is stored in stressed magnetic fields. What makes the problem so
  challenging is that the conversion process involves very small spatial
  scales. Simulations of, e.g., solar active regions cannot resolve the
  thin current sheets that separate the approximately 100,000 elemental
  magnetic flux strands that comprise a real active region. Heating
  in the simulations primarily takes the form of Ohmic dissipation of
  currents that are far less structured. Thus, the heating is only a
  proxy for the real heating mechanism. How good a proxy is it? Does
  it have the essential properties of the real mechanism? We suggest
  that the details of coronal heating matter. They determine not only
  the temporal behavior and spatial distribution of the heating, but
  also the total amount of energy release. We present some idealized
  MHD simulations that demonstrate this last point.

Title: Capabilities of a FOXSI Small Explorer
Authors: Inglis, A. R.; Christe, S.; Glesener, L.; Krucker, S.; Dennis,
   B. R.; Shih, A.; Wilson-Hodge, C.; Gubarev, M.; Hudson, H. S.; Kontar,
   E.; Buitrago Casas, J. C.; Drake, J. F.; Caspi, A.; Holman, G.; Allred,
   J. C.; Ryan, D.; Alaoui, M.; White, S. M.; Saint-Hilaire, P.; Klimchuk,
   J. A.; Hannah, I. G.; Antiochos, S. K.; Grefenstette, B.; Ramsey,
   B.; Jeffrey, N. L. S.; Reep, J. W.; Schwartz, R. A.; Ireland, J.
Bibcode: 2015AGUFMSH43B2456I
Altcode:
  We present the FOXSI (Focusing Optics X-ray Solar Imager) small explorer
  (SMEX) concept, a mission dedicated to studying particle acceleration
  and energy release on the Sun. FOXSI is designed as a 3-axis stabilized
  spacecraft in low-Earth orbit making use of state-of-the-art grazing
  incidence focusing optics, allowing for direct imaging of solar
  X-rays. The current design being studied features three telescope
  modules deployed in a low-inclination low-earth orbit (LEO). With a 15
  meter focal length enabled by a deployable boom, FOXSI will observe
  the Sun in the 3-50 keV energe range. The FOXSI imaging concept has
  already been tested on two sounding rocket flights, in 2012 and 2014
  and on the HEROES balloon payload flight in 2013. FOXSI will image
  the Sun with an angular resolution of 5'', a spectral resolution of
  0.5 keV, and sub-second temporal resolution using CdTe detectors. In
  this presentation we investigate the science objectives and targets
  which can be accessed from this mission. Because of the defining
  characteristic of FOXSI is true imaging spectroscopy with high dynamic
  range and sensitivity, a brand-new perspective on energy release on the
  Sun is possible. Some of the science targets discussed here include;
  flare particle acceleration processes, electron beams, return currents,
  sources of solar energetic particles (SEPs), as well as understanding
  X-ray emission from active region structures and the quiescent corona.

Title: How Gas-dynamic Flare Models Powered by Petschek Reconnection
    Differ from Those with Ad Hoc Energy Sources
Authors: Longcope, D. W.; Klimchuk, J. A.
Bibcode: 2015ApJ...813..131L
Altcode: 2015arXiv151005985L
  Aspects of solar flare dynamics, such as chromospheric evaporation
  and flare light curves, have long been studied using one-dimensional
  models of plasma dynamics inside a static flare loop, subjected
  to some energy input. While extremely successful at explaining
  the observed characteristics of flares, all such models so
  far have specified energy input ad hoc, rather than deriving it
  self-consistently. There is broad consensus that flares are powered
  by magnetic energy released through reconnection. Recent work has
  generalized Petschek’s basic reconnection scenario, topological
  change followed by field line retraction and shock heating, to permit
  its inclusion in a one-dimensional flare loop model. Here we compare
  the gas dynamics driven by retraction and shocking to those from more
  conventional static loop models energized by ad hoc source terms. We
  find significant differences during the first minute, when retraction
  leads to larger kinetic energies and produces higher densities at
  the loop top, while ad hoc heating tends to rarify the loop top. The
  loop-top density concentration is related to the slow magnetosonic
  shock, characteristic of Petschek’s model, but persists beyond
  the retraction phase occurring in the outflow jet. This offers an
  explanation for observed loop-top sources of X-ray and EUV emission,
  with advantages over that provided by ad hoc heating scenarios. The
  cooling phases of the two models are, however, notably similar to one
  another, suggesting that observations at that stage will yield little
  information on the nature of energy input.

Title: Chromospheric Nanoflares as a Source of Coronal
    Plasma. II. Repeating Nanoflares
Authors: Bradshaw, S. J.; Klimchuk, J. A.
Bibcode: 2015ApJ...811..129B
Altcode: 2016arXiv160306673B
  The million degree plasma of the solar corona must be supplied by
  the underlying layers of the atmosphere. The mechanism and location
  of energy release, and the precise source of coronal plasma, remain
  unresolved. In earlier work, we pursued the idea that warm plasma
  is supplied to the corona via direct heating of the chromosphere by
  nanoflares, contrary to the prevailing belief that the corona is heated
  in situ and the chromosphere is subsequently energized and ablated by
  thermal conduction. We found that single (low-frequency) chromospheric
  nanoflares could not explain the observed intensities, Doppler-shifts,
  and red/blue asymmetries in Fe xii and xiv emission lines. In the
  present work, we follow up on another suggestion that the corona could
  be powered by chromospheric nanoflares that repeat on a timescale
  substantially shorter than the cooling/draining timescale. That is,
  a single magnetic strand is re-supplied with coronal plasma before
  the existing plasma has time to cool and drain. We perform a series
  of hydrodynamic experiments and predict the Fe xii and xiv line
  intensities, Doppler-shifts, and red/blue asymmetries. We find that
  our predicted quantities disagree dramatically with observations and
  fully developed loop structures cannot be created by intermediate- or
  high-frequency chromospheric nanoflares. We conclude that the mechanism
  ultimately responsible for producing coronal plasma operates above the
  chromosphere, but this does not preclude the possibility of a similar
  mechanism powering the chromosphere, extreme examples of which may
  be responsible for heating chromospheric plasma to transition region
  temperatures (e.g., type II spicules).

Title: Loop observations and the coronal heating problem
Authors: López Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2015BAAA...57..231L
Altcode:
  Coronal heating continues to be one of the fundamental problems of solar
  physics. In recent years, instrumental advances and the availability of
  data from space observatories produced important progress, imposing
  restrictions to the models proposed. However, since the physical
  processes occur at spatial scales below the present instrumental
  resolution, definitive answers are still due. Since the corona is
  strongly dominated by the magnetic field, active region plasma is
  confined in closed structures or loops. These are the basic observable
  blocks of the corona, so the analysis of their structure and evolution
  is essential to understand the heating. In this report, mainly addressed
  to astronomers not necessarily familiarized with the subject, we review
  some of the proposed heating models and we pay special attention to
  the sometimes confusing and apparently contradictory observations of
  coronal loops. We discuss the consequences of these observations for
  some of the heating models proposed, in particular those based on
  impulsive events known as nanoflares.

Title: Division II: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Scrijver, Karel J.; Klimchuk,
   James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
   Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
   Vršnak, Bojan; Yan, Yihua
Bibcode: 2015IAUTB..28..106V
Altcode:
  The Business Meeting of Commission 10 was held as part of the Business
  Meeting of Division II (Sun and Heliosphere), chaired by Valentin
  Martínez-Pillet, the President of the Division. The President of
  Commission 10 (C10; Solar activity), Lidia van Driel-Gesztelyi, took
  the chair for the business meeting of C10. She summarised the activities
  of C10 over the triennium and the election of the incoming OC.

Title: Helicity Condensation During Reconnection of Twisted Flux
Tubes: Implications for Coronal Heating
Authors: Knizhnik, Kalman Joshua; Antiochos, Spiro K.; DeVore,
   C. Richard; Klimchuk, James A.; Wyper, Peter F.
Bibcode: 2015shin.confE..18K
Altcode:
  The nature of the heating of the Sun's corona has been a long-standing
  unanswered problem in solar physics. Beginning with the work of Parker
  (1972), many authors have argued that the corona is continuously
  heated through numerous small-scale reconnection events known as
  nanoflares. In these nanoflare models, braiding of magnetic flux tubes
  by surface motions causes the field to become misaligned. The current
  sheet separating the misaligned field eventually reconnects, converting
  the energy stored in the magnetic field into heat. A major challenge
  facing these models, however, is that the braiding required for this
  process injects magnetic helicity into the corona, and helicity is
  conserved under reconnection. In contrast, EUV and X-ray images of
  coronal loops reveal invariably smooth, laminar structures. The
  recently proposed helicity condensation model (Antiochos 2013)
  resolves this difficulty, explaining how reconnection transports
  helicity throughout the solar atmosphere and produces a smooth,
  hot corona. In this model, reconnection between adjacent flux tubes,
  twisted and tangled by surface convection, transports helicity to ever
  larger scales, where it ultimately condenses in filament channels. The
  reconnection that occurs throughout the solar atmosphere not only
  results in a smooth corona, but its net effect is to convert much of
  the magnetic energy injected by surface motions into heat. In this
  work, we use the Adaptively Refined MHD Solver (ARMS) to perform 3D MHD
  simulations that dynamically resolve regions of strong current to study
  the reconnection between multiple twisted flux tubes in a plane-parallel
  Parker configuration. We investigate the energetics of the process, and
  show that the flux tubes accumulate stress gradually before undergoing
  impulsive reconnection. We place constraints on the amount of heating
  expected from such reconnection. Finally, we study the motion of the
  individual field lines during the impulsive reconnection events.

Title: How gas-dynamic flare models powered by Petschek reconnection
    differ from those with ad hoc energy sources
Authors: Longcope, Dana; Klimchuk, Jim
Bibcode: 2015shin.confE...9L
Altcode:
  Aspects of solar flare dynamics, such as chromospheric evaporation
  and flare light-curves, have long been studied using one-dimensional
  models of plasma dynamics inside a static flare loop, subjected to some
  energy input. While extremely successful at explaining the observed
  characteristics of flares, all such models so far have specified energy
  input ad hoc, rather than deriving it self-consistently. There is broad
  consensus that flares are powered by magnetic energy released through
  reconnection. Recent work has generalized Petschek's basic reconnection
  scenario, topological change followed by field line retraction and
  shock heating, to permit its inclusion into a one-dimensional flare
  loop model. Here we compare the gas dynamics driven by retraction and
  shocking to those from more conventional static loop models energized
  by ad hoc source terms. We find significant differences during the
  first minute, when retraction leads to larger kinetic energies and
  produces higher densities at the loop top, while ad hoc heating tends
  to rarify the loop top. The loop-top density concentration is related
  to the slow magnetosonic shock, characteristic of Petschek's model,
  but persists beyond the retraction phase occurring in the outflow
  jet. This offers an explanation of observed loop-top sources of X-ray
  and EUV emission, with advantages over that provided by ad hoc heating
  scenarios. The cooling phases of the two models are, however, notably
  similar to one another, suggesting observations at that stage will
  yield little information on the nature of energy input.

Title: The Myth of Long Duration Flare Emission: Slow Heating or
    Slow Cooling?
Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A.
Bibcode: 2015TESS....130214Q
Altcode:
  Long duration flare emissions lasting for a few hours are likely
  governed by magnetic reconnection that continuously heats flare plasmas
  in continuously formed flare loops. In this study, we confirm that
  this process leads to the long-duration total emission for up to four
  hours in a C2.9 flare on 2011 September 13. Observed by AIA, the flare
  exhibits an ordered spread of flare UV ribbons along the polarity
  inversion line, followed by the sequential formation of post-flare
  loops in EUV emissions. We infer heating rates of thousands of flare
  loops from the UV light curves at the flare foot-points, and model
  the flare total emission with the 0d EBTEL model, which reproduces the
  global evolution pattern of the long-duration flare EUV emissions as
  the result of superposition of continuously formed and heated flare
  loops. However, observations at single loop pixels also show long
  duration EUV emission at 10 MK, long cooling time from 10 MK to 3 MK,
  and later on very short duration of EUV emission at 1-2 MK. All of these
  signatures cannot be produced by superposition of multiple impulsive
  heating events. Our experiments, with both the 0d EBTEL model and a
  1d hydrodynamic model, have demonstrated that a heating profile in
  a single loop consisting of two parts, an intense impulsive heating
  followed by a low-rate heating 1-2 orders of magnitude smaller that
  is attenuated over 20-30 minutes, is required to produce the observed
  time evolution signatures in a single loop. The total energy in the
  gradual heating phase is comparable with that in the impulsive heating
  phase in a flare loop. We discuss viable physical mechanisms for such
  two-phase heating in a post-reconnection flare loop.

Title: How gas-dynamic flare models powered by Petschek reconnection
    differ from thosewith ad hoc energy sources
Authors: Longcope, Dana; Klimchuk, James A.
Bibcode: 2015TESS....130212L
Altcode:
  Many aspects of solar flare dynamics including chromospheric evaporation
  have been, for more than thirty years, studied using one-dimensional
  models of static flaring loops. These models solve one-dimensional
  gas-dynamic equations for the dynamics of plasma inside a static
  loop, subjected to energy input through either non-thermal particles
  or heating. While they have been extremely successful at explaining
  the characteristics of emission observed in flares, none so far have
  been developed in which the energy input is derived self-consistently
  from the loop's dynamics. Instead the energy input is specified ad
  hoc. According to another line of theoretical investigation, flares
  derive their energy from magnetic energy released through fast magnetic
  reconnection. In the model due originally to Petschek reconnection
  occurring in a small diffusion region produces a bent flux tube whose
  retraction generates fast flows (an outflow jet) and shocks where flow
  energy is thermalized. In a recent line of work this scenario has been
  generalized so it may be incorporated into a one-dimensional loop model
  of the kind used so successfully in flare modeling. In this new model
  the flaring loop itself undergoes the retraction and shock formation,
  and thereby introduces the flare energy self-consistently. Here we
  compare the gas dynamics driven by retraction and shocking to those from
  more conventional static loop models. We find significant differences
  during the first minute, when retraction produces high densities at
  the loop top, while ad hoc heating tends to rarify the loop top.

Title: Key Aspects of Coronal Heating
Authors: Klimchuk, James A.
Bibcode: 2015TESS....120308K
Altcode:
  We highlight ten key aspects of coronal heating that must be understood
  before we can consider the problem to be solved. (1) All coronal
  heating is impulsive. (2) The details of coronal heating matter. (3)
  The corona is filled with elemental magnetic stands. (4) The corona is
  densely populated with current sheets. (5) The strands must reconnect
  to prevent an infinite buildup of stress. (6) Nanoflares repeat with
  different frequencies. (7) What is the characteristic magnitude of
  energy release? (8) What causes the collective behavior responsible
  for loops? (9) What are the onset conditions for energy release? (10)
  Chromospheric nanoflares are not a primary source of coronal
  plasma. Significant progress in solving the coronal heating problem
  will require a coordination of approaches: observational studies,
  field-aligned hydrodynamic simulations, large-scale and localized
  3D MHD simulations, and possibly also kinetic simulations. There is
  a unique value to each of these approaches, and the community must
  strive to coordinate better.

Title: Key aspects of coronal heating
Authors: Klimchuk, James A.
Bibcode: 2015RSPTA.37340256K
Altcode: 2014arXiv1410.5660K
  We highlight 10 key aspects of coronal heating that must be understood
  before we can consider the problem to be solved. (1) All coronal
  heating is impulsive. (2) The details of coronal heating matter. (3)
  The corona is filled with elemental magnetic stands. (4) The corona
  is densely populated with current sheets. (5) The strands must
  reconnect to prevent an infinite build-up of stress. (6) Nanoflares
  repeat with different frequencies. (7) What is the characteristic
  magnitude of energy release? (8) What causes the collective behaviour
  responsible for loops? (9) What are the onset conditions for energy
  release? (10) Chromospheric nanoflares are not a primary source of
  coronal plasma. Significant progress in solving the coronal heating
  problem will require coordination of approaches: observational studies,
  field-aligned hydrodynamic simulations, large-scale and localized
  three-dimensional magnetohydrodynamic simulations, and possibly
  also kinetic simulations. There is a unique value to each of these
  approaches, and the community must strive to coordinate better.

Title: Intensity Conserving Spline Interpolation (ICSI): A New Tool
    for Spectroscopic Analysis
Authors: Klimchuk, James A.; Patsourakos, Spiros; Tripathi, Durgesh
Bibcode: 2015TESS....120309K
Altcode: 2015arXiv150608102K
  Spectroscopy is an extremely powerful tool for diagnosing astrophysical
  and other plasmas. For example, the shapes of line profiles provide
  valuable information on the distribution of velocities along
  an optically thin line-of-sight and across the finite area of a
  resolution element. A number of recent studies have measured the
  asymmetries of line profiles in order to detect faint high-speed
  upflows, perhaps associated with coronal nanoflares or perhaps
  associated with chromospheric nanoflares and type II spicules. Over
  most of the Sun, these asymmetries are very subtle, so great care
  must be taken. A common technique is to perform a spline fit of the
  points in the profile in order to extract information at a spectral
  resolution higher than that of the original data. However, a fundamental
  problem is that the fits do not conserve intensity. We have therefore
  developed an iterative procedure called Intensity Conserving Spline
  Interpolation that does preserve the observed intensity within each
  wavelength bin. It improves the measurement of line asymmetries and
  can also help with the determination of line blends.

Title: The Onset of Magnetic Reconnection
Authors: Daldorff, Lars K. S.; Klimchuk, James A.; van der Holst, Bart
Bibcode: 2015TESS....110404D
Altcode:
  A fundamental question concerning magnetic energy release on the Sun is
  why the release occurs only after substantial stresses have been built
  up in the field. If reconnection were to occur readily, the released
  energy would be insufficient to explain coronal heating, CMEs, flares,
  jets, spicules, etc. How can we explain this switch-on property? What
  is the physical nature of the onset conditions? One idea involves the
  "secondary instability" of current sheets, which switches on when
  the rotation of the magnetic field across a current sheet reaches
  a critical angle. Such conditions would occur at the boundaries of
  flux tubes that become tangled and twisted by turbulent photospheric
  convection, for example. Other ideas involve a critical thickness
  for the current sheet. We report here on the preliminary results of
  our investigation of reconnect onset. Unlike our earlier work on the
  secondary instability (Dahlburg, Klimchuk, and Antiochos 2005), we
  treat the coupled chromosphere-corona system. Using the BATS-R-US MHD
  code, we simulate a single current sheet in a sheared magnetic field
  that extends from the chromosphere into the corona. Driver motions are
  applied at the base of the model. The configuration and chromosphere are
  both idealized, but capture the essential physics of the problem. The
  advantage of this unique approach is that it resolves the current sheet
  to the greatest extent possible while maintaining a realistic solar
  atmosphere. It thus bridges the gap between "reconnection in a box"
  studies and studies of large-scale systems such as active regions. One
  question we will address is whether onset conditions are met first
  in the chromosphere or corona. We will report on the work done on
  the project.

Title: Nanoflare Heating of the Quiet Sun
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2015TESS....121303V
Altcode:
  How the solar corona is heated to temperatures of over 1 MK, while
  the photosphere below is only ~ 6000 K remains one of the outstanding
  problems in all of space science. Solving this problem is crucial for
  understanding Sun-Earth connections, and will provide new insight into
  universal processes such as magnetic reconnection and wave-particle
  interactions. We use a new systematic technique to analyze the
  properties of coronal heating throughout the solar corona using data
  taken with the Atmospheric Imaging Assembly onboard the Solar Dynamics
  Observatory. Our technique computes cooling times of the coronal plasma
  on a pixel-by-pixel basis and has the advantage that it analyzes all
  of the coronal emission, including the diffuse emission surrounding
  distinguishable coronal features. We have already applied this technique
  to 15 different active regions, and find clear evidence for dynamic
  heating and cooling cycles that are consistent with the 'impulsive
  nanoflare' scenario. What about the rest of the Solar corona? Whether
  the quiet Sun is heated in a similar or distinct manner from active
  regions is a matter of great debate. In this paper, we apply our coronal
  heating analysis technique to quiet Sun locations. We find that the
  majority of the analyzed quiet Sun locations do undergo dynamic heating
  and cooling cycles, consistent with impulsive nanoflares. However, there
  are important characteristics that are distinct from those of active
  regions.This research was supported by a NASA Guest Investigator grant.

Title: Synthetic 3D modeling of active regions and simulation of
    their multi-wavelength emission
Authors: Nita, Gelu M.; Fleishman, Gregory; Kuznetsov, Alexey A.;
   Loukitcheva, Maria A.; Viall, Nicholeen M.; Klimchuk, James A.; Gary,
   Dale E.
Bibcode: 2015TESS....131204N
Altcode:
  To facilitate the study of solar active regions, we have created a
  synthetic modeling framework that combines 3D magnetic structures
  obtained from magnetic extrapolations with simplified 1D thermal
  models of the chromosphere, transition region, and corona. To handle,
  visualize, and use such synthetic data cubes to compute multi-wavelength
  emission maps and compare them with observations, we have
  undertaken a major enhancement of our simulation tools, GX_Simulator
  (ftp://sohoftp.nascom.nasa.gov/solarsoft/packages/gx_simulator/),
  developed earlier for modeling emission from flaring loops. The greatly
  enhanced, object-based architecture, which now runs on Windows, Mac,
  and UNIX platform, offers important new capabilities that include the
  ability to either import 3D density and temperature distribution models,
  or to assign to each individual voxel numerically defined coronal
  or chromospheric temperature and densities, or coronal Differential
  Emission Measure distributions. Due to these new capabilities, the
  GX_Simulator can now apply parametric heating models involving average
  properties of the magnetic field lines crossing a given voxel volume,
  as well as compute and investigate the spatial and spectral properties
  of radio (to be compared with VLA or EOVSA data), (sub-)millimeter
  (ALMA), EUV (AIA/SDO), and X-ray (RHESSI) emission calculated from the
  model. The application integrates shared-object libraries containing
  fast free-free, gyrosynchrotron, and gyroresonance emission codes
  developed in FORTRAN and C++, and soft and hard X-ray and EUV codes
  developed in IDL. We use this tool to model and analyze an active
  region and compare the synthetic emission maps obtained in different
  wavelengths with observations.This work was partially supported
  by NSF grants AGS-1250374, AGS-1262772, NASA grant NNX14AC87G, the
  Marie Curie International Research Staff Exchange Scheme "Radiosun"
  (PEOPLE-2011-IRSES-295272), RFBR grants 14-02-91157, 15-02-01089,
  15-02-03717, 15-02-03835, 15-02-08028.

Title: Two-dimensional Cellular Automaton Model for the Evolution
    of Active Region Coronal Plasmas
Authors: López Fuentes, Marcelo; Klimchuk, James A.
Bibcode: 2015ApJ...799..128L
Altcode: 2016arXiv160703883L
  We study a two-dimensional cellular automaton (CA) model for the
  evolution of coronal loop plasmas. The model is based on the idea that
  coronal loops are made of elementary magnetic strands that are tangled
  and stressed by the displacement of their footpoints by photospheric
  motions. The magnetic stress accumulated between neighbor strands
  is released in sudden reconnection events or nanoflares that heat
  the plasma. We combine the CA model with the Enthalpy Based Thermal
  Evolution of Loops model to compute the response of the plasma to
  the heating events. Using the known response of the X-Ray Telescope
  on board Hinode, we also obtain synthetic data. The model obeys
  easy-to-understand scaling laws relating the output (nanoflare
  energy, temperature, density, intensity) to the input parameters
  (field strength, strand length, critical misalignment angle). The
  nanoflares have a power-law distribution with a universal slope of
  -2.5, independent of the input parameters. The repetition frequency of
  nanoflares, expressed in terms of the plasma cooling time, increases
  with strand length. We discuss the implications of our results for
  the problem of heating and evolution of active region coronal plasmas.

Title: The Transition Region Response to a Coronal Nanoflare:
    Forward Modeling and Observations in SDO/AIA
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2015ApJ...799...58V
Altcode:
  The corona and transition region (TR) are fundamentally coupled
  through the processes of thermal conduction and mass exchange. It
  is not possible to understand one without the other. Yet the
  temperature-dependent emissions from the two locations behave quite
  differently in the aftermath of an impulsive heating event such as a
  coronal nanoflare. Whereas the corona cools sequentially, emitting
  first at higher temperatures and then at lower temperatures, the
  TR is multithermal and the emission at all temperatures responds in
  unison. We have previously applied the automated time lag technique
  of Viall &amp; Klimchuk to disk observations of an active region
  (AR) made by the Atmospheric Imaging Assembly (AIA) on the Solar
  Dynamics Observatory. Lines of sight passing through coronal plasma
  show clear evidence for post-nanoflare cooling, while lines of sight
  intersecting the TR footpoints of coronal strands show zero time lag. In
  this paper, we use the EBTEL hydrodynamics code to demonstrate that
  this is precisely the expected behavior when the corona is heated by
  nanoflares. We also apply the time lag technique for the first time
  to off-limb observations of an AR. Since TR emission is not present
  above the limb, the occurrence of zero time lags is greatly diminished,
  supporting the conclusion that zero time lags measured on the disk are
  due to TR plasma. Lastly, we show that the ''coronal'' channels in AIA
  can be dominated by bright TR emission. When defined in a physically
  meaningful way, the TR reaches a temperature of roughly 60% the peak
  temperature in a flux tube. The TR resulting from impulsive heating
  can extend to 3 MK and higher, well within the range of the ''coronal''
  AIA channels.

Title: Intensity Conserving Spline Interpolation (ICSI): A New Tool
    for Spectroscopic Analysis
Authors: Klimchuk, J. A.; Patsourakos, S.; Tripathi, D.
Bibcode: 2014AGUFMSH13B4109K
Altcode:
  Spectroscopy is an extremely powerful tool for diagnosing astrophysical
  and other plasmas. For example, the shapes of line profiles provide
  valuable information on the distribution of velocities along
  an optically thin line-of-sight and across the finite area of a
  resolution element. A number of recent studies have measured the
  asymmetries of line profiles in order to detect faint high-speed
  upflows, perhaps associated with coronal nanoflares or perhaps
  associated with chromospheric nanoflares and type II spicules. Over
  most of the Sun, these asymmetries are very subtle, so great care
  must be taken. A common technique is to perform a spline fit of the
  points in the profile in order to extract information at a spectral
  resolution higher than that of the original data. However, a fundamental
  problem is that the fits do not conserve intensity. We have therefore
  developed an iterative procedure called Intensity Conserving Spline
  Interpolation that does preserve the observed intensity within each
  wavelength bin. It improves the measurement of line asymmetries and
  can also help with the determination of line blends.

Title: The Onset of Magnetic Reconnection in the Solar Atmosphere
Authors: Evans, R. M.; Klimchuk, J. A.; van der Holst, B.
Bibcode: 2014AGUFMSH12A..02E
Altcode:
  A fundamental question concerning magnetic energy release on the Sun
  is why the release occurs only after substantial stresses have been
  built up in the field. If reconnection were to occur readily, then
  the released energy would be much less than the energy required for
  coronal heating, CMEs, flares, jets, spicules, etc. How can we explain
  this switch-on property? What is the physical nature of the onset
  conditions? One idea involves the secondary instability of current
  sheets, which switches on when the rotation of the magnetic field
  across a current sheet reaches a critical angle. Such conditions would
  occur at the boundaries of flux tubes that become tangled and twisted
  by turbulent photospheric convection, for example. Other ideas focus
  on a critical thickness for the current sheet. We report here on the
  preliminary results of our investigation of reconnection onset. Unlike
  our earlier work on the secondary instability (Dahlburg, Klimchuk,
  and Antiochos 2005), here we treat the coupled chromosphere-corona
  system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a
  single current sheet in a sheared magnetic field that extends from the
  chromosphere into the corona. Driver motions are applied at the base
  of the model. The configuration and chromosphere are both idealized,
  but capture the essential physics of the problem. The advantage of this
  unique approach is that it resolves the current sheet to the greatest
  extent possible while maintaining a realistic solar atmosphere. It thus
  bridges the gap between reconnection in a box studies and studies of
  large-scale systems such as active regions. One question we will address
  is whether onset conditions are met first in the chromosphere or corona.

Title: Hi-C Observations and the Structure of Coronal Loops
Authors: DeForest, C. E.; Klimchuk, J. A.
Bibcode: 2014AGUFMSH31C..04D
Altcode:
  Despite nearly four decades of study since the launch of Skylab, the
  physical structure of coronal loops remains an enigma. Loops are guided
  by the magnetic field and, in the common EUV emission lines, appear
  to be composed of stranded structures reminiscent of field lines. This
  stranded structure appears to have constant or nearly-constant width,
  at odds with naive understanding of flux tube behavior in a field
  gradient. Possible explanations range from physical solutions such as
  twisted magnetic structure or peculiar properties of separators and
  quasi-separators, to observation effects that invoke finite resolution
  or anisotropy of the field containing each strand. The uncertainty
  affects many aspects of basic coronal physics, because some of the
  possible explanations for stranded structure have strong implications
  for other mysteries such as the anomalously tall scale height of the
  EUV corona. The Hi-C EUV images are the highest resolution coronal
  images to date, and offer new insights into the structure of coronal
  loops. We present an overview of research to date, show results from
  a detailed analysis of several dozen well-presented loops that are
  visible in the Hi-C data set, and speculate on the implications for
  the rest of the corona.

Title: Emission Measure Distribution for Diffuse Regions in Solar
    Active Regions
Authors: Subramanian, Srividya; Tripathi, Durgesh; Klimchuk, James A.;
   Mason, Helen E.
Bibcode: 2014ApJ...795...76S
Altcode: 2014arXiv1409.1447S
  Our knowledge of the diffuse emission that encompasses active regions
  is very limited. In this paper we investigate two off-limb active
  regions, namely, AR 10939 and AR 10961, to probe the underlying heating
  mechanisms. For this purpose, we have used spectral observations from
  Hinode/EIS and employed the emission measure (EM) technique to obtain
  the thermal structure of these diffuse regions. Our results show that
  the characteristic EM distributions of the diffuse emission regions peak
  at log T = 6.25 and the coolward slopes are in the range 1.4-3.3. This
  suggests that both low- as well as high-frequency nanoflare heating
  events are at work. Our results provide additional constraints on the
  properties of these diffuse emission regions and their contribution to
  the background/foreground when active region cores are observed on-disk.

Title: Are Chromospheric Nanoflares a Primary Source of Coronal
    Plasma?
Authors: Klimchuk, J. A.; Bradshaw, S. J.
Bibcode: 2014ApJ...791...60K
Altcode: 2014arXiv1405.1708K
  It has been suggested that the hot plasma of the solar corona comes
  primarily from impulsive heating events, or nanoflares, that occur
  in the lower atmosphere, either in the upper part of the ordinary
  chromosphere or at the tips of type II spicules. We test this idea with
  a series of hydrodynamic simulations. We find that synthetic Fe XII
  (195) and Fe XIV (274) line profiles generated from the simulations
  disagree dramatically with actual observations. The integrated line
  intensities are much too faint; the blueshifts are much too fast; the
  blue-red asymmetries are much too large; and the emission is confined
  to low altitudes. We conclude that chromospheric nanoflares are not a
  primary source of hot coronal plasma. Such events may play an important
  role in producing the chromosphere and powering its intense radiation,
  but they do not, in general, raise the temperature of the plasma to
  coronal values. Those cases where coronal temperatures are reached
  must be relatively uncommon. The observed profiles of Fe XII and Fe
  XIV come primarily from plasma that is heated in the corona itself,
  either by coronal nanoflares or a quasi-steady coronal heating
  process. Chromospheric nanoflares might play a role in generating
  waves that provide this coronal heating.

Title: MHD modelling of coronal loops: injection of high-speed
    chromospheric flows
Authors: Petralia, A.; Reale, F.; Orlando, S.; Klimchuk, J. A.
Bibcode: 2014A&A...567A..70P
Altcode: 2014arXiv1405.2198P
  Context. Observations reveal a correspondence between chromospheric type
  II spicules and bright upward-moving fronts in the corona observed in
  the extreme-ultraviolet (EUV) band. However, theoretical considerations
  suggest that these flows are probably not the main source of heating
  in coronal magnetic loops. <BR /> Aims: We investigate the propagation
  of high-speed chromospheric flows into coronal magnetic flux tubes and
  the possible production of emission in the EUV band. <BR /> Methods: We
  simulated the propagation of a dense 10<SUP>4</SUP> K chromospheric jet
  upward along a coronal loop by means of a 2D cylindrical MHD model that
  includes gravity, radiative losses, thermal conduction, and magnetic
  induction. The jet propagates in a complete atmosphere including
  the chromosphere and a tenuous cool (~0.8 MK) corona, linked through
  a steep transition region. In our reference model, the jet initial
  speed is 70 km s<SUP>-1</SUP>, its initial density is 10<SUP>11</SUP>
  cm<SUP>-3</SUP>, and the ambient uniform magnetic field is 10 G. We
  also explored other values of jet speed and density in 1D and different
  magnetic field values in 2D, as well as the jet propagation in a hotter
  (~1.5 MK) background loop. <BR /> Results: While the initial speed of
  the jet does not allow it to reach the loop apex, a hot shock-front
  develops ahead of it and travels to the other extreme of the loop. The
  shock front compresses the coronal plasma and heats it to about
  10<SUP>6</SUP> K. As a result, a bright moving front becomes visible
  in the 171 Å channel of the SDO/AIA mission. This result generally
  applies to all the other explored cases, except for the propagation in
  the hotter loop. <BR /> Conclusions: For a cool, low-density initial
  coronal loop, the post-shock plasma ahead of upward chromospheric
  flows might explain at least part of the observed correspondence
  between type II spicules and EUV emission excess. <P />Movies
  associated to Figs. 3, 6, 7 are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201323012/olm">http://www.aanda.org</A>

Title: The Onset of Magnetic Reconnection in the Solar Atmosphere
Authors: Evans, Rebekah Minnel; Klimchuk, James; van der Holst, Bart
Bibcode: 2014shin.confE..65E
Altcode:
  A fundamental question concerning magnetic energy release on the Sun
  is why the release occurs only after substantial stresses have been
  built up in the field. If reconnection were to occur readily, then
  the released energy would be much less than the energy required for
  coronal heating, CMEs, flares, jets, spicules, etc. How can we explain
  this switch-on property? What is the physical nature of the onset
  conditions? One idea involves the secondary instability of current
  sheets, which switches on when the rotation of the magnetic field
  across a current sheet reaches a critical angle. Such conditions would
  occur at the boundaries of flux tubes that become tangled and twisted
  by turbulent photospheric convection, for example. Other ideas focus
  on a critical thickness for the current sheet. We report here on the
  preliminary results of our investigation of reconnection onset. Unlike
  our earlier work on the secondary instability (Dahlburg, Klimchuk,
  and Antiochos 2005), here we treat the coupled chromosphere-corona
  system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a
  single current sheet in a sheared magnetic field that extends from the
  chromosphere into the corona. Driver motions are applied at the base
  of the model. The configuration and chromosphere are both idealized,
  but capture the essential physics of the problem. The advantage of this
  unique approach is that it resolves the current sheet to the greatest
  extent possible while maintaining a realistic solar atmosphere. It thus
  bridges the gap between reconnection in a box studies and studies of
  large-scale systems such as active regions. One question we will address
  is whether onset conditions are met first in the chromosphere or corona.

Title: Modeling the response of the lower atmosphere to flare
    reconnection
Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, James A.
Bibcode: 2014AAS...22412324L
Altcode:
  It has long been recognized that energy release in a solar flare gives
  rise to ablation of material from the chromosphere (more commonly
  called evaporation). The prevailing view is that energy is initially
  transformed from stored magnetic energy by the process of magnetic
  reconnection. In some models reconnection accelerates electrons, either
  directly or indirectly, and these non-thermal electrons carry energy
  to the chromospheric footpoints. In others the reconnection converts
  magnetic energy into heat in the corona and thermal conduction carries
  that heat to the chromosphere. While no comprehensive, self-consistent
  model yet exists for the conversion of magnetic energy to non-thermal
  electron energy, models of the conversion to heat, via slow magnetosonic
  shocks, have been available since Petschek's 1964 paper. We present
  a numerical model encompassing the conversion of magnetic energy to
  shocks, to heat, and then to conduction-driven evaporation. We compare
  its results to those of more traditional conduction-driven models where
  reconnection is replaced by an ad hoc plasma heating. We consider, in
  particular, observable signatures such as doppler shifts and formation
  of flare ribbons.This work was supported by the NASA SR&amp;T program.

Title: A one-dimensional solar flare model capturing reconnection
    energy release, evaporation, and gradually cooling post-flare loops
Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, Jim
Bibcode: 2014shin.confE..32L
Altcode:
  The most obvious signature of energy release in a solar flare is the
  large amount of chromospheric material heated to coronal temperatures
  through a process known (inaccurately) as chromospheric evaporation. The
  thousand-fold increase in X-ray luminosity we associate with a
  flare is due entirely to evaporation. The most successful models of
  flare evaporation to date have come from one-dimensional flux tube
  simulations. These have provided the best means of resolving the
  very thin pre-flare transition region, and of easily accommodating
  the perfect field-alignment of the energy transport by either
  non-thermal electrons or thermal conduction. In traditional flux
  tube models magnetic reconnection is represented by an ad hoc heating
  term. This adds energy but no momentum, and represents only crudely
  known models of magnetic reconnection. Here we present a new approach
  which captures the physics of fast Petschek reconnection in a flux tube
  simulation. Following its creation by localized reconnection within a
  current sheet, the flux tube retracts under magnetic tension, converting
  magnetic energy into bulk flows; this is the outflow jet. These flows
  form a slow magnetosonic shock which heats the coronal plasma and
  drives a conduction front into the chromosphere. Our one-dimensional
  model captures the energy release, thermalization, and the evaporation
  it drives. We find observable signatures of the interplay between
  reconnection energy release and evaporation; signatures different
  from those found in conventional flux tube models with ad hoc heating,
  but similar to actual flare observations.

Title: The Onset of Magnetic Reconnection in the Solar Atmosphere
Authors: Evans, Rebekah M.; Klimchuk, James A.; Van Der Holst, Bart
Bibcode: 2014AAS...22432342E
Altcode:
  A fundamental question concerning magnetic energy release on the Sun
  is why the release occurs only after substantial stresses have been
  built up in the field. If reconnection were to occur readily, then
  the released energy would be much less than the energy required for
  coronal heating, CMEs, flares, jets, spicules, etc. How can we explain
  this switch-on property? What is the physical nature of the onset
  conditions? One idea involves the "secondary instability" of current
  sheets, which switches on when the rotation of the magnetic field
  across a current sheet reaches a critical angle. Such conditions would
  occur at the boundaries of flux tubes that become tangled and twisted
  by turbulent photopheric convection, for example. Other ideas focus
  on a critical thickness for the current sheet. We report here on the
  preliminary results of our investigation of reconnection onset. Unlike
  our earlier work on the secondary instability (Dahlburg, Klimchuk,
  and Antiochos 2005), here we treat the coupled chromosphere-corona
  system. Using the BATS-R-US MHD code (Toth et al. 2012), we simulate a
  single current sheet in a sheared magnetic field that extends from the
  chromosphere into the corona. Driver motions are applied at the base
  of the model. The configuration and chromosphere are both idealized,
  but capture the essential physics of the problem. The advantage of this
  unique approach is that it resolves the current sheet to the greatest
  extent possible while maintaining a realistic solar atmosphere. It thus
  bridges the gap between "reconnection in a box" studies and studies of
  large-scale systems such as active regions. One question we will address
  is whether onset conditions are met first in the chromosphere or corona.

Title: Long Duration Flare Emission by Sequential Reconnection
    and Heating
Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A.
Bibcode: 2014AAS...22412325Q
Altcode:
  Long duration flare emissions lasting for a few hours are likely
  produced by magnetic reconnection that continuously forms flare loops
  and heats plasma inside. In this study, we demonstrate that this
  process leads to the long duration emission in a C2.9 flare on 2011
  September 13. Observed by AIA, the flare exhibits an organized pattern
  of evolution with UV brightenings in flare ribbons spreading along the
  polarity inversion line, followed by sequential formation of post-flare
  loops seen in EUV emissions. The spatially resolved observation of
  flare ribbons can be used to infer heating rates insequentially formed
  and heated flare loops, with which we synthesize flare emission in
  these loops with hydrodynamic models. The 0d EBTEL model (Klimchuk
  et al. 2008) efficiently computes meanproperties of thousands of
  flare loops identified from flare ribbon signatures, and the synthetic
  tempo-spatial evolution of the total emission is in reasonable agreement
  with EUV observations. The 1d model applied on a few selected loops
  reveals physics of the heating mechanism and along-the-loop dynamics,
  particularly during the impulsive heating phase. During the four hours
  of this event, the estimated total energy in the heating amounts to
  2e30 erg, with the total reconnection flux about 1e21 Mx.

Title: A Survey of Coronal Heating Properties in Solar Active Regions
Authors: Viall, Nicholeen; Klimchuk, James A.
Bibcode: 2014AAS...22432315V
Altcode:
  We investigate the properties of coronal heating in solar active
  regions (AR) by systematically analyzing coronal light curves
  observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics
  Observatory. Our automated technique computes time-lags (cooling times)
  on a pixel-by-pixel basis, and has the advantage that it allows us
  to analyze all of the coronal AR emission, including the so-called
  diffuse emission between coronal loops. We recently presented results
  using this time-lag analysis on NOAA AR 11082 (Viall &amp; Klimchuk
  2012) and found that the majority of the pixels contained cooling
  plasma along their line of sight. This result is consistent with
  impulsive coronal nanoflare heating of both coronal loops and the
  surrounding diffuse emission in the AR. Here we present the results
  of our time-lag technique applied to a survey of 15 AR of different
  magnetic complexity, total unsigned magnetic flux, size and age. We
  show that the post-nanoflare cooling patterns identified in NOAA AR
  11082 are identified throughout all of the active regions in this
  survey, indicating that nanoflare heating is ubiquitous in solar
  active regions. However, some details of the nanoflare properties,
  such as the nanoflare energy, are different across these different
  active regions.We thank the SDO/AIA team for the use of these data,
  and the Coronal Heating ISSI team for helpful discussion of these
  topics. This research was supported by a NASA Heliophysics GI.

Title: Evidence for Impulsive Coronal Heating from EUNIS 2013
Authors: Daw, Adrian N.; Brosius, Jeffrey W.; Rabin, Douglas M.;
   Landi, Enrico; Klimchuk, James A.
Bibcode: 2014AAS...22431204D
Altcode:
  Pervasive, faint Fe XIX 592 Å line emission was observed in active
  regions by the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS)
  sounding rocket instrument on 23 April 2013. The broad spectral coverage
  (303-370 Å, 527-635 Å) and unprecedented dynamic range of the EUNIS
  observations includes emission lines of ionization stages from He I to
  Fe XX, and thus a wide temperature range of 0.03 to 10 MK. Comparison
  of observed line intensities with calculations demonstrates that the
  Fe XIX emission, formed at temperatures around 8 MK, is evidence of
  the faint hot emission predicted by impulsive heating models of the
  solar corona (such as nanoflares).

Title: Chromospheric Nanoflares
Authors: Klimchuk, James A.; Bradshaw, Stephen
Bibcode: 2014AAS...22430206K
Altcode:
  The traditional view is that coronal plasma results from energy
  release (reconnection, waves, etc.) that takes place above the
  chromosphere, in the corona itself. However, this need not be the
  case. Cool chromospheric plasma could be directly heated to high
  temperatures and rise upward to fill the corona. We here investigate
  this scenario in the context of impulsive nanoflares that occur in the
  upper chromosphere. Such events could represent the sudden heating
  of the tips of type II spicules, or they could take place in the
  ordinary chromosphere, completely unrelated to spicules. The results
  are general. We generate synthetic line profiles of Fe XIV (274) and
  Fe XII (195) based on a series of simulations performed with the HYDRAD
  1D hydro code, which includes non-equilibrium ionization. We find that
  the profiles are strongly blue-shifted and/or have large asymmetries
  that are grossly inconsistent with actual observations. The results
  are in agreement with our previous analytical predictions (Klimchuk
  2012). We conclude that most coronal plasma is a result of coronal
  energy release, not chromospheric nanoflares or type II spicules.

Title: MHD modeling of coronal loops: the transition region throat
Authors: Guarrasi, M.; Reale, F.; Orlando, S.; Mignone, A.; Klimchuk,
   J. A.
Bibcode: 2014A&A...564A..48G
Altcode: 2014arXiv1402.0338G
  Context. The expansion of coronal loops in the transition region
  may considerably influence the diagnostics of the plasma emission
  measure. The cross-sectional area of the loops is expected to depend
  on the temperature and pressure, and might be sensitive to the heating
  rate. <BR /> Aims: The approach here is to study the area response
  to slow changes in the coronal heating rate, and check the current
  interpretation in terms of steady heating models. <BR /> Methods: We
  study the area response with a time-dependent 2D magnetohydrodynamic
  (MHD) loop model, including the description of the expanding magnetic
  field, coronal heating and losses by thermal conduction, and radiation
  from optically thin plasma. We run a simulation for a loop 50 Mm
  long and quasi-statically heated to about 4 MK. <BR /> Results: We
  find that the area can change substantially with the quasi-steady
  heating rate, e.g., by ~40% at 0.5 MK as the loop temperature varies
  between 1 MK and 4 MK, and, therefore, affects the interpretation of
  the differential emission measure vs. temperature (DEM(T)) curves. <P
  />The movie associated to Fig. 4 is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201322848/olm">http://www.aanda.org</A>

Title: Core and Wing Densities of Asymmetric Coronal Spectral
Profiles: Implications for the Mass Supply of the Solar Corona
Authors: Patsourakos, S.; Klimchuk, J. A.; Young, P. R.
Bibcode: 2014ApJ...781...58P
Altcode: 2013arXiv1312.4842P
  Recent solar spectroscopic observations have shown that coronal spectral
  lines can exhibit asymmetric profiles, with enhanced emissions at their
  blue wings. These asymmetries correspond to rapidly upflowing plasmas
  at speeds exceeding ≈50 km s<SUP>-1</SUP>. Here, we perform a study
  of the density of the rapidly upflowing material and compare it with
  that of the line core that corresponds to the bulk of the plasma. For
  this task, we use spectroscopic observations of several active regions
  taken by the Extreme Ultraviolet Imaging Spectrometer of the Hinode
  mission. The density sensitive ratio of the Fe XIV lines at 264.78 and
  274.20 Å is used to determine wing and core densities. We compute the
  ratio of the blue wing density to the core density and find that most
  values are of order unity. This is consistent with the predictions for
  coronal nanoflares if most of the observed coronal mass is supplied
  by chromospheric evaporation driven by the nanoflares. However,
  much larger blue wing-to-core density ratios are predicted if most of
  the coronal mass is supplied by heated material ejected with type II
  spicules. Our measurements do not rule out a spicule origin for the
  blue wing emission, but they argue against spicules being a primary
  source of the hot plasma in the corona. We note that only about 40%
  of the pixels where line blends could be safely ignored have blue wing
  asymmetries in both Fe XIV lines. Anticipated sub-arcsecond spatial
  resolution spectroscopic observations in future missions could shed
  more light on the origin of blue, red, and mixed asymmetries.

Title: Cross-Sectional Properties of Coronal Loops
Authors: West, Matthew; Zhukov, Andrei; Klimchuk, James
Bibcode: 2014cosp...40E3620W
Altcode:
  In this work we assess if coronal loop cross sections observed in
  EUV images are symmetrical or asymmetrical in nature. To do this, we
  identified individual loop structures observed in EUV images taken
  with the EUVI instruments on the STEREO satellites. To image loops
  from two unique angles, we chose loops clearly discernible in both
  EUVI imagers during the period when the satellites were separated by
  approximately 90 degrees, allowing us to make observations of individual
  loops from two unique vantage points. Preference was given to loops
  which could be clearly identified in both satellites, especially
  those which were not crossed by other bright structures or loops,
  so reasonable background subtractions could be made. Once identified,
  the images were co-aligned and straightened, using a spline routine,
  for comparison. In total we identified 11 clearly discernible loops
  and derived the standard deviation and widths for both perspectives
  of the loop. It was found that within instrumental errors the loops
  can be considered circular in nature.

Title: EUV emission along observed coronal loops
Authors: Lopez Fuentes, Marcelo; Klimchuk, James
Bibcode: 2014cosp...40E1872L
Altcode:
  Theories based on steady heating concentrated at the footpoints of
  coronal loops predict a state of thermal nonequilibrium which produce,
  according to numerical studies, highly asymmetric intensity profiles. In
  this work we study a series of coronal loop observations obtained with
  the Transition Region and Coronal Explorer (TRACE) and the Atmospheric
  Imager Assembly (AIA) on board the Solar Dynamics Observatory (SDO)
  to explore how the intensity varies with position along the loops. As
  part of the data processing we apply a careful procedure to subtract
  the background contribution to the loop intensity. We find that the
  obtained intrinsic intensities have strong variations over short
  distances, due in part to a residual effect of the intense nonuniform
  background. Although it is not easy to separate the background
  contribution from the proper loop intensity fluctuations, our results
  show that in the majority of the cases the intensity structure is not
  consistent with the predicted profiles.

Title: Asymmetries in Coronal Spectral Lines and Emission Measure
    Distribution
Authors: Tripathi, Durgesh; Klimchuk, James A.
Bibcode: 2013ApJ...779....1T
Altcode: 2013arXiv1310.0168T
  It has previously been argued that (1) spicules do not provide enough
  pre-heated plasma to fill the corona, and (2) even if they did,
  additional heating would be required to keep the plasma hot as it
  expands upward. Here we address whether spicules play an important
  role by injecting plasma at cooler temperatures (&lt;2 MK), which
  then gets heated to coronal values at higher altitudes. We measure
  red-blue asymmetries in line profiles formed over a wide range of
  temperatures in the bright moss areas of two active regions. We derive
  emission measure distributions from the excess wing emission. We find
  that the asymmetries and emission measures are small and conclude
  that spicules do not inject an important (dominant) mass flux into
  the cores of active regions at temperatures &gt;0.6 MK (log T &gt;
  5.8). These conclusions apply not only to spicules but also to any
  process that suddenly heats and accelerates chromospheric plasma
  (e.g., a chromospheric nanoflare). The traditional picture of coronal
  heating and chromospheric evaporation appears to remain the most likely
  explanation of the active region corona.

Title: Can the Differential Emission Measure Constrain the Timescale
    of Energy Deposition in the Corona?
Authors: Guennou, C.; Auchère, F.; Klimchuk, J. A.; Bocchialini,
   K.; Parenti, S.
Bibcode: 2013ApJ...774...31G
Altcode: 2013arXiv1306.3114G
  In this paper, the ability of the Hinode/EIS instrument to detect
  radiative signatures of coronal heating is investigated. Recent
  observational studies of active region cores suggest that both
  the low and high frequency heating mechanisms are consistent with
  observations. Distinguishing between these possibilities is important
  for identifying the physical mechanism(s) of the heating. The
  differential emission measure (DEM) tool is one diagnostic that
  allows us to make this distinction, through the amplitude of the
  DEM slope coolward of the coronal peak. It is therefore crucial to
  understand the uncertainties associated with these measurements. Using
  proper estimations of the uncertainties involved in the problem
  of DEM inversion, we derive confidence levels on the observed DEM
  slope. Results show that the uncertainty in the slope reconstruction
  strongly depends on the number of lines constraining the slope. Typical
  uncertainty is estimated to be about ±1.0 in the more favorable cases.

Title: Ultraviolet and Extreme-ultraviolet Emissions at the Flare
    Footpoints Observed by Atmosphere Imaging Assembly
Authors: Qiu, Jiong; Sturrock, Zoe; Longcope, Dana W.; Klimchuk,
   James A.; Liu, Wen-Juan
Bibcode: 2013ApJ...774...14Q
Altcode: 2013arXiv1305.6899Q
  A solar flare is composed of impulsive energy release events by magnetic
  reconnection, which forms and heats flare loops. Recent studies have
  revealed a two-phase evolution pattern of UV 1600 Å emission at the
  feet of these loops: a rapid pulse lasting for a few seconds to a few
  minutes, followed by a gradual decay on timescales of a few tens of
  minutes. Multiple band EUV observations by the Atmosphere Imaging
  Assembly further reveal very similar signatures. These two phases
  represent different but related signatures of an impulsive energy
  release in the corona. The rapid pulse is an immediate response of
  the lower atmosphere to an intense thermal conduction flux resulting
  from the sudden heating of the corona to high temperatures (we rule
  out energetic particles due to a lack of significant hard X-ray
  emission). The gradual phase is associated with the cooling of hot
  plasma that has been evaporated into the corona. The observed footpoint
  emission is again powered by thermal conduction (and enthalpy), but now
  during a period when approximate steady-state conditions are established
  in the loop. UV and EUV light curves of individual pixels may therefore
  be separated into contributions from two distinct physical mechanisms to
  shed light on the nature of energy transport in a flare. We demonstrate
  this technique using coordinated, spatially resolved observations of
  UV and EUV emissions from the footpoints of a C3.2 thermal flare.

Title: Structure of solar coronal loops: from miniature to large-scale
Authors: Peter, H.; Bingert, S.; Klimchuk, J. A.; de Forest, C.;
   Cirtain, J. W.; Golub, L.; Winebarger, A. R.; Kobayashi, K.; Korreck,
   K. E.
Bibcode: 2013A&A...556A.104P
Altcode: 2013arXiv1306.4685P
  <BR /> Aims: We use new data from the High-resolution Coronal Imager
  (Hi-C) with its unprecedented spatial resolution of the solar corona
  to investigate the structure of coronal loops down to 0.2''. <BR />
  Methods: During a rocket flight, Hi-C provided images of the solar
  corona in a wavelength band around 193 Å that is dominated by emission
  from Fe xii showing plasma at temperatures around 1.5 MK. We analyze
  part of the Hi-C field-of-view to study the smallest coronal loops
  observed so far and search for the possible substructuring of larger
  loops. <BR /> Results: We find tiny 1.5 MK loop-like structures that
  we interpret as miniature coronal loops. Their coronal segments above
  the chromosphere have a length of only about 1 Mm and a thickness of
  less than 200 km. They could be interpreted as the coronal signature
  of small flux tubes breaking through the photosphere with a footpoint
  distance corresponding to the diameter of a cell of granulation. We
  find that loops that are longer than 50 Mm have diameters of about 2''
  or 1.5 Mm, which is consistent with previous observations. However, Hi-C
  really resolves these loops with some 20 pixels across the loop. Even
  at this greatly improved spatial resolution, the large loops seem to
  have no visible substructure. Instead they show a smooth variation in
  cross-section. <BR /> Conclusions: That the large coronal loops do not
  show a substructure on the spatial scale of 0.1'' per pixel implies that
  either the densities and temperatures are smoothly varying across these
  loops or it places an upper limit on the diameter of the strands the
  loops might be composed of. We estimate that strands that compose the
  2'' thick loop would have to be thinner than 15 km. The miniature loops
  we find for the first time pose a challenge to be properly understood
  through modeling. <P />Appendices are available in electronic form at
  <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: Modeling the Line-of-sight Integrated Emission in the Corona:
    Implications for Coronal Heating
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2013ApJ...771..115V
Altcode: 2013arXiv1304.5439V
  One of the outstanding problems in all of space science is uncovering
  how the solar corona is heated to temperatures greater than 1 MK. Though
  studied for decades, one of the major difficulties in solving this
  problem has been unraveling the line-of-sight (LOS) effects in the
  observations. The corona is optically thin, so a single pixel measures
  counts from an indeterminate number (perhaps tens of thousands)
  of independently heated flux tubes, all along that pixel's LOS. In
  this paper we model the emission in individual pixels imaging the
  active region corona in the extreme ultraviolet. If LOS effects are
  not properly taken into account, erroneous conclusions regarding both
  coronal heating and coronal dynamics may be reached. We model the corona
  as an LOS integration of many thousands of completely independently
  heated flux tubes. We demonstrate that despite the superposition of
  randomly heated flux tubes, nanoflares leave distinct signatures in
  light curves observed with multi-wavelength and high time cadence
  data, such as those data taken with the Atmospheric Imaging Assembly
  on board the Solar Dynamics Observatory. These signatures are readily
  detected with the time-lag analysis technique of Viall &amp; Klimchuk
  in 2012. Steady coronal heating leaves a different and equally distinct
  signature that is also revealed by the technique.

Title: Where is Coronal Plasma Heated?
Authors: Klimchuk, James A.; Bradshaw, S.; Patsourakos, S.; Tripathi,
   D.
Bibcode: 2013SPD....4420006K
Altcode:
  The coupling between the chromosphere and corona is a question of
  great current interest. It has long been understood that coronal mass
  originates in the chromosphere and that the energy which powers the
  corona flows up through the chromosphere. However, the details of
  how this happens are now being questioned. In the traditional view,
  “mechanical” energy flows into the corona in the form of waves
  or gradually increasing magnetic stresses. The waves and stresses
  dissipate and heat the plasma. The resulting downward thermal conduction
  flux causes material to evaporate from the chromosphere and fill
  the corona. If the heating is steady, an equilibrium is established
  whereby radiation and thermal conduction balance the energy input. If
  the heating is impulsive (a nanoflare), the evaporated plasma cools
  and drains, only to reappear during the next event. In either case,
  the heating occurs in the corona. A new idea is that the heating
  occurs instead in the chromosphere. Cold plasma is directly heated
  to coronal temperatures and then flows upward due to expansion and
  perhaps also an ejection process. The hot tips of type II spicules
  are one example, though spicules need not be involved. I will discuss
  these two fundamentally different scenarios and the observational
  predictions that they make. A comparison with actual observations
  leads to the conclusion that only a small fraction of the hot plasma
  in the corona comes from chromospheric heating. Most coronal plasma
  is a consequence of heating that occurs in the corona itself.

Title: UV and EUV Emissions at the Flare Foot-points Observed by AIA
Authors: Qiu, Jiong; Sturrock, Z.; Longcope, D.; Klimchuk, J. A.;
   Liu, W.
Bibcode: 2013SPD....44...53Q
Altcode:
  A solar flare is composed of impulsive energy release events by magnetic
  reconnection, which forms and heats flare loops. Recent studies have
  revealed a two-phase evolution pattern of UV 1600A emission at the
  feet of these loops: a rapid pulse lasting for a few seconds to a
  few minutes, followed by a gradual decay on timescales of a few tens
  of minutes. Multiple band EUV observations by AIA further reveal
  very similar signatures. These two phases represent different but
  related signatures of an impulsive energy release in the corona. The
  rapid pulse is an immediate response of the lower atmosphere to an
  intense thermal conduction flux resulting from the sudden heating of
  the corona to high temperatures (we rule out energetic particles due
  to a lack of significant hard X-ray emission). The gradual phase is
  associated with the cooling of hot plasma that has been evaporated
  into the corona. The observed footpoint emission is again powered
  by thermal conduction (and enthalpy), but now during a period when
  approximate steady state conditions are established in the loop. UV
  and EUV light curves of individual pixels may therefore be separated
  into contributions from two distinct physical mechanisms to shed
  light on the nature of energy transport in a flare. We demonstrate
  this technique using coordinated, spatially resolved observations of
  UV and EUV emission from the footpoints of a C3.2 thermal flare.

Title: A Survey of Nanoflare Properties in Solar Active Regions
Authors: Viall, Nicholeen; Klimchuk, J. A.
Bibcode: 2013SPD....44...16V
Altcode:
  We investigate the characteristics of coronal heating using a systematic
  technique that analyzes the properties of nanoflares in active regions
  (AR). Our technique computes cooling times, or time lags, using SDO/AIA
  light curves of all of the coronal AR emission, including the so-called
  diffuse emission. We recently presented results using this time-lag
  analysis on NOAA AR 11082 (Viall &amp; Klimchuk 2012). We found that
  the majority of the pixels had cooling plasma along their line of sight,
  consistent with impulsive coronal nanoflare heating. Additionally, our
  results using the AIA 94 channel data showed that the nanoflare energy
  is stronger in the AR core and weaker in the AR periphery. Are these
  results representative of the nanoflare characteristics exhibited in
  the majority of active regions, or is AR 11082 unique? Here we present
  the time-lag results for a survey of active regions and determine
  whether these nanoflare patterns are born out in other active regions
  as well. This research was supported by the NASA Heliophysics Guest
  Investigator program.

Title: Understanding Coronal Heating by Comparing SDO/AIA Observations
    with Modeled Light Curves
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2013enss.confE..18V
Altcode:
  An important signature of nanoflare heated coronal plasma is the
  sudden appearance of the plasma at hot temperatures, followed by
  a comparatively slow cooling and draining phase. This is due to
  the impulsive nature of nanoflare heating and the heat conduction
  and mass exchange between the corona and chromosphere. Identifying
  such nanoflare signatures is complicated by the fact that the solar
  corona is optically thin: many thousands of flux tubes which are
  heated completely independently are contributing to the total emission
  along a given line of sight. One approach has been to analyze isolated
  features such as coronal loops; however the diffuse emission between
  and around isolated features contribute as much, if not more to the
  EUV coronal emission, and therefore is crucial to the understanding of
  coronal heating. In this study we move beyond isolated features and
  analyze all of the emission in an entire active region and quiet Sun
  area. We investigate SDO/AIA light curves, systematically identifying
  nanoflare signatures. We compare the observations with a model of the
  corona as a line-of-sight integration of many thousands of completely
  independently heated flux tubes. We consider that the emission from
  these flux tubes may be due exclusively to impulsive nanoflare bursts,
  quasi-steady heating, or a mix of both, depending on the cadence of
  heat release. We demonstrate that despite the superposition of randomly
  heated flux tubes, different distributions of nanoflare cadences produce
  distinct signatures in light curves observed with multi-wavelength and
  high time cadence data, such as those from SDO/AIA. We find that much
  of the solar corona is heated through impulsive nanoflares.

Title: Can the Differential Emission Measure diagnostic be used to
    constrain the timescale of energy deposition in the corona?
Authors: Guennou, C.; Auchère, F.; Klimchuk, J. A.; Bocchialini,
   K.; Parenti, S.
Bibcode: 2013enss.confE..34G
Altcode:
  Differential emission measure (DEM) analysis is a widespread tool used
  to diagnose the thermal properties of coronal plasmas. The slope of
  the DEM distribution coolward of the coronal peak (near 3-4MK in active
  regions) can be used to diagnose the timescale for the energy deposition
  repeating on a given magnetic strand. Recent AR studies suggest that
  some active region cores are consistent with low frequency heating
  mechanisms, where the plasma cools completely before being reheated,
  while other show consistency with high frequency energy deposition,
  where rapid reheating causes the temperature to fluctuate about
  a particular value. Distinguishing between these possibilities is
  important for identifying the physical mechanism of the heating. It is
  therefore crucial to understand the uncertainties in measurements of
  observed DEM slopes. In this work, based on a probabilistic approach
  and Monte Carlo simulations, we carefully assess the errors in the
  slopes determined from EIS data. We consider both the random errors due
  to photon counting statistics, and the systematic errors associated
  with uncertainties in atomic physics and instrument calibration. The
  technique developed provides all the solutions consistent with the data
  and their associated probabilities. We demonstrate how the quality
  and the accuracy of the inversion are affected by the presence of
  noises and systematic errors, and we characterise the quality of the
  DEM inversion and its statistical properties. From these results,
  estimation of the uncertainties in the reconstructed slopes can be
  derived, thereby allowing a proper interpretation of the degree of
  agreement between observations and heating model predictions.

Title: Where is Coronal Plasma Heated?
Authors: Klimchuk, James A.
Bibcode: 2013enss.confE.105K
Altcode:
  The coupling between the chromosphere and corona is a question of
  great interest at the moment. It has long been understood that
  coronal mass originates in the chromosphere and that the energy
  which powers the corona flows up through the chromosphere. However,
  the details of how this happens are now being questioned. In the
  traditional view, "mechanical" energy flows into the corona in the
  form of waves or gradually increasing magnetic stresses. The waves or
  stresses dissipate and heat the plasma. The resulting downward thermal
  conduction flux causes material to evaporate from the chromosphere and
  fill the corona. If the heating is steady, an equilibrium is established
  whereby radiation and thermal conduction balance the energy input. If
  the heating is impulsive (a nanoflare), the evaporated plasma cools
  and drains, only to reappear during the next event. In either case,
  the heating occurs in the corona. A new idea is that the heating
  occurs instead in the chromosphere. Cold plasma is directly heated
  to coronal temperatures and then flows upward due to expansion and
  perhaps also an ejection process. The hot tips of type II spicules
  are one example, though spicules need not be involved. I will discuss
  these two fundamentally different scenarios and the observational
  predictions that they make. A comparison with actual observations
  leads to the conclusion that only a small fraction of the hot plasma
  in the corona comes from chromospheric heating. Most coronal plasma
  is a consequence of heating that occurs in the corona itself.

Title: Diagnosing the Time Dependence of Active Region Core Heating
    from the Emission Measure. II. Nanoflare Trains
Authors: Reep, J. W.; Bradshaw, S. J.; Klimchuk, J. A.
Bibcode: 2013ApJ...764..193R
Altcode: 2013arXiv1303.4466R
  The time dependence of heating in solar active regions can be studied
  by analyzing the slope of the emission measure distribution coolward
  of the peak. In a previous study we showed that low-frequency heating
  can account for 0% to 77% of active region core emission measures. We
  now turn our attention to heating by a finite succession of impulsive
  events for which the timescale between events on a single magnetic
  strand is shorter than the cooling timescale. We refer to this
  scenario as a "nanoflare train" and explore a parameter space of
  heating and coronal loop properties with a hydrodynamic model. Our
  conclusions are (1) nanoflare trains are consistent with 86% to 100%
  of observed active region cores when uncertainties in the atomic
  data are properly accounted for; (2) steeper slopes are found for
  larger values of the ratio of the train duration Δ<SUB> H </SUB>
  to the post-train cooling and draining timescale Δ<SUB> C </SUB>,
  where Δ<SUB> H </SUB> depends on the number of heating events,
  the event duration and the time interval between successive events
  (τ<SUB> C </SUB>); (3) τ<SUB> C </SUB> may be diagnosed from the
  width of the hot component of the emission measure provided that the
  temperature bins are much smaller than 0.1 dex (4) the slope of the
  emission measure alone is not sufficient to provide information about
  any timescale associated with heating—the length and density of the
  heated structure must be measured for Δ<SUB> H </SUB> to be uniquely
  extracted from the ratio Δ<SUB> H </SUB>/Δ<SUB> C </SUB>.

Title: Study of the EUV intensity variation along observed coronal
    loops
Authors: Lopez Fuentes, M.; Klimchuk, J. A.
Bibcode: 2013BAAA...56..399L
Altcode:
  The variation of intensity along coronal loops observed in X-rays and
  EUV can be used to constrain different coronal heating theories. For
  instance; thermal nonequilibrium that results from heating located at
  the footpoints of the loops predicts an intensity structure that is
  highly non uniform and asymmetric. We study the intensity variation
  along coronal loops observed with the Transition Region and Coronal
  Explorer (TRACE) in the 171 channel and find that loops present
  pronounced spatial fluctuations. Although during the data processing we
  carefully subtract the background contribution; part of the observed
  fluctuations is due to a residual effect of this contribution. Even
  allowing for the imperfect background subtraction; we conclude that
  most of the analyzed observations are inconsistent with the intensity
  profiles predicted by some numerical models.

Title: The role of type II spicules in the upper solar atmosphere
Authors: Klimchuk, J. A.
Bibcode: 2012JGRA..11712102K
Altcode: 2012arXiv1207.7048K
  We examine the suggestion that most of the hot plasma in the Sun's
  corona comes from type II spicule material that is heated as it is
  ejected from the chromosphere. This contrasts with the traditional view
  that the corona is filled via chromospheric evaporation that results
  from coronal heating. We explore the observational consequences of
  a hypothetical spicule dominated corona and conclude from the large
  discrepancy between predicted and actual observations that only a
  small fraction of the hot plasma can be supplied by spicules (&lt;2%
  in active regions, &lt;5% in the quiet Sun, and &lt;8% in coronal
  holes). The red-blue asymmetries of EUV spectral lines and the ratio
  of lower transition region (LTR;T ≤ 0.1 MK) to coronal emission
  measures are both predicted to be 2 orders of magnitude larger than
  observed. Furthermore, hot spicule material would cool dramatically by
  adiabatic expansion as it rises into the corona, so substantial coronal
  heating would be needed to maintain the high temperatures that are
  seen at all altitudes. We suggest that the corona contains a mixture
  of thin strands, some of which are populated by spicule injections,
  but most of which are not. A majority of the observed hot emission
  originates in non-spicule strands and is explained by traditional
  coronal heating models. However, since these models predict far too
  little emission from the LTR, most of this emission comes from the bulk
  of the spicule material that is only weakly heated and visible in He II
  (304 Å) as it falls back to the surface.

Title: The Role of Type II Spicules in the Upper Solar Atmosphere
Authors: Klimchuk, J. A.
Bibcode: 2012AGUFMSH31B..07K
Altcode:
  We examine the suggestion that most of the hot plasma in the Sun's
  corona comes from type II spicule material that is heated as it is
  ejected from the chromosphere. This contrasts with the traditional view
  that the corona is filled via chromospheric evaporation that results
  from coronal heating. We explore the observational consequences of
  a hypothetical spicule dominated corona and conclude from the large
  discrepancy between predicted and actual observations that only a
  small fraction of the hot plasma can be supplied by spicules (&lt;2% in
  active regions and &lt;5% in the quiet Sun). The red-blue asymmetries
  of EUV spectral lines and the ratio of lower transition region (LTR;
  T&lt;0.1 MK) to coronal emission measures are both predicted to be
  2 orders of magnitude larger than observed. Furthermore, hot spicule
  material would cool dramatically by adiabatic expansion as it rises
  into the corona, so coronal heating would likely be required to maintain
  the high temperatures that are seen at all altitudes. The necessity of
  coronal heating seems inescapable. Traditional coronal heating models
  predict far too little emission from the LTR, and we suggest that this
  emission comes primarily from the bulk of the spicule material that
  is heated to &lt;0.1 MK and is visible in He II (304 A) as it falls
  back to the surface.

Title: The Role of Spicules in Explaining the Corona and Transition
    Region
Authors: Klimchuk, J.
Bibcode: 2012IAUSS...6E.107K
Altcode:
  A portion of the material in the newly-discovered type II spicules
  is heated to coronal temperatures and contributes to the hot emission
  that we observe. What fraction of the coronal plasma can be attributed
  to spicules and what fraction must be explained by ordinary coronal
  heating? Is the as yet unexplained bright emission from the lower
  transition region (T &lt; 0.1 MK) due to spicules? I will address
  these questions and offer suggestions about future high-resolution
  observations, both space and ground-based, that can provide important
  information about this fascinating phenomenon.

Title: Diagnosing the Time-Dependence of Active Region Core Heating
    Using Emission Measures
Authors: Klimchuk, J. A.; Bradshaw, S. J.; Reep, J. W.
Bibcode: 2012AGUFMSH42A..01K
Altcode:
  It is widely believed that the cross-field spatial scale of coronal
  heating is small, so that the fundamental plasma structures
  (loop strands) are spatially unresolved. We therefore must
  appeal to diagnostic techniques that are not strongly affected by
  spatial averaging. One valuable observable is the emission measure
  distribution, EM(T), which indicates how much material is present
  at each temperature. The slope of the distribution coolward of its
  peak is related to the frequency of the presumed impulsive energy
  release. Nanoflares that have a long delay before repeating on the same
  loop strand give rise to shallow slopes, while nanoflares that repeat
  with a timescale shorter than a cooling time (or truly steady heating)
  give rise to steep slopes. Comparing recent Hinode observations with
  hydrodynamic loop simulations, we find that about 36% of active region
  cores are consistent with low-frequency nanoflares. The observational
  uncertainties are large, however, so as many as 77% or as few as none
  are consistent with low-frequency nanoflares when the uncertainties
  are taken into account. Constraining the time dependence of the heating
  is important for identifying the physical mechanism.

Title: Nanoflare Heating of the Solar Corona: Comparing SDO/AIA
    Observations with Modeled Light Curves
Authors: Viall, N. M.; Klimchuk, J. A.
Bibcode: 2012AGUFMSH42A..03V
Altcode:
  A significant outstanding issue in current solar and astrophysical
  research is that of the heating of the solar corona. Coronal plasma is
  typically measured to be at temperatures near ~1-3 MK. Is the majority
  of the coronal plasma maintained at these temperatures through a
  form of quasi-steady heating, or is this simply a measure of the
  average temperature of widely varying, impulsively heated coronal
  plasma? Addressing even this basic question is complicated by the
  fact that the corona is optically thin: many thousands of flux tubes
  which are heated completely independently are contributing to the
  total emission along a given line of sight. There is a large body of
  work focused on the heating of isolated features - coronal loops in
  active regions- which are impulsively heated, however understanding
  of the diffuse emission between loops and the emission from the quiet
  Sun are also crucial. Therefore in this study we move beyond isolated
  features and analyze all of the emission in an entire active region and
  quiet Sun area from all contributing flux tubes. We investigate light
  curves systematically using SDO/AIA observations. We also model the
  corona as a line-of-sight integration of many thousands of completely
  independently heated flux tubes. The emission from these flux tubes
  may be time dependent, quasi-steady, or a mix of both, depending on the
  cadence of heat release. We demonstrate that despite the superposition
  of randomly heated flux tubes, different distributions of nanoflare
  cadences produce distinct signatures in light curves observed with
  multi-wavelength and high time cadence data, such as those from
  SDO/AIA. We discuss the quiet Sun and active region emission in the
  context of these predicted nanoflare signatures.

Title: Exploring Small Spatial Scales in the Transition Region
    and Solar Corona with the Very High Angular Resolution Imaging
    Spectrometer (VERIS)
Authors: Chua, D. H.; Korendyke, C. M.; Vourlidas, A.; Brown, C. M.;
   Tun-Beltran, S.; Klimchuk, J. A.; Landi, E.; Seely, J.; Davila, J. M.;
   Hagood, R.; Roberts, D.; Shepler, E.; Feldman, R.; Moser, J.; Shea, J.
Bibcode: 2012AGUFMSH33A2217C
Altcode:
  Theoretical and experimental investigations of the transition region
  and coronal loops point to the importance of processes occurring on
  small spatial scales in governing the strong dynamics and impulsive
  energy release in these regions. As a consequence, high spatial,
  temporal, and temperature resolution over a broad temperature range,
  and accuracy in velocity and density determinations are all critical
  observational parameters. Current instruments lack one or more of these
  properties. These observational deficiencies have created a wide array
  of opposing descriptions of coronal loop heating and questions such
  as whether or not the plasma within coronal loops is multi-thermal or
  isothermal. High spectral and spatial resolution spectroscopic data
  are absolutely required to resolve these controversies and to advance
  our understanding of the dynamics within the solar atmosphere. We
  will achieve this with the Very High Angular Resolution Imaging
  Spectrometer (VERIS) sounding rocket payload. VERIS consists of an
  off-axis paraboloid telescope feeding a very high angular resolution,
  extreme ultraviolet (EUV) imaging spectrometer that will provide
  the first ever, simultaneous sub-arcsecond (0.16 arcsecond/pixel)
  spectra in bright lines needed to study plasma structures in the
  transition region, quiet corona, and active region core. It will do
  so with a spectral resolution of &gt;5000 to allow Doppler velocity
  determinations to better than 3 km/s. VERIS uses a novel two-element,
  normal incidence optical design with highly reflective, broad wavelength
  coverage EUV coatings to access a spectral range with broad temperature
  coverage (0.03-15 MK) and density-sensitive line ratios. Combined with
  Hinode Solar Optical Telescope (SOT) and ground based observatories,
  VERIS will deliver simultaneous observations of the entire solar
  atmosphere from the photosphere to the multi-million degree corona
  at sub-arcsecond resolution for the first time ever, allowing us to
  understand the missing link between chromospheric structures and the
  corona. VERIS will be launched from White Sands Missile Range in early
  2013. This paper presents a progress report on the VERIS payload and
  a summary of observations planned to further our understanding of
  the fine-scale structure of individual coronal loops and the heating
  mechanisms operating within them.

Title: Enthalpy-based Thermal Evolution of Loops. III. Comparison
    of Zero-dimensional Models
Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A.
Bibcode: 2012ApJ...758....5C
Altcode:
  Zero-dimensional (0D) hydrodynamic models provide a simple and quick
  way to study the thermal evolution of coronal loops subjected to
  time-dependent heating. This paper presents a comparison of a number
  of 0D models that have been published in the past and is intended to
  provide a guide for those interested in either using the old models
  or developing new ones. The principal difference between the models
  is the way the exchange of mass and energy between corona, transition
  region, and chromosphere is treated, as plasma cycles into and out of
  a loop during a heating-cooling cycle. It is shown that models based
  on the principles of mass and energy conservation can give satisfactory
  results at some or, in the case of the Enthalpy-based Thermal Evolution
  of Loops model, all stages of the loop evolution. Empirical models
  can have significant difficulties in obtaining accurate behavior due
  to invocation of assumptions incompatible with the correct exchange
  of mass and energy between corona, transition region, and chromosphere.

Title: Diagnosing the Time-dependence of Active Region Core Heating
    from the Emission Measure. I. Low-frequency Nanoflares
Authors: Bradshaw, S. J.; Klimchuk, J. A.; Reep, J. W.
Bibcode: 2012ApJ...758...53B
Altcode: 2012arXiv1209.0737B
  Observational measurements of active region emission measures contain
  clues to the time dependence of the underlying heating mechanism. A
  strongly nonlinear scaling of the emission measure with temperature
  indicates a large amount of hot plasma relative to warm plasma. A
  weakly nonlinear (or linear) scaling of the emission measure
  indicates a relatively large amount of warm plasma, suggesting that
  the hot active region plasma is allowed to cool and so the heating is
  impulsive with a long repeat time. This case is called low-frequency
  nanoflare heating, and we investigate its feasibility as an active
  region heating scenario here. We explore a parameter space of heating
  and coronal loop properties with a hydrodynamic model. For each model
  run, we calculate the slope α of the emission measure distribution
  EM(T)vpropT <SUP>α</SUP>. Our conclusions are: (1) low-frequency
  nanoflare heating is consistent with about 36% of observed active
  region cores when uncertainties in the atomic data are not accounted
  for; (2) proper consideration of uncertainties yields a range in
  which as many as 77% of observed active regions are consistent with
  low-frequency nanoflare heating and as few as zero; (3) low-frequency
  nanoflare heating cannot explain observed slopes greater than 3; (4)
  the upper limit to the volumetric energy release is in the region of
  50 erg cm<SUP>-3</SUP> to avoid unphysical magnetic field strengths;
  (5) the heating timescale may be short for loops of total length
  less than 40 Mm to be consistent with the observed range of slopes;
  (6) predicted slopes are consistently steeper for longer loops.

Title: Active Region Moss: Doppler Shifts from
    Hinode/Extreme-ultraviolet Imaging Spectrometer Observations
Authors: Tripathi, Durgesh; Mason, Helen E.; Klimchuk, James A.
Bibcode: 2012ApJ...753...37T
Altcode: 2012arXiv1204.6550T
  Studying the Doppler shifts and the temperature dependence of Doppler
  shifts in moss regions can help us understand the heating processes
  in the core of the active regions. In this paper, we have used an
  active region observation recorded by the Extreme-ultraviolet Imaging
  Spectrometer (EIS) on board Hinode on 2007 December 12 to measure the
  Doppler shifts in the moss regions. We have distinguished the moss
  regions from the rest of the active region by defining a low-density
  cutoff as derived by Tripathi et al. in 2010. We have carried out a
  very careful analysis of the EIS wavelength calibration based on the
  method described by Young et al. in 2012. For spectral lines having
  maximum sensitivity between log T = 5.85 and log T = 6.25 K, we find
  that the velocity distribution peaks at around 0 km s<SUP>-1</SUP>
  with an estimated error of 4-5 km s<SUP>-1</SUP>. The width of the
  distribution decreases with temperature. The mean of the distribution
  shows a blueshift which increases with increasing temperature and the
  distribution also shows asymmetries toward blueshift. Comparing these
  results with observables predicted from different coronal heating
  models, we find that these results are consistent with both steady
  and impulsive heating scenarios. However, the fact that there are a
  significant number of pixels showing velocity amplitudes that exceed
  the uncertainty of 5 km s<SUP>-1</SUP> is suggestive of impulsive
  heating. Clearly, further observational constraints are needed to
  distinguish between these two heating scenarios.

Title: Evidence for Widespread Cooling in an Active Region Observed
    with the SDO Atmospheric Imaging Assembly
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2012ApJ...753...35V
Altcode: 2012arXiv1202.4001V
  A well-known behavior of EUV light curves of discrete coronal loops
  is that the peak intensities of cooler channels or spectral lines
  are reached at progressively later times than hotter channels. This
  time lag is understood to be the result of hot coronal loop plasma
  cooling through these lower respective temperatures. However, loops
  typically comprise only a minority of the total emission in active
  regions (ARs). Is this cooling pattern a common property of AR coronal
  plasma, or does it only occur in unique circumstances, locations, and
  times? The new Solar Dynamics Observatory/Atmospheric Imaging Assembly
  (SDO/AIA) data provide a wonderful opportunity to answer this question
  systematically for an entire AR. We measure the time lag between pairs
  of SDO/AIA EUV channels using 24 hr of images of AR 11082 observed
  on 2010 June 19. We find that there is a time-lag signal consistent
  with cooling plasma, just as is usually found for loops, throughout
  the AR including the diffuse emission between loops for the entire 24
  hr duration. The pattern persists consistently for all channel pairs
  and choice of window length within the 24 hr time period, giving us
  confidence that the plasma is cooling from temperatures of greater
  than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower
  than ~0.8 MK. This suggests that the bulk of the emitting coronal
  plasma in this AR is not steady; rather, it is dynamic and constantly
  evolving. These measurements provide crucial constraints on any model
  which seeks to describe coronal heating.

Title: A cellular automaton model for coronal heating
Authors: López Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2012IAUS..286..433L
Altcode:
  We present a simple coronal heating model based on a cellular automaton
  approach. Following Parker's suggestion (1988), we consider the
  corona to be made up of elemental magnetic strands that accumulate
  magnetic stress due to the photospheric displacements of their
  footpoints. Magnetic energy is eventually released in small scale
  reconnection events. The model consists of a 2D grid in which strand
  footpoints travel with random displacements simulating convective
  motions. Each time two strands interact, a critical condition is
  tested (as in self-organized critical models), and if the condition
  is fulfilled, the strands reconnect and energy is released. We
  model the plasma response to the heating events and obtain synthetic
  observations. We compare the output of the model with real observations
  from Hinode/XRT and discuss the implications of our results for
  coronal heating.

Title: Enthalpy-based Thermal Evolution of Loops. II. Improvements
    to the Model
Authors: Cargill, P. J.; Bradshaw, S. J.; Klimchuk, J. A.
Bibcode: 2012ApJ...752..161C
Altcode: 2012arXiv1204.5960C
  This paper develops the zero-dimensional (0D) hydrodynamic coronal loop
  model "Enthalpy-based Thermal Evolution of Loops" (EBTEL) proposed by
  Klimchuk et al., which studies the plasma response to evolving coronal
  heating, especially impulsive heating events. The basis of EBTEL is
  the modeling of mass exchange between the corona and transition region
  (TR) and chromosphere in response to heating variations, with the key
  parameter being the ratio of the TR to coronal radiation. We develop new
  models for this parameter that now include gravitational stratification
  and a physically motivated approach to radiative cooling. A number of
  examples are presented, including nanoflares in short and long loops,
  and a small flare. The new features in EBTEL are important for accurate
  tracking of, in particular, the density. The 0D results are compared
  to a 1D hydro code (Hydrad) with generally good agreement. EBTEL
  is suitable for general use as a tool for (1) quick-look results of
  loop evolution in response to a given heating function, (2) extensive
  parameter surveys, and (3) situations where the modeling of hundreds
  or thousands of elemental loops is needed. A single run takes a few
  seconds on a contemporary laptop.

Title: Nanoflare Evidence from Analysis of the X-Ray Variability of
    an Active Region Observed with Hinode/XRT
Authors: Terzo, S.; Reale, F.; Miceli, M.; Kano, R.; Tsuneta, S.;
   Klimchuk, J. A.
Bibcode: 2012ASPC..455..245T
Altcode: 2012arXiv1201.5482T
  The heating of the solar corona is one of the big questions in
  astrophysics. Rapid pulses called nanoflares are among the best
  candidate mechanisms. The analysis of the time variability of coronal
  X-ray emission is potentially a very useful tool to detect impulsive
  events. We analyze the small-scale variability of a solar active
  region in a high cadence Hinode/XRT observation. The dataset allows
  us to detect very small deviations of emission fluctuations from the
  distribution expected for a constant rate. We discuss the deviations
  in the light of the pulsed-heating scenario.

Title: Nanoflare Properties throughout Active Regions: Comparing
    SDO/AIA Observations with Modeled Active Region Light Curves
Authors: Viall, Nicholeen; Klimchuk, J.
Bibcode: 2012AAS...22030904V
Altcode:
  Coronal plasma in active regions is typically measured to be at
  temperatures near 1-3 MK. Is the majority of the coronal plasma in
  hydrostatic equilibrium, maintained at these temperatures through
  a form of quasi-steady heating, or is this simply a measure of the
  average temperature of widely varying, impulsively heated coronal
  plasma? Addressing this question is complicated by the fact that
  the corona is optically thin: many thousands of flux tubes which
  are heated completely independently are contributing to the total
  emission along a given line of sight. There is a large body of work
  focused on the heating of isolated features - coronal loops - which are
  impulsively heated, however it is the diffuse emission between loops
  which often comprises the majority of active region emission. Therefore
  in this study we move beyond isolated features and analyze all of the
  emission in an entire active region from all contributing flux tubes. We
  investigate light curves systematically using SDO/AIA observations. We
  also model the active region corona as a line-of-sight integration
  of many thousands of completely independently heated flux tubes. The
  emission from these flux tubes may be time dependent, quasi-steady, or
  a mix of both, depending on the cadence of heat release. We demonstrate
  that despite the superposition of randomly heated flux tubes, different
  distributions of nanoflare cadences produce distinct signatures in
  light curves observed with multi-wavelength and high time cadence data,
  such as those from SDO/AIA. We conclude that the majority of the active
  region plasma is not maintained in hydrostatic equilibrium, rather
  it is undergoing dynamic heating and cooling cycles. The observed
  emission is consistent with heating through impulsive nanoflares,
  whose energy is a function of location within the active region. <P
  />This research was supported by an appointment to the NASA Postdoctoral
  Program at GSFC/NASA.

Title: Understanding Coronal Heating with Emission Measure
    Distributions
Authors: Klimchuk, James A.; Tripathi, D.; Bradshaw, S. J.; Mason,
   H. E.
Bibcode: 2012AAS...22042302K
Altcode:
  It is widely believed that the cross-field spatial scale of coronal
  heating is small, so that the fundamental plasma structures
  (loop strands) are spatially unresolved. We therefore must
  appeal to diagnostic techniques that are not strongly affected by
  spatial averaging. One valuable observable is the emission measure
  distribution, EM(T), which indicates how much material is present
  at each temperature. Using data from the Extreme-ultraviolet Imaging
  Spectrograph on the Hinode mission, we have determined emission measure
  distributions in the cores of two active regions. The distributions have
  power law slopes of approximately 2.4 coolward of the peak. We compare
  these slopes, as well as the amount of emission measure at very high
  temperature, with the predictions of a series of models. The models
  assume impulsive heating (nanoflares) in unresolved strands and take
  full account of nonequilibrium ionization.

Title: Division II: Sun and Heliosphere
Authors: Martínez Pillet, Valentín; Klimchuk, James A.; Melrose,
   Donald B.; Cauzzi, Gianna; van Driel-Gesztelyi, Lidia; Gopalswamy,
   Natchimuthuk; Kosovichev, Alexander; Mann, Ingrid; Schrijver,
   Carolus J.
Bibcode: 2012IAUTA..28...61M
Altcode: 2012IAUTA..28...61P
  The solar activity cycle entered a prolonged quiet phase that started
  in 2008 and ended in 2010. This minimum lasted for a year longer
  than expected and all activity proxies, as measured from Earth and
  from Space, reached minimum values never observed before (de Toma,
  2012). The number of spotless days from 2006 to 2009 totals 800, the
  largest ever recorded in modern times. Solar irradiance was at historic
  minimums. The interplanetary magnetic field was measured at values as
  low as 2.9 nT and the cosmic rays were observed at records-high. While
  rumors spread that the Sun could be entering a grand minimum quiet
  phase (such as the Maunder minimum of the XVII century), activity
  took over in 2010 and we are now well into Solar Cycle 24 (albeit,
  probably, a low intensity cycle), approaching towards a maximum due
  by mid 2013. In addition to bringing us the possibility to observe
  a quiet state of the Sun and of the Heliosphere that was previously
  not recorded with modern instruments, the Sun has also shown us how
  little we know about the dynamo mechanism that drives its activity as
  all solar cycle predictions failed to see this extended minimum coming.

Title: Commission 10: Solar Activity
Authors: van Driel-Gesztelyi, Lidia; Schrijver, Carolus J.; Klimchuk,
   James A.; Charbonneau, Paul; Fletcher, Lyndsay; Hasan, S. Sirajul;
   Hudson, Hugh S.; Kusano, Kanya; Mandrini, Cristina H.; Peter, Hardi;
   Vršnak, Bojan; Yan, Yihua
Bibcode: 2012IAUTA..28...69V
Altcode:
  Commission 10 of the International Astronomical Union has more than
  650 members who study a wide range of activity phenomena produced by
  our nearest star, the Sun. Solar activity is intrinsically related
  to solar magnetic fields and encompasses events from the smallest
  energy releases (nano- or even picoflares) to the largest eruptions
  in the Solar System, coronal mass ejections (CMEs), which propagate
  into the Heliosphere reaching the Earth and beyond. Solar activity is
  manifested in the appearance of sunspot groups or active regions, which
  are the principal sources of activity phenomena from the emergence of
  their magnetic flux through their dispersion and decay. The period
  2008-2009 saw an unanticipated extended solar cycle minimum and
  unprecedentedly weak polar-cap and heliospheric field. Associated with
  that was the 2009 historical maximum in galactic cosmic rays flux since
  measurements begun in the middle of the 20th Century. Since then Cycle
  24 has re-started solar activity producing some spectacular eruptions
  observed with a fleet of spacecraft and ground-based facilities. In
  the last triennium major advances in our knowledge and understanding
  of solar activity were due to continuing success of space missions as
  SOHO, Hinode, RHESSI and the twin STEREO spacecraft, further enriched
  by the breathtaking images of the solar atmosphere produced by the
  Solar Dynamic Observatory (SDO) launched on 11 February 2010 in the
  framework of NASA's Living with a Star program. In August 2012, at the
  time of the IAU General Assembly in Beijing when the mandate of this
  Commission ends, we will be in the unique position to have for the
  first time a full 3-D view of the Sun and solar activity phenomena
  provided by the twin STEREO missions about 120 degrees behind and
  ahead of Earth and other spacecraft around the Earth and ground-based
  observatories. These new observational insights are continuously
  posing new questions, inspiring and advancing theoretical analysis
  and modelling, improving our understanding of the physics underlying
  magnetic activity phenomena. Commission 10 reports on a vigorously
  evolving field of research produced by a large community. The number
  of refereed publications containing `Sun', `heliosphere', or a synonym
  in their abstracts continued the steady growth seen over the preceding
  decades, reaching about 2000 in the years 2008-2010, with a total of
  close to 4000 unique authors. This report, however, has its limitations
  and it is inherently incomplete, as it was prepared jointly by the
  members of the Organising Committee of Commission 10 (see the names
  of the primary contributors to the sections indicated in parentheses)
  reflecting their fields of expertise and interest. Nevertheless, we
  believe that it is a representative sample of significant new results
  obtained during the last triennium in the field of solar activity.

Title: Spectroscopic Diagnostics and Heating of Active Region Cores
Authors: Tripathi, D.; Mason, H. E.; Klimchuk, J. A.
Bibcode: 2012decs.confE..92T
Altcode:
  It is widely believed that we are still far from spatially resolving
  the fundamental plasma structures in solar corona. Therefore,
  we must use spectroscopic diagnostic techniques such as emission
  measure distribution (EM(T)) and Doppler shifts that are not affected
  by spatial averaging. Using observations recorded by the Extreme
  ultraviolet Imaging Spectrometer we have studies emission measure (EM)
  distribution and Doppler shift in the moss and inter-moss regions. The
  EM distributions obtained for moss regions cab be reproduced by
  considering strong coronal condensation scenario suggesting bulk
  downflow of the plasma. Doppler shift measurements for the moss regions
  show that almost all the moss regions are red-shifted with velocities
  up to 15km/s with mean velocity of 5 km/s. However, the uncertainty
  on the Doppler shift was large. The EM distributions obtained for
  inter-moss regions have power law slopes of approximately 2.4 coolward
  of the peak. We compare the EM for inter-moss region with that obtained
  from nanoflare model using EBTEL (Enthalpy-Based Thermal Evolution of
  Loops). Our results suggest that the EM distribution for both the moss
  as well as inter-moss regions and Doppler shift in the moss regions can
  be explained by nanoflare heating. IRIS will provide a better account
  of the Doppler shift in the moss regions, which will dramatically
  enhance our understanding of the heating of active region core.

Title: EBTEL: Enthalpy-Based Thermal Evolution of Loops
Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J.
Bibcode: 2012ascl.soft03007K
Altcode:
  Observational and theoretical evidence suggests that coronal heating
  is impulsive and occurs on very small cross-field spatial scales. A
  single coronal loop could contain a hundred or more individual strands
  that are heated quasi-independently by nanoflares. It is therefore
  an enormous undertaking to model an entire active region or the
  global corona. Three-dimensional MHD codes have inadequate spatial
  resolution, and 1D hydro codes are too slow to simulate the many
  thousands of elemental strands that must be treated in a reasonable
  representation. Fortunately, thermal conduction and flows tend to
  smooth out plasma gradients along the magnetic field, so "0D models"
  are an acceptable alternative. We have developed a highly efficient
  model called Enthalpy-Based Thermal Evolution of Loops (EBTEL) that
  accurately describes the evolution of the average temperature, pressure,
  and density along a coronal strand. It improves significantly upon
  earlier models of this type-in accuracy, flexibility, and capability. It
  treats both slowly varying and highly impulsive coronal heating;
  it provides the differential emission measure distribution, DEM(T),
  at the transition region footpoints; and there are options for heat
  flux saturation and nonthermal electron beam heating. EBTEL gives
  excellent agreement with far more sophisticated 1D hydro simulations
  despite using four orders of magnitude less computing time. It promises
  to be a powerful new tool for solar and stellar studies.

Title: The Pros and Cons of 1D vs. 3D Modeling
Authors: Klimchuk, James A.
Bibcode: 2012decs.confE..25K
Altcode:
  Advances in computing capability have led to tremendous improvements in
  3D modeling. Entire active regions are being simulated in what might
  be described as a first principles way, in which plasma heating is
  treated self consistently rather than through the specification of
  heating functions. There are limitations to this approach, however,
  as actual heating mechanisms on the Sun involve spatial scales orders
  of magnitude smaller than what these simulations can resolve. Other
  simulations begin to resolve these scales, but they only treat a
  tiny volume and do not include the all important coupling with larger
  scales or with other parts of the atmosphere, and so cannot be readily
  compared with observations. Finally, 1D hydrodynamic models capture the
  field-aligned evolution of the plasma extremely well and are ideally
  suited for data comparison, but they treat the heating in a totally
  ad hoc manner. All of these approaches have important contributions
  to make, but we must be aware of their limitations. I will highlight
  some of the strengths and weaknesses of each.

Title: Determining the Typical Nanoflare Cadence in Active Regions:
    Comparing SDO/AIA Observations with Modeled Active Region Light Curves
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2012decs.confE..40V
Altcode:
  Coronal plasma in active regions is typically measured to be at
  temperatures near 1-3 MK. Is the majority of the coronal plasma in
  hydrostatic equilibrium, maintained at these temperatures through a
  form of quasi-steady heating, or is this simply a measure of the average
  temperature of widely varying, impulsively heated coronal plasma which
  is continually undergoing heating and cooling cycles? Addressing this
  question is complicated by the fact that the corona is optically thin:
  many thousands of strands which are heated completely independently are
  contributing to the total emission along a given line of sight. There
  is a large body of work focused on the heating of coronal loops,
  which are impulsively heated, however it is the diffuse emission
  between loops which often comprises the majority of active region
  emission. Therefore, a different and necessary approach to analyzing
  active region heating is to analyze all of the emission in an active
  region, and account for emission along the line of sight from all of
  the contributing strands. We investigate light curves systematically in
  an entire active region using SDO/AIA observations. We also model the
  active region corona as a line-of-sight integration of many thousands
  of completely independently heated strands. The emission from these
  flux tubes may be time dependent, quasi-steady, or a mix of both,
  depending on the cadence of heat release on each strand. We examine a
  full range of heat cadences from effectively steady (heat pulse repeat
  time &lt;&lt; plasma cooling time) to fully impulsive (heat pulse repeat
  time &gt;&gt; plasma cooling time) and model the resulting emission
  when superposing strands undergoing these differing heat cycles. We
  demonstrate that despite the superposition of randomly heated strands,
  different distributions of heat cadences produce distinct signatures
  in light curves observed with multi-wavelength and high time cadence
  data, such as those from the AIA telescopes on SDO. Using these model
  predictions in conjunction with SDO/AIA observations, we evaluate the
  typical cadence of heat release in different active regions and patterns
  therein, which is a crucial constraint on coronal heating mechanisms.

Title: The Origin of the EUV Late Phase: A Case Study of the C8.8
    Flare on 2010 May 5
Authors: Hock, R. A.; Woods, T. N.; Klimchuk, J. A.; Eparvier, F. G.;
   Jones, A. R.
Bibcode: 2012arXiv1202.4819H
Altcode:
  Since the launch of NASA's Solar Dynamics Observatory on 2010 February
  11, the Extreme ultraviolet Variability Experiment (EVE) has observed
  numerous flares. One interesting feature observed by EVE is that a
  subset of flares exhibit an additional enhancement of the 2-3 million
  K emission several hours after the flare's soft X-ray emission. From
  the Atmospheric Imaging Assembly (AIA) images, we observe that this
  secondary emission, dubbed the EUV late phase, occurs in the same active
  region as the flare but not in the same coronal loops. Here, we examine
  the C8.8 flare that occurred on 2010 May 5 as a case study of EUV late
  phase flares. In addition to presenting detailed observations from
  both AIA and EVE, we develop a physical model of this flare and test
  it using the Enthalpy Based Thermal Evolution of Loops (EBTEL) model.

Title: Evidence of nanoflare heating in coronal loops observed with
    Hinode/XRT and SDO/AIA
Authors: López Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2012BAAA...55..103L
Altcode:
  We study a series of coronal loop lightcurves from X-ray and EUV
  observations. In search for signatures of nanoflare heating, we analyze
  the statistical properties of the observed lightcurves and compare them
  with synthetic cases obtained with a 2D cellular-automaton model based
  on nanoflare heating driven by photospheric motions. Our analysis shows
  that the observed and the model lightcurves have similar statistical
  properties. The asymmetries observed in the distribution of the
  intensity fluctuations indicate the possible presence of widespread
  cooling processes in sub-resolution magnetic strands.

Title: Understanding Coronal Heating with Emission Measure
    Distributions
Authors: Klimchuk, J. A.; Tripathi, D.; Bradshaw, S. J.; Mason, H. E.
Bibcode: 2011AGUFMSH43F..03K
Altcode:
  It is widely believed that the cross-field spatial scale of coronal
  heating is small, so that the fundamental plasma structures
  (loop strands) are spatially unresolved. We therefore must
  appeal to diagnostic techniques that are not strongly affected by
  spatial averaging. One valuable observable is the emission measure
  distribution, EM(T), which indicates how much material is present
  at each temperature. Using data from the Extreme-ultraviolet Imaging
  Spectrograph on the Hinode mission, we have determined emission measure
  distributions in the cores of two active regions. The distributions
  have power law slopes of approximately 2.4 coolward of the peak. We
  compare these slopes, as well as the amount of emission measure at
  very high temperature, with the predictions of a series of models. The
  models assume impulsive heating (nanoflares) in unresolved strands and
  take full account of nonequilibrium ionization. A variety of nanoflare
  properties and initial conditions are considered. We also comment on the
  selection of spectral lines for upcoming missions like Solar Orbiter.

Title: Determining the Typical Nanoflare Cadence in Active Regions:
    Modeling Light Curves of Active Regions
Authors: Viall, N. M.; Klimchuk, J. A.
Bibcode: 2011AGUFMSH33B2057V
Altcode:
  Active region coronal loops visible at 1MK are likely composed of many
  unresolved strands, heated by storms of impulsive nanoflares. Though
  well-studied, these loops often contribute only a fraction of the total
  emission in an active region; the degree to which the entire active
  region is heated in the same manner as loops are is highly debated. Is
  the majority of coronal active region plasma heated impulsively, or
  is the majority of the heating quasi-steady? Addressing this question
  is complicated by the fact that the corona is optically thin: many
  thousands of strands which are heated completely independently
  are contributing to the total emission along a given line of
  sight. Furthermore, certain geometries preclude even the best background
  subtraction methods from fully isolating the emission from even a
  single coronal loop. Therefore, a different and necessary approach
  to analyzing active region heating is to account for emission along
  the line of sight from all of the contributing strands. We model the
  active region corona as a line-of-sight integration of many thousands
  of completely independently heated strands. The emission from these
  flux tubes may be time dependent, quasi-steady, or a mix of both,
  depending on the cadence of heat release on each strand. We examine a
  full range of heat cadences from effectively steady (heat pulse repeat
  time &lt;&lt; plasma cooling time) to fully impulsive (heat pulse repeat
  time &gt;&gt; plasma cooling time) and model the resulting emission
  when superposing strands undergoing these differing heat cycles. We
  demonstrate that despite the superposition of randomly heated strands,
  different distributions of heat cadences produce distinct signatures
  in light curves observed with multi-wavelength and high time cadence
  data, such as those from the AIA telescopes on SDO. For this reason,
  high time cadence spectral information for lines sensitive to the 1-10
  MK range will be especially useful in future missions. Using these
  model predictions, we evaluate the typical cadence of heat release
  in different active regions and patterns therein, which is a crucial
  constraint on coronal heating mechanisms.

Title: New Solar Extreme-ultraviolet Irradiance Observations during
    Flares
Authors: Woods, Thomas N.; Hock, Rachel; Eparvier, Frank; Jones,
   Andrew R.; Chamberlin, Phillip C.; Klimchuk, James A.; Didkovsky,
   Leonid; Judge, Darrell; Mariska, John; Warren, Harry; Schrijver,
   Carolus J.; Webb, David F.; Bailey, Scott; Tobiska, W. Kent
Bibcode: 2011ApJ...739...59W
Altcode:
  New solar extreme-ultraviolet (EUV) irradiance observations from the
  NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide
  full coverage in the EUV range from 0.1 to 106 nm and continuously at
  a cadence of 10 s for spectra at 0.1 nm resolution and even faster,
  0.25 s, for six EUV bands. These observations can be decomposed into
  four distinct characteristics during flares. First, the emissions
  that dominate during the flare's impulsive phase are the transition
  region emissions, such as the He II 30.4 nm. Second, the hot coronal
  emissions above 5 MK dominate during the gradual phase and are highly
  correlated with the GOES X-ray. A third flare characteristic in the
  EUV is coronal dimming, seen best in the cool corona, such as the
  Fe IX 17.1 nm. As the post-flare loops reconnect and cool, many of
  the EUV coronal emissions peak a few minutes after the GOES X-ray
  peak. One interesting variation of the post-eruptive loop reconnection
  is that warm coronal emissions (e.g., Fe XVI 33.5 nm) sometimes exhibit
  a second large peak separated from the primary flare event by many
  minutes to hours, with EUV emission originating not from the original
  flare site and its immediate vicinity, but rather from a volume of
  higher loops. We refer to this second peak as the EUV late phase. The
  characterization of many flares during the SDO mission is provided,
  including quantification of the spectral irradiance from the EUV late
  phase that cannot be inferred from GOES X-ray diagnostics.

Title: Emission Measure Distribution and Heating of Two Active
    Region Cores
Authors: Tripathi, Durgesh; Klimchuk, James A.; Mason, Helen E.
Bibcode: 2011ApJ...740..111T
Altcode: 2011arXiv1107.4480T
  Using data from the Extreme-ultraviolet Imaging Spectrometer aboard
  Hinode, we have studied the coronal plasma in the core of two active
  regions. Concentrating on the area between opposite polarity moss, we
  found emission measure distributions having an approximate power-law
  form EMvpropT <SUP>2.4</SUP> from log T = 5.5 up to a peak at log T
  = 6.55. We show that the observations compare very favorably with
  a simple model of nanoflare-heated loop strands. They also appear
  to be consistent with more sophisticated nanoflare models. However,
  in the absence of additional constraints, steady heating is also a
  viable explanation.

Title: Patterns of Nanoflare Storm Heating Exhibited by an Active
    Region Observed with Solar Dynamics Observatory/Atmospheric Imaging
    Assembly
Authors: Viall, Nicholeen M.; Klimchuk, James A.
Bibcode: 2011ApJ...738...24V
Altcode: 2011arXiv1106.4196V
  It is largely agreed that many coronal loops—those observed at a
  temperature of about 1 MK—are bundles of unresolved strands that are
  heated by storms of impulsive nanoflares. The nature of coronal heating
  in hotter loops and in the very important but largely ignored diffuse
  component of active regions is much less clear. Are these regions also
  heated impulsively, or is the heating quasi-steady? The spectacular
  new data from the Atmospheric Imaging Assembly (AIA) telescopes on the
  Solar Dynamics Observatory offer an excellent opportunity to address
  this question. We analyze the light curves of coronal loops and the
  diffuse corona in six different AIA channels and compare them with the
  predicted light curves from theoretical models. Light curves in the
  different AIA channels reach their peak intensities with predictable
  orderings as a function the nanoflare storm properties. We show that
  while some sets of light curves exhibit clear evidence of cooling
  after nanoflare storms, other cases are less straightforward to
  interpret. Complications arise because of line-of-sight integration
  through many different structures, the broadband nature of the AIA
  channels, and because physical properties can change substantially
  depending on the magnitude of the energy release. Nevertheless, the
  light curves exhibit predictable and understandable patterns consistent
  with impulsive nanoflare heating.

Title: Widespread Nanoflare Variability Detected with Hinode/X-Ray
    Telescope in a Solar Active Region
Authors: Terzo, Sergio; Reale, Fabio; Miceli, Marco; Klimchuk, James
   A.; Kano, Ryouhei; Tsuneta, Saku
Bibcode: 2011ApJ...736..111T
Altcode: 2011arXiv1105.2506T
  It is generally agreed that small impulsive energy bursts called
  nanoflares are responsible for at least some of the Sun's hot corona,
  but whether they are the explanation for most of the multimillion-degree
  plasma has been a matter of ongoing debate. We present here evidence
  that nanoflares are widespread in an active region observed by the X-Ray
  Telescope on board the Hinode mission. The distributions of intensity
  fluctuations have small but important asymmetries, whether taken
  from individual pixels, multipixel subregions, or the entire active
  region. Negative fluctuations (corresponding to reduced intensity)
  are greater in number but weaker in amplitude, so that the median
  fluctuation is negative compared to a mean of zero. Using Monte Carlo
  simulations, we show that only part of this asymmetry can be explained
  by Poisson photon statistics. The remainder is explainable through
  a tendency for exponentially decreasing intensity, such as would be
  expected from a cooling plasma produced from a nanoflare. We suggest
  that nanoflares are a universal heating process within active regions.

Title: Heating of Active Regions by Impulsive Nanoflares
Authors: Viall, Nicholeen Mary; Klimchuk, James A.
Bibcode: 2011shin.confE..57V
Altcode:
  It has been proposed that plasma on active regions may be contributing
  plasma to the slow solar wind. If this is the case, then understanding
  the heating and dynamics of active regions adds vital knowledge to
  our understanding of the heating and acceleration of the slow solar
  wind. It seems largely agreed that many coronal loops--those observed
  at a temperature of about 1 MK--are bundles of unresolved strands that
  are heated by storms of impulsive nanoflares. The nature of coronal
  heating in hotter loops and in the very important but largely ignored
  diffuse component of active regions is much less clear. Are these
  regions also heated impulsively, or is the heating quasi steady? The
  spectacular new data from the Atmospheric Imaging Assembly (AIA)
  telescopes on the Solar Dynamics Observatory (SDO) offer an excellent
  opportunity to address this question. We analyze the light curves
  of coronal loops and the diffuse corona in 6 different AIA channels
  and compare them with the predicted light curves from theoretical
  models. Light curves in the different AIA channels reach their peak
  intensities with predictable orderings as a function of the nanoflare
  storm properties. These orderings, or time lags, are clearly exhibited
  in loop observations in all channels. What is especially exciting
  is that we identify these time lag patterns in observations of the
  seemingly steady diffuse corona as well. We model the diffuse corona as
  a line-of-sight integration of many thousands of completely independent,
  impulsively heated strands. The time lags of the simulated and actual
  observations are in excellent agreement. Our results suggest that
  impulsive nanoflare heating is ubiquitous within active regions.

Title: What Dominates the Coronal Emission Spectrum During the Cycle
    of Impulsive Heating and Cooling?
Authors: Bradshaw, S. J.; Klimchuk, J. A.
Bibcode: 2011ApJS..194...26B
Altcode:
  The "smoking gun" of small-scale, impulsive events heating the solar
  corona is expected to be the presence of hot (&gt;5 MK) plasma. Evidence
  for this has been scarce, but has gradually begun to accumulate due to
  recent studies designed to constrain the high-temperature part of the
  emission measure distribution. However, the detected hot component
  is often weaker than models predict and this is due in part to the
  common modeling assumption that the ionization balance remains in
  equilibrium. The launch of the latest generation of space-based
  observing instrumentation on board Hinode and the Solar Dynamics
  Observatory (SDO) has brought the matter of the ionization state of the
  plasma firmly to the forefront. It is timely to consider exactly what
  emission current instruments would detect when observing a corona heated
  impulsively on small scales by nanoflares. Only after we understand
  the full effects of nonequilibrium ionization can we draw meaningful
  conclusions about the plasma that is (or is not) present. We have
  therefore performed a series of hydrodynamic simulations for a variety
  of different nanoflare properties and initial conditions. Our study
  has led to several key conclusions. (1) Deviations from equilibrium
  are greatest for short-duration nanoflares at low initial coronal
  densities. (2) Hot emission lines are the most affected and are
  suppressed sometimes to the point of being invisible. (3) For the many
  scenarios we have considered, the emission detected in several of the
  SDO-AIA channels (131, 193, and 211 Å) would be dominated by warm,
  overdense, cooling plasma. (4) It is difficult not to create coronal
  loops that emit strongly at 1.5 MK and in the range 2-6 MK, which
  are the most commonly observed kind, for a broad range of nanoflare
  scenarios. (5) The Fe XV (284.16 Å) emission in most of our models
  is about 10 times brighter than the Ca XVII (192.82 Å) emission,
  consistent with observations. Our overarching conclusion is that
  small-scale, impulsive heating inducing a nonequilibrium ionization
  state leads to predictions for observable quantities that are entirely
  consistent with what is actually observed.

Title: Patterns of Nanoflare Heating Exhibited by Active Regions
    Observed with SDO/AIA
Authors: Viall, Nicholeen; Klimchuk, J.
Bibcode: 2011SPD....42.2103V
Altcode: 2011BAAS..43S.2103V
  It seems largely agreed that many coronal loops---those observed at a
  temperature of about 1 MK---are bundles of unresolved strands that are
  heated by storms of impulsive nanoflares. The nature of coronal heating
  in hotter loops and in the very important but largely ignored diffuse
  component of active regions is much less clear. Are these regions also
  heated impulsively, or is the heating quasi steady? The spectacular new
  data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar
  Dynamics Observatory (SDO) offer an excellent opportunity to address
  this question. We analyze the light curves of coronal loops and the
  diffuse corona in 6 different AIA channels and compare them with the
  predicted light curves from theoretical models. Light curves in the
  different AIA channels reach their peak intensities with predictable
  orderings as a function of the nanoflare storm properties. These
  orderings, or time lags, are clearly exhibited in loop observations in
  all channels. What is especially exciting is that we identify these time
  lag patterns in observations of the seemingly steady diffuse corona
  as well. We model the diffuse corona as a line-of-sight integration
  of many thousands of completely independent, impulsively heated
  strands. The time lags of the simulated and actual observations are
  in excellent agreement. Our results suggest that impulsive nanoflare
  heating is ubiquitous within active regions. <P />This research was
  supported through an appointment to the NASA Postdoctoral Program at
  the Goddard Space Flight Center, administered by Oak Ridge Associated
  Universities through a contract with NASA.

Title: Radiative Signatures of the Coronal Heating and Cooling Cycle
Authors: Bradshaw, Stephen; Klimchuk, J.
Bibcode: 2011SPD....42.0503B
Altcode: 2011BAAS..43S.0503B
  The 'smoking gun' of small-scale, impulsive heating in the non-flaring
  solar corona is expected to be emission from plasma at temperatures
  greater than 5 MK. Recent studies, designed to strongly constrain
  the high temperature part of the emission measure distribution, have
  begun to provide evidence for such a hot component to the emission
  spectrum. However, it is significantly weaker than predicted by
  numerical models. We propose that the discrepancy can be resolved by
  dropping the common modeling assumption of ionization equilibrium. <P
  />The launches of Hinode and the Solar Dynamics Observatory (SDO)
  have brought the matter of the ionization state of the plasma, which
  lies at the interface between models and observations, firmly to the
  forefront. <P />We present detailed, quantitative predictions for
  the spectral emission that the instruments EIS and AIA would actually
  detect, for a broad range of impulsive heating scenarios, derived from
  a combination of numerical hydrodynamic and forward modeling. <P />We
  demonstrate that a nonequilibrium ionization state, as induced by
  small-scale, impulsive heating, leads to predictions for observable
  quantities that are consistent with what is actually observed.

Title: Are Spicules the Primary Source of Hot Coronal Plasma?
Authors: Klimchuk, James A.
Bibcode: 2011SPD....42.1801K
Altcode: 2011BAAS..43S.1801K
  The recent discovery of Type II spicules has generated considerable
  excitement. It has even been suggested that these ejections can account
  for a majority of the hot plasma observed in the corona, thus obviating
  the need for "coronal” heating. If this is the case, however, then
  there should be observational consequences. We have begun to examine
  some of these consequences and find reason to question the idea that
  spicules are the primary source of hot coronal plasma.

Title: SDO/AIA Light Curves and Implications for Coronal Heating:
    Observations
Authors: Viall, N. M.; Klimchuk, J. A.
Bibcode: 2010AGUFMSH41E..02V
Altcode:
  It seems largely agreed that many coronal loops---those observed at a
  temperature of about 1 MK---are bundles of unresolved strands that are
  heated by storms of impulsive nanoflares. The nature of coronal heating
  in hotter loops and in the very important but largely ignored diffuse
  component of active regions is much less clear. Is it also impulsive
  or is it quasi steady? The spectacular new data from the Atmospheric
  Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory
  (SDO) offer an excellent opportunity to address this question. We
  analyze the light curves of coronal loops and the diffuse corona in
  6 different AIA channels and compare them with the predicted light
  curves from theoretical models. Light curves in the different AIA
  channels reach their peak intensities with predictable orderings as a
  function the nanoflare storm properties. We show that while some sets of
  light curves exhibit clear evidence of cooling after nanoflare storms,
  other cases are less straightforward to interpret. Complications arise
  because of line-of-sight integration through many different structures,
  the broadband nature of the AIA channels, and because physical
  properties can change substantially depending on the magnitude of the
  energy release. Nevertheless, the light curves exhibit predictable and
  understandable patterns. This presentation emphasizes the observational
  aspects of our study. A companion presentation emphasizes the models.

Title: Modeling the Secondary Flare Irradiance Measured by Solar
    Dynamic Observatory (SDO) Extreme ultraviolet Variability Experiment
    (EVE)
Authors: Hock, R. A.; Woods, T. N.; Klimchuk, J. A.; Eparvier, F. G.
Bibcode: 2010AGUFMSH13A..05H
Altcode:
  NASA’s Solar Dynamic Observatory (SDO) launched on 11 February
  2010 and normal operations for all three instruments began 1 May
  2010. Since then numerous small and moderate (C- and M-class) flares
  have been observed. One interesting feature observed by the Extreme
  ultraviolet Variability Experiment (EVE) is the enhancement of 2-3
  million K emission several hours after the flare’s soft x-ray
  emission. From the Atmospheric Imaging Assembly (AIA) images, we can
  tell that these secondary emissions occur in the same active region
  as the flare but not in same coronal loops. Here, we examine the C8.8
  flare that occurred on 5 May 2010. The flare occurred in Active Region
  11069, a small magnetically complex region near the western limb of
  the Sun. The gradual phase of the flare is clearly seen in both GOES
  soft X-rays and the hot coronal emissions (&gt;2 million K) measured
  by EVE. The secondary flare emission starts 30 minutes after the peak
  in gradual phase and slowly increases over an hour before decaying to
  the pre-flare levels. It is most strongly seen in Fe XV and Fe XVI (2-3
  million K). Using the Enthalpy-Based Thermal Evolution of Loops (EBTEL)
  model, we are able calculate the EUV irradiance of a set of coronal
  loops for a given heating function. This allows us to determine the
  best-fit heating profile as a function of time for the C8.8 flare. The
  heating profile for this event clearly shows that there are two separate
  phases of heating. The first phase involves traditional post-flare
  loops. The field reconnects after erupting, and the energy released
  during the reconnection heats the plasma to very high temperatures. As
  the loops cool, emissions are seen in progressively cooler lines from
  10 to 1 million K. The second phase is very different. A large number
  of coronal loops are heated only modestly-. The plasma in each loop,
  instead of reaching 10 million K, reaches 3 million K. The heating
  is also spread out over an hour generating the long secondary flare
  emission profile. We discuss the nature of this secondary flare
  emission, which seems to be an important component of many events.

Title: SDO/AIA Light Curves and Implications for Coronal Heating:
    Model Predictions
Authors: Klimchuk, J. A.; Viall, N. M.
Bibcode: 2010AGUFMSH41E..03K
Altcode:
  It seems largely agreed that many coronal loops---those observed
  at a temperature of about 1 MK---are bundles of unresolved strands
  that are heated by storms of impulsive nanoflares. The nature of
  coronal heating in hotter loops and in the very important but largely
  ignored diffuse component of active regions is much less clear. Is
  it also impulsive or is it quasi steady? The spectacular new data
  from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar
  Dynamics Observatory (SDO) offer an excellent opportunity to address
  this question. We analyze the light curves of coronal loops and the
  diffuse corona in 6 different AIA channels and compare them with the
  predicted light curves from theoretical models. Light curves in the
  different AIA channels reach their peak intensities with predictable
  orderings as a function the nanoflare storm properties. We show that
  while some sets of light curves exhibit clear evidence of cooling
  after nanoflare storms, other cases are less straightforward to
  interpret. Complications arise because of line-of-sight integration
  through many different structures, the broadband nature of the AIA
  channels, and because physical properties can change substantially
  depending on the magnitude of the energy release. Nevertheless, the
  light curves exhibit predictable and understandable patterns. This
  presentation emphasizes the modeling aspects of our study. A companion
  presentation emphasizes the observations.

Title: Science Objectives for an X-Ray Microcalorimeter Observing
    the Sun
Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M;
   Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse,
   N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.;
   DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra,
   A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg,
   C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.;
   Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska,
   J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres,
   G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.;
   Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz,
   J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.;
   Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B.
Bibcode: 2010arXiv1011.4052L
Altcode:
  We present the science case for a broadband X-ray imager with
  high-resolution spectroscopy, including simulations of X-ray spectral
  diagnostics of both active regions and solar flares. This is part of
  a trilogy of white papers discussing science, instrument (Bandler et
  al. 2010), and missions (Bookbinder et al. 2010) to exploit major
  advances recently made in transition-edge sensor (TES) detector
  technology that enable resolution better than 2 eV in an array that
  can handle high count rates. Combined with a modest X-ray mirror, this
  instrument would combine arcsecondscale imaging with high-resolution
  spectra over a field of view sufficiently large for the study of
  active regions and flares, enabling a wide range of studies such as
  the detection of microheating in active regions, ion-resolved velocity
  flows, and the presence of non-thermal electrons in hot plasmas. It
  would also enable more direct comparisons between solar and stellar
  soft X-ray spectra, a waveband in which (unusually) we currently have
  much better stellar data than we do of the Sun.

Title: On the Isothermality of Solar Plasmas
Authors: Landi, E.; Klimchuk, J. A.
Bibcode: 2010ApJ...723..320L
Altcode:
  Recent measurements have shown that the quiet unstructured solar corona
  observed at the solar limb is close to isothermal, at a temperature
  that does not appear to change over wide areas or with time. Some
  individual active region loop structures have also been found to
  be nearly isothermal both along their axis and across their cross
  section. Even a complex active region observed at the solar limb has
  been found to be composed of three distinct isothermal plasmas. If
  confirmed, these results would pose formidable challenges to the current
  theoretical understanding of the thermal structure and heating of the
  solar corona. For example, no current theoretical model can explain
  the excess densities and lifetimes of many observed loops if the loops
  are in fact isothermal. All of these measurements are based on the
  so-called emission measure (EM) diagnostic technique that is applied
  to a set of optically thin lines under the assumption of isothermal
  plasma. It provides simultaneous measurement of both the temperature and
  EM. In this work, we develop a new method to quantify the uncertainties
  in the technique and to rigorously assess its ability to discriminate
  between isothermal and multithermal plasmas. We define a formal measure
  of the uncertainty in the EM diagnostic technique that can easily be
  applied to real data. We here apply it to synthetic data based on a
  variety of assumed plasma thermal distributions and develop a method
  to quantitatively assess the degree of multithermality of a plasma.

Title: Evidence of Impulsive Heating in Active Region Core Loops
Authors: Tripathi, Durgesh; Mason, Helen E.; Klimchuk, James A.
Bibcode: 2010ApJ...723..713T
Altcode: 2010arXiv1009.0663T
  Using a full spectral scan of an active region from the
  Extreme-Ultraviolet Imaging Spectrometer (EIS) we have obtained emission
  measure EM(T) distributions in two different moss regions within the
  same active region. We have compared these with theoretical transition
  region EMs derived for three limiting cases, namely, static equilibrium,
  strong condensation, and strong evaporation from Klimchuk et al. The
  EM distributions in both the moss regions are strikingly similar and
  show a monotonically increasing trend from log T[K] = 5.15-6.3. Using
  photospheric abundances, we obtain a consistent EM distribution for
  all ions. Comparing the observed and theoretical EM distributions,
  we find that the observed EM distribution is best explained by the
  strong condensation case (EM<SUB>con</SUB>), suggesting that a downward
  enthalpy flux plays an important and possibly dominant role in powering
  the transition region moss emission. The downflows could be due to
  unresolved coronal plasma that is cooling and draining after having
  been impulsively heated. This supports the idea that the hot loops
  (with temperatures of 3-5 MK) seen in the core of active regions are
  heated by nanoflares.

Title: A Simple Model for the Evolution of Multi-stranded Coronal
    Loops
Authors: López Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2010ApJ...719..591L
Altcode: 2010arXiv1004.2061L
  We develop and analyze a simple cellular automaton model that reproduces
  the main properties of the evolution of soft X-ray coronal loops. We
  are motivated by the observation that these loops evolve in three
  distinguishable phases that suggest the development, maintenance,
  and decay of a self-organized system. The model is based on the
  idea that loops are made of elemental strands that are heated by the
  relaxation of magnetic stress in the form of nanoflares. In this vision,
  usually called the "Parker conjecture," the origin of stress is the
  displacement of the strand footpoints due to photospheric convective
  motions. Modeling the response and evolution of the plasma we obtain
  synthetic light curves that have the same characteristic properties
  (intensity, fluctuations, and timescales) as the observed cases. We
  study the dependence of these properties on the model parameters and
  find scaling laws that can be used as observational predictions of the
  model. We discuss the implications of our results for the interpretation
  of recent loop observations in different wavelengths.

Title: Determining the Temperature Structure of Solar Coronal Loops
    using their Temporal Evolution
Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden,
   M. J.; Klimchuk, J. A.
Bibcode: 2010AAS...21630001M
Altcode:
  Despite much progress toward understanding the dynamics of the
  corona, the physical properties of coronal loops are not yet fully
  understood. Recent investigations and observations from different
  instruments have yielded contradictory results about the true physical
  properties of coronal loops, specifically as to whether the observed
  loops are isothermal structures or the convolution of several
  multi-thermal strands. In this talk, we introduce a new technique
  to determine if an observed loop is isothermal or multi-thermal. We
  determine the evolution of ten selected loops in multiple filter
  images from the Transition Region and Coronal Explorer (TRACE). Our
  new technique calculates the delay, calculates a cooling time,
  and determines if that cooling time is consistent with the observed
  lifetime. If the observational lifetime of the loop agrees with the
  calculated lifetime, then we can conclude that the loop is a single
  "monolithic” structure that heats and cools as a homogeneous unit,
  with isothermal temperature over the cross-section. If not, the
  loop must be a bundle of multiple multi-thermal strands, all being
  heated and cooling independently. In the second part of the talk,
  we utilize the concept of nanoflare storms to understand the reason
  behind the extended lifetimes. By simulating the observed light curves
  of the loops using EBTEL (Enthalpy-Based Thermal Evolution of Loops),
  we find that the longer observed lifetimes can be reproduced by using
  a set of small-scale impulsively heated strands.

Title: The Existence and Origin of Turbulence in Solar Active Regions
Authors: Klimchuk, James A.; Nigro, G.; Dahlburg, R. B.; Antiochos,
   S. K.
Bibcode: 2010AAS...21630205K
Altcode:
  It has been suggested that turbulence plays a fundamental role in the
  heating of solar active regions, with its intermittent behavior being
  the explanation of impulsive energy release (nanoflares). We know that
  episodes of turbulence are produced in the final nonlinear stage of
  the secondary instability of electric current sheets. However, these
  current sheets must exist prior to the turbulence. Whether turbulence
  can dynamically produce current sheets that would not otherwise be
  present is a different and important question. <P />Turbulence occurs
  freely in the solar wind and in other situations where the magnetic
  field does not dominate. However, the magnetic field strongly resists
  being distorted in line-tied, low-beta environments such as active
  regions. Can turbulence develop naturally in these environments
  without being driven by an instability? To answer this question,
  we have performed a time-dependent MHD simulation of a slowly driven
  system that does not contain current sheets and is stable to applied
  perturbations. We find no evidence for bursty energy release, steep
  spatial gradients, or power-law energy spectra that are the typical
  signatures of turbulence. We conclude that the turbulence which occurs
  in active regions is an important yet secondary process and not the
  primary cause of heating.

Title: Division II: Sun and Heliosphere
Authors: Melrose, Donald B.; Martinez Pillet, Valentin; Webb, David
   F.; Bougeret, Jean-Louis; Klimchuk, James A.; Kosovichev, Alexander;
   van Driel-Gesztelyi, Lidia; von Steiger, Rudolf
Bibcode: 2010IAUTB..27..146M
Altcode:
  This report is on activities of the Division at the General Assembly
  in Rio de Janeiro. Summaries of scientific activities over the past
  triennium have been published in Transactions A, see Melrose et
  al. (2008), Klimchuk et al. (2008), Martinez Pillet et al. (2008) and
  Bougeret et al. (2008). The business meeting of the three Commissions
  were incorporated into the business meeting of the Division. This
  report is based in part on minutes of the business meeting, provided
  by the Secretary of the Division, Lidia van Driel-Gesztelyi, and it
  also includes reports provided by the Presidents of the Commissions
  (C10, C12, C49) and of the Working Groups (WGs) in the Division.

Title: Can Thermal Nonequilibrium Explain Coronal Loops?
Authors: Klimchuk, James A.; Karpen, Judy T.; Antiochos, Spiro K.
Bibcode: 2010ApJ...714.1239K
Altcode: 2009arXiv0912.0953K
  Any successful model of coronal loops must explain a number of observed
  properties. For warm (~1 MK) loops, these include (1) excess density,
  (2) flat temperature profile, (3) super-hydrostatic scale height,
  (4) unstructured intensity profile, and (5) 1000-5000 s lifetime. We
  examine whether thermal nonequilibrium can reproduce the observations
  by performing hydrodynamic simulations based on steady coronal heating
  that decreases exponentially with height. We consider both monolithic
  and multi-stranded loops. The simulations successfully reproduce
  certain aspects of the observations, including the excess density,
  but each of them fails in at least one critical way. Monolithic models
  have far too much intensity structure, while multi-strand models
  are either too structured or too long-lived. Our results appear to
  rule out the widespread existence of heating that is both highly
  concentrated low in the corona and steady or quasi-steady (slowly
  varying or impulsive with a rapid cadence). Active regions would have a
  very different appearance if the dominant heating mechanism had these
  properties. Thermal nonequilibrium may nonetheless play an important
  role in prominences and catastrophic cooling events (e.g., coronal rain)
  that occupy a small fraction of the coronal volume. However, apparent
  inconsistencies between the models and observations of cooling events
  have yet to be understood.

Title: Nanoflare heating of solar and stellar coronae
Authors: Klimchuk, James
Bibcode: 2010cosp...38.2897K
Altcode: 2010cosp.meet.2897K
  A combination of observational and theoretical evidence suggests
  that much, and perhaps most, of the Sun's corona is heated by small
  unresolved bursts of energy called nanoflares. It seems likely that
  stellar coronae are heated in a similar fashion. Nanoflares are here
  taken to mean any impulsive heating that occurs within a magnetic flux
  strand. Many mechanisms have this property, including waves, but we
  prefer Parker's picture of tangled magnetic fields. The tangling is
  caused by turbulent convection at the stellar surface, and magnetic
  energy is released when the stresses reach a critical level. We suggest
  that the mechanism of energy release is the "secondary instability"
  of electric current sheets that are present at the boundaries between
  misaligned strands. I will discuss the collective evidence for solar
  and stellar nanoflares and hopefully present new results from the
  Solar Dynamics Observatory that was just launched.

Title: Nanoflares, spicules, and other small-scale dynamic phenomena
    on the sun
Authors: Klimchuk, James
Bibcode: 2010cosp...38.2831K
Altcode: 2010cosp.meet.2831K
  There is abundant evidence of highly dynamic phenomena occurring
  on very small scales in the solar atmosphere. For example, the
  observed properties of many coronal loops can only be ex-plained
  if the loops are bundles of unresolved strands that are heated
  impulsively by nanoflares. Type II spicules recently discovered by
  Hinode are an example of small-scale impulsive events occurring in the
  chromosphere. The existence of these and other small-scale phenomena
  is not surprising given the highly structured nature of the magnetic
  field that is revealed by photo-spheric observations. Dynamic phenomena
  also occur on much larger scales, including coronal jets, flares,
  and CMEs. It is tempting to suggest that these different phenomena are
  all closely related and represent a continuous distribution of sizes
  and energies. However, this is a danger-ous over simplification in
  my opinion. While it is true that the phenomena all involve "magnetic
  reconnection" (the changing of field line connectivity) in some form,
  how this occurs depends strongly on the magnetic geometry. A nanoflare
  resulting from the interaction of tangled mag-netic strands within a
  confined coronal loop is much different from a major flare occurring
  at the current sheet formed when a CME rips open an active region. I
  will review the evidence for ubiquitous small-scale dynamic phenomena
  on the Sun and discuss why different phenomena are not all fundamentally
  the same.

Title: A cellular automaton nanoflare model of coronal loops
Authors: Lopez Fuentes, Marcelo; Klimchuk, James
Bibcode: 2010cosp...38.2833L
Altcode: 2010cosp.meet.2833L
  In Lopez-Fuentes, Klimchuk and Mandrini (2007) we found that the
  evolution of soft X-ray loops can be separated into three main
  phases that suggest the development, maintainance, and decay of a
  self-organized system. Here, we present a cellular automaton model that
  reproduces the main features of the observed evolution. The model is
  based on the idea that loops are made of multiple unresolved strands
  which have footpoints that are displaced by random photospheric motions
  (Parker 1988). In this scenario, there is a continuous increase of
  the magnetic stress between neighboring strands until a critical
  stress is reached and the accumulated energy is suddenly released by
  reconnection. Each of these reconnection "events" is associated with
  a nanoflare. Using the EBTEL hydrodynamic code (Klimchuk, Patsourakos
  and Cargill 2008) to model the plasma response we construct synthetic
  light curves that we compare with the observations. We also study how
  the properties of the light curves scale with the different parameters
  of the model. Finally, we discuss how the present model can be used
  to explain loop observations in different wavelengths.

Title: XRT Detection of Hot Plasma in Active Regions and Nanoflare
    Heating
Authors: Reale, F.; Klimchuk, J. A.; Parenti, S.; Testa, P.
Bibcode: 2009ASPC..415..256R
Altcode:
  Nanoflares occurring in sub-resolution strands have been long invoked
  as strong candidates for the heating of active region (AR) coronal
  loops. However, the frequent occurrence of nanoflares requires the
  steady presence of flare-hot plasma in the active region, which
  has been difficult to detect so far. We report on the analysis of
  multi-filter Hinode/XRT observations of an active region, which may
  show the widespread presence of 10 MK plasma.

Title: Coronal Loop Models and Those Annoying Observations! (Keynote)
Authors: Klimchuk, J. A.
Bibcode: 2009ASPC..415..221K
Altcode: 2009arXiv0904.1391K
  It was once thought that all coronal loops are in static equilibrium,
  but observational and modeling developments over the past decade
  have shown that this is clearly not the case. It is now established
  that warm (∼ 1 MK) loops observed in the EUV are explainable
  as bundles of unresolved strands that are heated impulsively by
  storms of nanoflares. A raging debate concerning the multi-thermal
  versus isothermal nature of the loops can be reconciled in terms
  of the duration of the storm. We show that short and long storms
  produce narrow and broad thermal distributions, respectively. We also
  examine the possibility that warm loops can be explained with thermal
  nonequilibrium, a process by which steady heating produces dynamic
  behavior whenever the heating is highly concentrated near the loop
  footpoints. We conclude that this is not a viable explanation for
  monolithic loops under the conditions we have considered, but that
  it may have application to multi-stranded loops. Serious questions
  remain, however.

Title: The Existence and Origin of Turbulence in Solar Active Regions
Authors: Klimchuk, J. A.; Nigro, G.; Dahlburg, R. B.; Antiochos, S. K.
Bibcode: 2009AGUFMSM42B..03K
Altcode:
  It has been suggested that turbulence plays a fundamental role in the
  heating of solar active regions, with its intermittent behavior being
  the explanation of impulsive energy release (nanoflares). We know that
  episodes of turbulence are produced in the final nonlinear stage of
  the secondary instability of electric current sheets. However, these
  current sheets must exist prior to the turbulence. Whether turbulence
  can dynamically produce current sheets that would not otherwise be
  present is a different and important question. Turbulence occurs
  freely in the solar wind and in other situations where the magnetic
  field does not dominate. However, the magnetic field strongly resists
  being distorted in line-tied, low-beta environments such as active
  regions. Can turbulence develop naturally in these environments
  without being driven by an instability? To answer this question,
  we have performed a time-dependent MHD simulation of a slowly driven
  system that does not contain current sheets and is stable to applied
  perturbations. We find no evidence for bursty energy release, steep
  spatial gradients, or power-law power spectra that are the typical
  signatures of turbulence. We conclude that the turbulence which occurs
  in active regions is an important yet secondary process and not the
  primary cause of heating.

Title: Explosive Instability and Coronal Heating
Authors: Dahlburg, R. B.; Liu, J. -H.; Klimchuk, J. A.; Nigro, G.
Bibcode: 2009ApJ...704.1059D
Altcode:
  The observed energy-loss rate from the solar corona implies that
  the coronal magnetic field has a critical angle at which energy is
  released. It has been hypothesized that at this critical angle an
  "explosive instability" would occur, leading to an enhanced conversion
  of magnetic energy into heat. In earlier investigations, we have shown
  that a shear-dependent magnetohydrodynamic process called "secondary
  instability" has many of the distinctive features of the hypothetical
  "explosive instability." In this paper, we give the first demonstration
  that this "secondary instability" occurs in a system with line-tied
  magnetic fields and boundary shearing—basically the situation
  described by Parker. We also show that, as the disturbance due to
  secondary instability attains finite amplitude, there is a transition
  to turbulence which leads to enhanced dissipation of magnetic and
  kinetic energy. These results are obtained from numerical simulations
  performed with a new parallelized, viscoresistive, three-dimensional
  code that solves the cold plasma equations. The code employs a Fourier
  collocation—finite difference spatial discretization, and uses a
  third-order Runge-Kutta temporal discretization.

Title: Evidence of Widespread Hot Plasma in a Nonflaring Coronal
    Active Region from Hinode/X-Ray Telescope
Authors: Reale, Fabio; Testa, Paola; Klimchuk, James A.; Parenti,
   Susanna
Bibcode: 2009ApJ...698..756R
Altcode: 2009arXiv0904.0878R
  Nanoflares, short and intense heat pulses within spatially unresolved
  magnetic strands, are now considered a leading candidate to solve
  the coronal heating problem. However, the frequent occurrence of
  nanoflares requires that flare-hot plasma be present in the corona at
  all times. Its detection has proved elusive until now, in part because
  the intensities are predicted to be very faint. Here, we report on the
  analysis of an active region observed with five filters by Hinode/X-Ray
  Telescope (XRT) in 2006 November. We have used the filter ratio method
  to derive maps of temperature and emission measure (EM) both in soft and
  hard ratios. These maps are approximate in that the plasma is assumed
  to be isothermal along each line of sight. Nonetheless, the hardest
  available ratio reveals the clear presence of plasma around 10 MK. To
  obtain more detailed information about the plasma properties, we have
  performed Monte Carlo simulations assuming a variety of nonisothermal
  EM distributions along the lines of sight. We find that the observed
  filter ratios imply bi-modal distributions consisting of a strong cool
  (log T ~ 6.3 - 6.5) component and a weaker (few percent) and hotter (6.6
  &lt; log T &lt; 7.2) component. The data are consistent with bi-modal
  distributions along all lines of sight, i.e., throughout the active
  region. We also find that the isothermal temperature inferred from a
  filter ratio depends sensitively on the precise temperature of the cool
  component. A slight shift of this component can cause the hot component
  to be obscured in a hard ratio measurement. Consequently, temperature
  maps made in hard and soft ratios tend to be anti-correlated. We
  conclude that this observation supports the presence of widespread
  nanoflaring activity in the active region.

Title: Spectroscopic Observations of Hot Lines Constraining Coronal
    Heating in Solar Active Regions
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2009ApJ...696..760P
Altcode: 2009arXiv0903.3880P
  Extreme-ultraviolet observations of warm coronal loops suggest that they
  are bundles of unresolved strands that are heated impulsively to high
  temperatures by nanoflares. The plasma would then have the observed
  properties (e.g., excess density compared with static equilibrium)
  when it cools into the 1-2MK range. If this interpretation is correct,
  then very hot emission should be present outside of proper flares. It
  is predicted to be very faint, however. A critical element for proving
  or refuting this hypothesis is the existence of hot, yet faint plasmas
  which should be at amounts predicted by impulsive heating models. We
  report on the first comprehensive spectroscopic study of hot plasmas
  in active regions (ARs). Data from the Extreme-ultraviolet Imaging
  Spectrometer on Hinode were used to construct emission measure (EM)
  distributions in quiescent ARs in the 1-5 MK temperature range. The
  distributions are flat or slowly increasing up to approximately 3 MK
  and then fall off rapidly at higher temperatures. We show that AR models
  based on impulsive heating can reproduce the observed EM distributions
  relatively well. Our results provide strong new evidence that coronal
  heating is impulsive in nature.

Title: Spectroscopic Observations of Hot Lines Constraining Coronal
    Heating in Solar Active Regions
Authors: Patsourakos, Spiros; Klimchuk, J. A.
Bibcode: 2009SPD....40.1211P
Altcode:
  EUV observations of warm coronal loops suggest that they are bundles of
  unresolved strands that are heated impulsively to high temperatures by
  nanoflares. The plasma would then have the observed properties (e.g.,
  excess density compared to static equilibrium) when it cools into
  the 1-2 MK range. If this interpretation is correct, then very hot
  emission should be present outside of proper flares. It is predicted
  to be vey faint, however. A critical element for proving or refuting
  this hypothesis is the existence of hot, very faint plasmas which
  should be at amounts predicted by impulsive heating. We report on
  the first comprehensive spectroscopic study of hot plasmas in active
  regions. Data from the EIS spectrometer on Hinode were used to construct
  emission measure distributions in quiescent active regions in the 1-5 MK
  temperature range. The distributions are flat or slowly increasing up to
  approximately 3 MK and then fall off rapidly at higher temperatures. We
  show that active region models based on impulsive heating can reproduce
  the observed EM distributions relatively well. Our results provide
  strong new evidence that coronal heating is impulsive in nature.

Title: Observations of Nanoflare Produced Hot ( 10 Mk) Plasma
Authors: Klimchuk, James A.; Reale, F.; Testa, P.; Parenti, S.
Bibcode: 2009SPD....40.1214K
Altcode:
  Indirect observational evidence suggests that some or most of
  the corona is heated impulsively on sub-resolution scales by
  nanoflares. Theoretical studies of possible heating mechanisms
  also support this picture. However, the most direct evidence of
  nanoflares---plasma hotter than 5 MK---has been difficult to obtain
  because the emission is expected to be very faint. The reason is
  two-fold: first, hot plasma cools very rapidly by thermal conduction;
  and second, densities are small because chromospheric evaporation
  has not had time to fill the corona. Recent observations from several
  instruments have now provided strong evidence of hot plasma. We report
  here on the detection of 10 MK plasma by the X-Ray Telescope (XRT)
  on Hinode. We show that the intensity of the emission is consistent
  with nanoflare models, but is extremely difficult to explain with
  steady heating.

Title: Models of Impulsively Heated Solar Active Regions
Authors: Airapetian, Vladimir; Klimchuk, J.
Bibcode: 2009SPD....40.1202A
Altcode:
  A number of attempts to model solar active regions with steady coronal
  heating have been modestly successful at reproducing the observed
  soft X-ray emission, but they fail dramatically at explaining EUV
  observations. Since impulsive heating (nanoflare) models can reproduce
  individual EUV loops, it seems reasonable to consider that entire active
  regions are impulsively heated. However, nanoflares are characterized
  by many parameters, such as magnitude, duration, and time delay between
  successive events, and these parameters may depend on the strength
  of the magnetic field or the length of field lines, for example, so a
  wide range of active region models must be examined. We have recently
  begun such a study. Each model begins with a magnetic "skeleton”
  obtained by extrapolating an observed photospheric magnetogram into
  the corona. Field lines are populated with plasma using our highly
  efficient hydro code called Enthalpy Based Thermal Evolution of Loops
  (EBTEL). We then produce synthetic images corresponding to emission
  line or broad-band observations. By determining which set of nanoflare
  parameters best reproduces actual observations, we hope to constrain
  the properties of the heating and ultimately to reveal the physical
  mechanism. We here report on the initial progress of our study.

Title: Hinode X-Ray Telescope Detection of Hot Emission from Quiescent
Active Regions: A Nanoflare Signature?
Authors: Schmelz, J. T.; Saar, S. H.; DeLuca, E. E.; Golub, L.;
   Kashyap, V. L.; Weber, M. A.; Klimchuk, J. A.
Bibcode: 2009ApJ...693L.131S
Altcode: 2009arXiv0901.3122S
  The X-Ray Telescope (XRT) on the Japanese/USA/UK Hinode (Solar-B)
  spacecraft has detected emission from a quiescent active region
  core that is consistent with nanoflare heating. The fluxes from 10
  broadband X-ray filters and filter combinations were used to construct
  differential emission measure (DEM) curves. In addition to the expected
  active region peak at log T = 6.3-6.5, we find a high-temperature
  component with significant emission measure at log T &gt; 7.0. This
  emission measure is weak compared to the main peak—the DEM is down
  by almost three orders of magnitude—which accounts of the fact
  that it has not been observed with earlier instruments. It is also
  consistent with spectra of quiescent active regions: no Fe XIX lines
  are observed in a CHIANTI synthetic spectrum generated using the XRT
  DEM distribution. The DEM result is successfully reproduced with a
  simple two-component nanoflare model.

Title: Multi-wavelength observations and modelling of a canonical
    solar flare
Authors: Raftery, C. L.; Gallagher, P. T.; Milligan, R. O.; Klimchuk,
   J. A.
Bibcode: 2009A&A...494.1127R
Altcode: 2008arXiv0812.0311R
  Aims: We investigate the temporal evolution of temperature, emission
  measure, energy loss, and velocity in a C-class solar flare from
  both observational and theoretical perspectives. <BR />Methods:
  The properties of the flare were derived by following the systematic
  cooling of the plasma through the response functions of a number of
  instruments - the Reuven Ramaty High Energy Solar Spectroscopic Imager
  (RHESSI; &gt;5 MK), GOES-12 (5-30 MK), the Transition Region and
  Coronal Explorer (TRACE 171 Å; 1 MK), and the Coronal Diagnostic
  Spectrometer (CDS; ~0.03-8 MK). These measurements were studied in
  combination with simulations from the 0-D enthalpy based thermal
  evolution of loops (EBTEL) model. <BR />Results: At the flare
  onset, upflows of ~90 km s<SUP>-1</SUP> and low-level emission were
  observed in Fe XIX, consistent with pre-flare heating and gentle
  chromospheric evaporation. During the impulsive phase, upflows of
  ~80 km s<SUP>-1</SUP> in Fe XIX and simultaneous downflows of ~20 km
  s<SUP>-1</SUP> in He I and O V were observed, indicating explosive
  chromospheric evaporation. The plasma was subsequently found to reach
  a peak temperature of ⪆13 MK in approximately 10 min. Using EBTEL,
  conduction was found to be the dominant loss mechanism during the
  initial ~300 s of the decay phase. It was also found to be responsible
  for driving gentle chromospheric evaporation during this period. As
  the temperature fell below ~8 MK, and for the next ~4000 s, radiative
  losses were determined to dominate over conductive losses. The
  radiative loss phase was accompanied by significant downflows of
  ≤40 km s<SUP>-1</SUP> in O V. <BR />Conclusions: This is the first
  extensive study of the evolution of a canonical solar flare using
  both spectroscopic and broad-band instruments in conjunction with a
  0-D hydrodynamic model. While our results are in broad agreement with
  the standard flare model, the simulations suggest that both conductive
  and non-thermal beam heating play important roles in heating the flare
  plasma during the impulsive phase of at least this event.

Title: Division II: Sun and Heliosphere
Authors: Melrose, Donald B.; Martínez Pillet, Valentin; Webb, David
   F.; van Driel-Gesztelyi, Lidia; Bougeret, Jean-Louis; Klimchuk,
   James A.; Kosovichev, Alexander; von Steiger, Rudolf
Bibcode: 2009IAUTA..27...73M
Altcode:
  Division II of the IAU provides a forum for astronomers and
  astrophysicists studying a wide range of phenomena related to the
  structure, radiation and activity of the Sun, and its interaction with
  the Earth and the rest of the solar system. Division II encompasses
  three Commissions, 10, 12 and 49, and four Working Groups.

Title: Commission 10: Solar Activity
Authors: Klimchuk, James A.; van Driel-Gesztelyi, Lidia; Schrijver,
   Carolus J.; Melrose, Donald B.; Fletcher, Lyndsay; Gopalswamy,
   Natchimuthuk; Harrison, Richard A.; Mandrini, Cristina H.; Peter,
   Hardi; Tsuneta, Saku; Vršnak, Bojan; Wang, Jing-Xiu
Bibcode: 2009IAUTA..27...79K
Altcode: 2008arXiv0809.1444K
  Commission 10 deals with solar activity in all of its forms,
  ranging from the smallest nanoflares to the largest coronal mass
  ejections. This report reviews scientific progress over the roughly
  two-year period ending in the middle of 2008. This has been an exciting
  time in solar physics, highlighted by the launches of the Hinode and
  STEREO missions late in 2006. The report is reasonably comprehensive,
  though it is far from exhaustive. Limited space prevents the inclusion
  of many significant results. The report is divided into the following
  sections: Photosphere and chromosphere; Transition region; Corona and
  coronal heating; Coronal jets; flares; Coronal mass ejection initiation;
  Global coronal waves and shocks; Coronal dimming; The link between low
  coronal CME signatures and magnetic clouds; Coronal mass ejections in
  the heliosphere; and Coronal mass ejections and space weather. Primary
  authorship is indicated at the beginning of each section.

Title: Static and Impulsive Models of Solar Active Regions
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2008ApJ...689.1406P
Altcode: 2008arXiv0808.2745P
  The physical modeling of active regions (ARs) and of the global corona
  is receiving increasing interest lately. Recent attempts to model ARs
  using static equilibrium models were quite successful in reproducing AR
  images of hot soft X-ray (SXR) loops. They however failed to predict
  the bright extreme-ultraviolet (EUV) warm loops permeating ARs: the
  synthetic images were dominated by intense footpoint emission. We
  demonstrate that this failure is due to the very weak dependence of
  loop temperature on loop length which cannot simultaneously account for
  both hot and warm loops in the same AR. We then consider time-dependent
  AR models based on nanoflare heating. We demonstrate that such models
  can simultaneously reproduce EUV and SXR loops in ARs. Moreover, they
  predict radial intensity variations consistent with the localized core
  and extended emissions in SXR and EUV AR observations, respectively. We
  finally show how the AR morphology can be used as a gauge of the
  properties (duration, energy, spatial dependence, and repetition time)
  of the impulsive heating.

Title: Highly Efficient Modeling of Dynamic Coronal Loops
Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J.
Bibcode: 2008ApJ...682.1351K
Altcode: 2007arXiv0710.0185K
  Observational and theoretical evidence suggests that coronal heating
  is impulsive and occurs on very small cross-field spatial scales. A
  single coronal loop could contain a hundred or more individual strands
  that are heated quasi-independently by nanoflares. It is therefore an
  enormous undertaking to model an entire active region or the global
  corona. Three-dimensional MHD codes have inadequate spatial resolution,
  and one-dimensional (1D) hydrodynamic codes are too slow to simulate
  the many thousands of elemental strands that must be treated in
  a reasonable representation. Fortunately, thermal conduction and
  flows tend to smooth out plasma gradients along the magnetic field,
  so zero-dimensional (0D) models are an acceptable alternative. We
  have developed a highly efficient model called "enthalpy-based thermal
  evolution of loops" (EBTEL), which accurately describes the evolution
  of the average temperature, pressure, and density along a coronal
  strand. It improves significantly on earlier models of this type—in
  accuracy, flexibility, and capability. It treats both slowly varying
  and highly impulsive coronal heating; it provides the time-dependent
  differential emission measure distribution, DEM(T), at the transition
  region footpoints; and there are options for heat flux saturation and
  nonthermal electron beam heating. EBTEL gives excellent agreement with
  far more sophisticated 1D hydrodynamic simulations despite using 4
  orders of magnitude less computing time. It promises to be a powerful
  new tool for solar and stellar studies.

Title: Hot Spectral Emissions in Quiescent Active Regions and
    Nanoflare Heating
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2008AGUSMSP43C..02P
Altcode:
  A leading candidate for the heating of active region (AR) coronal loops
  is the nanoflare model. This model treats coronal loops as collections
  of impulsively heated sub-resolution strands and explains several
  key observational aspects of warm (1-2 MK) coronal loops. However,
  the basic requirement of this model is that the strands initially reach
  very high temperatures of several MK before they cool down to canonical
  coronal temperatures. Therefore, the detection of hot plasmas in AR
  loops represents a stringest test of the nanoflare model. Previous
  work has shown that the best way to observe the postulated hot
  plasmas is by the means of spectroscopic observations in hot lines
  (T &gt; 3 MK). The emission is predicted to be quite faint, but the
  EIS spectrometer onboard Hinode has sufficient sensitivity to allow us
  to perform such a test for the first time. We will present an analysis
  of the emission characteristics of quiescent coronal loops in a number
  of hot lines spanning approximately 3-12 MK (Ni XVII, Ca XV, Fe XVII,
  Ca XVII, Fe XXIII). We will show that hot plasmas are ubiquitous over
  entire active regions, and we will compare the measured intensities
  of both hot and warm lines with predictions of nanoflare models.

Title: Are Constant Loop Widths an Artifact of the Background and
    the Spatial Resolution?
Authors: López Fuentes, M. C.; Démoulin, P.; Klimchuk, J. A.
Bibcode: 2008ApJ...673..586L
Altcode: 2007arXiv0704.0637L
  We study the effect of the coronal background in the determination
  of the diameter of EUV loops, and we analyze the suitability of
  the procedure followed in a previous paper for characterizing their
  expansion properties. For the analysis we create different synthetic
  loops, and we place them on real backgrounds from data obtained
  with the Transition Region and Coronal Explorer (TRACE). We apply to
  these loops the same procedure followed in our previous works, and we
  compare the results with real loop observations. We demonstrate that
  the procedure allows us to distinguish constant width loops from loops
  that expand appreciably with height, as predicted by simple force-free
  field models. This holds even for loops near the resolution limit. The
  procedure can easily determine when loops are below the resolution
  limit and therefore not reliably measured. We find that small-scale
  variations in the measured loop width are likely due to imperfections
  in the background subtraction. The greatest errors occur in especially
  narrow loops and in places where the background is especially bright
  relative to the loop. We stress, however, that these effects do not
  impact the ability to measure large-scale variations. The result that
  observed loops do not expand systematically with height is robust.

Title: Understanding Warm Coronal Loops
Authors: Klimchuk, J. A.; Karpen, J. T.; Patsourakos, S.
Bibcode: 2007AGUFMSH51C..05K
Altcode:
  One of the great mysteries of coronal physics that has come to light
  in the last few years is the discovery that warm (~ 1 MK) coronal loops
  are much denser than expected for quasi-static equilibrium. It has been
  shown that the excess density can be explained if loops are bundles
  of unresolved strands that are heated impulsively and quasi-randomly
  to very high temperatures. This picture of nanoflare heating predicts
  that neighboring strands of different temperature should coexist and
  therefore that loops should have multi-thermal cross sections. In
  particular, emission should be produced at temperatures hotter than 2
  MK. Such emission is sometimes but not always seen, however. We offer
  two possible explanations for the existence of over-dense warm loops
  without corresponding hot emission: (1) loops are bundles of nanoflare
  heated strands, but a significant fraction of the nanoflare energy takes
  the form of a nonthermal electron beam rather then direct heating;
  (2) loops are bundles of strands that undergo thermal nonequilibrium
  that results when steady heating is sufficiently concentrated near
  the footpoints. We verify these possibilities with numerical hydro
  simulations. Time permitting, we will show FeXVII line profile
  observations from EIS/Hinode that support the existence of nanoflare
  heating. Work supported by NASA and ONR.

Title: Division II: Sun and Heliosphere
Authors: Webb, David F.; Melrose, Donald B.; Benz, Arnold O.; Bogdan,
   Thomas J.; Bougeret, Jean-Louis; Klimchuk, James A.; Martinez-Pillet,
   Valentin
Bibcode: 2007IAUTB..26..101W
Altcode:
  Division II provides a forum for astronomers studying a wide range of
  problems related to the structure, radiation and activity of the Sun,
  and its interaction with the Earth and the rest of the solar system.

Title: Explosive Instability and Coronal Heating
Authors: Dahlburg, R. B.; Liu, J.; Klimchuk, J. A.; Nigro, G.
Bibcode: 2007AGUFMSH44A1726D
Altcode:
  The observed energy loss rate from the solar corona implies that
  the coronal magnetic field has a critical angle at which energy is
  released. It has been hypothesized that at this critical angle an
  "explosive instability" would occur, leading to an enhanced conversion
  of magnetic energy into heat. In earlier investigations we have shown
  that a shear-dependent process called "secondary instability" could
  account for many of the distinctive features of the hypothetical
  "explosive instability." Here we show that this "secondary
  instability" can occur in a system with line-tied magnetic fields
  and boundary shearing. We also show that, as the disturbance due to
  secondary instability attains finite amplitude, there is a transition
  to turbulence which leads to enhanced dissipation of magnetic and
  kinetic energy. Furthermore, after each dissipative burst, the system
  is able to reform itself so that a subsequent burst can occur. These
  results are obtained from numerical simulations performed with a new
  parallelized, viscoresistive, three-dimensional code that solves the
  cold plasma equations. The code employs a Fourier collocation -- finite
  difference spatial discretization, and uses a third-order Runge-Kutta
  temporal discretization.

Title: The Cross-Field Thermal Structure of Coronal Loops from
    Triple-Filter TRACE Observations
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2007ApJ...667..591P
Altcode:
  The highly suppressed thermal transport across the magnetic field
  in the solar corona makes the determination of the cross-field
  thermal distribution within coronal loops a powerful diagnostic
  of the properties of the heating process itself. The cross-field
  thermal structure is currently being strongly debated. Spectroscopic
  observations with high temperature fidelity but low spatial resolution
  indicate that some observed loops are multithermal, whereas imaging
  observations with high spatial resolution but low temperature fidelity
  indicate more isothermal conditions. We report here on triple filter
  observations of coronal loops made by the Transition Region and Coronal
  Explorer (TRACE), which has the best spatial resolution currently
  available. We tested the isothermal hypothesis using the emission
  measure loci technique and found that the loops are consistent with
  an isothermal plasma near 1.5 MK only if a generous estimate of
  the photometric uncertainties is used. A more restrictive estimate
  based on discussions with the TRACE experimenters rules out the
  isothermal hypothesis. The observations are much better explained by
  a multithermal plasma with significant emission measure throughout the
  range 1-3 MK. The details of the emission measure distribution are not
  well defined, however. Future subarcsecond spectroscopic observations
  covering a wide range of temperatures are the most promising means of
  unlocking the thermal structure of the corona.

Title: Modeling Active Regions with Steady and Impulsive Heating
Authors: Patsourakos, Spiros; Klimchuk, J.
Bibcode: 2007AAS...210.9124P
Altcode: 2007BAAS...39..208P
  There has been considerable recent interest in constructing physical
  models of active regions (ARs) and the global coronal. Models based on
  static equilibrium theory are quite successful at reproducing soft X-ray
  (SXR) images of active regions. They however fail to predict the warm
  ( 1 MK) loops that are seen to permeate ARs in the EUV. Instead, the
  synthetic EUV images are dominated by intense footpoint emission. We
  demonstrate that the failure of static models to predict EUV loops is
  associated with the very weak dependence of loop temperature on loop
  length in models that are based on a single heating mechanism and
  that match the SXR observations. The models predict either SXR loops
  or EUV loops, but not both. We therefore consider time-dependent AR
  models based on nanoflare heating. We demonstrate that such models
  can simultaneously reproduce both SXR and EUV loops. Moreover, they
  explain the general tendency for SXR emission to dominate in the cores
  of ARs and EUV emission to dominate in the periphery. We finally show
  how the properties of nanoflares (energy, duration, spatial dependence,
  repetition time) can affect the AR morphology. <P />Research supported
  by NASA and ONR.

Title: Coronal Loops Really Do Have Constant Cross Sections!
Authors: Klimchuk, James A.; Lopez Fuentes, M.; Demoulin, P.
Bibcode: 2007AAS...210.9111K
Altcode: 2007BAAS...39..205K
  The observation that coronal loops do not expand systematically
  with height has been one of the more intriguing puzzles in solar
  physics. Simple force-free magnetic field models based on extrapolated
  magnetograms predict a much larger expansion than is observed. It
  has been suggested that the cross section uniformity is an artifact
  of inadequate spatial resolution, complex background emission, or
  both. For example, loops that are everywhere thinner than the instrument
  point spread function (PSF) would be seen to have a nearly constant
  thickness even if they actually expand. We have argued previously
  that actual loops are wide enough to rule out this possibility. Our
  present work also rules out the background emission as a possible
  explanation. We have simulated TRACE observations in the following
  manner. We constructed synthetic loops with both uniform and expanding
  cross sections, convolved them with the PSF, and placed them on actual
  TRACE images. We then measured the widths of the loops using the same
  technique used in our earlier studies of real observations. We find
  that expanding loops can be readily distinguished from loops with
  a constant cross section. Thus, the enigma remains! We tentatively
  suggest that constant cross sections are a consequence of the complex
  internal structure of loops (e.g., loops as bundles of tangled elemental
  strands). We are confident that this can explain the observed symmetry
  of loops, but whether it can also explain the lack of systematic
  expansion with height is not at all clear.

Title: Energy Release in Tangled Magnetic Fields
Authors: Klimchuk, James A.; DeVore, C. R.
Bibcode: 2007AAS...210.5303K
Altcode: 2007BAAS...39..164K
  A highly promising picture of coronal heating, first advocated by
  G. Parker, involves elemental magnetic flux tubes that become tangled
  by the shuffling motions of photospheric convection. The tubes must
  “reconnect” in the corona in order to avoid a monotonic increase
  of magnetic stresses. The associated release of energy heats the
  plasma. In earlier work, we showed that a mechanism called the secondary
  instability is the likely mechanism of energy release (Dahlburg,
  Klimchuk, and Antiochos, 2005). The instability occurs within the
  electric current sheets that separate misaligned tubes and switches
  on only after the misalignment angle reaches a critical value. It is
  significant that this angle matches the angle implied by the observed
  heating requirements of the corona. Though very encouraging, our initial
  MHD simulations were highly idealized. We have therefore performed new
  simulations that relax several assumptions by including the effects of
  finite current sheet size, photospheric line-tying, and time-dependent
  shearing. These fully 3D simulations of interacting flux tubes provide
  further evidence for the fundamental role of the secondary instability
  in coronal heating. <P />Work supported by NASA and ONR.

Title: The Temporal Evolution of Coronal Loops Observed by GOES SXI
Authors: López Fuentes, M. C.; Klimchuk, J. A.; Mandrini, C. H.
Bibcode: 2007ApJ...657.1127L
Altcode: 2006astro.ph.11338L
  We study the temporal evolution of coronal loops using data from the
  Solar X-Ray Imager (SXI) on board the Geosynchronous Operational
  Environmental Satellite 12 (GOES-12). This instrument provides
  continuous soft X-ray observations of the solar corona at a high
  temporal cadence permitting detailed study of the full lifetime of
  each of several coronal loops. The observed light curves suggest
  three evolutionary phases: rise, main, and decay. The durations and
  characteristic timescales of these phases [I/(dI/dt), where I is the
  loop intensity] are much longer than a cooling time and indicate that
  the loop-averaged heating rate increases slowly, reaches a maintenance
  level, and then decreases slowly. This suggests that a single heating
  mechanism operates for the entire lifetime of the loop. For monolithic
  (uniformly filled) loops, the loop-averaged heating rate is the
  intrinsic energy release rate of the heating mechanism. For loops that
  are bundles of impulsively heated strands, it relates to the frequency
  of occurrence of individual heating events, or nanoflares. We show
  that the timescale of the loop-averaged heating rate is proportional
  to the timescale of the observed intensity variation, with a constant
  of proportionality of approximately 1.5 for monolithic loops and 1.0
  for multistranded loops. The ratios of the radiative to conductive
  cooling times in the loops are somewhat less than 1, putting them
  intermediate between the values measured previously for hotter and
  cooler loops. This provides further support for a trend suggesting
  that all loops are heated in a similar way.

Title: Division II: Sun and Heliosphere
Authors: Webb, David F.; Melrose, Donald B.; Benz, Arnold O.; Bogdan,
   Thomas J.; Bougeret, Jean-Louis; Klimchuk, James A.; Martinez Pillet,
   Valentin
Bibcode: 2007IAUTA..26...69W
Altcode:
  Division II of the IAU provides a forum for astronomers studying a wide
  range of phenomena related to the structure, radiation and activity
  of the Sun, and its interaction with the Earth and the rest of the
  solar system. Division II encompasses three Commissions, 10, 12 and
  49, and four working groups. During the last triennia the activities
  of the division involved some reorganization of the division and its
  working groups, developing new procedures for election of division and
  commission officers, promoting annual meetings from within the division
  and evaluating all the proposed meetings, evaluating the division's
  representatives for the IAU to international scientific organizations,
  and participating in general IAU business.

Title: Commission 10: Solar Activity
Authors: Melrose, Donald B.; Klimchuk, James A.; Benz, A. O.; Craig,
   I. J. D.; Gopalswamy, N.; Harrison, R. A.; Kozlovsky, B. Z.; Poletto,
   G.; Schrijver, K. J.; van Driel-Gesztelyi, L.; Wang, J. -X.
Bibcode: 2007IAUTA..26...75M
Altcode:
  Commission 10 aims at the study of various forms of solar activity,
  including networks, plages, pores, spots, fibrils, surges, jets,
  filaments/prominences, coronal loops, flares, coronal mass ejections
  (CMEs), solar cycle, microflares, nanoflares, coronal heating etc.,
  which are all manifestation of the interplay of magnetic fields and
  solar plasma. Increasingly important is the study of solar activities
  as sources of various disturbances in the interplanetary space
  and near-Earth "space weather".Over the past three years a major
  component of research on the active Sun has involved data from the
  RHESSI spacecraft. This review starts with an update on current and
  planned solar observations from spacecraft. The discussion of solar
  flares gives emphasis to new results from RHESSI, along with updates on
  other aspects of flares. Recent progress on two theoretical concepts,
  magnetic reconnection and magnetic helicity is then summarized, followed
  by discussions of coronal loops and heating, the magnetic carpet
  and filaments. The final topic discussed is coronal mass ejections
  and space weather.The discussions on each topic is relatively brief,
  and intended as an outline to put the extensive list of references
  in context.The review was prepared jointly by the members of the
  Organizing Committee, and the names of the primary contributors to
  the various sections are indicated in parentheses.

Title: Coronal Heating
Authors: Klimchuk, J. A.; López Fuentes, M. C.
Bibcode: 2006AIPC..848...55K
Altcode:
  Coronal heating is one of the most challenging and important problems
  in astrophysics. We here review some of the critical aspects of the
  problem and suggest that the most likely explanation is Parker's
  long-standing idea of nanoflares occurring in magnetic fields that
  become tangled by photospheric convection. An exciting new development
  is the identification of the ``secondary instability'' as the likely
  mechanism of energy release. We also present some preliminary new
  results indicating that the rise, main, and decay phases of the soft
  X-ray light curves of observed coronal loops might be associated
  with the development, maintenance, and destruction of self-organized
  critical systems.

Title: Nonthermal Spectral Line Broadening and the Nanoflare Model
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2006ApJ...647.1452P
Altcode:
  A number of theoretical and observational considerations suggest
  that coronal loops are bundles of unresolved, impulsively heated
  strands. This ``nanoflare'' model, as it is sometimes called,
  predicts high-speed evaporative upflows, which might be revealed as
  nonthermal broadening of spectral line profiles. We have therefore
  generated synthetic line profile observations based on one-dimensional
  hydrodynamic simulations for comparison with actual observations. The
  predicted profiles for Ne VIII (770.4 Å), a transition region line,
  and Mg X (624.9 Å), a warm coronal line, have modest broadening that
  agrees well with existing observations. The predicted profiles for
  Fe XVII (254.87 Å), a hot line that will be observed by the Extreme
  Ultraviolet Imaging Spectrometer (EIS) on the Solar-B mission, are
  somewhat broader and are also consistent with the limited number of hot
  line observations that are currently available. Moreover, depending on
  the properties of the assumed nanoflare and other parameters of the
  simulation, the Fe XVII profile can have distinctive enhancements in
  the line wing. This indicates a powerful diagnostic capability that
  can be exploited once Solar-B is launched.

Title: Summary of JD3: Solar Active Regions and 3D Magnetic Structure
Authors: Klimchuk, J. A.
Bibcode: 2006IAUJD...3E..57K
Altcode:
  In this summary, I will attempt to synthesize many of the individual
  results presented in JD3 into a more coherent picture of solar active
  regions and 3D magnetic structure. I will offer an assessment of our
  current state of understanding and suggest how we might best proceed
  to make further progress on this important topic.

Title: Heating of the Magnetically Closed Corona
Authors: Klimchuk, J. A.; López Fuentes, M. C.; DeVore, C. R.
Bibcode: 2006ESASP.617E...8K
Altcode: 2006soho...17E...8K
  No abstract at ADS

Title: Testing Nanoflare Heating in Coronal Loops With Observations
    From the Extreme Ultraviolet Imaging Spectrometer On-board the
    SOLAR-B Mission
Authors: Patsourakos, Spiros; Klimchuk, J. A.
Bibcode: 2006SPD....37.0124P
Altcode: 2006BAAS...38..219P
  A number of theoretical and observational considerations suggest
  that coronal loops are bundles of unresolved, impulsively heated
  strands. This "nanoflare" model, as it is sometimes called,
  predicts high-speed evaporative upflows, which might be revealed as
  non-thermal broadening of spectral line profiles. We have therefore
  generated synthetic line profile observations based on 1D hydrodynamic
  simulations of nanoflare heated loop bundles.We will show that hot lines
  (T&gt;5MK) hold the imprints of the heating process via the development
  of distinct enhancements in the line wings. These signatures do not
  appear in the profiles of cooler lines, which is fully consistent
  with existing observations. We will demonstrate how the spectra of hot
  lines from the Extreme Ultraviolet Imaging Spectrometer (EIS) on-board
  the upcoming SOLAR-B mission can be used to test the basic nanoflare
  picture and perhaps even to pinpoint the properties of the nanoflares,
  such as their energy content, duration, and spatial dependence. We
  will present sample observing programs for studying nanoflare heating
  in coronal loops that utilize EIS and other instrumentation on-board
  SOLAR-B and STEREO.Research supported by NASA and ONR.

Title: Why Are Coronal Loops So Symmetric?
Authors: Klimchuk, James A.; Lopez Fuentes, M. C.; Demoulin, P.
Bibcode: 2006SPD....37.1706K
Altcode: 2006BAAS...38..246K
  Coronal loops are observed to be very symmetric in the sense that the
  two legs have a comparable thickness. Magnetic flux tubes in magnetic
  field extrapolation models are typically much less symmetric. We
  have quantified these differences using 171 A images from TRACE and
  magnetograms from MDI/SOHO. For a sample of 20 different loops, we found
  the linear force-free field that best matches the observed loop. We
  then measured the plane-of-the-sky widths of the loops and corresponding
  flux tubes and computed footpoint-to-footpoint expansion factors (i.e.,
  asymmetry ratios). The mean expansion factor of the flux tubes is 2.62,
  whereas the mean expansion factor of the loops is only 1.35. Note that
  these expansion factors are different from the footpoint-to-midpoint
  expansion factors that we have presented previously.Evidence suggests
  that the coronal magnetic field is comprised elemental flux strands
  that are tangled by turbulent convection. These strands are so small
  that many tens of them are contained within a single TRACE loop. We
  suggest that this fine structure is a critical missing ingredient of
  the extrapolation models and that a combination of footpoint shuffling
  and coronal reconnection can explain the observed loop symmetry. This
  has important implications for coronal heating.Research supported by
  NASA and the Office of Naval Research.

Title: On the Temperature-Emission Measure Distribution in Stellar
    Coronae
Authors: Cargill, Peter J.; Klimchuk, James A.
Bibcode: 2006ApJ...643..438C
Altcode:
  Strong peaks in the emission measure-temperature (EM-T ) distributions
  in the coronae of some binary stars are associated with the presence of
  hot (10<SUP>7</SUP> K), dense (up to 10<SUP>13</SUP> cm<SUP> -3</SUP>)
  plasma. These peaks are very reminiscent of those predicted to arise in
  an impulsively heated solar corona. A coronal model comprised of many
  impulsively heated strands is adapted to stellar parameters. It is shown
  that the properties of the EM-T distribution can be accounted for in
  general terms provided the emission comes from many very small loops
  (length under 10<SUP>3</SUP> km) with intense magnetic fields (1 kG)
  distributed across part of the surface of the star. The heating requires
  events that generally dissipate between 10<SUP>26</SUP> and 10<SUP>
  28</SUP> ergs, which is in the range of solar microflares. This implies
  that such stars must be capable of generating regions of localized
  intense magnetic fields.

Title: The Magnetic Structure of Coronal Loops Observed by TRACE
Authors: López Fuentes, M. C.; Klimchuk, J. A.; Démoulin, P.
Bibcode: 2006ApJ...639..459L
Altcode: 2006ApJ...639..459F; 2005astro.ph..7462L
  Previous studies have found that coronal loops have a nearly uniform
  thickness, which seems to disagree with the characteristic expansion
  of active region magnetic fields. This is one of the most intriguing
  enigmas in solar physics. We here report on the first comprehensive
  one-to-one comparison of observed loops with corresponding magnetic flux
  tubes obtained from cotemporal magnetic field extrapolation models. We
  use EUV images from TRACE, magnetograms from the MDI instrument on
  SOHO, and linear force-free field extrapolations satisfying b.nabla
  XB=αB, with α equal to a constant. For each loop, we find the
  particular value of α that best matches the observed loop axis and
  then construct flux tubes using different assumed cross sections at
  one footpoint (circle and ellipses with different orientations). We
  find that the flux tubes expand with height by typically twice as much
  as the corresponding loops. We also find that many flux tubes are much
  wider at one footpoint than the other, whereas the corresponding loops
  are far more symmetric. It is clear that the actual coronal magnetic
  field is more complex than the models we have considered. We suggest
  that the observed symmetry of loops is related to the tangling of
  elemental magnetic flux strands produced by photospheric convection.

Title: On Solving the Coronal Heating Problem
Authors: Klimchuk, James A.
Bibcode: 2006SoPh..234...41K
Altcode: 2005astro.ph.11841K
  The question of what heats the solar corona remains one of the
  most important problems in astrophysics. Finding a definitive
  solution involves a number of challenging steps, beginning with an
  identification of the energy source and ending with a prediction
  of observable quantities that can be compared directly with actual
  observations. Critical intermediate steps include realistic modeling
  of both the energy release process (the conversion of magnetic stress
  energy or wave energy into heat) and the response of the plasma
  to the heating. A variety of difficult issues must be addressed:
  highly disparate spatial scales, physical connections between the
  corona and lower atmosphere, complex microphysics, and variability
  and dynamics. Nearly all of the coronal heating mechanisms that have
  been proposed produce heating that is impulsive from the perspective
  of elemental magnetic flux strands. It is this perspective that must
  be adopted to understand how the plasma responds and radiates. In our
  opinion, the most promising explanation offered so far is Parker's
  idea of nanoflares occurring in magnetic fields that become tangled
  by turbulent convection. Exciting new developments include the
  identification of the "secondary instability" as the likely mechanism
  of energy release and the demonstration that impulsive heating in
  sub-resolution strands can explain certain observed properties of
  coronal loops that are otherwise very difficult to understand. Whatever
  the detailed mechanism of energy release, it is clear that some form
  of magnetic reconnection must be occurring at significant altitudes
  in the corona (above the magnetic carpet), so that the tangling does
  not increase indefinitely. This article outlines the key elements of
  a comprehensive strategy for solving the coronal heating problem and
  warns of obstacles that must be overcome along the way.

Title: DC coronal heating and the nonlinear evolution of current
    sheets
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 2006AdSpR..37.1342D
Altcode:
  Recent theoretical developments have re-awakened interest
  in the role of electric current sheets in DC coronal heating
  [Parker. Astrophys. J. 330, 474, 1988; Priest et al. Astrophys. J. 576,
  522, 2002]. Dahlburg et al. [Dahlburg et al. Adv. Space Res. 32,
  1029, 2003; Dahlburg et al. Astrophys. J. 622, 1191, 2005] reported
  the existence of a "secondary instability" that could explain the
  required "switch-on" effect required for adequate energy storage. This
  ideal, three-dimensional instability also provided a straightforward
  explanation for the subsequent fast release of energy, as the
  rapid growth of the mode eventually results in a state of turbulent
  magnetic reconnection. Earlier studies of the secondary instability
  were limited to systems with relatively simple perturbations, viz.,
  resistive stability eigenmodes. A current sheet in the Sun is likely
  to be subject to much more complex perturbations involving a waves
  of various wavelengths and amplitudes. We describe the evolution
  of three-dimensional electric current sheets disturbed by random
  3D perturbations. We find that the significant characteristics of
  secondary instability are also observed in this case. The numerical
  results are compared to solar observations.

Title: Coronal loops as self-organized critical systems
Authors: López Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2006BAAA...49..108L
Altcode:
  We developed a numerical model that explains the evolution of coronal
  loops observed with GOES-SXI (see Lopez Fuentes, Klimchuk and Mandrini,
  ApJ, 2006, in press) in terms of Self-organized Critical Systems
  (SOC). We are inspired by the idea originally proposed by Parker (1988,
  ApJ, 330, 474), that coronal loops are made of elemental magnetic
  strands that wrap around each other due to photospheric convection. In
  our code the magnetic strength between neighbor strands increase until a
  threshold is reach and strands reconnect, releasing energy and heating
  the plasma. The number and intensity of these release events increase
  and a critical steady-state is reached. At some point, the photospheric
  dispersion makes the ``feeding'' mechanism inefficient and the loop
  decays. We model the plasma response and we obtain synthetic light
  curves that qualitatively reproduce the observed loop evolution.

Title: Magnetic structure and observed width of coronal loops
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.; Demoulin, P.
Bibcode: 2006cosp...36.2575L
Altcode: 2006cosp.meet.2575L
  Previous studies have found that coronal loops have a nearly uniform
  thickness which seems to disagree with the characteristic expansion
  of active region magnetic fields This is one of the most intriguing
  enigmas in solar physics We here report on the first comprehensive
  one-to-one comparison of observed loops with corresponding magnetic
  flux tubes obtained from cotemporal magnetic field extrapolation models
  We use EUV images from TRACE magnetograms from the MDI instrument
  on SOHO and linear force-free field extrapolations For each loop we
  find the particular value of the force-free parameter alpha that best
  matches the observed loop axis and then construct flux tubes using
  different assumed cross sections at one footpoint circle and ellipses
  with different orientations We find that the flux tubes expand with
  height by typically twice as much as the corresponding loops We also
  find that many flux tubes are much wider at one footpoint than the
  other whereas the corresponding loops are far more symmetric It is
  clear that the actual coronal magnetic field is more complex than the
  models we have considered We suggest that the observed symmetry of
  loops is related to the tangling of elemental magnetic flux strands
  produced by photospheric convection

Title: The Origin of High-Speed Motions and Threads in Prominences
Authors: Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.
Bibcode: 2006ApJ...637..531K
Altcode:
  Prominences are among the most spectacular manifestations of both
  quiescent and eruptive solar activity, yet the origins of their
  magnetic-field and plasma structures remain poorly understood. We
  have made steady progress toward a comprehensive model of prominence
  formation and evolution with our sheared three-dimensional arcade
  model for the magnetic field and our thermal nonequilibrium model for
  the cool, dense material suspended in the corona. According to the
  thermal nonequilibrium model, condensations form readily along long,
  low-lying magnetic field lines when the heating is localized near
  the chromosphere. In most cases this process yields a dynamic cycle
  in which condensations repetitively form, stream along the field,
  and ultimately disappear by falling onto the nearest footpoint. Two
  key observed features were not adequately explained by our earlier
  simulations of thermal nonequilibrium, however: the threadlike
  (i.e., elongated) horizontal structure and high-speed motions of
  many condensations. In this paper we discuss how simple modifications
  to the radiative loss function, the heating scale, and the geometry
  of our model largely eliminate these discrepancies. In particular,
  condensations in nearly horizontal flux tubes are most likely to
  develop both transient high-speed motions and elongated threads. These
  results strengthen the case for thermal nonequilibrium as the origin
  of prominence condensations and support low-twist models of prominence
  magnetic structure.

Title: Coronal heating and the need for high-resolution observations
Authors: Klimchuk, J.
Bibcode: 2006cosp...36.2524K
Altcode: 2006cosp.meet.2524K
  Despite excellent progress in recent years in understanding coronal
  heating there remain many crucial questions that are still unanswered
  Limitations in the observations are one important reason Both
  theoretical and observational considerations point to the importance
  of small spatial scales impulsive energy release strong dynamics and
  extreme plasma nonuniformity As a consequence high spatial resolution
  broad temperature coverage high temperature fidelity and sensitivity
  to velocities and densities are all critical observational parameters
  Current instruments lack one or more of these properties and this
  has led to considerable ambiguity and confusion In this talk I will
  discuss recent ideas about coronal heating and emphasize that high
  spatial resolution observations especially spectroscopic observations
  are needed to make major progress on this important problem

Title: Study of coronal loops observed by GOES-SXI
Authors: Lopez-Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A.
Bibcode: 2006cosp...36.2549L
Altcode: 2006cosp.meet.2549L
  We study the temporal evolution of coronal loops using data from
  the Solar X-ray Imager SXI on board the Geosynchronous Operational
  Environmental Satellite 12 GOES-12 This instrument has the advantage
  of providing continuous soft X-ray observations of the solar corona at
  a high temporal cadence which allows us to follow the full lifetime
  of a set of coronal loops from their brightening to their decay From
  the observed light curves we can divide the evolution of the loops in
  three phases rise main and decay For each of these phases we compute
  the corresponding evolutionary timescales and since we have full time
  coverage the real loop lifetime Using data in different filters we
  derive temperature and density averages The values found place SXI
  loops halfway between the typical ranges of physical parameters for
  loops observed by the Soft X-ray Telescope Yohkoh SXT and those for
  loops observed by the Transition Region and Coronal Explorer TRACE
  We compute radiative and conductive cooling times which turn out to
  be much shorter than the evolutionary timescales of the loops These
  results confirm previous findings Porter and Klimchuk 1995 based on
  observations covering partially the loop temporal evolution Our results
  can be interpreted in terms of two alternative coronal heating scenarios
  quasi-static heating of monolithic uniform loop structures or impulsive
  heating nanoflaring of multiple-stranded loops We present arguments
  based on recent observations and loop modelling that support the idea

Title: Evolution of coronal loops
Authors: López Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A.
Bibcode: 2006BAAA...49..107L
Altcode:
  We study the temporal evolution of coronal loops using data from the
  Solar X-ray Imager (SXI) on board the Geosynchronous Operational
  Environmental Satellite 12 (GOES-12). The observed light curves
  show three distinct evolutionary phases: rise, main, and decay. The
  durations and characteristic timescales of these phases are much
  longer than the cooling times, suggesting two possible scenarios:
  (1) loops are monolithic and evolve quasi-statically; or (2) they
  are made of unresolved strands that are impulsively heated. We show
  that the timescale of the loop-averaged heating rate is proportional
  to the timescale of the observed intensity variation. The constant
  of proportionality is approximately 1.5 for case (1) and 1.0 for case
  (2). The ratios of the radiative to conductive cooling times place these
  loops intermediate between previously measured hotter (Yohkoh-SXT)
  and cooler (TRACE) loops. Our results help to complete a trend that
  seems to support the impulsive heating hypothesis.

Title: Observing the Solar atmosphere with the Extreme Ultraviolet
    Imaging Spectrometer on Solar B
Authors: Korendyke, C. M.; Brown, C.; Dere, K.; Doschek, G.; Klimchuk,
   J.; Landi, E.; Mariska, J.; Warren, H.; Lang, J.
Bibcode: 2005AGUFMSH41B1124K
Altcode:
  The Extreme Ultraviolet Imaging Spectrometer (EIS) is part of the
  instrument complement on the Solar B satellite, scheduled for launch
  in the summer of 2006. The instrument has been calibrated and is
  presently mounted on the spacecraft. EIS is the most sensitive EUV
  solar spectrometer to be flown. The instrument is the first of a new
  generation of two optical element, solar spectrographs. Preliminary
  results from the laboratory focussing and calibration of the
  instrument will be shown. The instrument wavelength coverage includes
  reasonably bright spectral lines emitted by plasmas from 0.1 to 20 MK
  in temperature. The wavelength range also provides coronal density
  diagnostics. Temperature, density and velocity diagnostics will be
  discussed. An example observing program for exploring active region
  evolution and dynamics will be discussed.

Title: Why We Need Imaging Spectroscopy
Authors: Klimchuk, J. A.
Bibcode: 2005AGUFMSH44A..01K
Altcode:
  Despite excellent progress in recent years in understand fundamental
  solar physics problems such as coronal heating, solar wind acceleration,
  flares, and coronal mass ejections, there remain many crucial questions
  that are still unanswered. Limitations in the observations are one
  important reason. Both theoretical and observational considerations
  point to the importance of small spatial scales, impulsive energy
  release, strong dynamics, and extreme plasma nonuniformity. As a
  consequence, high spatial resolution, broad temperature coverage,
  high temperature fidelity, and sensitivity to velocities and densities
  are all critical observational parameters. Current instruments lack
  one or more of these properties, and this has led to considerable
  ambiguity and confusion. In this talk, I will discuss some of the
  ways that spectroscopic information is vital to further progress,
  giving particular emphasis to the coronal heating problem. I will
  argue that a high-speed imaging spectrometer must be a top priority
  instrument for the future.

Title: Coronal Loop Heating by Nanoflares: The Impact of the
    Field-aligned Distribution of the Heating on Loop Observations
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2005ApJ...628.1023P
Altcode:
  Nanoflares occurring at subresolution strands with repetition times
  longer than the coronal cooling time are a promising candidate
  for coronal loop heating. To investigate the impact of the spatial
  distribution of the nanoflare heating on loop observables, we compute
  hydrodynamic simulations with several different spatial distributions
  (uniform, loop top, randomly localized, and footpoint). The outputs
  of the simulations are then used to calculate density and temperature
  diagnostics from synthetic TRACE and SXT observations. We find that
  the diagnostics depend only weakly on the spatial distribution of the
  heating and therefore are not especially useful for distinguishing among
  the different possibilities. Observations of the very high temperature
  plasmas that are present only in the earliest stages of nanoflares
  can shed more light on the field-aligned distribution of the heating.

Title: Are Coronal Loops Self-organized Critical Systems?
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.; Mandrini, C. H.
Bibcode: 2005AGUSMSP14A..06L
Altcode:
  In Lopez-Fuentes et al. 2004 we studied a set of loops observed by
  GOES/SXI and found that loop evolution can be separated in three
  phases (rise, main and decay). We found that the time scales of all
  three phases are long compared to a cooling time. In this work we
  explore whether this evolution is consistent with the development
  of a self-organized critical (SOC) system. We compare the observed
  soft X-ray light curves with light curves predicted by a simple SOC
  model, and we examine how the rise and decay times are related to the
  properties of the system driver. The physical picture we have in mind
  is the shuffling of elemental flux tubes by photospheric motions, as
  first advocated by Parker. Work funded by NASA and ONR. Lopez Fuentes,
  M.C., Mandrini, C.H., &amp; Klimchuk, J.A., 2004, American Astronomical
  Society Meeting Abstracts, 204, 5602L

Title: Coronal Loop Heating by Nanoflares: Non-thermal Velocities
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2005AGUSMSP41A..06P
Altcode:
  Spectroscopic observations show non-negligible non-thermal velocities
  under coronal conditions. These motions place tight constraints on any
  coronal heating mechanism that should be able to reproduce them. We
  calculate the non-thermal velocities predicted by the nanoflare
  model. We perform 1D time-dependent hydrodynamic simulations of
  nanoflares occurring at sub-resolution strands, that make up the
  observed coronal loops and calculate profiles for representative
  spectral lines. We show that: (1) the calculated non-thermal velocities
  compare favorably with observations of cool and warm spectral
  lines and (2) the profiles of hot lines, that would be available in
  observations from the Extreme Ultraviolet Imaging Spectrometer (EIS)
  spectrometer onboard the SOLAR-B mission, can exhibit significant
  blue-wing asymmetries which can be used as a monitor of nanoflare
  properties. Research supported by NASA and ONR.

Title: The Origin of High-Speed Motions and Threads in Solar
    Prominences
Authors: Karpen, J.; Antiochos, S.; Klimchuk, J.
Bibcode: 2005AGUSMSP21B..02K
Altcode:
  Prominences are among the most spectacular manifestations of both
  quiescent and eruptive solar activity, yet the origins of their
  magnetic-field and plasma structures remain poorly understood. We
  have made steady progress toward a comprehensive model of prominence
  formation and evolution with our sheared 3D arcade model for the
  magnetic field and our thermal nonequilibrium model for the cool,
  dense material suspended in the corona. According to the thermal
  nonequilibrium model, condensations form readily along long,
  low-lying magnetic field lines if the heating is localized near the
  chromosphere. In most cases this process yields a dynamic cycle in
  which condensations repetitively form, stream along the field line,
  and ultimately disappear by falling onto the nearest footpoint. Two
  key observed features were not adequately explained by our earlier
  simulations of thermal nonequilibrium, however: the thread-like
  (i.e., elongated) horizontal structure and high-speed motions of many
  condensations. Here we discuss how simple modifications to our model
  largely eliminate these discrepancies, strengthening the case for
  thermal nonequilibrium as the origin of prominence condensations and
  for low-twist models of prominence magnetic structure. This work was
  supported by NASA and ONR.

Title: Coronal Loop Heating by Nanoflares: The Influence of the
    Field-aligned Distribution of the Heating on Observables
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2005AGUSMSP41A..05P
Altcode:
  We investigate the effect of the spatial distribution of nanoflare
  heating on loop observables. We perform 1D time-dependent hydrodynamic
  simulations of nanoflares occurring at sub-resolution strands, that make
  up the observed coronal loops. The simulations use different spatial
  forms for the nanoflare heating (randomly localized, footpoint, uniform
  loop top). The outputs of the simulations are then used to calculate
  diagnostics from synthetic TRACE and SXT observations. We find that
  the diagnostics depend only weakly on the spatial distribution of the
  heating, and therefore are not especially useful for distinguishing
  among the different possibilities. We propose that the best way to
  study the field-aligned spatial distribution of nanoflare heating is
  to observe the very high temperature plasmas that are present only in
  the earliest stages of an event. Research supported by NASA and ONR.

Title: Highly Efficient Modeling of Dynamic Coronal Loops
Authors: Klimchuk, J. A.; Patsourakos, S.; Cargill, P. J.
Bibcode: 2005AGUSMSP14A..03K
Altcode:
  It now seems clear that many coronal loops, especially those observed
  by TRACE and EIT, are inherently dynamic and composed of large numbers
  of impulsively-heated strands. Modeling these loops in full detail is
  extremely challenging, and modeling entire active regions or the whole
  Sun is completely out of the question unless approximate techniques are
  used. We have developed a simplified set of equations that is remarkably
  accurate at describing the evolution of the thermodynamic variables
  (T, P, n, v) averaged along the magnetic field of an individual
  strand. The equations can be solved ten thousand times more quickly
  than the full 1D hydro equations. This "0D" model relaxes two key
  assumptions of Cargill's (1994) nanoflare model: (1) the heating can
  have any time-dependent profile and need not be instantaneous; and
  (2) thermal conduction cooling and radiation cooling occur together
  at all times, in varying proportions. We here describe the essential
  features of the model and show examples of how well it works.

Title: An Explanation for the ``Switch-On'' Nature of Magnetic Energy
    Release and Its Application to Coronal Heating
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 2005ApJ...622.1191D
Altcode:
  A large class of coronal heating theories postulate that the random
  mixing of magnetic footpoints by photospheric motions leads to the
  formation of current sheets in the corona and, consequently, to energy
  release there via magnetic reconnection. Parker pointed out that in
  order for this process to supply the observed energy flux into the
  corona, the stress in the coronal magnetic field must have a fairly
  specific value at the time that the energy is released. In particular,
  he argued that the misalignment between reconnecting flux tubes must
  be roughly 30° in order to match the observed heating. No physical
  origin for this number was given, however. In this paper we propose
  that secondary instability is the mechanism that ``switches on'' the
  energy release when the misalignment angle in the corona reaches the
  correct value. We calculate both the three-dimensional linear and fully
  nonlinear development of the instability in current sheets corresponding
  to various misalignment angles. We find that no secondary instability
  occurs for angles less than about 45°, but for larger angles the
  instability grows at a rapid rate, and there is an explosive release
  of energy. We compare our results with the observed properties of the
  corona and discuss the implications for future observations.

Title: The Effect of the Spatial Distribution of Nanoflare Heating
    on Loop Observables
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2004ESASP.575..297P
Altcode: 2004soho...15..297P
  No abstract at ADS

Title: Coronal Seismology and the Propagation of Acoustic Waves
    along Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I.
Bibcode: 2004ApJ...616.1232K
Altcode: 2004astro.ph.12085K
  We use a combination of analytical theory, numerical simulation, and
  data analysis to study the propagation of acoustic waves along coronal
  loops. We show that the intensity perturbation of a wave depends
  on a number of factors, including dissipation of the wave energy,
  pressure and temperature gradients in the loop atmosphere, work action
  between the wave and a flow, and the sensitivity properties of the
  observing instrument. In particular, the scale length of the intensity
  perturbation varies directly with the dissipation scale length (i.e.,
  damping length) and the scale lengths of pressure, temperature, and
  velocity. We simulate wave propagation in three different equilibrium
  loop models and find that dissipation and pressure and temperature
  stratification are the most important effects in the low corona where
  the waves are most easily detected. Velocity effects are small and
  cross-sectional area variations play no direct role for lines of
  sight that are normal to the loop axis. The intensity perturbation
  scale lengths in our simulations agree very well with the scale
  lengths we measure in a sample of loops observed by TRACE. The median
  observed value is 4.35×10<SUP>9</SUP> cm. In some cases the intensity
  perturbation increases with height, which is likely an indication of
  a temperature inversion in the loop (i.e., temperature that decreases
  with height). Our most important conclusion is that thermal conduction,
  the primary damping mechanism, is accurately described by classical
  transport theory. There is no need to invoke anomalous processes to
  explain the observations.

Title: Coronal Seismology and the Propagation of Acoustic Waves
    Along Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E.; De Moortel, I.
Bibcode: 2004AGUFMSH24A..06K
Altcode:
  We use a combination of analytical theory, numerical simulation, and
  data analysis to study the propagation of acoustic waves along coronal
  loops. We show that the intensity perturbation of a wave depends
  on a number of factors, including dissipation of the wave energy,
  pressure and temperature gradients in the loop atmosphere, work action
  between the wave and a flow, and the sensitivity properties of the
  observing instrument. In particular, the scale length of the intensity
  perturbation varies directly with the dissipation scale length (i.e.,
  damping length) and the scale lengths of pressure, temperature, and
  velocity. We simulate wave propagation in three different equilibrium
  loop models and find that dissipation and pressure and temperature
  stratification are the most important effects in the low corona where
  the waves are most easily detected. Velocity effects are small, and
  cross-sectional area variations play no direct role for lines-of-sight
  that are normal to the loop axis. The intensity perturbation scale
  lengths in our simulations agree very well with the scale lengths we
  measure in a sample of loops observed by TRACE. The median observed
  value is 4.35×10<SUP>9</SUP> cm. In some cases the intensity
  perturbation increases with height, which is likely an indication of
  a temperature inversion in the loop (i.e., temperature that decreases
  with height). Our most important conclusion is that thermal conduction,
  the primary damping mechanism, is accurately described by classical
  transport theory. There is no need to invoke anomalous processes to
  explain the observations.

Title: A Model for Bright Extreme-Ultraviolet Knots in Solar Flare
    Loops
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
Bibcode: 2004ApJ...614.1022P
Altcode:
  EUV observations often indicate the presence of bright knots in flare
  loops. The temperature of the knot plasma is of the order of 1 MK,
  and the knots themselves are usually localized somewhere near the loop
  tops. We propose a model in which the formation of EUV knots is due
  to the spatial structure of the nonflare active region heating. We
  present the results of a series of one-dimensional hydrodynamic,
  flare-loop simulations, which include both an impulsive flare heating
  and a background, active region heating. The simulations demonstrate
  that the formation of the observed knots depends critically on
  the spatial distribution of the background heating during the decay
  phase. In particular, the heating must be localized far from the loop
  apex and have a magnitude comparable to the local radiative losses of
  the cooling loop. Our results, therefore, provide strong constraints
  on both coronal heating and postflare conditions.

Title: Bright EUV Knots in Solar Flare Loops: Constraints on Coronal
    Heating
Authors: Patsourakos, S.; Antiochos, S.; Klimchuk, J.
Bibcode: 2004AAS...204.8705P
Altcode: 2004BAAS...36Q.819P
  EUV observations often indicate the presence of bright knots in flare
  loops. The temperature of the knot plasma is of order 1MK, and the
  knots themselves are usually localized somewhere near the loop tops. We
  propose a model in which the formation of EUV knots is due to the
  spatial structure of the non-flare active region heating. We present
  the results of a series of 1D hydrodynamic, flare-loop simulations,
  which include both an impulsive flare heating and a background, active
  region heating. The simulations demonstrate that the formation of the
  observed knots depends critically on the spatial distribution of the
  background heating during the decay phase. In particular, the heating
  must: (1) be localized, (2) be situated far from the loop apex and (3)
  have a magnitude comparable with the local radiative losses of the
  cooling loop. Our results, therefore, provide strong constraints on
  both coronal heating and post-flare conditions. <P />Research supported
  by NASA and ONR.

Title: Evolution of Coronal Loops Observed by GOES-SXI
Authors: Lopez Fuentes, M. C.; Mandrini, C. H.; Klimchuk, J. A.
Bibcode: 2004AAS...204.5602L
Altcode: 2004BAAS...36R.761L
  Several years ago, Porter and Klimchuk (PK, 1995) studied a collection
  of coronal loops observed by the SXT instrument on Yohkoh and found
  that they have evolutionary timescales of typically 10<SUP>4</SUP>
  to 10<SUP>5</SUP> s. These very long timescales are roughly one or
  two orders of magnitude greater than the corresponding cooling times
  and therefore have important implications for loop heating. However,
  PK observed each loop for only a single spacecraft orbit ( 1 hr),
  and it has remained a question whether loops actually persist for as
  long as the ''instantaneous'' evolutionary timescales suggest. Perhaps
  loops turn on suddenly, remain nearly constant for a modest period, then
  decay on a cooling timescale. <P />We have begun to address the general
  issue of loop evolution using images from the Soft X-ray Imager (SXI)
  on board the GOES-12 satellite. This instrument has a coarser spatial
  resolution than TRACE or SXT, but has the advantage of continuous
  temporal coverage. Since GOES-12 is in geosynchronous orbit, there is
  no spacecraft night. We present here the light curves of several loops
  and compare them with the radiative cooling times inferred from the
  observed temperature and emission measure. We discuss heating rate
  requirements related to the observed turn on and maintenance of the
  loops, and we consider the implications for nanoflare versus steady
  heating scenarios. <P />This work is funded by NASA and the Office
  of Naval Research. <P />Porter, L. J., and Klimchuk, J. A. 1995, ApJ,
  454, 499

Title: Acoustic Wave Interpretation of Propagating Intensity
    Disturbances in Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I.
Bibcode: 2004AAS...204.9503K
Altcode: 2004BAAS...36..826K
  Intensity disturbances have been observed by TRACE and EIT to propagate
  upward along the legs of long active region coronal loops. The
  periodic nature and speed of these disturbances suggest that they
  are traveling acoustic waves. It is being debated, however, whether
  the damping of the perturbations is consistent with the acoustic
  wave interpretation. We here examine this issue in detail with a
  combination of numerical simulation, analytical theory, and improved
  analysis of the observations. Using our state-of-the-art 1D hydro code,
  we simulate the propagation of waves generated at the base of model
  coronal loops. We consider static equilibrium loops having constant and
  expanding cross-section, and an equilibrium loop with steady flow. We
  show that the amplitude of the intensity perturbation is affected by
  a number of factors: wave dissipation (direct plasma heating), work
  done by the wave on the flow, pressure stratification, nonuniform
  temperature, and temperature-dependent sensitivity of the observing
  instrument. We compare our theoretical results with intensity scale
  lengths measured in a sample of loops observed by TRACE. <P />Research
  supported by NASA and ONR.

Title: Comments on `Possible Role of MHD Waves in Heating the Solar
    Corona' by Dwivedi and Pandey
Authors: Klimchuk, J. A.; Porter, L. J.; Sturrock, P. A.
Bibcode: 2004SoPh..221...47K
Altcode:
  We comment on the recent paper by Dwivedi and Pandey (Solar Physics216,
  59, 2003). Parts of that paper closely reproduce, without reference,
  material that we had published previously, while other parts that
  deviate from our earlier analysis contain several critical flaws. We
  show that magnetoacoustic waves are capable of heating the corona with
  a modest enhancement in the coefficient of compressive viscosity.

Title: Nanoflare Heating of the Corona Revisited
Authors: Cargill, Peter J.; Klimchuk, James A.
Bibcode: 2004ApJ...605..911C
Altcode:
  The radiative signatures of the nanoflare model for coronal heating are
  investigated. If an observed coronal loop is assumed to consist of many
  small strands that cannot be distinguished spatially by EUV or X-ray
  observations, we are able to calculate differential emission-measure
  profiles and filling factors for a range of heating models. In this
  picture the strands undergo continual heating and cooling, leading
  to a corona comprising strands with a broad range of temperatures and
  densities. Thus, observations over a range of temperatures will show
  a multithermal coronal structure. The cyclical heating-cooling leads
  inevitably to loops that are underdense and overdense at high and low
  temperatures, respectively, compared to what would be expected from
  static equilibrium models, and in addition, we show that differential
  emission-measure profiles with shallow slopes can be obtained, as
  reported in recent observations. The differences between filling
  factors that can be seen by broadband and narrowband instruments are
  explored. Loops with broadband filling factors near unity can still have
  small narrowband factors, and the narrowband factor is shown to be a
  strong function of the local temperature. Nanoflare energy distributions
  that are constant, flat, or power laws are considered. Power laws lead
  to wide distributions of temperatures and densities in the corona,
  and steep power laws lead to larger filling factors.

Title: The Inability of Steady-Flow Models to Explain the
    Extreme-Ultraviolet Coronal Loops
Authors: Patsourakos, S.; Klimchuk, J. A.; MacNeice, P. J.
Bibcode: 2004ApJ...603..322P
Altcode:
  Recent observations from the Transition Region and Coronal Explorer
  (TRACE) and the EUV Imaging Telescope (EIT) show that warm (T~1-1.5 MK)
  EUV coronal loops in active regions generally have enhanced densities,
  enhanced pressure scale heights, and flat filter ratio (temperature)
  profiles in comparison with the predictions of static-equilibrium
  theory. It has been suggested that mass flows may explain these
  discrepancies. We investigate this conjecture using one-dimensional
  hydrodynamic simulations of steady flows in coronal loops. The flows
  are driven by asymmetric heating that decreases exponentially along
  the loop from one footpoint to the other. We find that a sufficiently
  large heating asymmetry can produce density enhancements consistent
  with a sizable fraction of the observed loops, but that the pressure
  scale heights are smaller than the corresponding gravitational scale
  heights, and that the filter ratio profiles are highly structured,
  in stark contrast to the observations. We conclude that most warm EUV
  loops cannot be explained by steady flows. It is thus likely that the
  heating in these loops is time dependent.

Title: An Observational Test for Coronal Heating Models
Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Mandrini, C. H.;
   Harra, L. K.; Klimchuk, J. A.
Bibcode: 2004IAUS..219..473V
Altcode: 2003IAUS..219E..97V
  We correlate the evolution of the mean X-ray flux emission measure
  and temperature (Yohkoh SXT &amp; BCS) with the magnetic flux density
  (SOHO/MDI) in active region NOAA 7978 from its birth throughout its
  decay for five solar rotations. We show that these plasma parameters
  together with other quantities deduced from them such as the density
  and the pressure follow power-law relationships with the mean magnetic
  flux density (bar{B}). We derive the dependence of the mean coronal
  heating rate on the magnetic flux density. We use the obtained scaling
  laws of coronal loops in thermal equilibrium to derive observational
  estimates of the scaling of the coronal heating with bar{B}. These
  results are used to test the validity of coronal heating models. We
  find that models invoking stochastic buildup of energy current layers
  and MHD turbulence are in best agreement with the observations. This
  narrows down the range of possible models retained by previous results
  obtained for individual coronal loops as well as for the global coronal
  emission of the Sun and cool stars.

Title: Coronal Loop Heating by Nanoflares: Some Observational
    Implications
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2004hell.conf...35P
Altcode:
  No abstract at ADS

Title: DC coronal heating and the nonlinear evolution of current
    sheets
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 2004cosp...35.2721D
Altcode: 2004cosp.meet.2721D
  Recent theoretical developments have re-awakened interest in the role
  of electric current sheets in DC coronal heating (Parker 1988; Priest
  et al. 2002). Dahlburg et al. (2003; 2004) reported the existence
  of a ``secondary instability'' that could explain the required
  ``switch-on'' effect required for adequate energy storage. This
  ideal, three-dimensional instability also provided a straightforward
  explanation for the subsequent fast release of energy, as the
  rapid growth of the mode eventually results in a state of turbulent
  magnetic reconnection. Earlier studies of the secondary instability
  were limited to systems with relatively simple perturbations, viz.,
  resistive stability eigenmodes. A current sheet in the Sun is likely
  to be subject to much more complex perturbations involving a waves
  of various wavelengths and amplitudes. We describe the evolution
  of three-dimensional electric current sheets disturbed by random 3D
  perturbations. We find that the significant characteristics of secondary
  instability are also observed in this case. The relative importance
  of subharmonic interactions, i.e., coalescence instability, will also
  be discussed. R. B. Dahlburg, J. A. Klimchuk and S. K. Antiochos
  Adv. Space Phys. 32, 1029 (2003). R. B. Dahlburg, J. A. Klimchuk
  and S. K. Antiochos Astrophys. J.. submitted, (2004). E. N. Parker
  Astrophys. J. 330, 474 (1988). E. R. Priest, J. F. Heyvaerts and
  A. M. TItle, Astrophys. J. 576, 533 (2002).

Title: Linear Force Free Field Models of Observed Coronal Loops
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2003AGUFMSH42B0515L
Altcode:
  Although active region magnetic fields have an overall expansion
  with height, soft X-ray and EUV loops are observed to have nearly
  constant cross sections. To investigate this apparent discrepancy, we
  have compared coronal loops observed by TRACE in the 171 A band with
  corresponding magnetic flux tubes obtained from linear force-free
  extrapolations (Demoulin et al. 1997) of nearly concurrent MDI
  magnetograms. The flux tubes were determined using a procedure that
  varies the force-free parameter alpha and searches for the field line
  that most closely coincides with the observed loop axis. Once the axis
  field line is identified, we construct flux tubes from the model field
  using a variety of assumed footpoint shapes and orientations. Our
  detailed comparison confirms the mystery of constant loop cross
  sections. Although the expansion of the extrapolated flux tube as seen
  projected onto the plane-of-the-sky varies depending on the footpoint
  shape and orientation, it is always considerably greater than the
  expansion of the corresponding TRACE loop. Furthermore, the extrapolated
  flux tubes are often highly asymmetric (with one leg much wider than
  the other), in stark contast to the TRACE loops. These results imply
  that the magnetic structure and possibly also the heating of coronal
  loops are more complex than we currently understand. Extrapolations of
  observed photospheric fields are very useful, but direct measurement
  of the coronal field may be necessary to make fundamental progress on
  this important problem. This work was funded by NASA and the Office
  of Naval Research.

Title: The Non-flare Emisson Measure Above 5 MK Observed By RHESSI
    and SXI
Authors: McTiernan, J. M.; Klimchuk, J. A.
Bibcode: 2003AGUFMSH21B0162M
Altcode:
  Since RHESSI was launched in February 2002, it has observed thousands
  of solar flares. It also observes solar emission above 3 keV when there
  are no observeable flares present. In this work we present measurements
  of the non-flare Temperature and Emission Measure for the period
  from October 2002 through July 2003. The temperature is relatively
  stable in the 6 - 8 MK range (this is not surprising considering
  that RHESSI cannot reliably measure temperatures less than about
  5 MK). The Emission Measure varies in the range from approximately
  1.0e49 to 1.0e50, with higher values associated with periods of more
  solar activity. Since RHESSI is an imaging spectrometer, we locate the
  source of the emission when possible. Preliminary results show that
  the source can be associated with active regions. We also present
  comparisons with SXI data, and measure the Differential Emission
  Measure for the range from 1 MK to 10 MK.

Title: Coronal energy release via ideal three-dimensional instability
    three-dimensional instability
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 2003AdSpR..32.1029D
Altcode:
  It is widely believed that most coronal phenomena involve the
  release of free energy that is stored within stressed magnetic field
  configurations. The availability of sufficient free energy to explain
  everything from coronal heating to flares and coronal mass ejections
  is well established. How this energy is released remains a major
  puzzle. Observations reveal that an important property of the energy
  release mechanism is its "switch on" character. The mechanism must
  remain dormant for long periods of time to allow the magnetic stresses
  to build, then it must operate very vigorously once it finally turns
  on. We discuss a mechanism called the "secondary instability" which
  exhibits this behavior. It is essentially an ideal instability of the
  thin twisted magnetic flux tubes that form from the resistive tearing
  of current sheets. We relate the mechanism to the coronal heating
  idea of Parker in which the coronal magnetic field becomes tangled by
  random motions of the photospheric footpoints. Global energy balance
  considerations imply that magnetic energy dissipation occurs at a
  particular angle in the field, and the secondary instability offers
  the first quantitative explanation for why this should be. It thus
  places Parker's popular idea on a much firmer physical footing.

Title: Constraints on the Magnetic Field Geometry in Prominences
Authors: Karpen, J. T.; Antiochos, S. K.; Klimchuk, J. A.; MacNeice,
   P. J.
Bibcode: 2003ApJ...593.1187K
Altcode:
  This paper discusses constraints on the magnetic field geometry of solar
  prominences derived from one-dimensional modeling and analytic theory
  of the formation and support of cool coronal condensations. In earlier
  numerical studies we identified a mechanism-thermal nonequilibrium-by
  which cool condensations can form on field lines heated at their
  footpoints. We also identified a broad range of field line shapes
  that can support condensations with the observed sizes and lifetimes:
  shallowly dipped to moderately arched field lines longer than several
  times the heating scale. Here we demonstrate that condensations formed
  on deeply dipped field lines, as would occur in all but the near-axial
  regions of twisted flux ropes, behave significantly differently than
  those on shallowly dipped field lines. Our modeling results yield
  a crucial observational test capable of discriminating between two
  competing scenarios for prominence magnetic field structure: the flux
  rope and sheared-arcade models.

Title: Hydrodynamic Simulations of Longitudinal Intensity Oscillations
    Observed in Coronal Loops by TRACE
Authors: Tanner, S. E.; Klimchuk, J. A.; Hood, A. W.; De Moortel, I.
Bibcode: 2003SPD....34.0406T
Altcode: 2003BAAS...35..811T
  Propagating intensity disturbances are often observed by TRACE in
  large coronal loops located at the perimeters of active regions
  (e.g., De Moortel et al., 2002, Solar Phys., 209, 61). On average,
  the disturbances have periods of 280 s, propagation speeds of 120
  km s<SUP>-1</SUP>, intensity amplitudes of 4%, and surprisingly
  small damping (detection) lengths of 9000 km. In addition, there
  is a positive correlation between damping length and period. The
  preliminary interpretation of these disturbances is that they are
  rapidly dissipating slow magneto-acoustic waves. <P />To investigate
  this interpretation more rigorously, we have performed a series of
  detailed coronal loop simulations using our 1D hydrodynamic code,
  ARGOS. We generate waves in the loop by imposing a spatially localized
  oscillating force at the loop footpoint, using a range of different
  oscillation periods. We here report on the results of our study and,
  in particular, whether the damping lengths have the properties observed
  by TRACE. <P />This work was supported by NASA and ONR.

Title: Linear force free field models of observed coronal loops
Authors: Lopez-Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2003SPD....34.0105L
Altcode: 2003BAAS...35R.805L
  Although active region magnetic fields have an overall expansion
  with height, soft X-ray and EUV loops are observed to have nearly
  constant cross sections. To investigate this apparent discrepancy, we
  have compared coronal loops observed by TRACE in the 171 A band with
  corresponding magnetic flux tubes obtained from linear force-free
  extrapolations (Demoulin et al. 1997) of nearly concurrent MDI
  magnetograms. The flux tubes were determined using a procedure that
  varies the force-free parameter alpha and searches for the field line
  that most closely coincides with the observed loop axis. Once the
  axis field line is identified, we construct flux tubes from the model
  field using a variety of assumed footpoint shapes and orientations. <P
  />Our detailed comparison confirms the mystery of constant loop cross
  sections. Although the expansion of the extrapolated flux tube as seen
  projected onto the plane-of-the-sky varies depending on the footpoint
  shape and orientation, it is always considerably greater than the
  expansion of the corresponding TRACE loop. This result implies that the
  magnetic structure and possibly also the heating of coronal loops are
  more complex than we currently understand. <P />This work was funded
  by NASA and the Office of Naval Research.

Title: The Non-flare Solar Temperature and Emission Measure Observed
    by RHESSI
Authors: McTiernan, J. M.; Klimchuk, J. A.
Bibcode: 2003SPD....34.1808M
Altcode: 2003BAAS...35..841M
  Since RHESSI was launched in February 2002, it has observed thousands
  of solar flares. It also observes solar emission above 3 keV when
  there are no observeable flares present. In this work we present
  measurements of the non-flare Temperature and Emission Measure for
  the period from October 2002 through May 2003. Preliminary results
  indicate that the temperature is relatively stable in the 6 - 8 MK
  range (this is not surprising considering that RHESSI cannot reliably
  measure temperatures less than about 5 MK). The emission measure varies
  in the range from approximately 1.0e49 to 1.0e50, with higher values
  associated with periods of more solar activity. <P />Since RHESSI is an
  imaging spectrometer, we will locate the source of the emission when
  possible. We also will discuss comparisons with TRACE and SXI data,
  and the possiblilty of measuring the Differential Emission Measure
  for the range from 1 MK to 10 MK, using the multiple data sets.

Title: Are All Coronal Loops Heated by Nanoflares?
Authors: Klimchuk, J. A.; Patsourakos, S.; Winebarger, A. R.
Bibcode: 2003SPD....34.1006K
Altcode: 2003BAAS...35R.825K
  Observations from TRACE, SOHO, and Yohkoh have revealed new details
  of coronal loops that make them more mysterious than ever. One of
  the biggest puzzles concerns the loop density. Hot (&gt; 2 MK) loops
  observed by Yohkoh tend to be under dense compared to the predictions
  of equilibrium theory, while warm ( 1 MK) loops observed by TRACE
  and EIT tend to be over dense. Some over dense loops can be explained
  by steady heating that is concentrated near one or both of the loop
  legs, but a majority of these loops cannot. <P />We here consider
  the possibility that observed loops are comprised of large numbers
  of unresolved strands that are heated impulsively and randomly by
  nanoflares. The loops appear quasi-steady even though the individual
  sub-strands are highly time dependent. When the strands are hot,
  they cool primarily by thermal conduction and are under dense, but
  when they are warm, they cool primarily by radiation and are over
  dense. Since Yohkoh and TRACE are sensitive to different strands,
  we might expect them to observe the under and over densities that
  they do. <P />To evaluate the feasibility of this universal model
  of coronal loops, we have performed 1D hydrodynamic simulations of
  impulsively heated strands and compared them with observations from
  Yohkoh and TRACE. The results are encouraging in many respects, but
  difficulties remain. In this presentation, we discuss the successes
  and failures of the model. <P />This work was supported by NASA and ONR.

Title: Coronal Energy Release via Explosive Three-Dimensional
    Instability
Authors: Dahlburg, R. B.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 2003SPD....34.0107D
Altcode: 2003BAAS...35..806D
  It is widely believed that most coronal phenomena involve the release
  of magnetic free energy that is stored within stressed magnetic field
  configurations. The availability of sufficient free energy to explain
  everything from coronal heating to flares and coronal mass ejections
  is well established, but how this energy is released remains a major
  puzzle. Observations reveal that an important property of the energy
  release mechanism is its ``switch on" character. The mechanism must
  remain dormant for long periods of time to allow the magnetic stresses
  build, then it must operate very vigorously once it finally turns on. <P
  />We discuss a mechanism called the ``secondary instability" which
  exhibits this behavior. It is essentially the ideal kinking of thin
  twisted magnetic flux tubes that form from the resistive instability
  of current sheets. We relate the mechanism to the coronal heating
  idea of Parker in which the coronal magnetic field becomes tangled by
  random motions of the photospheric footpoints. Global energy balance
  considerations imply that magnetic energy dissipation occurs at a
  particular angle in the field, and the secondary instability offers
  the first quantitative explanation for why this should be. It thus
  places Parker's popular idea on a much firmer physical footing. <P
  />This research was funded by NASA.

Title: Can Steady-state Mass Flows Explain the Non-hydrostatic Cool
    EUV Coronal Loops in Active Regions?
Authors: Patsourakos, S.; Klimchuk, J. A.
Bibcode: 2003SPD....34.1009P
Altcode: 2003BAAS...35..826P
  Recent EIT/TRACE observations of cool (≈ 1-1.5 MK) EUV coronal loops
  in active regions showed that these loops are very often characterized
  by greatly enhanced pressure scale-heights and densities compared to
  the predictions of static equilibrium theory. It has been suggested
  that mass flows may explain these over-dense and over-pressure
  loops. We investigate this conjecture by the means of 1D hydrodynamic
  simulations of steady-state mass flows in coronal loops. The mass flows
  in our calculations are driven by asymmetric heating that decreases
  exponentially along the loop from one footpoint all way to the
  other. By considering several representative cases for the magnitude
  and the length scale of the applied asymmetric heating, we determine
  how steady-state mass flows affect the thermodynamic structure of
  coronal loops and assess whether they can lead to enhanced pressure
  scale-heights and densities. Research supported in part by NASA and ONR.

Title: The Long-Term Evolution of AR 7978: The Scalings of the
    Coronal Plasma Parameters with the Mean Photospheric Magnetic Field
Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Mandrini, C. H.;
   Harra, L.; Klimchuk, J. A.
Bibcode: 2003ApJ...586..579V
Altcode:
  We analyze the evolution of the fluxes observed in X-rays and correlate
  them with the magnetic flux density in active region (AR) NOAA 7978
  from its birth throughout its decay, for five solar rotations. We
  use Solar and Heliospheric Observatory Michelson Doppler Imager
  (MDI) data, together with Yohkoh Soft X-Ray Telescope (SXT) and
  Yohkoh Bragg Crystal Spectrometer (BCS) data, to determine the global
  evolution of the temperature and the emission measure of the coronal
  plasma at times when no significant brightenings were observed. We
  show that the mean X-ray flux and derived parameters, temperature
  and emission measure (together with other quantities deduced from
  them, such as the density and the pressure), of the plasma in the AR
  follow power-law relationships with the mean magnetic flux density
  (B). The exponents (b) of these power-law functions (aB<SUP>b</SUP>)
  are derived using two different statistical methods, a classical
  least-squares method in log-log plots and a nonparametric method,
  which takes into account the fact that errors in the data may not be
  normally distributed. Both methods give similar exponents, within
  error bars, for the mean temperature and for both instruments (SXT
  and BCS); in particular, b stays in the range [0.27, 0.31] and [0.24,
  0.57] for full-resolution SXT images and BCS data, respectively. For
  the emission measure, the exponent b lies in the range [0.85, 1.35]
  and [0.45, 1.96] for SXT and BCS, respectively. The determination of
  such power-law relations, when combined with the results from coronal
  heating models, can provide us with powerful tools for determining the
  mechanism responsible for the existence of the high-temperature corona.

Title: The Long-Term Evolution of AR 7978: Testing Coronal Heating
    Models
Authors: Démoulin, P.; van Driel-Gesztelyi, L.; Mandrini, C. H.;
   Klimchuk, J. A.; Harra, L.
Bibcode: 2003ApJ...586..592D
Altcode:
  We derive the dependence of the mean coronal heating rate on the
  magnetic flux density. Our results are based on a previous study of
  the plasma parameters and the magnetic flux density (B) in the active
  region NOAA 7978 from its birth to its decay, throughout five solar
  rotations using the Solar and Heliospheric Observatory Michelson
  Doppler Imager, Yohkoh Soft X-Ray Telescope (SXT), and Yohkoh Bragg
  Crystal Spectrometer (BCS). We use the scaling laws of coronal loops
  in thermal equilibrium to derive four observational estimates of the
  scaling of the coronal heating with B (two from SXT and two from
  BCS observations). These results are used to test the validity of
  coronal heating models. We find that models based on the dissipation
  of stressed, current-carrying magnetic fields are in better agreement
  with the observations than models that attribute coronal heating to
  the dissipation of MHD waves injected at the base of the corona. This
  confirms, with smaller error bars, previous results obtained for
  individual coronal loops, as well as for the global coronal emission
  of the Sun and cool stars. Taking into account that the photospheric
  field is concentrated in thin magnetic flux tubes, both SXT and BCS
  data are in best agreement with models invoking a stochastic buildup
  of energy, current layers, and MHD turbulence.

Title: Riding the solar wind
Authors: Klimchuk, James A.
Bibcode: 2003Natur.421..894K
Altcode:
  No abstract at ADS

Title: How to test coronal heating models?
Authors: Mandrini, C. H.; Démoulin, P.; van Driel-Gesztelyi, L.;
   Klimchuk, J. A.; Harra, L. K.
Bibcode: 2003BAAA...46....5M
Altcode:
  We have tested coronal heating models following two different
  approaches. In the first case, we compared the dependence of the
  coronal heating rate predicted by theoretical models with the
  observed one, deriving the scalings of parameters, such as: the
  density, temperature and intensity of the coronal magnetic field,
  with the length of magnetic field lines. To do so, we combined density
  and temperature measurements for 47 coronal loops with magnetic field
  models for 14 active regions. In the second case, we analyzed the long
  term evolution of an active region observed during seven rotations
  on the solar disk and we determined the dependence of the observed
  heating rate with the magnetic field density (bar{B}), after finding
  the scalings of plasma parameters with bar{B}. In both cases, we found
  that models based on the dissipation of stressed, current-carrying
  magnetic fields (called direct current models) are in better agreement
  with observations than models that attribute coronal heating to the
  dissipation of MHD waves injected at the base of the corona (called
  alternate current models). Taking into account that the photospheric
  field is concentrated in thin magnetic flux tubes, observations are
  in best agreement with models invoking a stochastic buildup of energy,
  current layers and MHD turbulence, within direct current models.

Title: Coronal arcs and magnetic structure of the solar corona
Authors: López Fuentes, M. C.; Klimchuk, J. A.
Bibcode: 2003BAAA...46....2L
Altcode:
  Since the strength of the coronal magnetic field decreases with distance
  from the photosphere, it is expected that coronal magnetic structures
  expand with height. Nevertheless, loops observed in EUV and Soft X-rays
  have constant cross section (Klimchuk 2000). In this study we compare
  a set of TRACE loops with flux tubes obtained from the force-free
  extrapolation of the coronal magnetic field. As boundary condition
  we use MDI magnetograms cotemporal with the TRACE observations. The
  observed width of the coronal loops in the studied set is constant. On
  the other hand, the corresponding modeled flux tubes expand with
  distance from the photosphere. To compare with observed quantities, we
  compute the width of the flux tubes as seen from the line of sight. In
  contrast with the nearly constant thickness of the observed loops,
  the width of the modeled flux tubes is highly variable. These results
  imply that the magnetic structure of the corona and the mechanism of
  coronal heating are much more complex than currently understood.

Title: A Transient Heating Model for Coronal Structure and Dynamics
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
   Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J.
Bibcode: 2003ApJ...582..486S
Altcode:
  A wealth of observational evidence for flows and intensity variations in
  nonflaring coronal loops leads to the conclusion that coronal heating
  is intrinsically unsteady and concentrated near the chromosphere. We
  have investigated the hydrodynamic behavior of coronal loops undergoing
  transient heating with one-dimensional numerical simulations in which
  the timescale assumed for the heating variations (3000 s) is comparable
  to the coronal radiative cooling time and the assumed heating location
  and scale height (10 Mm) are consistent with the values derived from
  TRACE studies. The model loops represent typical active region loops:
  40-80 Mm in length, reaching peak temperatures up to 6 MK. We use ARGOS,
  our state-of-the-art numerical code with adaptive mesh refinement, in
  order to resolve adequately the dynamic chromospheric-coronal transition
  region sections of the loop. The major new results from our work are
  the following: (1) During much of the cooling phase, the loops exhibit
  densities significantly larger than those predicted by the well-known
  loop scaling laws, thus potentially explaining recent TRACE observations
  of overdense loops. (2) Throughout the transient heating interval,
  downflows appear in the lower transition region (T~0.1 MK) whose key
  signature would be persistent, redshifted UV and EUV line emission,
  as have long been observed. (3) Strongly unequal heating in the two
  legs of the loop drives siphon flows from the more strongly heated
  footpoint to the other end, thus explaining the substantial bulk flows
  in loops recently observed by the Coronal Diagnostic Spectrometer and
  the Solar Ultraviolet Measurement of Emission Radiation instrument. We
  discuss the implications of our studies for the physical origins of
  coronal heating and related dynamic phenomena.

Title: The mystery of Coronal Loops
Authors: Klimchuk, J. A.
Bibcode: 2003BAAA...46....2K
Altcode:
  Coronal loops, beautiful plasma structures that outline arched magnetic
  fields, are often referred to as the fundamental building blocks of
  the million degree solar corona. Despite their importance and years
  of intensive study, coronal loops are in many ways enigmatic. In
  particular, recent observations have forced us to abandon our old belief
  that they are static structures maintained by steady heating. In this
  talk we review the status of coronal loop physics and describe an
  emerging paradigm of nanoflare heating.

Title: Bright Knots in EUV Post-flare Loops : TRACE Observations
    and 1D Hydrodynamic Modeling
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
Bibcode: 2002AGUFMSH21C..04P
Altcode:
  EUV post-flare loops often possess bright knots along them. Some
  examples of such post-flare loops seen by TRACE will be shown, along
  with a brief outline of their properties. We will then present the
  results of a series of 1D hydrodynamic simulations of flaring loops,
  which employ different heating functions for the impulsive and decay
  phase of the simulated flares. It will be demonstrated that the creation
  of these knots depends crucially on the spatio-temporal distribution of
  the heating during the decay phase. This provides strong constraints
  on both post-flaring conditions and AR loop heating. We will finally
  briefly outline how SDO instrumentation could improve our knowledge
  of this topic. Research supported in part by NASA and ONR.

Title: Scaling Laws for Solar and Stellar Coronae
Authors: Klimchuk, James A.
Bibcode: 2002ASPC..277..321K
Altcode: 2002sccx.conf..321K
  No abstract at ADS

Title: Fuzzy hot post-flare loops versus sharp cool post-flare loops
Authors: Patsourakos, S.; Antiochos, S. K.; Klimchuk, J. A.
Bibcode: 2002ESASP.505..207P
Altcode: 2002solm.conf..207P; 2002IAUCo.188..207P
  By using high spatial resolution TRACE EUV observations we show that hot
  (≍2 MK) post-flare loops are fuzzier than the cooler (≍1 MK) ones. A
  simple 0d model of a cooling loop arcade, where different loops in the
  arcade start to cool down at slightly different initial conditions,
  is sufficient to reproduce qualitatively the observed behavior of the
  EUV post-flare loops.

Title: Hydrodynamic models of transiently heated coronal loops
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
   Antiochos, S. K.; Klimchuk, J. A.; MacNeice, P. J.
Bibcode: 2002ESASP.505..583S
Altcode: 2002solm.conf..583S; 2002IAUCo.188..583S
  We investigate the hydrodynamic behaviour of coronal loops
  undergoing transient heating. We adopt a 1-D loop model with space-
  and time-dependent heating, concentrated near the chromospheric
  footpoints. The timescale of heating variations is comparable with the
  radiative cooling time of the coronal plasma (~10<SUP>3</SUP>s). We
  use a new numerical code that has a fully adaptive grid, in order to
  properly resolve the chromospheric-coronal transition region sections of
  the loop. We simulate here the hydrodynamics of a loop with different
  effective gravity (i.e., loop geometry) and heating terms. We describe
  the temporal behaviour of the various physical quantities along the
  loop (plasma density, temperature, flow velocity), showing that the
  increase in heating produces a chromospheric evaporation, or a siphon
  flow if the loop heating is taken to be significantly different at
  the two footpoints, followed by long-lasting downflows with velocities
  of a few km s<SUP>-1</SUP> during the quiescent phases in between the
  episodic heatings. Moreover, in the case of considerable increase in
  heating, a catastrophic cooling of the loop plasma can occur, giving
  rise to downflows of several tens of km s<SUP>-1</SUP>.

Title: Hydrodynamic simulations of coronal loops subject to transient
    heating
Authors: Spadaro, D.; Lanza, A. F.; Lanzafame, A. C.; Karpen, J. T.;
   MacNeice, P. J.; Antiochos, S. K.; Klimchuk, J. A.
Bibcode: 2002ESASP.508..331S
Altcode: 2002soho...11..331S
  We investigate the hydrodynamic behaviour of coronal loops
  undergoing transient heating. We adopt a 1-D loop model with space-
  and time-dependent heating, concentrated near the chromospheric
  footpoints. The timescale of heating variations is comparable with the
  radiative cooling time of the coronal plasma (~10<SUP>3</SUP>s). We
  use a new numerical code that has a fully adaptive grid, in order to
  properly resolve the chromospheric-coronal transition region sections of
  the loop. We simulate here the hydrodynamics of a loop with different
  effective gravity (i.e., loop geometry) and heating terms. We describe
  the temporal behaviour of the various physical quantities along the loop
  (plasma density,temperature, flow velocity), showing that the increase
  in heating produces a chromospheric evaporation, or a siphon flow if
  the loop heating is taken to be significantly different at the two
  footpoints, followed by long-lasting downflows with velocities of a few
  km s<SUP>-1</SUP> during the quiescent phases in between the episodic
  heatings. Moreover, in the case of considerable increase in heating,
  a thermal instability can occur during the cooling phase of the loop
  plasma, giving rise to downflows of several tens of km s<SUP>-1</SUP>.

Title: Hot versus cool coronal loops
Authors: Patsourakos, S.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 2002AAS...200.0209P
Altcode: 2002BAAS...34..640P
  EUV and SXR observations show respectively that cool (1 MK) loops are
  finer and maybe more dynamic than hotter (2 MK) ones. Whether this
  reflects a fundamental difference in the properties of the heating
  mechanism in action in each loop class is not yet clear. We will address
  some aspects of this issue by combining EUV and SXR observations of
  such loops with eventually hydrodynamic simulations of a nano-flare
  heated corona. Research supported in part by ONR and NASA.

Title: An Explanation for the ``Switch On" Character of Magnetic
    Energy Release
Authors: Klimchuk, J. A.; Dahlburg, R. B.; Antiochos, S. K.
Bibcode: 2002AAS...200.1607K
Altcode: 2002BAAS...34..668K
  It is widely believed that most coronal phenomena involve the release
  of magnetic free energy that is stored in stressed magnetic field
  configurations. The availability of sufficient free energy to explain
  everything from coronal heating to flares and coronal mass ejections
  is well established, but how this energy is released remains a major
  puzzle. Observations reveal that an important property of the energy
  release mechanism is its ``switch on" character. The mechanism must
  remain dormant for long periods of time to allow the magnetic stresses
  to build, then it must operate very vigorously once it finally turns
  on. We discuss a mechanism called the ``secondary instability" which
  exhibits this behavior. It is essentially the ideal kinking of thin
  twisted magnetic flux tubes that form from the restive tearing of
  current sheets. We relate the mechanism to the coronal heating idea
  of Parker in which the coronal magnetic field becomes tangled by
  random motions of the photospheric footpoints. Global energy balance
  considerations imply that magnetic energy dissipation occurs at a
  particular angle in the field, and the secondary instability offers
  the first quantitative explanation for why this should be. It thus
  places Parker's popular idea on a much firmer physical footing.

Title: Coronal loops
Authors: Klimchuk, James
Bibcode: 2002ocnd.confE..17K
Altcode:
  No abstract at ADS

Title: Observations and Modeling of Solar Coronal Loops
Authors: Klimchuk, J.
Bibcode: 2002cosp...34E1208K
Altcode: 2002cosp.meetE1208K
  Coronal loops are often described as the fundamental building blocks
  of solar and stellar coronae. Clearly, therefore, a comprehensive
  understanding of coronae requires an explanation of the nature and
  origin of these loops, including the mechanism of their heating. Certain
  general aspects of coronal loops are reasonably well understood. For
  example, we know that the plasma is structured by the magnetic field
  and that strongly heated flux tubes tend to be hotter and denser than
  weakly heated flux tubes. Some observations suggest that loops are in
  quasi-static equilibrium, and scaling laws have been used to describe
  the relationships among physical variables and to test competing
  theories of coronal heating. Other observations raise serious doubts
  about whether the quasi-static description is valid. At this point,
  we cannot say with any certainty whether loops are isothermal or
  multithermal (i.e., monolithic structures or collections of unresolved
  strands) or whether they are heated steadily or in a highly episodic
  fashion (e.g., by nanoflares). This talk will address what we can learn
  about these important questions from a combination of observations
  and theoretical modeling.

Title: Modeling the coronal loop of an X-ray bright point
Authors: McMullen, R.; Longcope, D.; McKenzie, D.; Kankelborg, C.;
   Klimchuk, J.
Bibcode: 2002ocnd.confE..28M
Altcode:
  No abstract at ADS

Title: Observation and Theory of Coronal Loop Structure
Authors: Klimchuk, J. A.
Bibcode: 2002mwoc.conf...65K
Altcode:
  Following up on an initial study of 10 soft X-ray loops observed
  by Yohkoh (Klimchuk et al. 1992), we have carefully examined 43
  additional Yohkoh loops and 24 EUV loops observed by TRACE, and
  we confirm our original finding that most coronal loops have a
  nearly uniform thickness. This implies that: 1. the magnetic field
  in these loops expands with height much less than standard coronal
  models would predict; and 2. the shape of the loop cross section is
  approximately circular. We have investigated whether these surprising
  results can be explained by locally enhanced twist in the field,
  so that observed loops correspond to twisted coronal flux tubes. Our
  approach is to construct numerical models of fully three-dimensional
  force-free magnetic fields. To resolve the internal structure of an
  individual loop embedded within a much larger dipole configuration,
  we use a nonuniform numerical grid of size 609 times 513 times 593, the
  largest ever applied to a solar problem, to our knowledge. Our models
  indicate that twist does indeed promote circular cross sections in the
  corona, even when the footpoint cross section is irregular. However,
  twist does not seem to be a likely explanation for the observed minimal
  expansion with height.

Title: Coronal energy release via explosive magnetic reconnection
Authors: Dahlburg, R.; Klimchuk, J.; Antiochos, S.
Bibcode: 2002cosp...34E1264D
Altcode: 2002cosp.meetE1264D
  It is widely believed that most coronal phenomena involve the release
  of magnetic free energy that is stored within stressed magnetic field
  configurations. The availability of sufficient free energy to explain
  everything from coronal heating to flares and coronal mass ejections
  is well established, but how this energy is released remains a major
  puzzle. Observations reveal that an important property of the energy
  release mechanism is its "switch on" character. The mechanism must
  remain dormant for long periods of time to allow the magnetic stresses
  to build, then it must operate very vigorously once it finally turns
  on. We discuss a mechanism called the "secondary instability" which
  exhibits this behavior. It is essentially the ideal kinking of thin
  twisted magnetic flux tubes that form from the resistive tearing of
  current sheets. We relate the mechanism to the coronal heating idea
  of Parker in which the coronal magnetic field becomes tangled by
  random motions of the photospheric footpoints. Global energy balance
  considerations imply that magnetic energy dissipation occurs at a
  particular angle in the field, and the secondary instability offers
  the first quantitative explanation for why this should be. It thus
  places Parker's popular idea on a much firmer physical footing.

Title: On the Correlation between Coronal and Lower Transition Region
    Structures at Arcsecond Scales
Authors: Vourlidas, A.; Klimchuk, J. A.; Korendyke, C. M.; Tarbell,
   T. D.; Handy, B. N.
Bibcode: 2001ApJ...563..374V
Altcode:
  We compare the morphology of active region structures observed in
  the 171 Å (T~9×10<SUP>5</SUP> K) and Lyα (T~2×10<SUP>4</SUP> K)
  lines. The coronal data were obtained by the Transition Region and
  Coronal Explorer (TRACE) in support of the Very High Angular Resolution
  Ultraviolet Telescope (VAULT) sounding rocket launch, which acquired
  subarcsecond resolution images of an active region in the Lyα line,
  on 1999 May 7. Using a pair of calibrated, nearly simultaneous images,
  we find that: (i) a very good correlation exists between the Lyα and
  171 Å intensities in the TRACE moss regions, (ii) we can identify
  several identical structures in some (but not all) moss areas, and
  (iii) the correlations are greatly reduced at the footpoints of the
  171 Å large-scale loops. We derive a lower limit for the Lyα emission
  measure, under the assumption of effectively optically thin emission,
  and compare it to the 171 Å emission measure. As in previous studies,
  we find an excess of Lyα material compared to the amount expected
  for a thermal conduction-dominated corona-chromosphere transition
  region, even for structures that appear to be identical in the two
  wavelengths. This result implies that some other mechanism besides
  classical heat conduction from the corona must contribute to the
  observed Lyα intensities. The observations do not support the idea
  of a physically distinct cool loop component within active regions.

Title: Spectroscopic Diagnostics of Nanoflare-heated Loops
Authors: Klimchuk, J. A.; Cargill, P. J.
Bibcode: 2001ApJ...553..440K
Altcode:
  To evaluate the usefulness of spectroscopic techniques for diagnosing
  realistic solar plasmas and to better understand the physical origin of
  coronal heating, we have simulated observations of model coronal loops
  that are heated randomly and impulsively by nanoflares. We find that
  the emission measures, densities, and filling factors that are inferred
  from spectral line intensities (EM<SUB>s</SUB>, n<SUB>s</SUB>, and
  φ<SUB>s</SUB>, respectively) are generally an excellent representation
  of the properties of the nanoflare-heated plasma. To better than 25%
  in most cases, EM<SUB>s</SUB> indicates the amount of material present
  in the ΔlogT=0.3 temperature interval centered on the peak of the line
  contribution function, n<SUB>s</SUB> indicates the average density of
  this material, and φ<SUB>s</SUB> indicates the fraction of the total
  volume that the material occupies. Measurements with lithium-like lines
  are much less accurate, however. We provide diagnostic values and line
  intensities for many different spectral lines that can be compared
  directly with observations from the Coronal Diagnostic Spectrometer
  and Solar Ultraviolet Measurements of Emitted Radiation instruments
  on SOHO and from the future Extreme Ultraviolet Imaging Spectrometer
  instrument on Solar-B. Such comparisons will provide the first ever
  rigorous test of the nanoflare concept, which has enormous implications
  for understanding the mechanism of coronal heating.

Title: Ultra-High Resolution Observations of the Upper Chromosphere:
    First Results From the NRL VAULT Sounding Rocket Payload
Authors: Vourlidas, A.; Korendyke, C. M.; Dere, K. P.; Klimchuk, J. A.
Bibcode: 2001AGUSM..SP61A03V
Altcode:
  The Very high resolution Advanced ULtraviolet Telescope (VAULT) is
  a new spectroscopic imaging instrument. The instrument was launched
  on May 7, 1999 as a sounding rocket payload. The goal of the first
  VAULT flight was to obtain sub-arcsecond images of the Sun in the
  light of Lya (1216 Å). VAULT directly imaged an active region plage,
  fliaments and the fine structures in the supergranule boundaries and
  network with the unprecented spatial resolution of 0.33 arcseconds. We
  present the VAULT images and the first results from the comparison of
  the Lya data to observations from other instruments and in particular
  with a sequence of TRACE 171 Å images taken during the VAULT flight.

Title: Are Magnetic Dips Necessary for Prominence Formation?
Authors: Karpen, J. T.; Antiochos, S. K.; Hohensee, M.; Klimchuk,
   J. A.; MacNeice, P. J.
Bibcode: 2001ApJ...553L..85K
Altcode:
  The short answer: No.

Title: Observational Signatures of Nanoflare-Heated Loops
Authors: Klimchuk, J. A.; Cargill, P. J.
Bibcode: 2001AGUSM..SP52B01K
Altcode:
  After many years of intense study, the heating of the corona remains
  a largely unsolved problem. A number of ideas have been proposed
  involving the episodic release of small bursts of energy, termed
  ``nanoflares" by Parker based on his rough estimate of the energies
  involved. Adopting this concept, we investigate a model whereby
  individual coronal loops like those observed by Yohkoh, SOHO, and
  TRACE are comprised of many thousands of unresolved strands that are
  heated randomly and independently by nanoflares. The strands cool
  by thermal conduction and radiation until they are heated again by
  the next event. We simulate the observation of these model loops by
  broadband and spectroscopic instruments such as SXT, CDS, SUMER, and
  EIS. By varying the nanoflare parameters (primarily the mean energy
  of the events) and comparing with actual observations, we are able to
  test the model and constrain the parameters. We find that many real
  loops are compatible with the model, especially those hotter than 2 MK,
  but that many others are not. These latter loops are also incompatible
  with steady heating, and we suggest that they contain coronal material
  that is injected into the loop by a mechanism not directly related to
  coronal heating (e.g., not chromospheric evaporation).

Title: Properties of EUV and X-ray emission in solar active regions
Authors: Matthews, S. A.; Klimchuk, J. A.; Harra, L. K.
Bibcode: 2001A&A...365..186M
Altcode:
  Using observations from the Coronal Diagnostic Spectrometer (CDS) on
  SoHO and the Soft X-ray Telescope (SXT) on Yohkoh we investigate how the
  spatial properties of active region emission observed in the EUV and
  X-ray range varies with temperature. We examine the contrast per unit
  area of the EUV emission from a number of active regions, and employ
  correlation techniques and Fourier methods with which we obtain the two
  dimensional power spectrum of the intensity distribution for a number of
  images in emission lines formed at different temperatures. Integrating
  this over polar angle we find isotropic power-law behaviour at all
  temperatures in a number of topologically different active regions,
  with a tendency for flatter spectra at lower temperatures. The
  existence of power-law spectra indicates that there is no preferred
  length scale within the regions, at least not a resolvable one, while
  flatter spectra at lower temperatures indicate that the structures are
  relatively smaller in this temperature range, possibly providing support
  for the idea of a multi-component transition region (TR). Implications
  for various heating models are discussed.

Title: Theory of Coronal Mass Ejections
Authors: Klimchuk, J. A.
Bibcode: 2001GMS...125..143K
Altcode:
  Coronal mass ejections (CMEs) are extremely important phenomena,
  both for understanding the evolution of the global corona and for
  understanding and predicting space weather. Despite this importance,
  the physical explanation of CMEs remains largely confused. A variety
  of theoretical models have been proposed, and we here attempt to
  organize them according to a classification scheme that identifies and
  differentiates their essential physical attributes. We propose five
  distinct classes of models, which we present with the aid of simple
  analogues involving springs, ropes, and weights. We also indicate how
  some of the models appear to be inconsistent with certain observations.

Title: Twisted Coronal Magnetic Loops
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.
Bibcode: 2000ApJ...542..504K
Altcode:
  Observed coronal loops have a surprisingly uniform thickness that
  cannot be easily understood in terms of standard coronal magnetic
  field models. We investigate the possibility that the uniform
  thickness can be explained by locally enhanced twist in the field,
  so that observed loops correspond to twisted coronal flux tubes. Our
  approach is to construct numerical models of fully three-dimensional
  force-free magnetic fields. To resolve the internal structure of an
  individual loop embedded within a much larger dipole configuration,
  we use a nonuniform numerical grid of size 609×513×593, the largest
  ever applied to a solar problem to our knowledge. Our models show that
  twist promotes circular cross sections in loops. Such cross sections are
  typically assumed, and have recently been verified from observations,
  but their physical cause has been heretofore unexplained.

Title: Observation and theory of coronal loop structure.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A.
Bibcode: 2000BAAS...32R.809K
Altcode:
  No abstract at ADS

Title: Observation and Theory of Coronal Loop Structure
Authors: Klimchuk, J. A.; Antiochos, S. K.; Norton, D.; Watko, J. A.
Bibcode: 2000SPD....31.0144K
Altcode:
  We have carefully examined 43 soft X-ray loops observed by Yohkoh and
  24 EUV loops observed by TRACE and find that the large majority have
  a nearly uniform thickness. This implies that: 1. the magnetic field
  in these loops expands with height much less than standard coronal
  models would predict; and 2. the shape of the loop cross section is
  approximately circular. We have investigated whether these surprising
  results can be explained by locally enhanced twist in the field,
  so that observed loops correspond to twisted coronal flux tubes. Our
  approach is to construct numerical models of fully three-dimensional
  force-free magnetic fields. To resolve the internal structure of an
  individual loop embedded within a much larger dipole configuration,
  we use a nonuniform numerical grid of size 609 x 513 x 593, the largest
  ever applied to a solar problem, to our knowledge. Our models indicate
  that twist does indeed promote circular cross sections in the corona,
  even when the footpoint cross section is irregular. However, twist does
  not seem to be a likely explanation for the observed minimal expansion
  with height. This work was supported by the NASA Sun-Earth Connection
  Theory and Guest Investigator Programs.

Title: Cross-Sectional Properties of Coronal Loops
Authors: Klimchuk, J. A.
Bibcode: 2000SoPh..193...53K
Altcode:
  Careful examination of 43 soft X-ray loops observed by Yohkoh
  has revealed a number of interesting properties of the loop cross
  section. First, the loops tend to be only slightly (≈ 30%) wider at
  their midpoints than at their footpoints, implying less-than-expected
  expansion of the magnetic field. Second, the variation of width along
  each loop tends to be modest, implying that the cross section has an
  approximately circular shape. And third, cross-axis intensity profiles
  tend to be singly-peaked and simple, implying that the cross section
  is approximately uniformly filled on resolvable scales. We conclude
  that the energy which heats the plasma is either dissipated axially
  symmetrically on a scale equal to a loop diameter (≈ 11 000 km)
  or else is dissipated with any spatial structure, but on a scale
  much smaller than a loop diameter, and then transported laterally in
  an axisymmetric fashion (perhaps via conduction along chaotic field
  lines). In their present form, none of the theoretical ideas concerning
  the magnetic structure and heating of loops are obviously capable of
  explaining all of the observed properties.

Title: Width Variations along Coronal Loops Observed by TRACE
Authors: Watko, J. A.; Klimchuk, J. A.
Bibcode: 2000SoPh..193...77W
Altcode:
  We have measured width variations along coronal loops observed by TRACE
  in the 171, 195, and 284 Å bandpasses. The loops are not significantly
  thicker in the middle compared to near the footpoints, and there is
  no correlation between the footpoint-to-midpoint expansion and the
  loop length. This applies to both post-flare and non-flare loops. The
  observations conflict with our present understanding of active region
  magnetic fields, and they have important implications for the structure
  and heating of the corona.

Title: Three-dimensional Stereoscopic Analysis of Solar Active Region
    Loops. II. SOHO/EIT Observations at Temperatures of 1.5-2.5 MK
Authors: Aschwanden, Markus J.; Alexander, David; Hurlburt, Neal;
   Newmark, Jeffrey S.; Neupert, Werner M.; Klimchuk, J. A.; Gary,
   G. Allen
Bibcode: 2000ApJ...531.1129A
Altcode:
  In this paper we study the three-dimensional structure of hot
  (T<SUB>e</SUB>~1.5-2.5 MK) loops in solar active region NOAA
  7986, observed on 1996 August 30 with the Extreme-ultraviolet
  Imaging Telescope (EIT) on board the Solar and Heliospheric
  Observatory (SOHO). This complements a first study (Paper I) on
  cooler (T<SUB>e</SUB>~1.0-1.5 MK) loops of the same active region,
  using the same method of Dynamic Stereoscopy to reconstruct the
  three-dimensional geometry. We reconstruct the three-dimensional
  coordinates x(s), y(s), z(s), the density n<SUB>e</SUB>(s), and
  temperature profile T<SUB>e</SUB>(s) of 35 individual loop segments
  (as a function of the loop coordinate s) using EIT 195 and 284 Å
  images. The major findings are as follows. (1) All loops are found
  to be in hydrostatic equilibrium, in the entire temperature regime
  of T<SUB>e</SUB>=1.0-2.5 MK. (2) The analyzed loops have a height of
  2-3 scale heights, and thus only segments extending over about one
  vertical scale height have sufficient emission measure contrast for
  detection. (3) The temperature gradient over the lowest scale height
  is of order dT/ds~1-10 K km<SUP>-1</SUP>. (4) The radiative loss
  rate is found to exceed the conductive loss rate by about two orders
  or magnitude in the coronal loop segments, implying that the loops
  cannot be in quasi-static equilibrium, since standard steady-state loop
  models show that radiative and conductive losses are comparable. (5) A
  steady state could only be maintained if the heating rate E<SUB>H</SUB>
  matches exactly the radiative loss rate in hydrostatic equilibrium,
  requiring a heat deposition length λ<SUB>H</SUB> of the half density
  scale height λ. (6) We find a correlation of p~L<SUP>-1</SUP> between
  loop base pressure and loop length, which is not consistent with the
  scaling law predicted from steady-state models of large-scale loops. All
  observational findings indicate consistently that the energy balance
  of the observed EUV loops cannot be described by steady-state models.

Title: Magnetic Field and Plasma Scaling Laws: Their Implications
    for Coronal Heating Models
Authors: Mandrini, C. H.; Démoulin, P.; Klimchuk, J. A.
Bibcode: 2000ApJ...530..999M
Altcode:
  In order to test different models of coronal heating, we have
  investigated how the magnetic field strength of coronal flux
  tubes depends on the end-to-end length of the tube. Using
  photospheric magnetograms from both observed and idealized
  active regions, we computed potential, linear force-free, and
  magnetostatic extrapolation models. For each model, we then
  determined the average coronal field strength, &lt;B&gt;, in
  approximately 1000 individual flux tubes with regularly spaced
  footpoints. Scatter plots of &lt;B&gt; versus length, L, are
  characterized by a flat section for small L and a steeply declining
  section for large L. They are well described by a function of the form
  log=C<SUB>1</SUB>+C<SUB>2</SUB>logL+C<SUB>3</SUB>/2log(L<SUP>2</SUP>+S<SUP>2</SUP>),
  where C<SUB>2</SUB>~0, -3&lt;=C<SUB>3</SUB>&lt;=-1, and 40&lt;=S&lt;=240
  Mm is related to the characteristic size of the active region. There
  is a tendency for the magnitude of C<SUB>3</SUB> to decrease as the
  magnetic complexity of the region increases. The average magnetic
  energy in a flux tube, &lt;B<SUP>2</SUP>&gt;, exhibits a similar
  behavior, with only C<SUB>3</SUB> being significantly different. For
  flux tubes of intermediate length, 50&lt;=L&lt;=300 Mm, corresponding
  to the soft X-ray loops in a study by Klimchuk &amp; Porter (1995),
  we find a universal scaling law of the form ~L<SUP>δ</SUP>, where
  δ=-0.88+/-0.3. By combining this with the Klimchuk &amp; Porter result
  that the heating rate scales as L<SUP>-2</SUP>, we can test different
  models of coronal heating. We find that models involving the gradual
  stressing of the magnetic field, by slow footpoint motions, are in
  generally better agreement with the observational constraints than are
  wave heating models. We conclude, however, that the theoretical models
  must be more fully developed and the observational uncertainties must
  be reduced before any definitive statements about specific heating
  mechanisms can be made.

Title: The spatial distribution of EUV emission in active regions
Authors: Matthews, S. A.; Klimchuk, J. A.; Harra, L. K.
Bibcode: 2000ssls.work...53M
Altcode:
  The full version of this paper will be published elsewhere. We give
  here only an extended abstract.

Title: Test on the parameter dependence of coronal heating models
Authors: Démoulin, P.; Mandrini, C. H.; Klimchuk, J. A.
Bibcode: 2000ssls.work...85D
Altcode:
  The motivation of this work has been to provide observational
  constraints on coronal heating models by testing their predictions
  for the heating rate as a function of several physical parameters. In
  Mandrini et al. (1999), we have investigated how the magnetic field
  strength, , of coronal flux tubes depends on the end-to-end length,
  L, of the tube. For flux tubes of intermediate length, 50 ≤ L ≤
  300 Mm, corresponding to the soft X-ray loops in a study by Klimchuk
  &amp; Porter (1995), we find a universal scaling law of the form
  ∝L<SUP>δ</SUP>, where δ= -0.88±0.3. By combining this with
  the Klimchuk &amp; Porter result that the heating rate scales as
  L<SUP>-2</SUP>, we can test different models of coronal heating. We
  find that models involving the gradual stressing of the magnetic field,
  by slow footpoint motions, are in generally better agreement with the
  observational constraints than are wave heating models.

Title: Properties of Transition Region and Coronal Loops
Authors: Matthews, S. A.; Klimchuk, J. A.; Harra-Murnion, L. K.
Bibcode: 1999ESASP.446..489M
Altcode: 1999soho....8..489M
  The magnetic field plays a vital role in governing the size, shape
  and dynamics of loops in the solar atmosphere and as such it seems
  reasonable to ask whether differences in the spatial distribution
  of these structures are indicative of differences in the form
  of the heating. Using observations from the Coronal Diagnostic
  Spectrometer (CDS) on SoHO and the Soft X-ray Telescope (SXT) on
  Yohkoh we investigate how the spatial distribution of EUV and X-ray
  emission in active regions varies with temperature. We examine various
  correlations between emission at different temperatures, the variation
  of contrast with temperature and employ Fourier methods to obtain
  the two dimensional power spectrum of the intensity distribution for
  a number of lines at different temperatures. Integrating this over
  polar angle we find isotropic power-law behaviour at all temperatures
  in a number of topologically different active regions, with a tendency
  for flatter spectra at lower temperatures. The existence of power-law
  spectra indicates that there is no preferred length scale within the
  regions. Flatter spectra at lower temperatures are consistent with
  emission predominantly from smaller scale structures such as low-lying
  loops or footpoints.

Title: Temperature and density in a polar plume - measurements
    from CDS/SOHO
Authors: Young, P. R.; Klimchuk, J. A.; Mason, H. E.
Bibcode: 1999A&A...350..286Y
Altcode:
  A detailed analysis of a particularly intense polar plume observed
  on the 25th of October, 1996, by the Coronal Diagnostic Spectrometer
  (CDS) on board the Solar and Heliospheric Observatory (SOHO) is
  presented. Above the limb, emission measure distributions derived for
  both the plume and a section of coronal hole background are found to
  be sharply peaked at approximately 1.0-1.1 million degrees in both
  regions. The temperature rises with height in the background, but no
  evidence is found for a rising temperature in the plume. The density
  of the background is approximately 10(8) electrons/cm(3) and falls
  with height. In the plume the density is between 3.8 and 9.5x 10(8)
  electrons/cm(3) , and exhibits no decrease with height up to 70 000
  km. The plume base is visible on the solar surface and shows a strong
  brightening lying directly below the main body of the plume. This
  brightening has a temperature of 2 000 000 K, and a density of
  2.5-5.6x 10(9) electrons/cm(3) . Images from lines formed at different
  temperatures suggest that the morphology of the base is consistent with
  an emerged bipole in a region of unipolar magnetic flux. A measurement
  of the Mg/Ne relative abundance is made at two transition region
  brightenings at the base of the plume. An enhancement of only 1.5 is
  found over the photospheric value. Considerations of the geometry of
  both the high temperature brightening at the base of the plume and
  the off-limb section give filling factors of 0.5 and 1.0, respectively.

Title: Magnetic Field Scaling Laws and Their Implications for
    Coronal Heating
Authors: Klimchuk, J. A.; Demoulin, P.; Mandrini, C. H.
Bibcode: 1999AAS...194.2304K
Altcode: 1999BAAS...31..861K
  Ever since it was realized, some 60 years ago, that the solar
  corona is two orders of magnitude hotter than the underlying
  photosphere, scientists have puzzled over the reason for these
  extreme conditions. A number of plausible ideas have been proposed,
  including the dissipation of MHD waves (AC models) and the dissipation
  of stressed, current-carrying magnetic fields (DC models), but it has
  proved difficult to establish which, if any, is correct. One approach
  to answering this fundamental question is to determine scaling laws
  relating the heating rate to observable physical parameters. Klimchuk
  &amp; Porter (1995, Nature, 377, 131) showed that the heating rate
  varies inversely with the square of the length of coronal loops observed
  by Yohkoh. To compare this with the predictions of coronal heating
  theories, it is necessary to know also how the magnetic field strength
  in the loops varies with their length. By computing magnetic field
  extrapolation models based on both observed and synthetic distributions
  of active region surface fields, we have found that B ~ ( L(2) + S(2)
  )(c/2) , where B is the coronal field strength averaged along a loop,
  L is the loop length, S is the characteristic size of the active region,
  and -3 &lt;= c &lt;= -1, depending on the complexity of the region. More
  importantly, for the range of loop lengths studied by Klimchuk &amp;
  Porter, 50 &lt; L &lt; 300 Mm, there is a universal scaling law of
  the form B ~ L(delta ,) where delta = -0.98 +/- 0.3. The details of
  these results will be presented, and their implications for theories
  of coronal heating will be discussed. It will be shown that DC models
  are in better agreement with the observations than are AC models. This
  work was supported in part by NASA grant W-19,200.

Title: The Solar Flotilla
Authors: Kahler, S. W.; Klimchuk, J. A.; Szabo, A.; Galvin, A. B.
Bibcode: 1999AAS...194.6507K
Altcode: 1999BAAS...31R.928K
  The Solar Flotilla is one of two candidate missions for the NASA
  Roadmap which would use a multispacecraft mission to explore the
  inner heliosphere. The Solar Flotilla will make particles and fields
  measurements at 10 or more points around the Sun within a distance
  of 0.2 to 0.4 AU. Identically instrumented microsatellites will be
  placed in orbit after a journey to Mercury where they will be injected
  into three principal orbit planes by 3-body interactions with the
  planet. Solar Flotilla measurements will address the question of
  how the solar output of plasma and magnetic flux varies with space
  and time, similar to the far better understood variations of radiant
  energy outputs, which can be measured remotely. Fundamental questions
  of magnetic flux and helicity expulsion can only be studied with
  such measurements. Additionally, mass flows and their variations
  can be understood on a global scale with measurements from the Solar
  Flotilla. The radial and azimuthal variations of turbulence, waves,
  and particle temperatures and abundances all relate to the heating of
  the solar corona and the acceleration of the solar wind. The spatial
  extents and shapes of interplanetary shocks and energetic particle
  distributions can be studied and used for improved space weather
  predictions. Together with imaging instruments at 1 AU we can explore
  the interplanetary extensions of short time-scale coronal features
  such as blobs and coronal mass ejections and long time-scale features
  such as plumes and streamers. The number of the spacecraft, their
  near-solar environment and great distance from Earth pose significant
  technological problems for operations and communications to be solved
  before deployment of the mission. We discuss further benefits and
  requirements of the proposed mission.

Title: The Structure of Solar Prominences
Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A.
Bibcode: 1999AAS...194.3102A
Altcode: 1999BAAS...31Q.868A
  With the advent of new high-spatial and high-temporal resolution
  observations from SOHO and TRACE, prominences/filaments have once again
  become a major focus of study for solar physics. Prominences/filaments
  are also important for their role in space weather. They yield
  key information on the type of magnetic structure that leads to
  eruptive flares and coronal mass ejections. We present results from
  our calculations of the 3D magnetic structure of prominences and the
  origin of the prominence mass. We show that many of the well-know
  features of their global structure, such as the prominence legs and
  barbs, the inverse polarity, and the sinistral-dextral property,
  can be easily understood as due to the geometry of a sheared bipolar
  field. Both fully time-dependent 3D MHD simulations and 3D force-free
  field equilibrium calculations demonstrate this conclusion. Furthermore,
  we discuss results showing that the magnetic structure of a sheared
  3D bipole leads naturally to the formation of cool condensations and
  to their observed motions. (*) This work is supported by NASA and ONR.

Title: Three-dimensional Stereoscopic Analysis of Solar Active
    Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5)
    × 10<SUP>6</SUP> K
Authors: Aschwanden, Markus J.; Newmark, Jeffrey S.; Delaboudinière,
   Jean-Pierre; Neupert, Werner M.; Klimchuk, J. A.; Gary, G. Allen;
   Portier-Fozzani, Fabrice; Zucker, Arik
Bibcode: 1999ApJ...515..842A
Altcode:
  The three-dimensional structure of solar active region NOAA 7986
  observed on 1996 August 30 with the Extreme-Ultraviolet Imaging
  Telescope (EIT) on board the Solar and Heliospheric Observatory
  (SOHO) is analyzed. We develop a new method of dynamic stereoscopy to
  reconstruct the three-dimensional geometry of dynamically changing
  loops, which allows us to determine the orientation of the mean
  loop plane with respect to the line of sight, a prerequisite to
  correct properly for projection effects in three-dimensional loop
  models. With this method and the filter-ratio technique applied
  to EIT 171 and 195 Å images we determine the three-dimensional
  coordinates [x(s), y(s), z(s)], the loop width w(s), the electron
  density n<SUB>e</SUB>(s), and the electron temperature T<SUB>e</SUB>(s)
  as a function of the loop length s for 30 loop segments. Fitting the
  loop densities with an exponential density model n<SUB>e</SUB>(h)
  we find that the mean of inferred scale height temperatures,
  T<SUP>λ</SUP><SUB>e</SUB>=1.22+/-0.23 MK, matches closely that of EIT
  filter-ratio temperatures, T<SUP>EIT</SUP><SUB>e</SUB>=1.21+/-0.06
  MK. We conclude that these cool and rather large-scale loops (with
  heights of h~30-225 Mm) are in hydrostatic equilibrium. Most of the
  loops show no significant thickness variation w(s), but we measure
  for most of them a positive temperature gradient (dT/ds&gt;0) across
  the first scale height above the footpoint. Based on these temperature
  gradients we find that the conductive loss rate is about 2 orders of
  magnitude smaller than the radiative loss rate, which is in strong
  contrast to hot active region loops seen in soft X-rays. We infer a
  mean radiative loss time of τ<SUB>rad</SUB>~40 minutes at the loop
  base. Because thermal conduction is negligible in these cool EUV
  loops, they are not in steady state, and radiative loss has entirely
  to be balanced by the heating function. A statistical heating model
  with recurrent heating events distributed along the entire loop can
  explain the observed temperature gradients if the mean recurrence time
  is &lt;~10 minutes. We computed also a potential field model (from
  SOHO/MDI magnetograms) and found a reasonable match with the traced
  EIT loops. With the magnetic field model we determined also the height
  dependence of the magnetic field B(h), the plasma parameter β(h),
  and the Alfvén velocity v<SUB>A</SUB>(h). No correlation was found
  between the heating rate requirement E<SUB>H0</SUB> and the magnetic
  field B<SUB>foot</SUB> at the loop footpoints.

Title: The Dynamic Formation of Prominence Condensations
Authors: Antiochos, S. K.; MacNeice, P. J.; Spicer, D. S.; Klimchuk,
   J. A.
Bibcode: 1999ApJ...512..985A
Altcode: 1998astro.ph..8199A
  We present simulations of a model for the formation of a prominence
  condensation in a coronal loop. The key idea behind the model is that
  the spatial localization of loop heating near the chromosphere leads
  to a catastrophic cooling in the corona. Using a new adaptive grid
  code, we simulate the complete growth of a condensation and find that
  after ~5000 s it reaches a quasi-steady state. We show that the size
  and growth time of the condensation are in good agreement with data
  and discuss the implications of the model for coronal heating and for
  observations of prominences and the surrounding corona.

Title: A Model for Solar Coronal Mass Ejections
Authors: Antiochos, S. K.; DeVore, C. R.; Klimchuk, J. A.
Bibcode: 1999ApJ...510..485A
Altcode: 1998astro.ph..7220A
  We propose a new model for the initiation of a solar coronal mass
  ejection (CME). The model agrees with two properties of CMEs and
  eruptive flares that have proved to be very difficult to explain with
  previous models: (1) very low-lying magnetic field lines, down to the
  photospheric neutral line, can open toward infinity during an eruption;
  and (2) the eruption is driven solely by magnetic free energy stored
  in a closed, sheared arcade. Consequently, the magnetic energy of the
  closed state is well above that of the posteruption open state. The key
  new feature of our model is that CMEs occur in multipolar topologies
  in which reconnection between a sheared arcade and neighboring flux
  systems triggers the eruption. In this ``magnetic breakout'' model,
  reconnection removes the unsheared field above the low-lying, sheared
  core flux near the neutral line, thereby allowing this core flux to
  burst open. We present numerical simulations that demonstrate our
  model can account for the energy requirements for CMEs. We discuss
  the implication of the model for CME/flare prediction.

Title: Report on new mission concept study: Stereo X-Ray Corona
    Imager mission
Authors: Liewer, Paulett C.; Davis, John M.; de Jong, E. M.; Gary,
   G. A.; Klimchuk, James A.; Reinert, Richard P.
Bibcode: 1998SPIE.3442...53L
Altcode:
  Studies of the 3D structure and dynamics of the solar corona
  have been severely limited by the constraint of single viewpoint
  observations. The Stereo X-Ray Coronal Imager (SXCI) mission will send
  a single instrument, an X-ray telescope, into deep space expressly to
  record stereoscopic images of the solar corona. The SXCI spacecraft
  will be inserted into an approximately 1 ZAU heliocentric orbit leading
  Earth by approximately 25 degrees at the end of nine months. The SXCI
  x-ray telescope forms one element of a stereo pair, the second element
  being an identical x-ray telescope in Earth orbit placed there as part
  of the NOAA GOES program. X-ray emission is a powerful diagnostic of
  the corona and its magnetic fields, and 3D information on the coronal
  magnetic structure would be obtained by combining the data from the
  two x-ray telescopes. This information can be used to address the major
  solar physics questions of (1) what causes explosive coronal events such
  as coronal mass ejections, eruptive flares and prominence eruptions and
  (2) what causes the transient heating of coronal loops. Stereoscopic
  views of the optically thin corona will resolve some ambiguities
  inherent in single line-of-sight observations. Triangulation gives 3D
  solar coordinates of features which can be seen in the simultaneous
  images form both telescopes. As part of this study, tools were
  developed for determining the 3D geometry of coronal features using
  triangulation. Advanced technologies for visualization and analysis of
  stereo images were tested. Results of mission and spacecraft studies
  are also reported.

Title: Prominence Formation by Localized Heating
Authors: Dahlburg, Russell B.; Antiochos, Spiro K.; Klimchuk, James A.
Bibcode: 1998ApJ...495..485D
Altcode:
  We describe a model for the formation of the cool condensed material
  that comprises a coronal filament or prominence. Numerical calculations
  are presented which demonstrate that large condensations form in
  a coronal loop if the loop satisfies two key requirements: (1) the
  loop heating must be localized near the chromospheric footpoints,
  and (2) the loop must have a dipped geometry in order to support the
  prominence condensation against gravity. We calculate one-dimensional
  equilibrium solutions for the equations of force and energy balance
  assuming optically thin radiative losses and a parameterized form for
  the coronal heating. This physical situation is modeled as a boundary
  value problem, which we solve numerically using a B-spline collocation
  scheme. The relation of our solutions to the well-known loop scaling
  laws is discussed, and the implications of our model for active region
  and quiescent prominences are discussed.

Title: 3D-Stereoscopic Analysis of Solar Active Region Loops Observed
    with SOHO/EIT
Authors: Aschwanden, M. J.; Newmark, J. S.; Delaboudiniere, J. -P.;
   Neupert, W. M.; Klimchuk, J. A.; Gary, G. Allen; Portier-Fozzani,
   F.; Zucker, A.
Bibcode: 1998cee..workE..19A
Altcode:
  The three-dimensional (3D) structure of solar active region NOAA
  7986 observed on 1996 August 30 with the Extrem-ultraviolet Imaging
  Telescope (EIT) onboard the Solar and Heliospheric Observatory
  (SoHO) is analyzed. We develop a new method of Dynamic Stereoscopy to
  reconstruct the 3D geometry of dynamically changing loops, which allows
  us to determine the orientation of the loop plane with respect to
  the line-of-sight, a prerequisite to correct properly for projection
  effects in 3D loop models. With this method and the filter-ratio
  technique applied to EIT 171 angle and 195 angle images we determine
  the 3D coordinates [x(s),y(s),z(s)], the loop width w(s), the electron
  density n_e(s), and the electron temperature T_e(s) as function of
  the loop length s for 30 loop segments. Fitting the loop densities
  with an exponential density model n_e(h) we find that the mean of
  inferred scale height temperatures, T_e<SUP>lambda</SUP> = 1.22 plus or
  minus 0.23 MK, matches closely that of EIT filter-ratio temperatures,
  T_e<SUP>EIT</SUP> = 1.21 plus or minus 0.06 MK. We conclude that these
  cool and rather large-scale loops (with heights of h ~30-225 Mm), which
  dominate the EIT 171 angle images, are in hydrostatic equilibrium. Most
  of the loops show no significant thickness variation w(s), but we can
  measure for most of them a positive temperature gradient (dT/ds &gt;
  0) across the first scale height above the footpoint. Based on these
  temperature gradients we find that the conductive loss rate is about
  two orders of magnitude smaller than the radiative loss rate, which is
  in strong contrast to hot active region loops seen in SXR. We infer a
  mean radiative loss time of τ<SUB>rad</SUB> ~40 minutes. For steady
  state models, the heating rate has to balance the radiative loss,
  i.e. the heating rate has to scale with the squared density (E_H
  propto n_e^2). From potential-field extrapolations we determine also
  the magnetic field strength B(s), the plasma beta-parameter beta(s),
  and the Alfven velocity v_A(s) along the loops, and discuss the findings
  in the context of coronal heating models.

Title: Theory of spicules, jets, plumes and other solar eruptions
Authors: Klimchuk, J. A.
Bibcode: 1998ESASP.421..233K
Altcode: 1998sjcp.conf..233K
  No abstract at ADS

Title: A Self-consistent Model for the Resonant Heating of Coronal
Loops: The Effects of Coupling with the Chromosphere
Authors: Ofman, L.; Klimchuk, J. A.; Davila, J. M.
Bibcode: 1998ApJ...493..474O
Altcode:
  We present the first model of resonant heating of coronal loops that
  incorporates the dependence of the loop density on the heating rate. By
  adopting the quasi-static equilibrium scaling law ρ ~ Q<SUP>5/7</SUP>,
  where ρ is the density and Q is the volumetric heating rate, we
  are able to approximate the well-known phenomena of chromospheric
  evaporation and chromospheric condensation, which regulate the coronal
  density. We combine this scaling law with a quasi-nonlinear MHD model
  for the resonant absorption of Alfvén waves in order to study the
  spatial and temporal dependence of the heating. We find that the heating
  is concentrated in multiple resonance layers, rather than in the single
  layer of previous models, and that these layers drift throughout the
  loop to heat the entire volume. These newfound properties are in much
  better agreement with coronal observations.

Title: A Self-Consistent Model for the Resonant Heating of Coronal
Loops: the Effects of Coupling with the Chromosphere
Authors: Klimchuk, J. A.; Ofman, L.; Davila, J. M.
Bibcode: 1997SPD....28.0504K
Altcode: 1997BAAS...29..909K
  The physical nature of coronal heating remains one of the great problems
  of solar physics. One of the several theories that are being pursued
  is the resonant absorption of MHD waves. While promising in several
  respects, this theory has suffered from a glaring deficiency: the
  computed heating is incompatible with both the assumed density and
  the observed structure of coronal loops. We present the first model
  of resonant heating of coronal loops that incorporates the dependence
  of the loop density on the heating rate. By adopting the quasi-static
  equilibrium scaling law rho ~ Q(5/7) , where rho is the density and Q is
  the volumetric heating rate, we are able to approximate the well-known
  phenomena of chromospheric evaporation and chromospheric condensation,
  which regulate the coronal density. We combine this scaling law with
  a linearized MHD model for the resonant absorption of Alfven waves
  to study the spatial and temporal dependence of the heating. We find
  that the heating is concentrated in multiple resonance layers, rather
  than the single layer of previous models, and that these layers drift
  throughout the loop to heat the entire volume. These new properties
  are in much better agreement with coronal observations, including
  recent observations from the CDS and EIT instruments on SOHO, as well
  as earlier observations from the SXT instrument on Yohkoh.

Title: A Nanoflare Explanation for the Heating of Coronal Loops
    Observed by Yohkoh
Authors: Cargill, P. J.; Klimchuk, J. A.
Bibcode: 1997ApJ...478..799C
Altcode:
  The nanoflare model of Cargill (1994a) is used to model active
  region loops observed by the Yohkoh Soft X-ray Telescope (SXT). Using
  observed information concerning the dimensions and energy-loss rate
  of each loop, a range of loop models with different temperatures,
  emission measures, and filling factors is generated. For hot loops
  (T &gt; 4 × 10<SUP>6</SUP> K), it is shown that filling factors less
  than 0.1 can fit the data, although the uncertainties can be quite
  large. For cool loops (T ~ 2 × 10<SUP>6</SUP> K), the model cannot
  reproduce the observed temperature and emission measure for any value
  of the filling factor. Earlier work of Porter &amp; Klimchuk suggested
  that some of these loops cannot be explained by a steady state heating
  model either. It is proposed that there may exist two distinct classes
  of loops and that coronal material is injected into the cooler loops
  by a mechanism that is not directly related to heating (e.g., not
  chromospheric evaporation).

Title: The Occurrence Rate of Soft X-Ray Flares as a Function of
    Solar Activity
Authors: Feldman, U.; Doschek, G. A.; Klimchuk, J. A.
Bibcode: 1997ApJ...474..511F
Altcode:
  In this paper we investigate the occurrence rate of soft X-ray
  solar flares observed by the Geostationary Operational Environmental
  Satellites (GOES). The analysis includes all flares classified as equal
  to or brighter than A1 and covers the time period from 1993 November
  to 1995 July. We find a power-law relationship between the number of
  flares per hour and peak X-ray brightness in the 1-8 Å range. The
  average power-law index for dN/dF, where N is the number of events
  per hour and F, the GOES flux, is about -1.88 +/- 0.21. A similar
  result was found from previous work based on uncollimated GOES-type
  observations concerning flares brighter than about C2. This index is
  independent of the background flux level (which is related to the solar
  activity level) to within our statistical uncertainties. We obtain the
  FWHM distribution of flare lifetimes from our sample and find that the
  distribution is independent of X-ray brightness class. We extrapolate
  the soft X-ray flare occurrence rate obtained for the Sun to other
  very active solar-like stars.

Title: STEREO: a solar terrestrial event observer mission concept
Authors: Socker, Dennis G.; Antiochos, S. K.; Brueckner, Guenter E.;
   Cook, John W.; Dere, Kenneth P.; Howard, Russell A.; Karpen, J. T.;
   Klimchuk, J. A.; Korendyke, Clarence M.; Michels, Donald J.; Moses,
   J. Daniel; Prinz, Dianne K.; Sheely, N. R.; Wu, Shi T.; Buffington,
   Andrew; Jackson, Bernard V.; Labonte, Barry; Lamy, Philippe L.;
   Rosenbauer, H.; Schwenn, Rainer; Burlaga, L.; Davila, Joseph M.; Davis,
   John M.; Goldstein, Barry; Harris, H.; Liewer, Paulett C.; Neugebauer,
   Marcia; Hildner, E.; Pizzo, Victor J.; Moulton, Norman E.; Linker,
   J. A.; Mikic, Z.
Bibcode: 1996SPIE.2804...50S
Altcode:
  A STEREO mission concept requiring only a single new spacecraft has been
  proposed. The mission would place the new spacecraft in a heliocentric
  orbit and well off the Sun- Earth line, where it can simultaneously view
  both the solar source of heliospheric disturbances and their propagation
  through the heliosphere all the way to the earth. Joint observations,
  utilizing the new spacecraft and existing solar spacecraft in earth
  orbit or L1 orbit would provide a stereographic data set. The new
  and unique aspect of this mission lies in the vantage point of the
  new spacecraft, which is far enough from Sun-Earth line to allow an
  entirely new way of studying the structure of the solar corona, the
  heliosphere and solar-terrestrial interactions. The mission science
  objectives have been selected to take maximum advantage of this new
  vantage point. They fall into two classes: those possible with the
  new spacecraft alone and those possible with joint measurements using
  the new and existing spacecraft. The instrument complement on the new
  spacecraft supporting the mission science objectives includes a soft
  x-ray imager, a coronagraph and a sun-earth imager. Telemetry rate
  appears to be the main performance determinant. The spacecraft could
  be launched with the new Med-Lite system.

Title: Magnetic Reconnection Following Coronal Mass Ejections
Authors: Klimchuk, J. A.
Bibcode: 1996AAS...188.3306K
Altcode: 1996BAAS...28..868K
  It is well known that large-scale soft X-ray arcades form and grow
  in the aftermath of coronal mass ejections. This phenomenon is
  usually interpreted as evidence for magnetic reconnection occurring
  at progressively greater heights along the vertical current sheet
  that is created when the coronal field is stretched open by the
  eruption. Closed magnetic loops are formed and heated by the
  reconnection, and they subsequently fill with hot, dense plasma
  evaporated from the chromosphere. The loops then cool and fade from
  view, but as they do, new hot loops are created above. In this way the
  arcade slowly grows, shell by shell. This picture is very appealing and
  seems to agree qualitatively with many aspects of the observations,
  but is it correct? We have studied the disk event of 1993 Jan 26 and
  found a startling inconsistency: the rate at which the arcade plasma
  is observed to be heated is two orders of magnitude smaller than the
  rate at which energy is expected to be extracted from the magnetic
  field. Is the standard interpretation fundamentally wrong? We suggest
  that it is essentially correct, but that only a small fraction ( ~ 1%)
  of the open magnetic field actually reconnects. We discuss a possible
  reason for this involving 3D effects, as well as the interesting
  consequences that it has for the detailed magnetic structure of fully
  formed coronal arcades.

Title: Broadband Imaging Spectroscopy with the Solar Radio Telescope
Authors: Bastian, T. S.; Gary, D. E.; Hurford, G. J.; Hudson, H. S.;
   Klimchuk, J. A.; Petrosian, V.; White, S. M.
Bibcode: 1996ASPC...93..430B
Altcode: 1996ress.conf..430B
  No abstract at ADS

Title: The Heating of Soft X-ray Coronal Loops
Authors: Klimchuk, J. A.; Porter, L. J.
Bibcode: 1996mpsa.conf...39K
Altcode: 1996IAUCo.153...39K
  No abstract at ADS

Title: Post-Eruption Arcades and 3-D Magnetic Reconnection (Invited)
Authors: Klimchuk, James A.
Bibcode: 1996ASPC..111..319K
Altcode: 1997ASPC..111..319K
  Soft X-ray emitting arcades are often observed to form and grow in
  the aftermath of coronal eruptions. They are generally interpreted
  as evidence for magnetic reconnection occurring at progressively
  greater heights along a vertical current sheet that is created by the
  eruption. This "standard model" explains many of the morphological
  aspects of the observations very well. The author has examined the
  energetics of an event observed by Yohkoh and finds that the rate
  of magnetic energy conversion expected from the model exceeds the
  observed rate of arcade plasma heating by perhaps as much as a factor
  of 100. He takes this as evidence that reconnection is being inhibited,
  and suggests that the cause may be three-dimensional effects involving
  the interlinking of field lines.

Title: Soft X-Ray Loops and Coronal Heating
Authors: Porter, Lisa J.; Klimchuk, James A.
Bibcode: 1995ApJ...454..499P
Altcode:
  We have measured the temperatures, pressures, and lengths of 47
  nonflaring coronal loops observed by the Soft X-Ray Telescope on the
  Yohkoh satellite. The median temperature is 5.7 x 10<SUP>6</SUP> K,
  and the median pressure is 1.6 x 10<SUP>16</SUP> cm<SUP>-3</SUP> K. We
  have carefully examined the possible random and systematic errors in
  the measurements and have found, through simulated observations, that
  the errors produced by photon statistical noise can sometimes greatly
  exceed the values given by commonly used error expressions, derived
  here and elsewhere. Furthermore, the measurements are not normally
  distributed and therefore are not amenable to standard statistical
  analysis. We have used nonparametric methods to look for statistical
  relationships and find that temperature and length are uncorrelated
  and that pressure varies inversely with length to approximately the
  first power. <P />The observed lifetimes of the loops are much longer
  than their computed cooling times, suggesting that the loops are in a
  state of quasi-static equilibrium. This has allowed us to use simple
  scaling law theory to infer that the volumetric heating rate in the
  loops (averaged along the loop axis) varies inversely with length to
  approximately the second power. This is an important constraint for
  distinguishing among competing theories of coronal heating, and we
  discuss the results in the context of three specific models.

Title: Solar Rotation Stereoscopy in Microwaves
Authors: Aschwanden, Markus J.; Lim, Jeremy; Gary, Dale E.; Klimchuk,
   James A.
Bibcode: 1995ApJ...454..512A
Altcode:
  We present here the first stereoscopic altitude measurements of active
  region sources observed at microwave frequencies (10-14 GHz The active
  region NOAA 7128 was observed with the Owens Valley Radio Observatory
  (OVRO) on 1992 April 13, 14, 15, and 16 as it passed through the central
  meridian. From white-light data of the underlying sunspot we determined
  the rotation rate of the active region, which was found to have a
  relative motion of dL/dt = +0°.240 day<SUP>-1</SUP> with respect to the
  standard photospheric differential rotation rate. Based on this rotation
  rate we determine for the microwave sources stereoscopic altitudes of
  3.3-11.0 Mm above the photosphere. The altitude spectrum h(v) of the
  right circular polarization (RCP) main source shows a discontinuity
  at 12 GHz and can be satisfactorily fitted with a dipole model with a
  transition from the second to the third harmonic level at 12 GHz. The
  dominance of the third harmonic for frequencies above 12 GHz occurs
  because the second harmonic level drops below the transition region, at
  a height of 2.6±0.6 Mm according to the microwave data. The altitude
  spectrum h(v) serves also to invert the temperature profile T(h)
  from the optically thick parts of the radio brightness temperature
  spectrum T<SUB>B</SUB>(ν[h]). The microwave emission in both circular
  polarizations can be modeled with gyroresonance emission, with x-mode
  for RCP and o-mode in LCP, with the same harmonics at each frequency,
  but different emission angles in both modes. The contributions from
  free-free emission are negligible in both polarizations, based on the
  peak emission measure of EM ≍ 6 × 10<SUP>28</SUP> cm<SUP>-5</SUP>
  observed in soft X-rays by Yohkoh/SXT. <P />This study demonstrates
  that the height dependence of the coronal magnetic field B(h) and
  the plasma temperature T(h) in an active region can be inverted from
  the stereoscopic altitude spectra h(v) and the observed brightness
  temperature spectra T<SUB>B</SUB>(ν).

Title: Scaling of heating rates in solar coronal loops
Authors: Klimchuk, James A.; Porter, Lisa J.
Bibcode: 1995Natur.377..131K
Altcode:
  THE gas of the solar corona is at a temperature of several million
  degrees, orders of magnitude hotter than the underlying photosphere. The
  nature of the physical process that heats the solar corona (and the
  coronae of solar-type stars more generally) has been a long-standing
  puzzle. A number of plausible heating mechanisms have been proposed,
  but observations have so far been unable to discriminate between
  them<SUP>1</SUP>. Here we show that coronal heating exhibits scaling
  properties that should provide a powerful diagnostic of the underlying
  mechanism. The coronal magnetic field organizes the coronal plasma
  into loop-like features, which form the basic structural elements of
  the corona<SUP>2</SUP>. We demonstrate that the pressures and lengths
  of the coronal loops are statistically related, suggesting that the
  heating rate scales inversely with approximately the square of the loop
  length. Existing coronal heating theories make different predictions
  about what this scaling should be, and a model<SUP>3-4</SUP> of energy
  dissipation by stressed coronal magnetic fields appears at present to
  be the most consistent with our observational result.

Title: A Comparison of Active Region Temperatures and Emission
    Measures Observed in Soft X-Rays and Microwaves and Implications
    for Coronal Heating
Authors: Klimchuk, J. A.; Gary, D. E.
Bibcode: 1995ApJ...448..925K
Altcode:
  We have determined active region temperatures and emission measures
  using both broad-band soft X-ray images from the Yohkoh satellite and
  spatially and spectrally resolved microwave data from the Owens Valley
  Radio Observatory (OVRO). This work differs from previous work in that
  the microwave temperatures and emission measures are directly measured
  from the microwave spectrum, and are not model-dependent. The soft
  X-ray temperatures and emission measures are ≍2.5 times greater
  than the corresponding microwave values, on average. Detailed
  error analysis indicates that the temperature differences are real,
  but that the emission measure differences may not be. <P />We have
  simulated Yohkoh and OVRO observations of idealized plane-parallel and
  nested-loop coronal models. The plane-parallel model reproduces the
  observed temperature differences if the coronal temperature decreases
  exponentially with height from a maximum value of 4 × 10<SUP>6</SUP>
  K at the base to an asymptotic value of ≍10<SUP>6</SUP> K. The
  nested-loop model, which assumes quasi-static loop equilibrium, also
  reproduces the observed temperature differences and indicates that the
  volumetric coronal heating rate varies inversely with loop length to a
  power greater than 2. Both models predict microwave emission measures
  that are larger than observed. We suggest that a more complex model
  is required to explain the observed emission measures and that more
  than one coronal heating mechanism may be operative in solar active
  regions. <P />We present derivations of the temperature and emission
  measure uncertainties that result from random and systematic errors in
  the Yohkoh observations. The expression for the random error emission
  measure uncertainty is different from that used previously and is
  especially important for observations of nonflaring plasmas.

Title: The Magnetic Field of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.; Dahlburg, R. B.
Bibcode: 1995SPD....26..717A
Altcode: 1995BAAS...27..969A
  No abstract at ADS

Title: A Solar Radio Telescope for the Future: Science Summary from
    the SRT Workshop
Authors: Gary, D. E.; Bastian, T. S.; Hudson, H. S.; Hurford, G. J.;
   Klimchuk, J. A.; Petrosian, V.; White, S. M.
Bibcode: 1995SPD....26..801G
Altcode: 1995BAAS...27..971G
  No abstract at ADS

Title: The Cross Sectional Properties of Coronal Loops
Authors: Klimchuk, J. A.
Bibcode: 1995SPD....26..705K
Altcode: 1995BAAS...27Q.966K
  No abstract at ADS

Title: A Solar Radio Telescope for the Future: Strawman Concept from
    the SRT Workshop
Authors: Hurford, G. J.; Bastian, T. S.; Gary, D. E.; Hudson, H. S.;
   Klimchuk, J. A.; Petrosian, V.; White, S. M.
Bibcode: 1995SPD....26..802H
Altcode: 1995BAAS...27..971H
  No abstract at ADS

Title: The Possible Role of MHD Waves in Heating the Solar Corona
Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A.
Bibcode: 1994ApJ...435..482P
Altcode:
  The possible role of waves in the heating of the solar corona has
  been investigated. A general dispersion relation has been derived for
  waves propagating in a homogeneous plasma subject to dissipation by
  viscosity and thermal conduction. The dissipation mechanisms have been
  incorporated self-consistently into the equations, and no assumptions
  about the strength of the damping have been made. Solutions of the
  sixth-order dispersion relation provide information on how the damping
  of both slow and fast mode waves depends upon the plasma density,
  temperature, field strength, and angle of propagation relative to the
  background magnetic field. We provide a detailed comparison to the
  standard approach, which is to solve for the wave quantities in the
  absence of dissipation and then to use these quantities in expressions
  for the heating due to viscosity and thermal conduction.

Title: The Possible Role of High-Frequency Waves in Heating Solar
    Coronal Loops
Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A.
Bibcode: 1994ApJ...435..502P
Altcode:
  We investigate the role of high-frequency waves in the heating of
  solar active region coronal loops. We assume a uniform background
  magnetic field, and we introduce a density stratification in a
  direction perpendicular to this field. We focus on ion compressive
  viscosity as the damping mechanism of the waves. We incorporate
  viscosity self-consistently into the equations, and we derive a
  dispersion relation by adopting a slab model, where the density inside
  the slab is greater than that outside. Such a configuration supports
  two types of modes: surface waves and trapped body waves. In order
  to determine under what conditions these waves may contribute to the
  heating of active regions, we solve our dispersion relation for a
  range of densities, temperatures, magnetic field strengths, density
  ratios, wavevector magnitudes, wavevector ratios, and slab widths. We
  find that surface waves exhibit very small damping, but body waves can
  potentially damp at rates needed to balance radiative losses. However,
  the required frequencies of these body waves are very high. For
  example, the wave frequency must be at least 5.0/s for a slab density
  of 10<SUP>9,5</SUP>/cc, a slab temperature of 10<SUP>6,5</SUP> K, a
  field strength of 100 G, and a density ratio of 5. For a slab density
  of 10<SUP>10</SUP>/cc, this frequency increases to 8.8/s. Although
  these frequencies are very high, there in no observational evidence
  to rule out their existence, and they may be generated both below the
  corona and at magnetic reconnection sites in the corona. However, we
  do find that, for slab densities of 10<SUP>10</SUP>/cc or less, the
  dissipation of high-frequency waves will be insufficient to balance
  the radiative losses if the magnetic field strength exceeds roughly
  200 G. Because the magnetic field is known to exceed 200 G in many
  active region loops, particularly low-lying loops and loops emanating
  from sunspots, it is unlikely that high-frequency waves can provide
  sufficient heating in these regions.

Title: Asymptotic Forms for the Energy of Force-free Magnetic Field
    Configurations of Translational Symmetry
Authors: Sturrock, P. A.; Antiochos, S. K.; Klimchuk, J. A.;
   Roumeliotis, G.
Bibcode: 1994ApJ...431..870S
Altcode:
  It is known from computer calculations that if a force-free
  magnetic field configuration is stressed progressively by footpoint
  displacements, the configuration expands and approaches the open
  configuration with the same surface flux distribution and the
  energy of the field increases progressively. For configurations of
  translational symmetry, it has been found empirically that the energy
  tends asymptotically to a certain functional form. It is here shown
  that analysis of a simple model of the asymptotic form of force-free
  fields of translational symmetry leads to and therefore justifies
  this functional form. According to this model, the field evolves in
  a well-behaved manner with no indication of instability or loss of
  equilibrium.

Title: The Magnetic Field of Solar Prominences
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A.
Bibcode: 1994ApJ...420L..41A
Altcode:
  A model is presented which accounts for the formation of coronal
  magnetic field lines with the appropriate 'dipped' structure to
  support prominences. The critical ingredients of the model are that the
  prominence magnetic field is a truly three-dimensional structure with
  significant variation along the prominence length, and the magnetic
  field is strongly sheared near the photospheric neutral line. Numerical
  calculations are presented which demonstrate that these two features
  lead to dip formation. In addition our model is able to account for the
  long-puzzling observation of inverse polarity in quiescent prominences.

Title: The Asymptotic Behavior of Force-Free Magnetic-Field
    Configurations
Authors: Sturrock, P. A.; Klimchuk, J. A.; Roumeliotis, G.; Antiochos,
   S. K.
Bibcode: 1994ASPC...68..219S
Altcode: 1994sare.conf..219S
  No abstract at ADS

Title: Coronal Eruptions Observed by YOHKOH
Authors: Klimchuk, J. A.; Acton, L. W.; Harvey, K. L.; Hudson, H. S.;
   Kluge, K. L.; Sime, D. G.; Strong, K. T.; Watanabe, Ta.
Bibcode: 1994xspy.conf..181K
Altcode:
  No abstract at ADS

Title: Interplanetary Consequences of Transient Coronal Events
Authors: Watanabe, Ta.; Kojima, M.; Kozuka, Y.; Tsuneta, S.; Lemen,
   J. R.; Hudson, H.; Joselyn, J. A.; Klimchuk, J. A.
Bibcode: 1994xspy.conf..207W
Altcode:
  No abstract at ADS

Title: Photospheric Magnetic Field Measurement Errors and the Inferred
    Properties of Coronal Magnetic Fields
Authors: Klimchuk, James A.; Canfield, Richard C.
Bibcode: 1994ASPC...68..233K
Altcode: 1994sare.conf..233K
  No abstract at ADS

Title: Eruptive-Prominence Related Coronal Disturbances Observed
    with YOHKOH SXT
Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi,
   K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton,
   L. W.; Klimchuk, J. A.
Bibcode: 1994step.conf...85W
Altcode:
  No abstract at ADS

Title: Coronal/Interplanetary Disturbances Associated with a Solar
    Filament Disappearance on September 28, 1991
Authors: Watanabe, T.; Kozuka, Y.; Ohyama, M.; Kojima, M.; Yamaguchi,
   K.; Watari, S.; Tsuneta, S.; Joselyn, J. A.; Harvey, K. L.; Acton,
   L. W.; Klimchuk, J. A.
Bibcode: 1994step.conf...89W
Altcode:
  No abstract at ADS

Title: Collisional Damping of Magnetoacoustic Waves in the Solar
    Corona
Authors: Porter, Lisa A.; Sturrock, Peter A.; Klimchuk, James A.
Bibcode: 1993BAAS...25.1203P
Altcode:
  No abstract at ADS

Title: The Structure of Prominence Magnetic Fields
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J. A.
Bibcode: 1993BAAS...25.1206A
Altcode:
  No abstract at ADS

Title: Comparison of Coronal Temperatures and Emission Measures
    Determined from X-Ray and Microwave Observations
Authors: Klimchuk, J. A.; Gary, D. E.
Bibcode: 1993BAAS...25.1179K
Altcode:
  No abstract at ADS

Title: Static and dynamic loop models and their observational
    signatures.
Authors: Klimchuk, James A.
Bibcode: 1992ESASP.348..167K
Altcode: 1992cscl.work..167K
  The magnetically-closed regions of the outer solar atmosphere can be
  studied in terms of one-dimensional hydrodynamic loop models. These
  regions include the bright plasma loops that are readily visible in
  EUV and X-ray images as well as the fainter, more diffuse-appearing
  plasma that surrounds them. The author reviews the basic theoretical
  properties of static, steady-state, and time-dependent loop models,
  and relates these properties to observations of emission measure
  distributions and Doppler shifts. Both existing observations and future
  observations from the Solar and Heliospheric Observatory (SOHO) are
  considered. In particular, the dependence of the models on the form
  of the energy input is emphasized.

Title: Thickness Variations along Coronal Loops Observed by the Soft
    X-Ray Telescope on YOHKOH
Authors: Klimchuk, James A.; Lemen, James R.; Feldman, Uri; Tsuneta,
   Saku; Uchida, Yutaka
Bibcode: 1992PASJ...44L.181K
Altcode:
  It has been suggested that observed coronal loops have constant
  thicknesses. However, if plasma loops coincide with magnetic loops,
  then we might expect many loops to be significantly broader at their
  tops than at their footpoints (since, on average, magnetic fields
  must diverge with height in the solar corona). It is important to
  understand how the thicknesses of loops vary along their lengths, since
  such a variation is related to the distribution of electric currents
  in the corona and is therefore relevant to solar flares and coronal
  heating. We here present preliminary results of our investigation of
  thickness variations along coronal loops observed with the Soft X-ray
  Telescope (SXT) on board the Yohkoh satellite.

Title: Coronal/Interplanetary Disturbances Associated with
    Disappearing Solar Filaments
Authors: Watanabe, Takashi; Kozuka, Yukio; Ohyama, Masamitsu; Kojima,
   Masayoshi; Yamaguchi, Kisuke; Watari, Shin-Ichi; Tsuneta, Saku;
   Joselyn, Jo A.; Harvey, Karen L.; Acton, Loren W.; Klimchuk, James. A.
Bibcode: 1992PASJ...44L.199W
Altcode:
  We discuss two examples of coronal/interplanetary disturbances
  associated with the disappearance of a 35(deg) long quiescent filament
  occurring near the solar disk center on 1991 September 28 (McAllister
  et al. 1992, Publ. Astron. Soc. Japan, 44, L205) and with a 25(deg)
  long eruptive prominence at the eastern solar limb taking place on
  1991 November 7. Bright soft X-ray arcades were observed for both
  cases with the Yohkoh SXT, about 2--3 hr after the onset of each
  Hα event. For the erupting prominence on November 7, the arcade
  did not appear before the prominence reached a height of about 0.3
  solar radii above the limb. This suggests that magnetic reconnection
  occurred below the relevant Hα structures. A transient coronal hole
  was formed in the immediate vicinity of the disappearing filament on
  September 28. Formation of the new coronal hole is suggested to be a
  cause of the filament disappearance. An interplanetary disturbance was
  detected by radio scintillation (IPS) observations immediately after
  the filament disappeared.

Title: A Model for the Magnetic Fields of Solar Prominences
Authors: Antiochos, S. K.; Dahlburg, R. B.; Klimchuk, J.
Bibcode: 1992AAS...180.1205A
Altcode: 1992BAAS...24..748A
  No abstract at ADS

Title: Thickness Variations Along Coronal Loops Observed by Yohkoh
Authors: Klimchuk, J. A.; Kluge, K.; Lemen, J. R.; Feldman, U.;
   Uchida, Y.
Bibcode: 1992AAS...180.2304K
Altcode: 1992BAAS...24Q.760K
  No abstract at ADS

Title: Cylindrically Symmetric Force-free Magnetic Fields
Authors: Porter, Lisa J.; Klimchuk, James A.; Sturrock, Peter A.
Bibcode: 1992ApJ...385..738P
Altcode:
  The magnetofrictional method was used to study the energy buildup
  in stressed coronal fields possessing cylindrical symmetry. Four
  different nonlinear, force-free magnetic-field configurations were
  examined. It was determined that, in all cases, a reasonable amount
  of twist in the field lines can produce enough free magnetic energy
  to power a typical flare. Furthermore, it was found that the rate of
  energy buildup is enhanced if the greatest twist and/or the magnetic
  flux is concentrated closer to the neutral line. It is thought that
  the open-field configuration (a configuration for which the field
  lines extend to infinity and the current is confined to a current
  sheet separating outgoing and incoming field lines) is the limiting
  state as one imposes infinite shear. The results of this work do not
  contradict this theory once numerical errors are taken into account.

Title: The Practical Application of the Magnetic Virial Theorem
Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E.
Bibcode: 1992ApJ...385..327K
Altcode:
  The magnetic energy of solar active regions is computed via the
  application of the magnetic virial theorem together with vector
  magnetograph data. In order to investigate how errors in the vector
  magnetograph measurements produce errors in the virial theorem
  energy, the effects of realistic errors on known magnetic fields are
  simulated numerically. These include errors due to random polarization
  noise, crosstalk between different polarization signals, systematic
  polarization bias, and seeing-induced crosstalk. Analytical expressions
  for the energy errors which apply under certain idealized conditions are
  derived. The results are useful for evaluating the ability of vector
  magnetographs to provide suitable data for the accurate determination
  of magnetic energies using the virial theorem.

Title: Three-dimensional Force-free Magnetic Fields and Flare
    Energy Buildup
Authors: Klimchuk, J. A.; Sturrock, P. A.
Bibcode: 1992ApJ...385..344K
Altcode:
  The 'magneto-frictional' method is used to compute fully 3D models
  of force-free magnetic fields. Beginning with a potential field
  produced by a point dipole buried below the solar surface, the magnetic
  footpoints at the photosphere are displaced to investigate the buildup
  of magnetic energy. Reasonable footpoint shearing displacements are
  found to be able to increase the total magnetic energy by at least
  one-third. The energy buildup is greater when the shearing displacements
  are concentrated closer to the magnetic neutral line. Roughly half
  of the energy buildup is free magnetic energy. The absolute quantity
  of free magnetic energy (10 exp 30-33 ergs, depending on the scaling
  of the models) is sufficient to explain solar flares. No evidence for
  'loss of equilibrium' was found.

Title: A Model for the Formation of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.
Bibcode: 1991ApJ...378..372A
Altcode:
  A model for the formation of prominence condensations in hot coronal
  loops is proposed. Previous studies have concentrated on cooling the
  hot plasma by decreasing the coronal heating rate. The difficulty
  with such models is that when the heating decreases, most of the
  loop mass is lost by draining onto the chromosphere. It is argued
  that a prominence condensation is likely to be due to an increase in
  the heating. The key idea of the model is that the heating increase is
  spatially dependent so that it is localized nearer to the chromospheric
  footpoints than to the loop midpoint. Results are presented of
  numerical simulations of hot loops that are initially heated uniformly,
  and then undergo heating increases that are concentreated away from
  the loop midpoint. The temperature at the midpoint first increases,
  but eventually it collapses to chromospheric values as a result of
  chromospheric evaporation. Hence, a curious result is obtained, that
  increasing the heating causes cooling. The resulting densities and
  time scales agree well with observations. The implications of this
  model for coronal heating and prominence structure are discussed.

Title: The Practical Application of the Magnetic Virial Therom:
    Analytical Results
Authors: Rhoads, J. E.; Klimchuk, J. A.; Canfield, R. C.
Bibcode: 1991BAAS...23.1055R
Altcode:
  No abstract at ADS

Title: The Practical Application of the Magnetic Virial Theorm:
    Simulated Magnetograph Observations
Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E.
Bibcode: 1991BAAS...23.1031K
Altcode:
  No abstract at ADS

Title: Simulated SXT Observations of Coronal Loops
Authors: Dixon, W. W.; Klimchuk, J. A.; Sturrock, P. A.; Lemen, J. R.
Bibcode: 1991LNP...387..297D
Altcode: 1991fpsa.conf..297D
  We have simulated the appearance of two static coronal loops as they
  might be observed by the Soft X-ray Telescope onboard the Solar-A
  spacecraft. One loop corresponds to a non-flaring active region loop,
  and the other corresponds to a post-flare loop. We find that the
  loops have fundamentally different appearances: the quiescent loop is
  brightest at its apex, while the hotter post-flare loop is brightest
  at, or near, its base (depending on the particular X-ray filter assumed
  for the observations).

Title: The Practical Application of the Magnetic Virial Theorem
Authors: Klimchuk, J. A.; Canfield, R. C.; Rhoads, J. E.
Bibcode: 1991LNP...387..219K
Altcode: 1991fpsa.conf..219K
  We have performed simulated vector magnetograph observations to study
  the effects of random and systematic magnetic field measurement errors
  on the magnetic energies that may be estimated using the virial theorem.

Title: Episodic Coronal Heating
Authors: Sturrock, P. A.; Dixon, W. W.; Klimchuk, J. A.; Antiochos,
   S. K.
Bibcode: 1990ApJ...356L..31S
Altcode:
  A study is made of the observational consequences of the hypothesis
  that there is no steady coronal heating, the solar corona instead
  being heated episodically, such that each short burst of heating
  is followed by a long period of radiative cooling. The form of the
  resulting contribution to the differential emission measure (DEM), and
  to a convenient related function (the differential energy flux, DEF) is
  calculated. Observational data for the quiet solar atmosphere indicate
  that the upper branch of the DEM, corresponding to temperatures above
  100,000 K, can be interpreted in terms of episodic energy injection
  at coronal temperatures.

Title: Shear-induced Inflation of Coronal Magnetic Fields
Authors: Klimchuk, James A.
Bibcode: 1990ApJ...354..745K
Altcode:
  Using numerical models of force-free magnetic fields, the shearing
  of footprints in arcade geometries leading to an inflation of the
  coronal magnetic field was examined. For each of the shear profiles
  considered, all of the field lines become elevated compared with the
  potential field. This includes cases where the shear is concentrated
  well away from the arcade axis, such that B<SUB>z</SUB>, the component
  of field parallel to the axis, increases outward to produce an inward
  B<SUB>z</SUB> squared/8 pi magnetic pressure gradient force. These
  results contrast with an earlier claim, shown to be incorrect, that
  field lines can sometimes become depressed as a result of shear. It is
  conjectured that an inflation of the entire field will always result
  from the shearing of simple arcade configurations. These results have
  implications for prominence formation, the interplanetary magnetic flux,
  and possibly also coronal holes.

Title: Flare Energy Buildup and the Stressing of 3-D Coronal
    Magnetic Fields
Authors: Klimchuk, J. A.; Sturrock, P. A.
Bibcode: 1990BAAS...22..900K
Altcode:
  No abstract at ADS

Title: Cylindrically-Symmetric Force-Free Magnetic Fields
Authors: Porter, L. J.; Klimchuk, J. A.; Sturrock, P. A.
Bibcode: 1990BAAS...22..853P
Altcode:
  No abstract at ADS

Title: Force-free Magnetic Fields: Is There a ``Loss of Equilibrium''?
Authors: Klimchuk, J. A.; Sturrock, P. A.
Bibcode: 1989ApJ...345.1034K
Altcode:
  This paper examines concept in solar physics that is known as loss of
  equilibrium in which a sequence of force-free magnetic fields, said to
  represent a possible quasi-static evolution of solar magnetic fields,
  reaches a critical configuration beyond which no acceptable solution
  of the prescribed form exists. This concept is used to explain eruptive
  phenomena ranging from solar flares to coronal mass ejections. Certain
  sequences of force-free configurations are discussed that exhibit
  a loss of equilibrium, and it is argued that the concept is devoid
  of physical significance since each sequence is defined a way that
  does not represent an acceptable thought experiment. For example, the
  sequence may be defined in terms of a global constraint on the boundary
  conditions, or the evolution of the sequence may require the creation
  of mgnetic flux that is not connected to the photosphere and is not
  present in the original configuration. The global constraints typically
  occur in using the so-called generating function method. An acceptance
  thought experiment is proposed to specify the field configuration
  in terms of photospheric boundary conditions comprising the normal
  component of the field and the field-line connectivity. Consider a
  magnetic-field sequence that, when described in terms of a generating
  function, exhibits a loss of equilibrium and show that, when one instead
  defines the sequence in terms of the corresponding boundary conditions,
  the sequence is well behaved.

Title: Episodic Coronal Heating and the Solar Differential Emission
    Measure
Authors: Sturrock, P. A.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 1989BAAS...21R1186S
Altcode:
  No abstract at ADS

Title: The Formation of Solar Prominences
Authors: Antiochos, S. K.; Klimchuk, J. A.
Bibcode: 1989BAAS...21.1185A
Altcode:
  No abstract at ADS

Title: Shear-induced inflation of coronal magnetic fields
Authors: Klimchuk, James A.
Bibcode: 1989STIN...9014178K
Altcode:
  Using numerical models of force-free magnetic fields, the shearing
  of footprints in arcade geometries leading to an inflation of the
  coronal magnetic field was examined. For each of the shear profiles
  considered, all of the field lines become elevated compared with the
  potential field. This includes cases where the shear is concentrated
  well away from the arcade axis, such that B<SUB>z</SUB>, the component
  of field parallel to the axis, increases outward to produce an inward
  B<SUB>z</SUB>squared/8 pi magnetic pressure gradient force. These
  results contrast with an earlier claim, shown to be incorrect, that
  field lines can sometimes become depressed as a result of shear. It is
  conjectured that an inflation of the entire field will always result
  from the shearing of simple arcade configurations. These results have
  implications for prominence formation, the interplanetary magnetic flux,
  and possibly also coronal holes.

Title: Shear-Induced Inflation of Coronal Magnetic Fields
Authors: Klimchuk, J. A.
Bibcode: 1989BAAS...21..864K
Altcode:
  No abstract at ADS

Title: Magnetic properties of  Civ Doppler shift patterns
Authors: Klimchuk, James A.
Bibcode: 1989SoPh..119...19K
Altcode:
  The relationship between Doppler shift patterns observed in the
  transition region and magnetic field patterns observed in the
  photosphere is studied using coaligned pairs of CIV Dopplergrams
  and FeI magnetograms. Categories of magnetic features are defined -
  including neutral lines, unipolar regions, strong field regions, weak
  field regions, and magnetic boundaries - and from these, magnetic
  associations are determined for 159 V<SUB>0</SUB> lines separating
  areas of relative blueshift and redshift observed in and around active
  regions. The cases are subdivided on the basis of whether blueshifts
  or redshifts are observed on the side of the V<SUB>0</SUB> line nearest
  the solar limb.

Title: Force-Free Magnetic Fields: Is there a "Loss of Equilibrium?"
Authors: Klimchuk, J. A.; Sturrock, P. A.
Bibcode: 1989BAAS...21R.855K
Altcode:
  No abstract at ADS

Title: Largescale Magnetic Field Phenomena
Authors: Harrison, R. A.; Bentley, R. D.; Brosius, J.; Dwivedi,
   B. N.; Jardine, M.; Klimchuk, J. A.; Kundu, M. R.; Pearce, G.; Saba,
   J.; Sakurai, T.; Schmahl, E. J.; Schmelz, J.; Sime, D. G.; Steele,
   C. D. C.; Sun, M. T.; Tappin, S. J.; Waljeski, K.; Wang, A. H.; Wu,
   S. T.
Bibcode: 1989tnti.conf....1H
Altcode:
  No abstract at ADS

Title: Coronal Magnetic Fields Produced by Photospheric Shear
Authors: Klimchuk, J. A.; Sturrock, P. A.; Yang, W. -H.
Bibcode: 1988ApJ...335..456K
Altcode:
  The magnetofrictional method for computing force-free fields is used
  to examine the evolution of the magnetic field of a line dipole,
  when there is relative shearing motion between the two polarities. It
  is found that the energy of the sheared field can be arbitrarily
  large compared with the potential field. It is also found that it is
  possible to fit the magnetic energy, as a function of shear amplitude,
  by a simple functional form. The fit parameters depend only on the
  distribution of normal field in the photosphere and the form of the
  shearing displacement. They show that the energy is relatively more
  enhanced if the shear occurs: (1) where the normal field is strongest;
  and/or (2) in the inner region of the dipole, near the axis; and/or
  (3) over a large fraction of the dipole area.

Title: Force-free magnetic fields: Is there a loss of equilibrium
Authors: Klimchuk, James A.; Sturrock, Peter A.
Bibcode: 1988STIN...8921717K
Altcode:
  This paper examines concept in solar physics that is known as loss of
  equilibrium in which a sequence of force-free magnetic fields, said to
  represent a possible quasi-static evolution of solar magnetic fields,
  reaches a critical configuration beyond which no acceptable solution
  of the prescribed form exists. This concept is used to explain eruptive
  phenomena ranging from solar flares to coronal mass ejections. Certain
  sequences of force-free configurations are discussed that exhibit
  a loss of equilibrium, and it is argued that the concept is devoid
  of physical significance since each sequence is defined a way that
  does not represent an acceptable thought experiment. For example, the
  sequence may be defined in terms of a global constraint on the boundary
  conditions, or the evolution of the sequence may require the creation
  of magnetic flux that is not connected to the photosphere and is not
  present in the original configuration. The global constraints typically
  occur in using the so-called generating function method. An acceptable
  thought experiment is proposed to specify the field configuration
  in terms of photospheric boundary conditions comprising the normal
  component of the field and the field-line connectivity. Consider a
  magnetic-field sequence that, when described in terms of a generating
  function, exhibits a loss of equilibrium and show that, when one instead
  defines the sequence in terms of the corresponding boundary conditions,
  the sequence is well behaved.

Title: Heating-related Flows in Cool Solar Loops
Authors: Klimchuk, J. A.; Mariska, J. T.
Bibcode: 1988ApJ...328..334K
Altcode:
  The authors have investigated the effects of spatial and temporal
  variations in the heating of cool loop models in an attempt
  to explain the net redshifts that are observed on the Sun. In
  none of the situations considered are the induced flows able to
  satisfactorily reproduce the observations. In the case of asymmetric
  heating, the end-to-end flows can be as fast as 20 km s<SUP>-1</SUP>,
  but the downflowing leg is neither appreciably faster nor appreciably
  brighter than the upflowing leg; no net redshift is produced. In the
  case of symmetric heating, the downflows can also be large, but they are
  restricted to temperatures that are well below 10<SUP>5</SUP>K. Neither
  situation would give rise to the ⪆7 km s<SUP>-1</SUP> disk-averaged
  redshifts seen in emission lines of species like C IV.

Title: Coronal magnetic fields produced by photospheric shear
Authors: Klimchuk, James A.; Sturrock, Peter A.; Yang, Wei-Hong
Bibcode: 1988cmfp.book.....K
Altcode:
  The magneto-frictional method for computing force-free fields examines
  the evolution of the magnetic field of a line dipole, when there is
  relative shearing motion between the two polarities. The energy of the
  sheared field can be arbitrarily large compared with the potential
  field. It is possible to fit the magnetic energy, as a function
  of shear amplitude, by a simple functional form. The fit parameters
  depend only on the distribution of normal field in the photosphere and
  the form of the shearing displacement. The energy is relatively more
  enhanced if the shear occurs: (1) where the normal field is strongest;
  (2) in the inner region of the dipole, near the axis; or (3) over a
  large fraction of the dipole area.

Title: Magnetic properties of C 4 Doppler shift patterns
Authors: Klimchuk, James A.
Bibcode: 1988STIN...8915062K
Altcode:
  The relationship between Doppler shift patterns observed in the
  transition region and magnetic field patterns observed in the
  photosphere is studied using coaligned pairs C IV Dopplergrams and Fe
  I magnetograms. Categories of magnetic features are defined--including
  neutral lines, unipolar regions, strong field regions, weak field
  regions, and magnetic boundaries--and from these, magnetic associations
  are determined for 159 V0 lines separating areas of relative blueshift
  and redshift observed in and nearby to active regions. The cases
  are subdivided on the basis of whether blueshifts or redshifts are
  observed on the side of the V0 line nearest the limb. Two of the main
  results are that V0 lines associated with neutral lines tend to have
  limbward blueshifts, while V0 lines associated with unipolar regions
  tend to have limbward redshifts. These and other results provide
  supportive evidence for the active region model proposed recently by
  Klimchuk, in which relative redshifts occur where strong vertical
  fields penetrate the surface, and relative blueshifts occur where
  these same fields have spread out to become horizontal. It is likely
  that the relative blueshifts correspond to absolute Doppler shifts of
  very small amplitude, possibly even redshifts.

Title: Coronal Magnetic Fields Produced by Photospheric Shear
Authors: Klimchuk, J. A.; Sturrock, P. A.; Yang, W. -H.
Bibcode: 1988BAAS...20..716K
Altcode:
  No abstract at ADS

Title: On the Large-Scale Dynamics and Magnetic Structure of Solar
    Active Regions
Authors: Klimchuk, James A.
Bibcode: 1987ApJ...323..368K
Altcode:
  The author has studied sets of carefully coaligned C IV Dopplergrams,
  photospheric magnetograms, and Hα filtergrams to infer the flow
  properties of active regions and the relationship of these flows
  to the active region magnetic fields. The combined data show that
  active regions can be naturally divided into three basic parts:
  strong field regions, weak field corridors between strong fields of
  opposite polarity, and surrounding weak field areas. An idealized
  topological model in which the vertical fields of strong field regions
  diverge very rapidly with height to become essentially horizontal in
  the adjacent low-lying areas is proposed. The picture is similar to
  canopy structures.

Title: A Numerical Study of the Nonlinear Thermal Stability of
    Solar Loops
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
Bibcode: 1987ApJ...320..409K
Altcode:
  A time-dependent numerical model is used to investigate the nonlinear
  thermal stability of static loops of various heights. Simulations show
  that the instability of a hot state with loop heights of less than about
  1000 km is physically significant, with an initially hot atmosphere
  in low-lying compact loops evolving to an extended atmosphere with
  temperatures far below 100,000 K. Results also show that high-lying
  loops are stable to all reasonable perturbations, including those of
  large initial amplitude and long wavelength. The simulation results
  suggest that low-lying compact loops should not be common to the sun,
  and that cool loops with temperatures near 100,000 K must be formed
  in the cool state initially and cannot evolve from preexisiting loops.

Title: A numerical study of the thermal stability of solar loops.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
Bibcode: 1987NASCP2483..113K
Altcode: 1987tphr.conf..113K
  An important property of all loops is their thermal stability. If low
  lying hot loops were thermally unstable, for example, a great majority
  of the low loops on the Sun might be expected to be cool. How small
  perturbations evolve in low lying, linearly unstable hot loops was
  determined and how high lying, linearly stable hot loops respond to
  large amplitude disturbances such as might be expected on the Sun were
  examined. Only general descriptions and results are given.

Title: The Magnetic and Velocity Structure Adjacent to Solar Active
    Regions
Authors: Athay, R. Grant; Klimchuk, J. A.
Bibcode: 1987ApJ...318..437A
Altcode:
  Results from a number of earlier papers relating velocity patterns
  observed in the C IV line at 154.8 nm to photospheric magnetic-field
  patterns are combined to develop a qualitative model of the
  magnetic-field geometry outside of the strong field areas of active
  regions. The motion is assumed to originate at the crests of magnetic
  arcades and flow downward along field lines, which are assumed to
  be elliptical in shape with the major axis in the photosphere. It is
  found that the ratio of the major axis to the minor axis of the ellipse
  must be less than two for fields under 100 G. Also, it is concluded
  that the magnetic neutral surfaces defined by the loci of horizontal
  field lines are often tilted at a large angle to the vertical at the
  altitude of the C IV emission.

Title: Heating Related Flows in Cool Loops
Authors: Klimchuk, J. A.; Mariska, J. T.
Bibcode: 1987BAAS...19..932K
Altcode:
  No abstract at ADS

Title: C IV Doppler shifts observed in active region filaments.
Authors: Klimchuk, J. A.
Bibcode: 1986NASCP2442..183K
Altcode: 1986copp.nasa..183K
  The Doppler shift properties of 21 active region filaments have
  been studied using C IV Dopplergram data. Most are associated with
  corridors of weak magnetic field that separate opposite polarity strong
  fields seen in photospheric magnetograms. A majority of the filaments
  are relatively blue shifted, although several lie very close to the
  dividing lines between relative blue and red shift. Only one filament
  in our sample is clearly red shifted. A new calibration procedure
  for Dopplergrams indicates that sizable zero point offsets are often
  required. The center-to-limb behavior of the resulting absolute
  Doppler shifts suggests that filament flows are usually quite small
  (&lt;3 km/s). It is possible that they vanish.

Title: Numerical simulations of a siphon mechanism for quiescent
    prominence formation.
Authors: Poland, A. I.; Mariska, J. T.; Klimchuk, J. A.
Bibcode: 1986NASCP2442...57P
Altcode: 1986copp.nasa...57P
  Quiescent prominences represent a significant challenge to our
  understanding of the flow of mass and energy in the outer layers of
  the solar atmosphere. A small number of quiescent prominences contain
  as much mass as the entire corona (Athay, 1976). The problem then is
  how to get that much material into the relatively small volume of
  a prominence and maintain it at a temperature of 10,000 K in close
  proximity to material at one million K. The thermal insulation to
  conduction provided by the magnetic field explains the disparate
  temperatures. The mass source problem is less well understood. One
  method for supplying mass to the prominence is to siphon it from the
  chromosphere. The siphon mechanism begins with a magnetic loop that
  evolves into a configuration with a gravitational well, such as that
  described by Kippenhahn and Schluter (1957). This could be formed, for
  example, by a twist in the magnetic field. A gravitational well could
  also be formed by a condensation induced sag in the field. This could
  further enhance the condensation process. Once this well has formed,
  or as it is forming, the material in the well area of the loop must
  cool and condense to the point where radiative losses exceed any
  heat input. Additional material must also flow into the well from
  the underlying chromosphere to supply the mass required to form the
  prominence. One example from a series of numerical simulations that were
  performed to study the formation of quiescent prominences is presented.

Title: A numerical study of the thermal stability of low-lying
    coronal loops.
Authors: Klimchuk, J. A.; Antiochos, S. K.; Mariska, J. T.
Bibcode: 1986NASCP2442..389K
Altcode: 1986copp.nasa..389K
  The nonlinear evolution of loops that are subjected to a variety
  of small but finite perturbations was studied. Only the low-lying
  loops are considered. The analysis was performed numerically using a
  one-dimensional hydrodynamical model developed at the Naval Research
  Laboratory. The computer codes solve the time-dependent equations
  for mass, momentum, and energy transport. The primary interest is
  the active region filaments, hence a geometry appropriate to those
  structures was considered. The static solutions were subjected to a
  moderate sized perturbation and allowed to evolve. The results suggest
  that both hot and cool loops of the geometry considered are thermally
  stable against amplitude perturbations of all kinds.

Title: Magnetic Shear. IV. Hale Regions 16740, 16815, and 16850
Authors: Athay, R. G.; Klimchuk, J. A.; Jones, H. P.; Zirin, H.
Bibcode: 1986ApJ...303..884A
Altcode:
  Dopplergrams made in C IV 1548 A are studied for evidence of velocity
  shear near H-alpha dark filaments and for large-scale flow convergent on
  active regions. The three regions studied support earlier conclusions
  that shear is a common property of active regions and that active
  regions may be the foci of converging plasma flow. Flow patterns
  near filaments show divergence or convergence as well as shear. Also
  the sense of the shear can be either cyclonic or anticyclonic. No
  preference is noted for convergence or divergence or for a particular
  sense of shear, and there appears to be no correlation between the
  sense of the shear and the sign of the velocity gradient normal to
  the filament. The close association of H-alpha dark filaments with
  shear lines leads to the suggestion that the filaments may arise from
  a cooling instability induced by the Bernoulli effect.

Title: A Numerical Study of the Stability of Low-Lying Solar Loops
Authors: Mariska, J. T.; Klimchuk, J. A.; Antiochos, S. K.
Bibcode: 1986BAAS...18Q.708M
Altcode:
  No abstract at ADS

Title: The Large-Scale Dynamics and Structure of Solar Active Regions
    Observed in C IV
Authors: Klimchuk, J. A.
Bibcode: 1986BAAS...18R.702K
Altcode:
  No abstract at ADS

Title: Large-scale structure and dynamics of solar active regions
    observed in the far ultraviolet
Authors: Klimchuk, James Andrew
Bibcode: 1985PhDT.......145K
Altcode:
  No abstract at ADS

Title: Large-Scale Structure and Dynamics of Solar Active Regions
    Observed in the Far Ultraviolet.
Authors: Klimchuk, J. A.
Bibcode: 1985PhDT.........6K
Altcode:
  In this thesis we examine high resolution Dopplergrams in CIV (1548) for
  the purpose of understanding the large -scale dynamics of solar active
  regions and the closely related problem of magnetic structure. The
  Dopplergrams are compared with photospheric magnetograms and H(,(alpha))
  filtergrams in order to determine the relationship of the flows to
  the overall magnetic field topology. These observations sample three
  different heights in the atmosphere and can potentially be used to map
  the field from the photosphere up through the transition region. Vector
  properties of the flow and field are inferred from center-to-limb
  variations in the measured Doppler shift and longitudinal field
  component. We find that spatial correlations between features seen in
  Dopplergrams, magnetograms, and filtergrams are quite close, and that
  active regions can be naturally divided into three basic parts: strong
  field regions, weak field corridors between opposite polarities, and
  surrounding weak field areas. Strong field regions are relatively red
  shifted and contain magnetic fields that penetrate the photosphere in a
  nearly vertical fashion. Corridors and surrounding areas are relatively
  blue shifted, on the other hand, and contain fields that are mostly
  horizontal in the upper photosphere and chromosphere. The transition
  from vertical to horizontal field appears to be quite sharp and implies
  a magnetic topology that diverges very rapidly with height. Sizable
  uncertainties in the Dopplergram interpretation have prevented us
  from describing the transition region flows unambiguously. We can,
  however, identify two possible scenarios that are consistent with the
  data. In the most likely of the two, the flows vanish within corridors
  and surrounding areas, and they produce constant (across the disk)
  absolute red shifts of about 18 km/s within strong field regions. The
  physical causes of this last result are unclear, and it remains an
  important unsolved problem.

Title: Observed Associations Between CIV Doppler Shifts and
    Photospheric Magnetic Fields in Active Regions
Authors: Klimchuk, J. A.
Bibcode: 1984BAAS...16..532K
Altcode:
  No abstract at ADS

Title: Outflow from the sun's polar corona
Authors: Orrall, F. Q.; Rottman, G. J.; Klimchuk, J. A.
Bibcode: 1983ApJ...266L..65O
Altcode:
  New observations of systematic Doppler shifts of EUV resonance lines
  formed both in the low corona and transition region are reported. They
  were made with an improved, high-resolution, stable, rocket-borne
  spectrometer flown on 1981 November 23. The chord of the solar disk
  scanned by the spectrometer crossed the north polar cap hole and its
  low-latitude extension, and also a compact low-latitude hole near
  sun center. Within both holes, the lines were systematically shifted
  to shorter wavelengths relative to the rest of the solar disk. These
  observations strengthen the association of negative Doppler shifts
  with coronal holes and indicate that this 'blueshift signature',
  previously observed only within small low-latitude holes, is also
  characteristic of the low-density polar corona. The mean relative
  blueshift measured in lambda 625 Mg X (T = 10 to the 6.15 power K)
  within the polar hole was about 8 km/s.

Title: Measurements of outflow from the base of solar coronal holes
Authors: Rottman, G. J.; Orrall, F. Q.; Klimchuk, J. A.
Bibcode: 1982ApJ...260..326R
Altcode:
  New evidence is presented that EUV emission lines formed at the levels
  of the base of the corona and the transition region are systematically
  shifted to shorter wavelengths within coronal holes relative to the
  rest of the solar disk, and that moreover this shift increases with
  height in the atmosphere. Measurements were made with a rocket-borne
  EUV spectrometer having high spectroscopic resolution and stability
  flown on July 15, 1980. Repeated measurements were made along a chord
  of the solar disk that crossed a compact coronal hole near sun center
  identified on gamma 10830 He I spectroheliograms. The maximum measured
  shift corresponded to a velocity of 12 km/sec in gamma 625 Mg X and 7
  km/sec in gamma 629 O V. If these velocities correspond to a true mass
  flux, they provide important data on the acceleration of coronal plasma
  in open magnetic field regions. These observed Doppler displacements
  are a strong and significant signature of coronal holes, now measured
  on three rocket flights.

Title: Measurement of systematic outflow from the solar transition
    region underlying a coronal hole
Authors: Rottman, G. J.; Klimchuk, J. A.; Orrall, F. Q.
Bibcode: 1981ApJ...247L.135R
Altcode:
  This letter presents measurements of small Doppler shifts in the line
  center position of 629A OV obtained with a new high-resolution EUV
  spectrometer flown aboard a sounding rocket. A major result is the
  detection of an apparent systematic outflow (relative to the quiet
  sun) of approximately 3 km/s average and 5 km/s maximum in the solar
  transition region underlying a well-defined low-latitude coronal
  hole. This is reminiscent of a similar apparent outflow observed by
  Cushman and Rense in the coronal line 303A Si XI. The hypothesis that
  this is evidence for acceleration of the high speed solar wind deep
  in the transition region and inner corona is explored briefly.

Title: EUV Observations of High-Speed Downflows Over Sunspots
Authors: Klimchuk, J. A.; Rottman, G. J.
Bibcode: 1981BAAS...13..914K
Altcode:
  No abstract at ADS

Title: EUV Observations of Solar Mass Loss from the Lower Solar
    Atmosphere
Authors: Rottman, G. J.; Orrall, F. Q.; Klimchuk, J. A.
Bibcode: 1981BAAS...13..812R
Altcode:
  No abstract at ADS

Title: Mass Flux within Coronal Holes
Authors: Orrall, F. Q.; Rottman, G. J.; Klimchuk, J.
Bibcode: 1980BAAS...12..919O
Altcode:
  No abstract at ADS

Title: Velocity Fields Observed in Coronal Holes and the Underlying
    Transition Region
Authors: Rottman, G. J.; Klimchuk, J. A.; Orrall, F. Q.
Bibcode: 1980BAAS...12..919R
Altcode:
  No abstract at ADS

