Author name code: chitta
ADS astronomy entries on 2022-09-14
author:"Chitta, Lakshmi Pradeep" 
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Title: What drives decayless kink oscillations in active region
    coronal loops on the Sun?
Authors: Mandal, Sudip; Chitta, Lakshmi P.; Antolin, Patrick; Peter,
   Hardi; Solanki, Sami K.; Auchère, Frédéric; Berghmans, David;
   Zhukov, Andrei N.; Teriaca, Luca; Cuadrado, Regina A.; Schühle,
   Udo; Parenti, Susanna; Buchlin, Éric; Harra, Louise; Verbeeck, Cis;
   Kraaikamp, Emil; Long, David M.; Rodriguez, Luciano; Pelouze, Gabriel;
   Schwanitz, Conrad; Barczynski, Krzysztof; Smith, Phil J.
Bibcode: 2022arXiv220904251M
Altcode:
  We study here the phenomena of decayless kink oscillations in a system
  of active region (AR) coronal loops. Using high resolution observations
  from two different instruments, namely the Extreme Ultraviolet Imager
  (EUI) on board Solar Orbiter and the Atmospheric Imaging Assembly
  (AIA) on board the Solar Dynamics Observatory, we follow these AR
  loops for an hour each on three consecutive days. Our results show
  significantly more resolved decayless waves in the higher-resolution
  EUI data compared with the AIA data. Furthermore, the same system of
  loops exhibits many of these decayless oscillations on Day-2, while on
  Day-3, we detect very few oscillations and on Day-1, we find none at
  all. Analysis of photospheric magnetic field data reveals that at most
  times, these loops were rooted in sunspots, where supergranular flows
  are generally absent. This suggests that supergranular flows, which
  are often invoked as drivers of decayless waves, are not necessarily
  driving such oscillations in our observations. Similarly, our findings
  also cast doubt on other possible drivers of these waves, such as a
  transient driver or mode conversion of longitudinal waves near the loop
  footpoints. In conclusion, through our analysis we find that none of
  the commonly suspected sources proposed to drive decayless oscillations
  in active region loops seems to be operating in this event and hence,
  the search for that elusive wave driver needs to continue.

Title: Defining the Middle Corona
Authors: West, Matthew J.; Seaton, Daniel B.; Wexler, David B.;
   Raymond, John C.; Del Zanna, Giulio; Rivera, Yeimy J.; Kobelski,
   Adam R.; DeForest, Craig; Golub, Leon; Caspi, Amir; Gilly, Chris R.;
   Kooi, Jason E.; Alterman, Benjamin L.; Alzate, Nathalia; Banerjee,
   Dipankar; Berghmans, David; Chen, Bin; Chitta, Lakshmi Pradeep; Downs,
   Cooper; Giordano, Silvio; Higginson, Aleida; Howard, Russel A.; Mason,
   Emily; Mason, James P.; Meyer, Karen A.; Nykyri, Katariina; Rachmeler,
   Laurel; Reardon, Kevin P.; Reeves, Katharine K.; Savage, Sabrina;
   Thompson, Barbara J.; Van Kooten, Samuel J.; Viall, Nicholeen M.;
   Vourlidas, Angelos
Bibcode: 2022arXiv220804485W
Altcode:
  The middle corona, the region roughly spanning heliocentric altitudes
  from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential
  physical transitions and processes that govern the behavior of
  coronal outflow into the heliosphere. Eruptions that could disrupt
  the near-Earth environment propagate through it. Importantly, it
  modulates inflow from above that can drive dynamic changes at lower
  heights in the inner corona. Consequently, this region is essential
  for comprehensively connecting the corona to the heliosphere and for
  developing corresponding global models. Nonetheless, because it is
  challenging to observe, the middle corona has been poorly studied by
  major solar remote sensing missions and instruments, extending back to
  the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent
  advances in instrumentation, observational processing techniques,
  and a realization of the importance of the region, interest in the
  middle corona has increased. Although the region cannot be intrinsically
  separated from other regions of the solar atmosphere, there has emerged
  a need to define the region in terms of its location and extension
  in the solar atmosphere, its composition, the physical transitions
  it covers, and the underlying physics believed to be encapsulated by
  the region. This paper aims to define the middle corona and give an
  overview of the processes that occur there.

Title: A highly dynamic small-scale jet in a polar coronal hole
Authors: Mandal, Sudip; Chitta, Lakshmi Pradeep; Peter, Hardi;
   Solanki, Sami K.; Cuadrado, Regina Aznar; Teriaca, Luca; Schühle,
   Udo; Berghmans, David; Auchère, Frédéric
Bibcode: 2022A&A...664A..28M
Altcode: 2022arXiv220602236M
  We present an observational study of the plasma dynamics at the base
  of a solar coronal jet, using high resolution extreme ultraviolet
  imaging data taken by the Extreme Ultraviolet Imager on board Solar
  Orbiter, and by the Atmospheric Imaging Assembly on board Solar
  Dynamics Observatory. We observed multiple plasma ejection events
  over a period of ∼1 h from a dome-like base that is ca. 4 Mm wide
  and is embedded in a polar coronal hole. Within the dome below the
  jet spire, multiple plasma blobs with sizes around 1−2 Mm propagate
  upwards to the dome apex with speeds of the order of the sound speed
  (ca. 120 km s<SUP>−1</SUP>). Upon reaching the apex, some of these
  blobs initiate flows with similar speeds towards the other footpoint
  of the dome. At the same time, high speed super-sonic outflows
  (∼230 km s<SUP>−1</SUP>) are detected along the jet spire. These
  outflows as well as the intensity near the dome apex appear to be
  repetitive. Furthermore, during its evolution, the jet undergoes
  many complex morphological changes, including transitions between
  the standard and blowout type eruption. These new observational
  results highlight the underlying complexity of the reconnection
  process that powers these jets and they also provide insights into
  the plasma response when subjected to rapid energy injection. <P
  />Movies associated to Figs. 1, 2, and 4 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243765/olm">https://www.aanda.org</A>

Title: Tracing the Drivers of Slow Solar Wind in the Middle Corona
Authors: Chitta, Lakshmi Pradeep; DeForest, Craig; Downs, Cooper;
   Seaton, Daniel; Higginson, Aleida
Bibcode: 2022cosp...44.1328C
Altcode:
  Compared to the so-called fast solar wind that originates from polar
  coronal holes during the solar minimum, low-latitude wind streams
  generally have lower speeds ($\le$ 500 km/s). These slow solar wind
  streams closer to the ecliptic plane are characterised by their high
  spatial structuring, temporal variability, and coronal compositions. The
  magnetic driver responsible for the origin of this slow solar wind
  and its characteristics, however, is a subject of active debate. Using
  coronal observations from the 2018 off-pointing campaign of the GOES
  Solar Ultraviolet Imager (SUVI) as well as images from SOHO/LASCO, we
  found signatures of solar wind streams driven by magnetic reconnection
  in the highly structured middle corona. In particular, elongated
  coronal loops in the middle corona over a coronal-hole-active-region
  complex are observed to reconnect and retract while in the process,
  some plasma is propelled away from the Sun as streams or blobs. Using
  STEREO observations we found that similar streams forming over the
  same complex escape into the heliosphere. In this talk, we will
  present these new results and discuss them in the context of models
  of slow solar wind sources and drivers. our findings emphasise the
  key necessity of having routine extreme ultraviolet observations of
  the middle coronal processes from ecliptic and polar vantage points,
  along with extended coronal diagnostics to develop better understanding
  of the heliospheric impact of Sun's activity.

Title: Small-scale coronal brightenings as seen by Solar Orbiter
Authors: Peter, Hardi; Berghmans, David; Chitta, Lakshmi Pradeep
Bibcode: 2022cosp...44.1323P
Altcode:
  The corona of the Sun shows variability over a wide range of scales,
  in space, time and energy. The power-law-like distributions small-scale
  coronal brightenings events have been used to propose self-similarity
  of the involved processes. Already during the cruise phase, Solar
  Orbiter was close enough to the Sun so that images provided by the
  Extreme Ultraviolet Imager (EUI) are among the highest resolution
  coronal data acquired so far. The small brightenings found in the
  quiet Sun, now often termed campfires, could be considered as the small
  end of the distribution of coronal transients. Mostly, these coronal
  brightenings occur very low in the atmosphere, essentially just above
  the chromosphere. Still they show a variety of morphology, ranging
  from dot-like to loop-like with propagating disturbances, small jets,
  or miniature flux-rope eruptions. This variety of the phenomenology on
  the smallest resolvable scales points towards a conclusion that there is
  not one single process that can drive small-scale brightenings. This is
  supported by studies relating the EUV brightenings to the underlying
  magnetic field: a part of the cases shows a relation to changes of
  the surface magnetic flux, while in other cases it is very hard to
  find any connection to the magnetic field. In the quiet Sun these
  small brightenings are mostly found at the edges of bright elements
  of the chromospheric network, which they have in common with another
  class of brightenings seen at lower temperatures in the transition
  region, namely explosive events. These transients have been proposed
  as being due to reconnection, have a similar size and lifetime as
  the small brightenings, but mostly seem to lack a component at high
  temperatures. It might well be that these explosive events are related
  to one particular type of the campfires, e.g. the jet-types, but that
  remains to be studied. Modelling work on small brightenings is not
  yet very abundant. One 3D MHD model of the quiet Sun shows coronal
  brightenings that share properties with the observations. Here the
  brightening is caused (mostly) by component reconnection, but also
  one case of a twisted flux rope is found in the simulation data,
  other (future) models most likely will reveal that also different
  processes could produce similar brightenings. Because of timing and
  the mission profile, so far remote sensing observations have been
  taken mostly in regions of quiet Sun. With the perihelion in March
  2022 Solar Orbiter will not only be closer than 0.3 AU from the Sun,
  providing coronal observations at even higher resolution than before,
  but also active region observations are planned. These might extend
  the zoo of the small-scale coronal transients to small features in
  active regions that might show properties similar or different from
  the quiet Sun coronal brightenings. Either way, this will provide a
  challenge for our understanding of the small-scale corona.

Title: Parallel Plasma Loops and the Energization of the Solar Corona
Authors: Peter, Hardi; Chitta, Lakshmi Pradeep; Chen, Feng; Pontin,
   David I.; Winebarger, Amy R.; Golub, Leon; Savage, Sabrina L.;
   Rachmeler, Laurel A.; Kobayashi, Ken; Brooks, David H.; Cirtain,
   Jonathan W.; De Pontieu, Bart; McKenzie, David E.; Morton, Richard J.;
   Testa, Paola; Tiwari, Sanjiv K.; Walsh, Robert W.; Warren, Harry P.
Bibcode: 2022ApJ...933..153P
Altcode: 2022arXiv220515919P
  The outer atmosphere of the Sun is composed of plasma heated to
  temperatures well in excess of the visible surface. We investigate
  short cool and warm (&lt;1 MK) loops seen in the core of an active
  region to address the role of field-line braiding in energizing these
  structures. We report observations from the High-resolution Coronal
  imager (Hi-C) that have been acquired in a coordinated campaign with
  the Interface Region Imaging Spectrograph (IRIS). In the core of the
  active region, the 172 Å band of Hi-C and the 1400 Å channel of IRIS
  show plasma loops at different temperatures that run in parallel. There
  is a small but detectable spatial offset of less than 1″ between
  the loops seen in the two bands. Most importantly, we do not see
  observational signatures that these loops might be twisted around each
  other. Considering the scenario of magnetic braiding, our observations
  of parallel loops imply that the stresses put into the magnetic field
  have to relax while the braiding is applied: the magnetic field never
  reaches a highly braided state on these length scales comparable to
  the separation of the loops. This supports recent numerical 3D models
  of loop braiding in which the effective dissipation is sufficiently
  large that it keeps the magnetic field from getting highly twisted
  within a loop.

Title: Automatic detection of small-scale EUV brightenings observed
    by the Solar Orbiter/EUI
Authors: Alipour, N.; Safari, H.; Verbeeck, C.; Berghmans, D.;
   Auchère, F.; Chitta, L. P.; Antolin, P.; Barczynski, K.; Buchlin,
   É.; Aznar Cuadrado, R.; Dolla, L.; Georgoulis, M. K.; Gissot, S.;
   Harra, L.; Katsiyannis, A. C.; Long, D. M.; Mandal, S.; Parenti,
   S.; Podladchikova, O.; Petrova, E.; Soubrié, É.; Schühle, U.;
   Schwanitz, C.; Teriaca, L.; West, M. J.; Zhukov, A. N.
Bibcode: 2022A&A...663A.128A
Altcode: 2022arXiv220404027A
  Context. Accurate detections of frequent small-scale extreme ultraviolet
  (EUV) brightenings are essential to the investigation of the physical
  processes heating the corona. <BR /> Aims: We detected small-scale
  brightenings, termed campfires, using their morphological and
  intensity structures as observed in coronal EUV imaging observations
  for statistical analysis. <BR /> Methods: We applied a method based
  on Zernike moments and a support vector machine (SVM) classifier
  to automatically identify and track campfires observed by Solar
  Orbiter/Extreme Ultraviolet Imager (EUI) and Solar Dynamics Observatory
  (SDO)/Atmospheric Imaging Assembly (AIA). <BR /> Results: This method
  detected 8678 campfires (with length scales between 400 km and 4000 km)
  from a sequence of 50 High Resolution EUV telescope (HRI<SUB>EUV</SUB>)
  174 Å images. From 21 near co-temporal AIA images covering the same
  field of view as EUI, we found 1131 campfires, 58% of which were
  also detected in HRI<SUB>EUV</SUB> images. In contrast, about 16%
  of campfires recognized in HRI<SUB>EUV</SUB> were detected by AIA. We
  obtain a campfire birthrate of 2 × 10<SUP>−16</SUP> m<SUP>−2</SUP>
  s<SUP>−1</SUP>. About 40% of campfires show a duration longer than 5
  s, having been observed in at least two HRI<SUB>EUV</SUB> images. We
  find that 27% of campfires were found in coronal bright points and
  the remaining 73% have occurred out of coronal bright points. We
  detected 23 EUI campfires with a duration greater than 245 s. We found
  that about 80% of campfires are formed at supergranular boundaries,
  and the features with the highest total intensities are generated at
  network junctions and intense H I Lyman-α emission regions observed
  by EUI/HRI<SUB>Lya</SUB>. The probability distribution functions for
  the total intensity, peak intensity, and projected area of campfires
  follow a power law behavior with absolute indices between 2 and 3. This
  self-similar behavior is a possible signature of self-organization,
  or even self-organized criticality, in the campfire formation
  process. <P />Supplementary material (S1-S3) is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243257/olm">https://www.aanda.org</A>

Title: Observation of Magnetic Switchback in the Solar Corona
Authors: Telloni, Daniele; Zank, Gary P.; Stangalini, Marco;
   Downs, Cooper; Liang, Haoming; Nakanotani, Masaru; Andretta,
   Vincenzo; Antonucci, Ester; Sorriso-Valvo, Luca; Adhikari, Laxman;
   Zhao, Lingling; Marino, Raffaele; Susino, Roberto; Grimani, Catia;
   Fabi, Michele; D'Amicis, Raffaella; Perrone, Denise; Bruno, Roberto;
   Carbone, Francesco; Mancuso, Salvatore; Romoli, Marco; Da Deppo, Vania;
   Fineschi, Silvano; Heinzel, Petr; Moses, John D.; Naletto, Giampiero;
   Nicolini, Gianalfredo; Spadaro, Daniele; Teriaca, Luca; Frassati,
   Federica; Jerse, Giovanna; Landini, Federico; Pancrazzi, Maurizio;
   Russano, Giuliana; Sasso, Clementina; Berghmans, David; Auchère,
   Frédéric; Aznar Cuadrado, Regina; Chitta, Lakshmi P.; Harra, Louise;
   Kraaikamp, Emil; Long, David M.; Mandal, Sudip; Parenti, Susanna;
   Pelouze, Gabriel; Peter, Hardi; Rodriguez, Luciano; Schühle, Udo;
   Schwanitz, Conrad; Smith, Phil J.; Verbeeck, Cis; Zhukov, Andrei N.
Bibcode: 2022arXiv220603090T
Altcode:
  Switchbacks are sudden, large radial deflections of the solar wind
  magnetic field, widely revealed in interplanetary space by the Parker
  Solar Probe. The switchbacks' formation mechanism and sources are still
  unresolved, although candidate mechanisms include Alfvénic turbulence,
  shear-driven Kelvin-Helmholtz instabilities, interchange reconnection,
  and geometrical effects related to the Parker spiral. This Letter
  presents observations from the Metis coronagraph onboard Solar Orbiter
  of a single large propagating S-shaped vortex, interpreted as first
  evidence of a switchback in the solar corona. It originated above
  an active region with the related loop system bounded by open-field
  regions to the East and West. Observations, modeling, and theory provide
  strong arguments in favor of the interchange reconnection origin of
  switchbacks. Metis measurements suggest that the initiation of the
  switchback may also be an indicator of the origin of slow solar wind.

Title: The magnetic drivers of campfires seen by the Polarimetric
    and Helioseismic Imager (PHI) on Solar Orbiter
Authors: Kahil, F.; Hirzberger, J.; Solanki, S. K.; Chitta, L. P.;
   Peter, H.; Auchère, F.; Sinjan, J.; Orozco Suárez, D.; Albert,
   K.; Albelo Jorge, N.; Appourchaux, T.; Alvarez-Herrero, A.; Blanco
   Rodríguez, J.; Gandorfer, A.; Germerott, D.; Guerrero, L.; Gutiérrez
   Márquez, P.; Kolleck, M.; del Toro Iniesta, J. C.; Volkmer, R.;
   Woch, J.; Fiethe, B.; Gómez Cama, J. M.; Pérez-Grande, I.; Sanchis
   Kilders, E.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Calchetti,
   D.; Carmona, M.; Deutsch, W.; Fernández-Rico, G.; Fernández-Medina,
   A.; García Parejo, P.; Gasent-Blesa, J. L.; Gizon, L.; Grauf, B.;
   Heerlein, K.; Lagg, A.; Lange, T.; López Jiménez, A.; Maue, T.;
   Meller, R.; Michalik, H.; Moreno Vacas, A.; Müller, R.; Nakai,
   E.; Schmidt, W.; Schou, J.; Schühle, U.; Staub, J.; Strecker, H.;
   Torralbo, I.; Valori, G.; Aznar Cuadrado, R.; Teriaca, L.; Berghmans,
   D.; Verbeeck, C.; Kraaikamp, E.; Gissot, S.
Bibcode: 2022A&A...660A.143K
Altcode: 2022arXiv220213859K
  Context. The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter
  (SO) spacecraft observed small extreme ultraviolet (EUV) bursts,
  termed campfires, that have been proposed to be brightenings near the
  apexes of low-lying loops in the quiet-Sun atmosphere. The underlying
  magnetic processes driving these campfires are not understood. <BR
  /> Aims: During the cruise phase of SO and at a distance of 0.523
  AU from the Sun, the Polarimetric and Helioseismic Imager on Solar
  Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI,
  offering the possibility to investigate the surface magnetic field
  dynamics underlying campfires at a spatial resolution of about 380
  km. <BR /> Methods: We used co-spatial and co-temporal data of the
  quiet-Sun network at disc centre acquired with the High Resolution
  Imager of SO/EUI at 17.4 nm (HRI<SUB>EUV</SUB>, cadence 2 s) and the
  High Resolution Telescope of SO/PHI at 617.3 nm (HRT, cadence 2.5
  min). Campfires that are within the SO/PHI−SO/EUI common field
  of view were isolated and categorised according to the underlying
  magnetic activity. <BR /> Results: In 71% of the 38 isolated events,
  campfires are confined between bipolar magnetic features, which seem to
  exhibit signatures of magnetic flux cancellation. The flux cancellation
  occurs either between the two main footpoints, or between one of the
  footpoints of the loop housing the campfire and a nearby opposite
  polarity patch. In one particularly clear-cut case, we detected the
  emergence of a small-scale magnetic loop in the internetwork followed
  soon afterwards by a campfire brightening adjacent to the location
  of the linear polarisation signal in the photosphere, that is to
  say near where the apex of the emerging loop lays. The rest of the
  events were observed over small scattered magnetic features, which
  could not be identified as magnetic footpoints of the campfire hosting
  loops. <BR /> Conclusions: The majority of campfires could be driven
  by magnetic reconnection triggered at the footpoints, similar to the
  physical processes occurring in the burst-like EUV events discussed
  in the literature. About a quarter of all analysed campfires, however,
  are not associated to such magnetic activity in the photosphere, which
  implies that other heating mechanisms are energising these small-scale
  EUV brightenings.

Title: Spectroscopic observation of a transition region network jet
Authors: Gorman, J.; Chitta, L. P.; Peter, H.
Bibcode: 2022A&A...660A.116G
Altcode: 2022arXiv220211375G
  <BR /> Aims: Ubiquitous transition region (TR) network jets are
  considered to be substantial sources of mass and energy to the corona
  and solar wind. We conduct a case study of a network jet to better
  understand the nature of mass flows along its length and the energetics
  involved in its launch. <BR /> Methods: We present an observation of a
  jet with the Interface Region Imaging Spectrograph (IRIS), while also
  using data from the Solar Dynamics Observatory (SDO) to provide further
  context. The jet was located within a coronal hole close to the disk
  center. <BR /> Results: We find that a blueshifted secondary component
  of TR emission is associated with the jet and is persistent along its
  spire. This component exhibits upward speeds of approximately 20-70 km
  s<SUP>−1</SUP> and shows enhanced line broadening. However, plasma
  associated with the jet in the upper chromosphere shows downflows of
  5-10 km s<SUP>−1</SUP>. Finally, the jet emanates from a seemingly
  unipolar magnetic footpoint. <BR /> Conclusions: While a definitive
  magnetic driver is not discernible for this event, we infer that
  the energy driving the network jet is deposited at the top of the
  chromosphere, indicating that TR network jets are driven from the
  mid-atmospheric layers of the Sun. The energy flux associated with the
  line broadening indicates that the jet could be powered all the way into
  the solar wind. <P />Movie associated with Fig. 3 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202142995/olm">https://www.aanda.org</A>

Title: Coronal condensation as the source of transition-region
    supersonic downflows above a sunspot
Authors: Chen, Hechao; Tian, Hui; Li, Leping; Peter, Hardi; Chitta,
   Lakshmi Pradeep; Hou, Zhenyong
Bibcode: 2022A&A...659A.107C
Altcode: 2021arXiv211201354C
  Context. Plasma loops or plumes rooted in sunspot umbrae often harbor
  downflows with speeds of 100 km s<SUP>−1</SUP>. These downflows
  are supersonic at transition region temperatures of ∼0.1 MK. The
  source of these flows is not well understood. <BR /> Aims: We aim
  to investigate the source of sunspot supersonic downflows (SSDs)
  in active region 12740 using simultaneous spectroscopic and imaging
  observations. <BR /> Methods: We identified SSD events from multiple
  raster scans of a sunspot by the Interface Region Imaging Spectrograph,
  and we calculated the electron densities, mass fluxes, and velocities
  of these SSDs. The extreme-ultraviolet (EUV) images provided by the
  Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and
  the EUV Imager onboard the Solar Terrestrial Relations Observatory were
  employed to investigate the origin of these SSDs and their associated
  coronal rain. <BR /> Results: Almost all the identified SSDs appear
  at the footpoints of sunspot plumes and are temporally associated
  with the appearance of chromospheric bright dots inside the sunspot
  umbra. Dual-perspective EUV imaging observations reveal a large-scale
  closed magnetic loop system spanning the sunspot region and a remote
  region. We observed that the SSDs are caused by repeated coronal
  rain that forms and flows along these closed magnetic loops toward the
  sunspot. One episode of coronal rain clearly indicates that reconnection
  near a coronal X-shaped structure first leads to the formation of
  a magnetic dip. Subsequently, hot coronal plasma catastrophically
  cools from ∼2 MK in the dip region via thermal instability. This
  results in the formation of a transient prominence in the dip, from
  which the cool gas mostly slides into the sunspot along inclined
  magnetic fields under the gravity. This drainage process manifests
  as a continuous rain flow, which lasts for ∼2 h and concurrently
  results in a nearly steady SSD event. The total mass of condensation
  (1.3 × 10<SUP>14</SUP> g) and condensation rate (1.5 × 10<SUP>10</SUP>
  g s<SUP>−1</SUP>) in the dip region were found to be sufficient to
  sustain this long-lived SSD event, which has a mass transport rate of
  0.7 − 1.2 × 10<SUP>10</SUP> g s<SUP>−1</SUP>. <BR /> Conclusions:
  Our results demonstrate that coronal condensation in magnetic dips
  can result in the quasi-steady sunspot supersonic downflows. <P
  />Movies associated to Figs. 1, 3, 6, 7, and 11 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202142093/olm">https://www.aanda.org</A>

Title: A solar coronal loop in a box: Energy generation and heating
Authors: Breu, C.; Peter, H.; Cameron, R.; Solanki, S. K.; Przybylski,
   D.; Rempel, M.; Chitta, L. P.
Bibcode: 2022A&A...658A..45B
Altcode: 2021arXiv211211549B
  Context. Coronal loops are the basic building block of the upper solar
  atmosphere as seen in the extreme UV and X-rays. Comprehending how
  these are energized, structured, and evolve is key to understanding
  stellar coronae. <BR /> Aims: Here we investigate how the energy
  to heat the loop is generated by photospheric magneto-convection,
  transported into the upper atmosphere, and how the internal
  structure of a coronal magnetic loop forms. <BR /> Methods: In a 3D
  magnetohydrodynamics model, we study an isolated coronal loop rooted
  with both footpoints in a shallow layer within the convection zone
  using the MURaM code. To resolve its internal structure, we limited
  the computational domain to a rectangular box containing a single
  coronal loop as a straightened magnetic flux tube. Field-aligned heat
  conduction, gray radiative transfer in the photosphere and chromosphere,
  and optically thin radiative losses in the corona were taken into
  account. The footpoints were allowed to interact self-consistently
  with the granulation surrounding them. <BR /> Results: The loop is
  heated by a Poynting flux that is self-consistently generated through
  small-scale motions within individual magnetic concentrations in
  the photosphere. Turbulence develops in the upper layers of the
  atmosphere as a response to the footpoint motions. We see little
  sign of heating by large-scale braiding of magnetic flux tubes
  from different photospheric concentrations at a given footpoint. The
  synthesized emission, as it would be observed by the Atmospheric Imaging
  Assembly or the X-Ray Telescope, reveals transient bright strands that
  form in response to the heating events. Overall, our model roughly
  reproduces the properties and evolution of the plasma as observed
  within (the substructures of) coronal loops. <BR /> Conclusions:
  With this model we can build a coherent picture of how the energy
  flux to heat the upper atmosphere is generated near the solar surface
  and how this process drives and governs the heating and dynamics of
  a coronal loop. <P />Movie associated to Fig. 2 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141451/olm">https://www.aanda.org</A>

Title: Tracing the Drivers of Slow Solar Wind in the Middle Corona
Authors: Chitta, Lakshmi Pradeep; Seaton, Daniel; Downs, Cooper;
   DeForest, Craig; Higginson, Aleida
Bibcode: 2021AGUFMSH24C..01C
Altcode:
  Compared to the so-called fast solar wind that originates from polar
  coronal holes during the solar minimum, low-latitude wind streams
  generally have lower speeds ( 500 km/s). These slow solar wind streams
  closer to the ecliptic plane are characterized by their high spatial
  structuring, temporal variability, and coronal compositions. The
  magnetic driver responsible for the origin of this slow solar wind and
  its characteristics, however, is not well understood. Using coronal
  observations from the 2018 off-pointing campaign of the GOES Solar
  Ultraviolet Imager (SUVI) as well as images from SOHO/LASCO, we found
  signatures of solar wind streams driven by magnetic reconnection
  in the highly structured middle corona. In particular, elongated
  coronal loops in the middle corona over a coronal-hole-active-region
  complex are observed to reconnect and retract while in the process,
  some plasma is propelled away from the Sun as streams or blobs. Using
  STEREO observations we found that similar streams forming over the same
  complex escape into the heliosphere. Our observations of reconnection
  effects, however, are limited by the time resolution of the SUVI data
  in that they clearly represent the tail of a distribution of event
  durations, with a strong likelihood that many more events remain
  unresolved in the current data. In this talk, we will present these
  new results and discuss them in the context of models of slow solar
  wind sources and drivers. our findings emphasize the key necessity of
  having routine extreme ultraviolet observations of the middle coronal
  processes from ecliptic and polar vantage points, along with extended
  coronal diagnostics to develop better understanding of the heliospheric
  impact of Sun's activity.

Title: Campfires observed by EUI: What have we learned so far?
Authors: Berghmans, David; Auchere, F.; Zhukov, Andrei; Mierla,
   Marilena; Chen, Yajie; Peter, Hardi; Panesar, Navdeep; Chitta, Lakshmi
   Pradeep; Antolin, Patrick; Aznar Cuadrado, Regina; Tian, Hui; Hou,
   Zhenyong; Podladchikova, Olena
Bibcode: 2021AGUFMSH21A..02B
Altcode:
  Since its very first light images of the corona, the EUI/HRIEUV
  telescope onboard Solar Orbiter has observed small localised
  brightenings in the Quiet Sun. These small localised brightenings,
  have become known as campfires, and are observed with length scales
  between 400 km and 4000 km and durations between 10 sec and 200
  sec. The smallest and weakest of these HRIEUV brightenings have
  not been previously observed. Simultaneous observations from the
  EUI High-resolution Lyman- telescope (HRILYA) do not show localised
  brightening events, but the locations of the HRIEUV events clearly
  correspond to the chromospheric network. Comparisons with simultaneous
  AIA images shows that most events can also be identified in the
  17.1 nm, 19.3 nm, 21.1 nm, and 30.4 nm pass-bands of AIA, although
  they appear weaker and blurred. Some of the larger campfires have
  the appearance of small interacting loops with the brightening
  expanding from the contact point of the loops. Our differential
  emission measure (DEM) analysis indicated coronal temperatures. We
  determined the height for a few of these campfires to be between 1
  and 5 Mm above the photosphere. We interpret these events as a new
  extension to the flare-microflare-nanoflare family. Given their low
  height, the EUI campfires could stand as a new element of the fine
  structure of the transition region-low corona, that is, as apexes
  of small-scale loops that undergo internal heating all the way up to
  coronal temperatures. 3D MHD simulations with the MURaM code revealed
  brightenings that are in many ways similar to the campfires by EUI. The
  brightenings in the simulations suggest that campfires are triggered by
  component reconnection inside flux bundles rather than flux emergence
  or cancellation. Nevertheless, some of the observed campfires can
  be clearly linked to flux cancellation events and, intriguingly,
  are preceded by an erupting cool plasma structure. Analysis of the
  dynamics of campfires revealed that some have the appearance of coronal
  microjets, the smallest coronal jets observed in the quiet Sun. The
  HRIEUV images also reveal transient jets on a somewhat bigger scale
  with repeated outflows on the order of 100 km s1. In this paper we
  will provide an overview of the campfire related phenomena that EUI
  has observed and discuss the possible relevance for coronal heating.

Title: Propagating brightenings in small loop-like structures in
the quiet-Sun corona: Observations from Solar Orbiter/EUI
Authors: Mandal, Sudip; Peter, Hardi; Chitta, Lakshmi Pradeep;
   Solanki, Sami K.; Aznar Cuadrado, Regina; Teriaca, Luca; Schühle,
   Udo; Berghmans, David; Auchère, Frédéric
Bibcode: 2021A&A...656L..16M
Altcode: 2021arXiv211108106M
  Brightenings observed in solar extreme-ultraviolet images are generally
  interpreted as signatures of micro- or nanoflares occurring in the
  transition region or at coronal temperatures. Recent observations
  with the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have
  revealed the smallest of such brightenings (called campfires) in the
  quiet-Sun corona. Analyzing EUI 174 Å data obtained at a resolution
  of about 400 km on the Sun with a cadence of 5 s on 30 May 2020,
  we report here a number of cases in which these campfires exhibit
  propagating signatures along their apparently small (3-5 Mm) loop-like
  structures. The measured propagation speeds are generally between 25
  km s<SUP>−1</SUP> and 60 km s<SUP>−1</SUP>. If the loop plasma is
  assumed to be at a million Kelvin, these apparent motions would be
  slower than the local sound speed. Furthermore, these brightenings
  exhibit nontrivial propagation characteristics such as bifurcation,
  merging, reflection, and repeated plasma ejections. We suggest that
  these features are manifestations of the internal dynamics of these
  small-scale magnetic structures and could provide important insights
  into the dynamic response (∼40 s) of the loop plasma to the heating
  events and also into the locations of the heating events themselves. <P
  />Movies associated to Figs 2-5, A.1, and B.1 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202142041/olm">https://www.aanda.org</A>

Title: Capturing transient plasma flows and jets in the solar corona
Authors: Chitta, L. P.; Solanki, S. K.; Peter, H.; Aznar Cuadrado,
   R.; Teriaca, L.; Schühle, U.; Auchère, F.; Berghmans, D.; Kraaikamp,
   E.; Gissot, S.; Verbeeck, C.
Bibcode: 2021A&A...656L..13C
Altcode: 2021arXiv210915106C
  Intensity bursts in ultraviolet (UV) to X-ray wavelengths and plasma
  jets are typical signatures of magnetic reconnection and the associated
  impulsive heating of the solar atmospheric plasma. To gain new insights
  into the process, high-cadence observations are required to capture
  the rapid response of plasma to magnetic reconnection as well as the
  highly dynamic evolution of jets. Here, we report the first 2 s cadence
  extreme-UV observations recorded by the 174 Å High Resolution Imager of
  the Extreme Ultraviolet Imager on board the Solar Orbiter mission. These
  observations, covering a quiet-Sun coronal region, reveal the onset
  signatures of magnetic reconnection as localized heating events. These
  localized sources then exhibit repeated plasma eruptions or jet
  activity. Our observations show that this spatial morphological change
  from localized sources to jet activity could occur rapidly on timescales
  of about 20 s. The jets themselves are intermittent and are produced
  from the source region on timescales of about 20 s. In the initial
  phases of these events, plasma jets are observed to exhibit speeds,
  as inferred from propagating intensity disturbances, in the range of
  100 km s<SUP>−1</SUP> to 150 km s<SUP>−1</SUP>. These jets then
  propagate to lengths of about 5 Mm. We discuss examples of bidirectional
  and unidirectional jet activity observed to have been initiated from
  the initially localized bursts in the corona. The transient nature
  of coronal bursts and the associated plasma flows or jets along with
  their dynamics could provide a benchmark for magnetic reconnection
  models of coronal bursts and jets. <P />Movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202141683/olm">https://www.aanda.org</A>

Title: First observations from the SPICE EUV spectrometer on Solar
    Orbiter
Authors: Fludra, A.; Caldwell, M.; Giunta, A.; Grundy, T.; Guest,
   S.; Leeks, S.; Sidher, S.; Auchère, F.; Carlsson, M.; Hassler, D.;
   Peter, H.; Aznar Cuadrado, R.; Buchlin, É.; Caminade, S.; DeForest,
   C.; Fredvik, T.; Haberreiter, M.; Harra, L.; Janvier, M.; Kucera, T.;
   Müller, D.; Parenti, S.; Schmutz, W.; Schühle, U.; Solanki, S. K.;
   Teriaca, L.; Thompson, W. T.; Tustain, S.; Williams, D.; Young, P. R.;
   Chitta, L. P.
Bibcode: 2021A&A...656A..38F
Altcode: 2021arXiv211011252F
  <BR /> Aims: We present first science observations taken during the
  commissioning activities of the Spectral Imaging of the Coronal
  Environment (SPICE) instrument on the ESA/NASA Solar Orbiter
  mission. SPICE is a high-resolution imaging spectrometer operating at
  extreme ultraviolet (EUV) wavelengths. In this paper we illustrate
  the possible types of observations to give prospective users a
  better understanding of the science capabilities of SPICE. <BR />
  Methods: We have reviewed the data obtained by SPICE between April
  and June 2020 and selected representative results obtained with
  different slits and a range of exposure times between 5 s and 180
  s. Standard instrumental corrections have been applied to the raw
  data. <BR /> Results: The paper discusses the first observations
  of the Sun on different targets and presents an example of the full
  spectra from the quiet Sun, identifying over 40 spectral lines from
  neutral hydrogen and ions of carbon, oxygen, nitrogen, neon, sulphur,
  magnesium, and iron. These lines cover the temperature range between
  20 000 K and 1 million K (10 MK in flares), providing slices of the
  Sun's atmosphere in narrow temperature intervals. We provide a list
  of count rates for the 23 brightest spectral lines. We show examples
  of raster images of the quiet Sun in several strong transition region
  lines, where we have found unusually bright, compact structures in the
  quiet Sun network, with extreme intensities up to 25 times greater
  than the average intensity across the image. The lifetimes of these
  structures can exceed 2.5 hours. We identify them as a transition
  region signature of coronal bright points and compare their areas and
  intensity enhancements. We also show the first above-limb measurements
  with SPICE above the polar limb in C III, O VI, and Ne VIII lines, and
  far off limb measurements in the equatorial plane in Mg IX, Ne VIII,
  and O VI lines. We discuss the potential to use abundance diagnostics
  methods to study the variability of the elemental composition that can
  be compared with in situ measurements to help confirm the magnetic
  connection between the spacecraft location and the Sun's surface,
  and locate the sources of the solar wind. <BR /> Conclusions: The
  SPICE instrument successfully performs measurements of EUV spectra
  and raster images that will make vital contributions to the scientific
  success of the Solar Orbiter mission.

Title: An Analysis of Spikes in Atmospheric Imaging Assembly
    (AIA) Data
Authors: Young, Peter R.; Viall, Nicholeen M.; Kirk, Michael S.;
   Mason, Emily I.; Chitta, Lakshmi Pradeep
Bibcode: 2021SoPh..296..181Y
Altcode: 2021arXiv210802624Y
  The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
  Observatory (SDO) returns high-resolution images of the solar atmosphere
  in seven extreme ultraviolet (EUV) wavelength channels. The images
  are processed on the ground to remove intensity spikes arising from
  energetic particles hitting the instrument, and the despiked images
  are provided to the community. In this article, a three-hour series of
  images from the 171 Å channel obtained on 28 February 2017 was studied
  to investigate how often the despiking algorithm gave false positives
  caused by compact brightenings in the solar atmosphere. The latter
  were identified through spikes appearing in the same detector pixel
  for three consecutive frames. 1096 examples were found from the 900
  image frames. These "three-spikes" were assigned to 126 dynamic solar
  features, and it is estimated that the three-spike method identifies
  19% of the total number of features affected by despiking. For any
  ten-minute sequence of AIA 171 Å images there are around 37 solar
  features that have their intensity modified by despiking. The features
  are found in active regions, quiet Sun, and coronal holes and, in
  relation to solar surface area, there is a greater proportion within
  coronal holes. In 96% of the cases, the despiked structure is a compact
  brightening with a size of two arcsec or less, and the remaining 4%
  have narrow, elongated structures. By applying an EUV burst detection
  algorithm, we found that 96% of the events could be classified as EUV
  bursts. None of the spike events are rendered invisible by the AIA
  processing pipeline, but the total intensity over an event's lifetime
  can be reduced by up to 67%. Users are recommended to always restore
  the original intensities in AIA data when studying short-lived or
  rapidly evolving features that exhibit fine-scale structure.

Title: Revisiting the formation mechanism for coronal rain from
    previous studies
Authors: Li, Le-Ping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
   Hong-Qiang
Bibcode: 2021RAA....21..255L
Altcode: 2021arXiv210701339L
  Solar coronal rain is classified generally into two categories:
  flare-driven and quiescent coronal rain. Th latter is observed to form
  along both closed and open magnetic field structures. Recently, we
  proposed that some of the quiescent coronal rain events, detected in the
  transition region and chromospheric diagnostics, along loop-like paths
  could be explained by the formation mechanism for quiescent coronal
  rain facilitated by interchange magnetic reconnection between open and
  closed field lines. In this study, we revisited 38 coronal rain reports
  from the literature. From theseearlier works, we picked 15 quiescent
  coronal rain events out of the solar limb, mostly suggested to occur in
  active region closed loops due to thermal nonequilibrium, to scrutinize
  their formation mechanism. Employing the extreme ultraviolet images
  and line-of-sight magnetograms, the evolution of the quiescent coronal
  rain events and their magnetic fields and context coronal structures
  is examined. We find that six, comprising 40%, of the 15 quiescent
  coronal rain events could be totally or partially interpreted by the
  formation mechanism for quiescent coronal rain along open structures
  facilitated by interchange reconnection. The results suggest that the
  quiescent coronal rain facilitated by interchange reconnection between
  open and closed field lines deserves more attention.

Title: Formation of a Solar Filament by Magnetic Reconnection and
    Coronal Condensation
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
   Hongqiang
Bibcode: 2021ApJ...919L..21L
Altcode: 2021arXiv210905669L
  In solar filament formation mechanisms, magnetic reconnection
  between two sets of sheared arcades forms helical structures of the
  filament with numerous magnetic dips, and cooling and condensation
  of plasma trapped inside the helical structures supply mass to
  the filament. Although each of these processes, namely, magnetic
  reconnection and coronal condensation have been separately reported,
  observations that show the whole process of filament formation are
  rare. In this Letter, we present the formation of a sigmoid via
  reconnection between two sets of coronal loops, and the subsequent
  formation of a filament through cooling and condensation of plasma
  inside the newly formed sigmoid. On 2014 August 27, a set of loops
  in the active region 12151 reconnected with another set of loops
  that are located to the east. A longer twisted sigmoidal structure
  and a set of shorter lower-lying loops then formed. The observations
  coincide well with the tether-cutting model. The newly formed sigmoid
  remains stable and does not erupt as a coronal mass ejection. From the
  eastern endpoint, signatures of injection of material into the sigmoid
  (as brightenings) are detected, which closely outline the features of
  increasing emission measure at these locations. This may indicate the
  chromospheric evaporation caused by reconnection, supplying heated
  plasma into the sigmoid. In the sigmoid, thermal instability occurs,
  and rapid cooling and condensation of plasma take place, forming a
  filament. The condensations then flow bidirectionally to the filament
  endpoints. Our results provide a clear observational evidence of the
  filament formation via magnetic reconnection and coronal condensation.

Title: Magnetic imaging of the outer solar atmosphere (MImOSA)
Authors: Peter, H.; Ballester, E. Alsina; Andretta, V.; Auchère, F.;
   Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.;
   Chitta, L. P.; Dalmasse, K.; Alemán, T. del Pino; Feller, A.; Froment,
   C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski,
   D.; Solanki, S. K.; Štěpán, J.; Teriaca, L.; Bueno, J. Trujillo
Bibcode: 2021ExA...tmp...95P
Altcode:
  The magnetic activity of the Sun directly impacts the Earth and human
  life. Likewise, other stars will have an impact on the habitability of
  planets orbiting these host stars. Although the magnetic field at the
  surface of the Sun is reasonably well characterised by observations,
  the information on the magnetic field in the higher atmospheric layers
  is mainly indirect. This lack of information hampers our progress in
  understanding solar magnetic activity. Overcoming this limitation would
  allow us to address four paramount long-standing questions: (1) How
  does the magnetic field couple the different layers of the atmosphere,
  and how does it transport energy? (2) How does the magnetic field
  structure, drive and interact with the plasma in the chromosphere and
  upper atmosphere? (3) How does the magnetic field destabilise the outer
  solar atmosphere and thus affect the interplanetary environment? (4)
  How do magnetic processes accelerate particles to high energies? New
  ground-breaking observations are needed to address these science
  questions. We suggest a suite of three instruments that far exceed
  current capabilities in terms of spatial resolution, light-gathering
  power, and polarimetric performance: (a) A large-aperture UV-to-IR
  telescope of the 1-3 m class aimed mainly to measure the magnetic
  field in the chromosphere by combining high spatial resolution
  and high sensitivity. (b) An extreme-UV-to-IR coronagraph that is
  designed to measure the large-scale magnetic field in the corona with
  an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter
  based on a 30 cm telescope that combines high throughput in the
  extreme UV with polarimetry to connect the magnetic measurements
  of the other two instruments. Placed in a near-Earth orbit, the data
  downlink would be maximised, while a location at L4 or L5 would provide
  stereoscopic observations of the Sun in combination with Earth-based
  observatories. This mission to measure the magnetic field will finally
  unlock the driver of the dynamics in the outer solar atmosphere and
  thereby will greatly advance our understanding of the Sun and the
  heliosphere.

Title: Characterizing a "Solar FRB"
Authors: Hudson, H.; Briggs, M.; Chitta, L.; Fletcher, L.; Gary, D.;
   Monstein, C.; Nimmo, K.; Saint-Hilaire, P.; White, S.
Bibcode: 2021AAS...23812716H
Altcode:
  A remarkable solar microwave (1.4 GHz) burst,
  SOL2019-05-06T17:47:35.385, has been reported by the STARE2 fast cosmic
  transient survey (Bochenek et al., 2020). Its behavior resembles
  that of the Fast Radio Burst (FRB) extragalactic events in having a
  relatively broad spectral bandwidth and brief (19-msec) duration. It
  also had no measureable dispersion. The associated flare, GOES class
  C1, had a relatively hard X-ray spectrum as observed by Fermi/GBM,
  but no temporal association at the msec time scale suggested by the
  microwaves. Although msec variability in the microwave domain has
  been known to solar radio astronomy since the 1970s, the brightness
  and isolation of this event (both spatial and temporal) suggests
  novelty. Accordingly we survey the available correlative data from
  many sources and discuss possible interpretations in terms of type
  III-like behavior, electron cyclotron masering, and gyrosynchrotron
  emission. We note that the radio data (e-Callisto and EOVSA) revealed
  abundant type III activity in the vicinity, and the related flares
  as observed by GOES had exceptionally short time scales, suggesting
  burst origins in the lower solar atmosphere.

Title: Dynamic Evolution Of A Solar Flare Current Sheet
Authors: Chitta, L.; Priest, E. R.; Cheng, X.
Bibcode: 2021AAS...23830301C
Altcode:
  Current sheets play a key role in solar flares as they are the locations
  where magnetic energy is liberated through reconnection and is converted
  to other forms. Yet, their formation and evolution during the impulsive
  phase of a flare remain elusive. In this talk, we will report new
  observations of a current-sheet formation and subsequent evolution
  in the early stages of a solar flare. In particular, we will present
  multi-phase evolution of a dynamic current sheet from its formation
  to quasi-stable evolution and disruption. Implications for the onset
  and evolution of reconnection will be discussed.

Title: Coronal loops in a box: 3D models of their internal structure,
    dynamics and heating
Authors: Breu, C. A.; Peter, H.; Cameron, R.; Solanki, S.; Przybylski,
   D.; Chitta, L.
Bibcode: 2021AAS...23810606B
Altcode:
  The corona of the Sun, and probably also of other stars, is built
  up by loops defined through the magnetic field. They vividly appear
  in solar observations in the extreme UV and X-rays. High-resolution
  observations show individual strands with diameters down to a few 100
  km, and so far it remains open what defines these strands, in particular
  their width, and where the energy to heat them is generated. <P />The
  aim of our study is to understand how the magnetic field couples the
  different layers of the solar atmosphere, how the energy generated
  by magnetoconvection is transported into the upper atmosphere and
  dissipated, and how this process determines the scales of observed
  bright strands in the loop. <P />To this end, we conduct 3D resistive
  MHD simulations with the MURaM code. We include the effects of heat
  conduction, radiative transfer and optically thin radiative losses.We
  study an isolated coronal loop that is rooted with both footpoints
  in a shallow convection zone layer. To properly resolve the internal
  structure of the loop while limiting the size of the computational box,
  the coronal loop is modelled as a straightened magnetic flux tube. By
  including part of the convection zone, we drive the evolution of
  the corona self-consistently by magnetoconvection. <P />We find that
  the energy injected into the loop is generated by internal coherent
  motions within strong magnetic elements. The resulting Poynting
  flux is channelled into the loop in vortex tubes forming a magnetic
  connection between the photosphere and corona, where it is dissipated
  and heats the upper atmosphere. <P />The coronal emission as it would
  be observed in solar extreme UV or X-ray observations, e.g. with AIA
  or XRT, shows transient bright strands.The widths of these strands are
  consistent with observations. From our model we find that the width
  of the strands is governed by the size of the individual photospheric
  magnetic field concentrations where the field line through these strands
  are rooted. Essentially, each coronal strand is rooted in a single
  magnetic patch in the photosphere, and the energy to heat the strand is
  generated by internal motions within this magnetic concentration. <P
  />With this model we can build a coherent picture of how energy and
  matter are transported into the upper solar atmosphere and how these
  processes structure the interior of coronal loops.

Title: From Formation to Disruption: Observing the Multiphase
    Evolution of a Solar Flare Current Sheet
Authors: Chitta, L. P.; Priest, E. R.; Cheng, X.
Bibcode: 2021ApJ...911..133C
Altcode: 2021arXiv210302352C
  A current sheet, where magnetic energy is liberated through
  reconnection and converted to other forms, is thought to play
  the central role in solar flares, the most intense explosions in
  the heliosphere. However, the evolution of a current sheet and its
  subsequent role in flare-related phenomena such as particle acceleration
  is poorly understood. Here we report observations obtained with
  NASA's Solar Dynamics Observatory that reveal a multiphase evolution
  of a current sheet in the early stages of a solar flare, from its
  formation to quasi-stable evolution and disruption. Our observations
  have implications for the understanding of the onset and evolution of
  reconnection in the early stages of eruptive solar flares.

Title: On-disk Solar Coronal Condensations Facilitated by Magnetic
    Reconnection between Open and Closed Magnetic Structures
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
   Hongqiang
Bibcode: 2021ApJ...910...82L
Altcode: 2021arXiv210204605L
  Coronal condensation and rain are a crucial part of the mass cycle
  between the corona and chromosphere. In some cases, condensation and
  subsequent rain originate in the magnetic dips formed during magnetic
  reconnection. This provides a new and alternative formation mechanism
  for coronal rain. Until now, only off-limb, rather than on-disk,
  condensation events during reconnection have been reported. In
  this paper, employing extreme-ultraviolet (EUV) images of the
  Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics
  Observatory (SDO), we investigate the condensations facilitated by
  reconnection from 2011 July 14-15, when STEREO was in quadrature with
  respect to the Sun-Earth line. Above the limb, in STEREO/EUV Imager
  (EUVI) 171 Å images, higher-lying open structures move downward,
  reconnect with the lower-lying closed loops, and form dips. Two
  sets of newly reconnected structures then form. In the dips, bright
  condensations occur in the EUVI 304 Å images repeatedly, which
  then flow downward to the surface. In the on-disk observations by
  SDO/Atmospheric Imaging Assembly (AIA) in the 171 Å channel, these
  magnetic structures are difficult to identify. Dark condensations
  appear in the AIA 304 Å images, and then move to the surface as
  on-disk coronal rain. The cooling and condensation of coronal plasma
  is revealed by the EUV light curves. If only the on-disk observations
  were be available, the relation between the condensations and
  reconnection, shown clearly by the off-limb observations, could not
  be identified. Thus, we suggest that some on-disk condensation events
  seen in transition region and chromospheric lines may be facilitated
  by reconnection.

Title: Extreme-ultraviolet bursts and nanoflares in the quiet-Sun
    transition region and corona
Authors: Chitta, L. P.; Peter, H.; Young, P. R.
Bibcode: 2021A&A...647A.159C
Altcode: 2021arXiv210200730C
  The quiet solar corona consists of myriads of loop-like features, with
  magnetic fields originating from network and internetwork regions on
  the solar surface. The continuous interaction between these different
  magnetic patches leads to transient brightenings or bursts that might
  contribute to the heating of the solar atmosphere. The literature
  on a variety of such burst phenomena in the solar atmosphere is
  rich. However, it remains unclear whether such transients, which are
  mostly observed in the extreme ultraviolet (EUV), play a significant
  role in atmospheric heating. We revisit the open question of these
  bursts as a prelude to the new high-resolution EUV imagery expected
  from the recently launched Solar Orbiter. We use EUV image sequences
  recorded by the Atmospheric Imaging Assembly (AIA) on board the Solar
  Dynamics Observatory (SDO) to investigate statistical properties of
  the bursts. We detect the bursts in the 171 Å filter images of AIA in
  an automated way through a pixel-wise analysis by imposing different
  intensity thresholds. By exploiting the high cadence (12 s) of the
  AIA observations, we find that the distribution of lifetimes of these
  events peaks at about 120 s. However, a significant number of events
  also have lifetimes shorter than 60 s. The sizes of the detected bursts
  are limited by the spatial resolution, which indicates that a larger
  number of events might be hidden in the AIA data. We estimate that about
  100 new bursts appear per second on the whole Sun. The detected bursts
  have nanoflare-like energies of 10<SUP>24</SUP> erg per event. Based
  on this, we estimate that at least 100 times more events of a similar
  nature would be required to account for the energy that is required
  to heat the corona. When AIA observations are considered alone, the
  EUV bursts discussed here therefore play no significant role in the
  coronal heating of the quiet Sun. If the coronal heating of the quiet
  Sun is mainly bursty, then the high-resolution EUV observations from
  Solar Orbiter may be able to reduce the deficit in the number of EUV
  bursts seen with SDO/AIA at least partly by detecting more such events.

Title: Magnetic Reconnection between Loops Accelerated By a Nearby
    Filament Eruption
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
   Hongqiang; Ji, Kaifan; Xiang, YongYuan
Bibcode: 2021ApJ...908..213L
Altcode: 2020arXiv201208710L
  Magnetic reconnection modulated by nonlocal disturbances in the
  solar atmosphere has been investigated theoretically, but rarely
  observed. In this study, employing Hα and extreme ultraviolet (EUV)
  images and line-of-sight magnetograms, we report the acceleration of
  reconnection by an adjacent filament eruption. In Hα images, four
  groups of chromospheric fibrils are observed to form a saddle-like
  structure. Among them, two groups of fibrils converge and reconnect. Two
  newly reconnected fibrils then form and retract away from the
  reconnection region. In EUV images, similar structures and evolution
  of coronal loops are identified. The current sheet forms repeatedly
  at the interface of reconnecting loops, with a width and length of
  1-2 and 5.3-7.2 Mm and a reconnection rate of 0.18-0.3. It appears
  in the EUV low-temperature channels, with an average differential
  emission measure (DEM) weighed temperature and EM of 2 MK and 2.5
  × 10<SUP>27</SUP> cm<SUP>-5</SUP>. Plasmoids appear in the current
  sheet and propagate along it, and then further along the reconnection
  loops. The filament, located to the southeast of the reconnection
  region, erupts and pushes away the loops covering the reconnection
  region. Thereafter, the current sheet has a width and length of 2
  and 3.5 Mm and a reconnection rate of 0.57. It becomes much brighter
  and appears in the EUV high-temperature channels, with an average
  DEM-weighed temperature and EM of 5.5 MK and 1.7 × 10<SUP>28</SUP>
  cm<SUP>-5</SUP>. In the current sheet, more hotter plasmoids form. More
  thermal and kinetic energy is hence converted. These results suggest
  that the reconnection is significantly accelerated by the propagating
  disturbance caused by the nearby filament eruption.

Title: Magnetic Imaging of the Outer Solar Atmosphere (MImOSA):
    Unlocking the driver of the dynamics in the upper solar atmosphere
Authors: Peter, H.; Alsina Ballester, E.; Andretta, V.; Auchere, F.;
   Belluzzi, L.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Calcines, A.;
   Chitta, L. P.; Dalmasse, K.; del Pino Aleman, T.; Feller, A.; Froment,
   C.; Harrison, R.; Janvier, M.; Matthews, S.; Parenti, S.; Przybylski,
   D.; Solanki, S. K.; Stepan, J.; Teriaca, L.; Trujillo Bueno, J.
Bibcode: 2021arXiv210101566P
Altcode:
  The magnetic activity of the Sun directly impacts the Earth and human
  life. Likewise, other stars will have an impact on the habitability
  of planets orbiting these host stars. The lack of information on the
  magnetic field in the higher atmospheric layers hampers our progress in
  understanding solar magnetic activity. Overcoming this limitation would
  allow us to address four paramount long-standing questions: (1) How
  does the magnetic field couple the different layers of the atmosphere,
  and how does it transport energy? (2) How does the magnetic field
  structure, drive and interact with the plasma in the chromosphere and
  upper atmosphere? (3) How does the magnetic field destabilise the outer
  solar atmosphere and thus affect the interplanetary environment? (4)
  How do magnetic processes accelerate particles to high energies? New
  ground-breaking observations are needed to address these science
  questions. We suggest a suite of three instruments that far exceed
  current capabilities in terms of spatial resolution, light-gathering
  power, and polarimetric performance: (a) A large-aperture UV-to-IR
  telescope of the 1-3 m class aimed mainly to measure the magnetic
  field in the chromosphere by combining high spatial resolution and high
  sensitivity. (b) An extreme-UV-to-IR coronagraph that is designed to
  measure the large-scale magnetic field in the corona with an aperture
  of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30
  cm telescope that combines high throughput in the extreme UV with
  polarimetry to connect the magnetic measurements of the other two
  instruments. This mission to measure the magnetic field will unlock
  the driver of the dynamics in the outer solar atmosphere and thereby
  greatly advance our understanding of the Sun and the heliosphere.

Title: Spectroscopic Observations of the Eruption of an Filament
    and Associated Magnetic Reconnection
Authors: Hu, H.; Liu, Y. D.; Peter, H.; Chitta, L. P.; Wang, R.
Bibcode: 2020AGUFMSH0010013H
Altcode:
  We analyze the spectroscopic data from Interface Region Imaging
  Spectrograph (IRIS) and images from Solar Dynamics Observatory (SDO)
  to investigate the eruption and associated magnetic reconnection of
  a filament structure in an active region (AR). Doppler maps derived
  from Si IV 1394 Å based on single Gaussian fitting indicate that the
  filament structure ascended rapidly after the birth of the situated
  AR. The ascent apparently ceased ~2 days after the rapid ascent. The
  filament structure gradually ascended again ~1 day before the
  eruption. Blue and red shifts of ~30 km/s are observed in a narrow flare
  region between two filament threads ~2 hours before the eruption, which
  are probably the signatures of the outflows of magnetic reconnection
  between the two filament threads. Eventually the upper filament thread
  erupted and the lower filament thread remained. Downward motion with a
  red shift of ~30 km/s and density enhancement are also observed in the
  regions of flare ribbons during the eruption. This work pictures the
  evolution of a filament structure before an eruption and the associated
  magnetic reconnection between two filament threads.

Title: Relation of Coronal Rain Originating from Coronal Condensations
    to Interchange Magnetic Reconnection
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Song,
   Hongqiang
Bibcode: 2020ApJ...905...26L
Altcode: 2020arXiv201100709L
  Using extreme-ultraviolet images, we recently proposed a new and
  alternative formation mechanism for coronal rain along magnetically
  open field lines due to interchange magnetic reconnection. In this
  paper we report coronal rain at chromospheric and transition region
  temperatures originating from the coronal condensations facilitated
  by reconnection between open and closed coronal loops. For this,
  we employ the Interface Region Imaging Spectrograph (IRIS) and
  the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics
  Observatory. Around 2013 October 19, a coronal rain along curved
  paths was recorded by IRIS over the southeastern solar limb. Related
  to this, we found reconnection between a system of higher-lying open
  features and lower-lying closed loops that occurs repeatedly in AIA
  images. In this process, the higher-lying features form magnetic
  dips. In response, two sets of newly reconnected loops appear and
  retract away from the reconnection region. In the dips, seven events
  of cooling and condensation of coronal plasma repeatedly occur due
  to thermal instability over several days, from October 18 to 20. The
  condensations flow downward to the surface as coronal rain, with a
  mean interval between condensations of ∼6.6 hr. In the cases where
  IRIS data were available we found the condensations to cool all the
  way down to chromospheric temperatures. Based on our observations we
  suggest that some of the coronal rain events observed at chromospheric
  temperatures could be explained by the new and alternative scenario for
  the formation of coronal rain, where the condensation is facilitated
  by interchange reconnection.

Title: Impulsive coronal heating during the interaction of surface
    magnetic fields in the lower solar atmosphere
Authors: Chitta, L. P.; Peter, H.; Priest, E. R.; Solanki, S. K.
Bibcode: 2020A&A...644A.130C
Altcode: 2020arXiv201012560C
  Coronal plasma in the cores of solar active regions is impulsively
  heated to more than 5 MK. The nature and location of the magnetic
  energy source responsible for such impulsive heating is poorly
  understood. Using observations of seven active regions from the Solar
  Dynamics Observatory, we found that a majority of coronal loops hosting
  hot plasma have at least one footpoint rooted in regions of interacting
  mixed magnetic polarity at the solar surface. In cases when co-temporal
  observations from the Interface Region Imaging Spectrograph space
  mission are available, we found spectroscopic evidence for magnetic
  reconnection at the base of the hot coronal loops. Our analysis suggests
  that interactions of magnetic patches of opposite polarity at the
  solar surface and the associated energy release during reconnection
  are key to impulsive coronal heating. <P />Movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202039099/olm">https://www.aanda.org</A>

Title: The Solar Orbiter Science Activity Plan. Translating solar
    and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
   Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
   A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
   Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
   Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
   Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
   Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
   Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
   L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
   A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
   F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
   Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
   Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
   van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
   L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
   D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
   S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
   G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
   D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
   K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
   J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
   I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
   Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
   G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
   Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
   Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
   K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
   H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
   Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
   Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
   J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
   Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
   Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
   Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
   Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
   Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
   G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
   A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
   Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
   T.; Young, P. R.; Zhukov, A. N.
Bibcode: 2020A&A...642A...3Z
Altcode: 2020arXiv200910772Z
  Solar Orbiter is the first space mission observing the solar plasma
  both in situ and remotely, from a close distance, in and out of the
  ecliptic. The ultimate goal is to understand how the Sun produces
  and controls the heliosphere, filling the Solar System and driving
  the planetary environments. With six remote-sensing and four in-situ
  instrument suites, the coordination and planning of the operations are
  essential to address the following four top-level science questions:
  (1) What drives the solar wind and where does the coronal magnetic field
  originate?; (2) How do solar transients drive heliospheric variability?;
  (3) How do solar eruptions produce energetic particle radiation that
  fills the heliosphere?; (4) How does the solar dynamo work and drive
  connections between the Sun and the heliosphere? Maximising the
  mission's science return requires considering the characteristics
  of each orbit, including the relative position of the spacecraft
  to Earth (affecting downlink rates), trajectory events (such
  as gravitational assist manoeuvres), and the phase of the solar
  activity cycle. Furthermore, since each orbit's science telemetry
  will be downloaded over the course of the following orbit, science
  operations must be planned at mission level, rather than at the level
  of individual orbits. It is important to explore the way in which those
  science questions are translated into an actual plan of observations
  that fits into the mission, thus ensuring that no opportunities are
  missed. First, the overarching goals are broken down into specific,
  answerable questions along with the required observations and the
  so-called Science Activity Plan (SAP) is developed to achieve this. The
  SAP groups objectives that require similar observations into Solar
  Orbiter Observing Plans, resulting in a strategic, top-level view of
  the optimal opportunities for science observations during the mission
  lifetime. This allows for all four mission goals to be addressed. In
  this paper, we introduce Solar Orbiter's SAP through a series of
  examples and the strategy being followed.

Title: Non-thermal line broadening due to braiding-induced turbulence
    in solar coronal loops
Authors: Pontin, D. I.; Peter, H.; Chitta, L. P.
Bibcode: 2020A&A...639A..21P
Altcode: 2020arXiv200811915P
  <BR /> Aims: Emission line profiles from solar coronal loops exhibit
  properties that are unexplained by current models. We investigate
  the non-thermal broadening associated with plasma heating in coronal
  loops that is induced by magnetic field line braiding. <BR /> Methods:
  We describe the coronal loop by a 3D magnetohydrodynamic model of
  the turbulent decay of an initially-braided magnetic field. From
  this, we synthesised the Fe XII line at 193 Å that forms around
  1.5 MK. <BR /> Results: The key features of current observations of
  extreme ultraviolet (UV) lines from the corona are reproduced in the
  synthesised spectra: (i) Typical non-thermal widths range from 15 to
  20 km s<SUP>-1</SUP>. (ii) The widths are approximately independent
  of the size of the field of view. (iii) There is a correlation between
  the line intensity and non-thermal broadening. (iv) Spectra are found
  to be non-Gaussian, with enhanced power in the wings of the order
  of 10-20%. <BR /> Conclusions: Our model provides an explanation
  that self-consistently connects the heating process to the observed
  non-thermal line broadening. The non-Gaussian nature of the spectra
  is a consequence of the non-Gaussian nature of the underlying velocity
  fluctuations, which is interpreted as a signature of intermittency in
  the turbulence.

Title: Solar Photosphere
Authors: Chitta, L. P.; Smitha, H. N.; Solanki, S. K.
Bibcode: 2020orep.bookE...1C
Altcode:
  No abstract at ADS

Title: Onset of Turbulent Fast Magnetic Reconnection Observed in
    the Solar Atmosphere
Authors: Chitta, L. P.; Lazarian, A.
Bibcode: 2020ApJ...890L...2C
Altcode: 2020arXiv200108595C
  Fast magnetic reconnection powers explosive events throughout
  the universe, from gamma-ray bursts to solar flares. Despite its
  importance, the onset of astrophysical fast reconnection is the subject
  of intense debate and remains an open question in plasma physics. Here
  we report high-cadence observations of two reconnection-driven solar
  microflares obtained by the Interface Region Imaging Spectrograph
  that show persistent turbulent flows preceding flaring. The speeds
  of these flows are comparable to the local sound speed initially,
  suggesting the onset of fast reconnection in a highly turbulent
  plasma environment. Our results are in close quantitative agreement
  with the theory of turbulence-driven reconnection as well as with
  numerical simulations in which fast magnetic reconnection is induced
  by turbulence.

Title: Repeated Coronal Condensations Caused by Magnetic Reconnection
    between Solar Coronal Loops
Authors: Li, Leping; Peter, Hardi; Chitta, Lakshmi Pradeep; Zhang,
   Jun; Su, Jiangtao; Song, Hongqiang; Hou, Yijun; Xia, Chun
Bibcode: 2019ApJ...884...34L
Altcode:
  We recently presented coronal condensations, caused by magnetic
  reconnection (MR) between coronal loops from extreme ultraviolet
  observations, over the course of one day, on 2012 January 19. In
  this paper, by investigating the loops over an extended period of
  time from January 16 to 20, we present a case for repeated coronal
  condensations caused by repeated MR between them. In these five days,
  MR between higher-lying open loops and lower-lying closed loops occurs
  repeatedly, forming magnetic dips in the higher-lying open loops. During
  the MR process, cooling and condensation of coronal plasma occur
  repeatedly. Early on January 16, cooling, but not condensation, of
  coronal plasma happens. Later, condensation appears at the edge of the
  dips and falls down along the loops as coronal rains. On January 17,
  a similar condensation happens at the edge of the higher-lying dips
  and falls down along the loops. However, another condensation appears
  in the lower-lying dips and rains down across them. From January 18
  to 19, multiple condensations mostly occur at the edge of the dips
  and fall down both along the loops and across the dips. On January
  20, five condensations sequentially appear and rain down across the
  dips. Overall, 15 condensation events occur in five days, lasting from
  0.5 to 15.6 hr. We suggest that the formation of coronal condensations
  by MR between loops is common in the solar corona. The repeated MR
  between loops thus plays an essential role in the mass cycle of coronal
  plasma by initiating repeated catastrophic cooling and condensation.

Title: Plasmoid-mediated reconnection in solar UV bursts
Authors: Peter, H.; Huang, Y. -M.; Chitta, L. P.; Young, P. R.
Bibcode: 2019A&A...628A...8P
Altcode: 2019arXiv190704335P
  Context. Ultraviolet bursts are transients in the solar atmosphere with
  an increased impulsive emission in the extreme UV lasting for one to
  several tens of minutes. They often show spectral profiles indicative
  of a bi-directional outflow in response to magnetic reconnection. <BR
  /> Aims: To understand UV bursts, we study how motions of magnetic
  elements at the surface can drive the self-consistent formation
  of a current sheet resulting in plasmoid-mediated reconnection. In
  particular, we want to study the role of the height of the reconnection
  in the atmosphere. <BR /> Methods: We conducted numerical experiments
  solving the 2D magnetohydrodynamic equations from the solar surface
  to the upper atmosphere. Motivated by observations, we drove a small
  magnetic patch embedded in a larger system of magnetic field of opposite
  polarity. This type of configuration creates an X-type neutral point
  in the initial potential field. The models are characterized by the
  (average) plasma-β at the height of this X point. <BR /> Results:
  The driving at the surface stretches the X-point into a thin current
  sheet, where plasmoids appear, accelerating the reconnection, and a
  bi-directional jet forms. This is consistent with what is expected
  for UV bursts or explosive events, and we provide a self-consistent
  model of the formation of the reconnection region in such events. The
  gravitational stratification gives a natural explanation for why
  explosive events are restricted to a temperature range around a few
  0.1 MK, and the presence of plasmoids in the reconnection process
  provides an understanding of the observed variability during the
  transient events on a timescale of minutes. <BR /> Conclusions: Our
  numerical experiments provide a comprehensive understanding of UV bursts
  and explosive events, in particular of how the atmospheric response
  changes if the reconnection happens at different plasma-β, that is,
  at different heights in the atmosphere. This analysis also gives
  new insight into how UV bursts might be related to the photospheric
  Ellerman bombs. <P />Movie attached to Fig. 2 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201935820/olm">https://www.aanda.org</A>

Title: Hot prominence spicules launched from turbulent cool solar
    prominences
Authors: Chitta, L. P.; Peter, H.; Li, L.
Bibcode: 2019A&A...627L...5C
Altcode: 2019arXiv190609125C
  A solar filament is a dense cool condensation that is supported and
  thermally insulated by magnetic fields in the rarefied hot corona. Its
  evolution and stability, leading to either an eruption or disappearance,
  depend on its coupling with the surrounding hot corona through a
  thin transition region, where the temperature steeply rises. However,
  the heating and dynamics of this transition region remain elusive. We
  report extreme-ultraviolet observations of quiescent filaments from the
  Solar Dynamics Observatory that reveal prominence spicules propagating
  through the transition region of the filament-corona system. These thin
  needle-like jet features are generated and heated to at least 0.7 MK
  by turbulent motions of the material in the filament. We suggest that
  the prominence spicules continuously channel the heated mass into the
  corona and aid in the filament evaporation and decay. Our results shed
  light on the turbulence-driven heating in magnetized condensations
  that are commonly observed on the Sun and in the interstellar
  medium. <P />The movie associated to Fig. 1 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201936027/olm">https://www.aanda.org</A>.

Title: Energetics of magnetic transients in a solar active region
    plage
Authors: Chitta, L. P.; Sukarmadji, A. R. C.; Rouppe van der Voort,
   L.; Peter, H.
Bibcode: 2019A&A...623A.176C
Altcode: 2019arXiv190201650C
  Context. Densely packed coronal loops are rooted in photospheric
  plages in the vicinity of active regions on the Sun. The photospheric
  magnetic features underlying these plage areas are patches of mostly
  unidirectional magnetic field extending several arcsec on the solar
  surface. <BR /> Aims: We aim to explore the transient nature of
  the magnetic field, its mixed-polarity characteristics, and the
  associated energetics in the active region plage using high spatial
  resolution observations and numerical simulations. <BR /> Methods:
  We used photospheric Fe I 6173 Å spectropolarimetric observations of
  a decaying active region obtained from the Swedish 1-m Solar Telescope
  (SST). These data were inverted to retrieve the photospheric magnetic
  field underlying the plage as identified in the extreme-ultraviolet
  emission maps obtained from the Atmospheric Imaging Assembly (AIA)
  on board the Solar Dynamics Observatory (SDO). To obtain better
  insight into the evolution of extended unidirectional magnetic field
  patches on the Sun, we performed 3D radiation magnetohydrodynamic
  simulations of magnetoconvection using the MURaM code. <BR />
  Results: The observations show transient magnetic flux emergence
  and cancellation events within the extended predominantly unipolar
  patch on timescales of a few 100 s and on spatial scales comparable
  to granules. These transient events occur at the footpoints of active
  region plage loops. In one case the coronal response at the footpoints
  of these loops is clearly associated with the underlying transient. The
  numerical simulations also reveal similar magnetic flux emergence and
  cancellation events that extend to even smaller spatial and temporal
  scales. Individual simulated transient events transfer an energy
  flux in excess of 1 MW m<SUP>-2</SUP> through the photosphere. <BR
  /> Conclusions: We suggest that the magnetic transients could play
  an important role in the energetics of active region plage. Both in
  observations and simulations, the opposite-polarity magnetic field
  brought up by transient flux emergence cancels with the surrounding
  plage field. Magnetic reconnection associated with such transient events
  likely conduits magnetic energy to power the overlying chromosphere
  and coronal loops.

Title: A Cancellation Nanoflare Model for Solar Chromospheric and
    Coronal Heating. II. 2D Theory and Simulations
Authors: Syntelis, P.; Priest, E. R.; Chitta, L. P.
Bibcode: 2019ApJ...872...32S
Altcode: 2019arXiv190102798S
  Recent observations at high spatial resolution have shown that magnetic
  flux cancellation occurs on the solar surface much more frequently than
  previously thought, and so this led Priest et al. (2018) to propose
  magnetic reconnection driven by photospheric flux cancellation as
  a mechanism for chromospheric and coronal heating. In particular,
  they estimated analytically the amount of energy released as heat
  and the height of the energy release during flux cancellation. In
  the present work, we take the next step in the theory by setting up a
  two-dimensional resistive MHD simulation of two canceling polarities
  in the presence of a horizontal external field and a stratified
  atmosphere in order to check and improve upon the analytical
  estimates. Computational evaluation of the energy release during
  reconnection is found to be in good qualitative agreement with the
  analytical estimates. In addition, we go further and undertake an
  initial study of the atmospheric response to reconnection. We find
  that, during the cancellation, either hot ejections or cool ones or a
  combination of both hot and cool ejections can be formed, depending
  on the height of the reconnection location. The hot structures can
  have the density and temperature of coronal loops, while the cooler
  structures are suggestive of surges and large spicules.

Title: Quasi-periodic Fast Propagating Magnetoacoustic Waves during
    the Magnetic Reconnection Between Solar Coronal Loops
Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Chitta, Lakshmi Pradeep;
   Su, Jiangtao; Song, Hongqiang; Xia, Chun; Hou, Yijun
Bibcode: 2018ApJ...868L..33L
Altcode: 2018arXiv181108553L
  Employing Solar Dynamics Observatory/Atmospheric Imaging Assembly
  (AIA) multi-wavelength images, we have presented coronal condensations
  caused by magnetic reconnection between a system of open and closed
  solar coronal loops. In this Letter, we report the quasi-periodic fast
  magnetoacoustic waves propagating away from the reconnection region
  upward across the higher-lying open loops during the reconnection
  process. On 2012 January 19, reconnection between the higher-lying
  open loops and lower-lying closed loops took place, and two sets of
  newly reconnected loops formed. Thereafter, cooling and condensations
  of coronal plasma occurred in the magnetic dip region of higher-lying
  open loops. During the reconnection process, disturbances originating
  from the reconnection region propagate upward across the magnetic
  dip region of higher-lying loops with the mean speed and mean speed
  amplitude of 200 and 30 km s<SUP>-1</SUP>, respectively. The mean speed
  of the propagating disturbances decreases from ∼230 km s<SUP>-1</SUP>
  to ∼150 km s<SUP>-1</SUP> during the coronal condensation process,
  and then increases to ∼220 km s<SUP>-1</SUP>. This temporal evolution
  of the mean speed anti-correlates with the light curves of the AIA 131
  and 304 Å channels that show the cooling and condensation process
  of coronal plasma. Furthermore, the propagating disturbances appear
  quasi-periodically with a peak period of 4 minutes. Our results suggest
  that the disturbances represent the quasi-periodic fast propagating
  magnetoacoustic (QFPM) waves originating from the magnetic reconnection
  between coronal loops.

Title: Solar Ultraviolet Bursts
Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.;
   Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal
   J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria
   S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark
   C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr
Bibcode: 2018SSRv..214..120Y
Altcode: 2018arXiv180505850Y
  The term "ultraviolet (UV) burst" is introduced to describe small,
  intense, transient brightenings in ultraviolet images of solar active
  regions. We inventorize their properties and provide a definition
  based on image sequences in transition-region lines. Coronal signatures
  are rare, and most bursts are associated with small-scale, canceling
  opposite-polarity fields in the photosphere that occur in emerging flux
  regions, moving magnetic features in sunspot moats, and sunspot light
  bridges. We also compare UV bursts with similar transition-region
  phenomena found previously in solar ultraviolet spectrometry and
  with similar phenomena at optical wavelengths, in particular Ellerman
  bombs. Akin to the latter, UV bursts are probably small-scale magnetic
  reconnection events occurring in the low atmosphere, at photospheric
  and/or chromospheric heights. Their intense emission in lines with
  optically thin formation gives unique diagnostic opportunities
  for studying the physics of magnetic reconnection in the low solar
  atmosphere. This paper is a review report from an International Space
  Science Institute team that met in 2016-2017.

Title: Observations of solar chromospheric heating at sub-arcsec
    spatial resolution
Authors: Smitha, H. N.; Chitta, L. P.; Wiegelmann, T.; Solanki, S. K.
Bibcode: 2018A&A...617A.128S
Altcode: 2018arXiv180701078S
  A wide variety of phenomena such as gentle but persistent
  brightening, dynamic slender features (∼100 km), and compact
  (∼1″) ultraviolet (UV) bursts are associated with the heating of
  the solar chromosphere. High spatio-temporal resolution is required to
  capture the finer details of the likely magnetic reconnection-driven,
  rapidly evolving bursts. Such observations are also needed to reveal
  their similarities to large-scale flares, which are also thought to be
  reconnection driven, and more generally their role in chromospheric
  heating. Here we report observations of chromospheric heating in
  the form of a UV burst obtained with the balloon-borne observatory
  SUNRISE. The observed burst displayed a spatial morphology similar
  to that of a large-scale solar flare with a circular ribbon. While
  the co-temporal UV observations at 1.5″ spatial resolution and
  24 s cadence from the Solar Dynamics Observatory showed a compact
  brightening, the SUNRISE observations at diffraction-limited spatial
  resolution of 0.1″ at 7 s cadence revealed a dynamic substructure
  of the burst that it is composed of an extended ribbon-like feature
  and a rapidly evolving arcade of thin (∼0.1″) magnetic loop-like
  features, similar to post-flare loops. Such a dynamic substructure
  reveals the small-scale nature of chromospheric heating in these
  bursts. Furthermore, based on magnetic field extrapolations, this
  heating event is associated with a complex fan-spine magnetic
  topology. Our observations strongly hint at a unified picture of
  magnetic heating in the solar atmosphere from some large-scale
  flares to small-scale bursts, all associated with such a magnetic
  topology. <P />The movie associated to Fig. 2 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201833276/olm">https://www.aanda.org/</A>

Title: Emission of solar chromospheric and transition region features
    related to the underlying magnetic field
Authors: Barczynski, K.; Peter, H.; Chitta, L. P.; Solanki, S. K.
Bibcode: 2018A&A...619A...5B
Altcode: 2018arXiv180702372B
  Context. The emission of the upper atmosphere of the Sun is closely
  related to magnetic field concentrations at the solar surface. <BR />
  Aims: It is well established that this relation between chromospheric
  emission and magnetic field is nonlinear. Here we investigate
  systematically how this relation, characterised by the exponent
  of a power-law fit, changes through the atmosphere, from the upper
  photosphere through the temperature minimum region and chromosphere
  to the transition region. <BR /> Methods: We used spectral maps from
  the Interface Region Imaging Spectrograph (IRIS) covering Mg II and
  its wings, C II, and Si IV together with magnetograms and UV continuum
  images from the Solar Dynamics Observatory. After a careful alignment
  of the data we performed a power-law fit for the relation between each
  pair of observables and determine the power-law index (or exponent) for
  these. This was done for different spatial resolutions and different
  features on the Sun. <BR /> Results: While the correlation between
  emission and magnetic field drops monotonically with temperature,
  the power-law index shows a hockey-stick-type variation: from the
  upper photosphere to the temperature-minimum it drops sharply and then
  increases through the chromosphere into the transition region. This
  is even seen through the features of the Mg II line, this is,
  from k1 to k2 and k3. It is irrespective of spatial resolution or
  whether we investigate active regions, plage areas, quiet Sun, or
  coronal holes. <BR /> Conclusions: In accordance with the general
  picture of flux-flux relations from the chromosphere to the corona,
  above the temperature minimum the sensitivity of the emission to the
  plasma heating increases with temperature. Below the temperature
  minimum a different mechanism has to govern the opposite trend of
  the power-law index with temperature. We suggest four possibilities,
  in other words, a geometric effect of expanding flux tubes filling the
  available chromospheric volume, the height of formation of the emitted
  radiation, the dependence on wavelength of the intensity-temperature
  relationship, and the dependence of the heating of flux tubes on the
  magnetic flux density.

Title: Coronal Condensations Caused by Magnetic Reconnection between
    Solar Coronal Loops
Authors: Li, Leping; Zhang, Jun; Peter, Hardi; Chitta, Lakshmi Pradeep;
   Su, Jiangtao; Xia, Chun; Song, Hongqiang; Hou, Yijun
Bibcode: 2018ApJ...864L...4L
Altcode: 2018arXiv180809626L
  Employing Solar Dynamics Observatory/Atmospheric Imaging Assembly
  (AIA) multi-wavelength images, we report the coronal condensation
  during the magnetic reconnection (MR) between a system of open and
  closed coronal loops. Higher-lying magnetically open structures,
  observed in AIA 171 Å images above the solar limb, move downward and
  interact with the lower-lying closed loops, resulting in the formation
  of dips in the former. An X-type structure forms at the interface. The
  interacting loops reconnect and disappear. Two sets of newly reconnected
  loops then form and recede from the MR region. During the MR process,
  bright emission appears sequentially in the AIA 131 and 304 Å channels
  repeatedly in the dips of higher-lying open structures. This indicates
  the cooling and condensation process of hotter plasma from ∼0.9
  MK down to ∼0.6 MK, and then to ∼0.05 MK, also supported by the
  light curves of the AIA 171, 131, and 304 Å channels. The part of
  higher-lying open structures supporting the condensation participate
  in the successive MR. Without support from underlying loops, the
  condensation then rains back to the solar surface along the newly
  reconnected loops. Our results suggest that the MR between coronal loops
  leads to the condensation of hotter coronal plasma and its downflows. MR
  thus plays an active role in the mass cycle of coronal plasma because it
  can initiate the catastrophic cooling and condensation. This underlines
  that the magnetic and thermal evolution has to be treated together
  and cannot be separated, even in the case of catastrophic cooling.

Title: A Cancellation Nanoflare Model for Solar Chromospheric and
    Coronal Heating
Authors: Priest, E. R.; Chitta, L. P.; Syntelis, P.
Bibcode: 2018ApJ...862L..24P
Altcode: 2018arXiv180708161P
  Nanoflare models for heating the solar corona usually assume magnetic
  braiding and reconnection as the source of the energy. However,
  recent observations at record spatial resolution from the SUNRISE
  balloon mission suggest that photospheric magnetic flux cancellation
  is much more common than previously realized. We therefore examine
  the possibility of three-dimensional reconnection driven by flux
  cancellation as a cause of chromospheric and coronal heating. In
  particular, we estimate how the heights and amount of energy release
  produced by flux cancellation depend on flux size, flux cancellation
  speed, and overlying field strength.

Title: Nature of the energy source powering solar coronal loops
    driven by nanoflares
Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.
Bibcode: 2018A&A...615L...9C
Altcode: 2018arXiv180611045C
  Context. Magnetic energy is required to heat the corona, the outer
  atmosphere of the Sun, to millions of degrees. <BR /> Aims: We study the
  nature of the magnetic energy source that is probably responsible for
  the brightening of coronal loops driven by nanoflares in the cores of
  solar active regions. <BR /> Methods: We consider observations of two
  active regions (ARs), 11890 and 12234, in which nanoflares have been
  detected. To this end, we use ultraviolet (UV) and extreme ultraviolet
  (EUV) images from the Atmospheric Imaging Assembly (AIA) onboard the
  Solar Dynamics Observatory (SDO) for coronal loop diagnostics. These
  images are combined with the co-temporal line-of-sight magnetic
  field maps from the Helioseismic and Magnetic Imager (HMI) onboard
  SDO to investigate the connection between coronal loops and their
  magnetic roots in the photosphere. <BR /> Results: The core of
  these ARs exhibit loop brightening in multiple EUV channels of AIA,
  particularly in its 9.4 nm filter. The HMI magnetic field maps reveal
  the presence of a complex mixed polarity magnetic field distribution
  at the base of these loops. We detect the cancellation of photospheric
  magnetic flux at these locations at a rate of about 10<SUP>15</SUP>
  Mx s<SUP>-1</SUP>. The associated compact coronal brightenings directly
  above the cancelling magnetic features are indicative of plasma heating
  due to chromospheric magnetic reconnection. <BR /> Conclusions:
  We suggest that the complex magnetic topology and the evolution of
  magnetic field, such as flux cancellation in the photosphere and
  the resulting chromospheric reconnection, can play an important role
  in energizing active region coronal loops driven by nanoflares. Our
  estimate of magnetic energy release during flux cancellation in the
  quiet Sun suggests that chromospheric reconnection can also power the
  quiet corona. <P />The movie associated to Fig. 1 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201833404/olm">https://www.aanda.org</A>

Title: Compact solar UV burst triggered in a magnetic field with a
    fan-spine topology
Authors: Chitta, L. P.; Peter, H.; Young, P. R.; Huang, Y. -M.
Bibcode: 2017A&A...605A..49C
Altcode: 2017arXiv170608059C
  Context. Solar ultraviolet (UV) bursts are small-scale features
  that exhibit intermittent brightenings that are thought to be due to
  magnetic reconnection. They are observed abundantly in the chromosphere
  and transition region, in particular in active regions. <BR /> Aims:
  We investigate in detail a UV burst related to a magnetic feature that
  is advected by the moat flow from a sunspot towards a pore. The moving
  feature is parasitic in that its magnetic polarity is opposite to that
  of the spot and the pore. This comparably simple photospheric magnetic
  field distribution allows for an unambiguous interpretation of the
  magnetic geometry leading to the onset of the observed UV burst. <BR />
  Methods: We used UV spectroscopic and slit-jaw observations from the
  Interface Region Imaging Spectrograph (IRIS) to identify and study
  chromospheric and transition region spectral signatures of said UV
  burst. To investigate the magnetic topology surrounding the UV burst,
  we used a two-hour-long time sequence of simultaneous line-of-sight
  magnetograms from the Helioseismic and Magnetic Imager (HMI) and
  performed data-driven 3D magnetic field extrapolations by means of
  a magnetofrictional relaxation technique. We can connect UV burst
  signatures to the overlying extreme UV (EUV) coronal loops observed
  by the Atmospheric Imaging Assembly (AIA). <BR /> Results: The UV
  burst shows a variety of extremely broad line profiles indicating
  plasma flows in excess of ±200 km s<SUP>-1</SUP> at times. The whole
  structure is divided into two spatially distinct zones of predominantly
  up- and downflows. The magnetic field extrapolations show a persistent
  fan-spine magnetic topology at the UV burst. The associated 3D magnetic
  null point exists at a height of about 500 km above the photosphere
  and evolves co-spatially with the observed UV burst. The EUV emission
  at the footpoints of coronal loops is correlated with the evolution of
  the underlying UV burst. <BR /> Conclusions: The magnetic field around
  the null point is sheared by photospheric motions, triggering magnetic
  reconnection that ultimately powers the observed UV burst and energises
  the overlying coronal loops. The location of the null point suggests
  that the burst is triggered low in the solar chromosphere. <P
  />Movies associated to Figs. 2 and 4 are available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201730830/olm">http://www.aanda.org</A>

Title: Solar Coronal Loops Associated with Small-scale Mixed Polarity
    Surface Magnetic Fields
Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.; Barthol, P.;
   Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van
   Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco
   Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M.
Bibcode: 2017ApJS..229....4C
Altcode: 2016arXiv161007484C
  How and where are coronal loops rooted in the solar lower
  atmosphere? The details of the magnetic environment and its evolution
  at the footpoints of coronal loops are crucial to understanding the
  processes of mass and energy supply to the solar corona. To address
  the above question, we use high-resolution line-of-sight magnetic
  field data from the Imaging Magnetograph eXperiment instrument on the
  Sunrise balloon-borne observatory and coronal observations from the
  Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory
  of an emerging active region. We find that the coronal loops are
  often rooted at the locations with minor small-scale but persistent
  opposite-polarity magnetic elements very close to the larger dominant
  polarity. These opposite-polarity small-scale elements continually
  interact with the dominant polarity underlying the coronal loop through
  flux cancellation. At these locations we detect small inverse Y-shaped
  jets in chromospheric Ca II H images obtained from the Sunrise Filter
  Imager during the flux cancellation. Our results indicate that magnetic
  flux cancellation and reconnection at the base of coronal loops due
  to mixed polarity fields might be a crucial feature for the supply of
  mass and energy into the corona.

Title: Coronal loop footpoints threaded with small-scale mixed
    polarity surface magnetic fields
Authors: Chitta, L. P.
Bibcode: 2017psio.confE..62C
Altcode:
  No abstract at ADS

Title: A closer look at a coronal loop rooted in a sunspot umbra
Authors: Chitta, L. P.; Peter, H.; Young, P. R.
Bibcode: 2016A&A...587A..20C
Altcode: 2015arXiv151203831C
  Context. Extreme UV (EUV) and X-ray loops in the solar corona connect
  regions of enhanced magnetic activity, but they are not usually rooted
  in the dark umbrae of sunspots because the strong magnetic field
  found there suppresses convection. This means that the Poynting flux
  of magnetic energy into the upper atmosphere is not significant within
  the umbra as long as there are no light bridges or umbral dots. <BR />
  Aims: Here we report a rare observation of a coronal loop rooted in the
  dark umbra of a sunspot without any traces of light bridges or umbral
  dots. This allows us to investigate the loop without much confusion
  from background or line-of-sight integration effects. <BR /> Methods:
  We used the slit-jaw images and spectroscopic data from the Interface
  Region Imaging Spectrograph (IRIS) and concentrate on the line profiles
  of O iv and Si iv that show persistent strong redshifted components in
  the loop rooted in the umbra. Using the ratios of O iv, we can estimate
  the density and thus investigate the mass flux. The coronal context
  and temperature diagnostics of these observations is provided through
  the EUV channels of the Atmospheric Imaging Assembly (AIA). <BR />
  Results: The coronal loop, embedded within cooler downflows, hosts
  supersonic downflows. The speed of more than 100 km s<SUP>-1</SUP> is
  on the same order of magnitude in the transition region lines of O iv
  and Si iv, and is even seen at comparable speed in the chromospheric
  Mg II lines. At a projected distance of within 1'' of the footpoint,
  we see a shock transition to smaller downflow speeds of about 15
  km s<SUP>-1</SUP> being consistent with mass conservation across a
  stationary isothermal shock. <BR /> Conclusions: We see no direct
  evidence for energy input into the loop because the loop is rooted
  in the dark uniform part of the umbra with no light bridges or umbral
  dots near by. Thus one might conclude that we are seeing a siphon flow
  driven from the footpoint at the other end of the loop. However, for a
  final result data of similar quality at the other footpoint are needed,
  but this is too far away to be covered by the IRIS field of view.

Title: Limitations of force-free magnetic field extrapolations:
    Revisiting basic assumptions
Authors: Peter, H.; Warnecke, J.; Chitta, L. P.; Cameron, R. H.
Bibcode: 2015A&A...584A..68P
Altcode: 2015arXiv151004642P
  Context. Force-free extrapolations are widely used to study the magnetic
  field in the solar corona based on surface measurements. <BR /> Aims:
  The extrapolations assume that the ratio of internal energy of the
  plasma to magnetic energy, the plasma β, is negligible. Despite the
  widespread use of this assumption observations, models, and theoretical
  considerations show that β is of the order of a few percent to more
  than 10%, and thus not small. We investigate what consequences this
  has for the reliability of extrapolation results. <BR /> Methods: We
  use basic concepts starting with force and energy balance to infer
  relations between plasma β and free magnetic energy to study the
  direction of currents in the corona with respect to the magnetic
  field, and to estimate the errors in the free magnetic energy by
  neglecting effects of the plasma (β ≪ 1). A comparison with a 3D
  magneto-hydrodynamics (MHD) model supports our basic considerations. <BR
  /> Results: If plasma β is of the order of the relative free energy
  (the ratio of the free magnetic energy to the total magnetic energy)
  then the pressure gradient can balance the Lorentz force. This is the
  case in solar corona, and therefore the currents are not properly
  described. In particular, the error in terms of magnetic energy by
  neglecting the plasma is of the order of the free magnetic energy, so
  that the latter cannot be reliably determined by an extrapolation. <BR
  /> Conclusions: While a force-free extrapolation might capture the
  magnetic structure and connectivity of the coronal magnetic field,
  the derived currents and free magnetic energy are not reliable. Thus
  quantitative results of extrapolations on the location and amount of
  heating in the corona (through current dissipation) and on the energy
  storage of the magnetic field (e.g. for eruptive events) are limited.

Title: VLBI Imaging of the Double Peaked Emission Line Seyfert
    KISSR 1494
Authors: Kharb, P.; Das, M.; Paragi, Z.; Subramanian, S.; Chitta, L. P.
Bibcode: 2015ApJ...799..161K
Altcode: 2014arXiv1412.0400K
  We present here the results from dual-frequency phase-referenced Very
  Long Baseline Interferometry observations of the Seyfert galaxy KISSR
  1494, which exhibits double peaked emission lines in its Sloan Digital
  Sky Survey spectrum. We detect a single radio component at 1.6 GHz,
  but not at 5 GHz, implying a spectral index steeper than -1.5 ± 0.5
  (S <SUB>ν</SUB>vpropν<SUP>α</SUP>). The high brightness temperature
  of the radio component (~1.4 × 10<SUP>7</SUP> K) and the steep radio
  spectrum support a non-thermal synchrotron origin. A crude estimate of
  the black hole mass derived from the M <SUB>BH</SUB>-σ<SUB>sstarf</SUB>
  relation is ~1.4 ± 1.0 × 10<SUP>8</SUP> M <SUB>⊙</SUB> it is
  accreting at an Eddington rate of ~0.02. The radio data are consistent
  with either the radio emission coming from the parsec-scale base of
  a synchrotron wind originating in the magnetized corona above the
  accretion disk, or from the inner ionized edge of the accretion disk
  or torus. In the former case, the narrow line region (NLR) clouds may
  form a part of the broad outflow, while in the latter, the NLR clouds
  may form a part of an extended disk beyond the torus. The radio and NLR
  emission may also be decoupled so that the radio emission originates
  in an outflow while the NLR is in a disk and vice versa. While
  with the present data it is not possible to clearly distinguish
  between these scenarios, there appears to be greater circumstantial
  evidence supporting the coronal wind picture in KISSR 1494. From
  the kiloparsec-scale radio emission, the time-averaged kinetic power
  of this outflow is estimated to be Q ≈ 1.5 × 10<SUP>42</SUP> erg
  s<SUP>-1</SUP>, which is typical of radio outflows in low-luminosity
  active galactic nuclei. This supports the idea that radio "jets"
  and outflowing coronal winds are indistinguishable in Seyfert galaxies.

Title: Nonlinear Force-free Field Modeling of the Solar Magnetic
    Carpet and Comparison with SDO/HMI and Sunrise/IMaX Observations
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
   DeLuca, E. E.; Solanki, S. K.
Bibcode: 2014ApJ...793..112C
Altcode: 2014arXiv1408.0497C
  In the quiet solar photosphere, the mixed polarity fields form a
  magnetic carpet that continuously evolves due to dynamical interaction
  between the convective motions and magnetic field. This interplay is a
  viable source to heat the solar atmosphere. In this work, we used the
  line-of-sight (LOS) magnetograms obtained from the Helioseismic and
  Magnetic Imager on the Solar Dynamics Observatory, and the Imaging
  Magnetograph eXperiment instrument on the Sunrise balloon-borne
  observatory, as time-dependent lower boundary conditions, to study the
  evolution of the coronal magnetic field. We use a magneto-frictional
  relaxation method, including hyperdiffusion, to produce a time series
  of three-dimensional nonlinear force-free fields from a sequence
  of photospheric LOS magnetograms. Vertical flows are added up to a
  height of 0.7 Mm in the modeling to simulate the non-force-freeness
  at the photosphere-chromosphere layers. Among the derived quantities,
  we study the spatial and temporal variations of the energy dissipation
  rate and energy flux. Our results show that the energy deposited in
  the solar atmosphere is concentrated within 2 Mm of the photosphere and
  there is not sufficient energy flux at the base of the corona to cover
  radiative and conductive losses. Possible reasons and implications are
  discussed. Better observational constraints of the magnetic field in
  the chromosphere are crucial to understand the role of the magnetic
  carpet in coronal heating.

Title: Segmentation of coronal features to understand the solar EUV
    and UV irradiance variability
Authors: Kumara, S. T.; Kariyappa, R.; Zender, J. J.; Giono, G.;
   Delouille, V.; Chitta, L. P.; Damé, L.; Hochedez, J. -F.; Verbeeck,
   C.; Mampaey, B.; Doddamani, V. H.
Bibcode: 2014A&A...561A...9K
Altcode:
  Context. The study of solar irradiance variability is of great
  importance in heliophysics, the Earth's climate, and space weather
  applications. These studies require careful identifying, tracking
  and monitoring of active regions (ARs), coronal holes (CHs), and the
  quiet Sun (QS). <BR /> Aims: We studied the variability of solar
  irradiance for a period of two years (January 2011-December 2012)
  using the Large Yield Radiometer (LYRA), the Sun Watcher using APS and
  image Processing (SWAP) on board PROBA2, and the Atmospheric Imaging
  Assembly (AIA) on board the Solar Dynamics Observatory (SDO). <BR />
  Methods: We used the spatial possibilistic clustering algorithm (SPoCA)
  to identify and segment coronal features from the EUV observations of
  AIA. The AIA segmentation maps were then applied on SWAP images, and
  parameters such as the intensity, fractional area, and contribution
  of ARs/CHs/QS features were computed and compared with the full-disk
  integrated intensity and LYRA irradiance measurements. <BR /> Results:
  We report the results obtained from SDO/AIA and PROBA2/SWAP images
  taken from January 2011 to December 2012 and compare the resulting
  integrated full-disk intensity with PROBA2/LYRA irradiance. We
  determine the contributions of the segmented features to EUV and UV
  irradiance variations. The variations of the parameters resulting
  from the segmentation, namely the area, integrated intensity, and
  relative contribution to the solar irradiance, are compared with LYRA
  irradiance. We find that the active regions have a great impact on the
  irradiance fluctuations. In the EUV passbands considered in this study,
  the QS is the greatest contributor to the solar irradiance, with up
  to 63% of total intensity values. Active regions, on the other hand,
  contribute to about 10%, and off-limb structures to about 24%. We
  also find that the area of the features is highly variable suggesting
  that their area has to be taken into account in irradiance models,
  in addition to their intensity variations. <BR /> Conclusions:
  We successfully show that the feature extraction allows us to use
  EUV telescopes to measure irradiance fluctuations and to quantify
  the contribution of each part to the EUV spectral solar irradiance
  observed with a calibrated radiometer. This study also shows that
  SPoCA is viable, and that the segmentation of images can be a useful
  tool. We also provide the measurement correlation between SWAP and
  AIA during this analysis.

Title: Observations and Modeling of the Emerging Extreme-ultraviolet
    Loops in the Quiet Sun as Seen with the Solar Dynamics Observatory
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
   DeLuca, E. E.; Hasan, S. S.; Hanslmeier, A.
Bibcode: 2013ApJ...768...32C
Altcode: 2013arXiv1303.3426C
  We used data from the Helioseismic and Magnetic Imager (HMI) and the
  Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
  (SDO) to study coronal loops at small scales, emerging in the quiet
  Sun. With HMI line-of-sight magnetograms, we derive the integrated
  and unsigned photospheric magnetic flux at the loop footpoints in the
  photosphere. These loops are bright in the EUV channels of AIA. Using
  the six AIA EUV filters, we construct the differential emission measure
  (DEM) in the temperature range 5.7-6.5 in log T (K) for several hours
  of observations. The observed DEMs have a peak distribution around
  log T ≈ 6.3, falling rapidly at higher temperatures. For log T &lt;
  6.3, DEMs are comparable to their peak values within an order of
  magnitude. The emission-weighted temperature is calculated, and its
  time variations are compared with those of magnetic flux. We present
  two possibilities for explaining the observed DEMs and temperatures
  variations. (1) Assuming that the observed loops are composed of
  a hundred thin strands with certain radius and length, we tested
  three time-dependent heating models and compared the resulting DEMs
  and temperatures with the observed quantities. This modeling used
  enthalpy-based thermal evolution of loops (EBTEL), a zero-dimensional
  (0D) hydrodynamic code. The comparisons suggest that a medium-frequency
  heating model with a population of different heating amplitudes can
  roughly reproduce the observations. (2) We also consider a loop model
  with steady heating and non-uniform cross-section of the loop along
  its length, and find that this model can also reproduce the observed
  DEMs, provided the loop expansion factor γ ~ 5-10. More observational
  constraints are required to better understand the nature of coronal
  heating in the short emerging loops on the quiet Sun.

Title: Dynamics of the Solar Magnetic Bright Points Derived from
    Their Horizontal Motions
Authors: Chitta, L. P.; van Ballegooijen, A. A.; Rouppe van der Voort,
   L.; DeLuca, E. E.; Kariyappa, R.
Bibcode: 2012ApJ...752...48C
Altcode: 2012arXiv1204.4362C
  The subarcsecond bright points (BPs) associated with the small-scale
  magnetic fields in the lower solar atmosphere are advected by
  the evolution of the photospheric granules. We measure various
  quantities related to the horizontal motions of the BPs observed in
  two wavelengths, including the velocity autocorrelation function. A
  1 hr time sequence of wideband Hα observations conducted at the
  Swedish 1 m Solar Telescope (SST) and a 4 hr Hinode G-band time
  sequence observed with the Solar Optical Telescope are used in this
  work. We follow 97 SST and 212 Hinode BPs with 3800 and 1950 individual
  velocity measurements, respectively. For its high cadence of 5 s as
  compared to 30 s for Hinode data, we emphasize more the results from
  SST data. The BP positional uncertainty achieved by SST is as low as 3
  km. The position errors contribute 0.75 km<SUP>2</SUP> s<SUP>-2</SUP>
  to the variance of the observed velocities. The raw and corrected
  velocity measurements in both directions, i.e., (v<SUB>x</SUB> ,
  v<SUB>y</SUB> ), have Gaussian distributions with standard deviations
  of (1.32, 1.22) and (1.00, 0.86) km s<SUP>-1</SUP>, respectively. The
  BP motions have correlation times of about 22-30 s. We construct the
  power spectrum of the horizontal motions as a function of frequency,
  a quantity that is useful and relevant to the studies of generation
  of Alfvén waves. Photospheric turbulent diffusion at timescales less
  than 200 s is found to satisfy a power law with an index of 1.59.

Title: Dynamics of the Photospheric Bright Points Observed With SST
    and Hinode
Authors: Chitta, Lakshmi Pradeep; van Ballegooijen, A.; Rouppe van
   der Voort, L.; DeLuca, E.; Kariyappa, R.
Bibcode: 2012AAS...22020614C
Altcode: 2012AAS...22020614P
  The horizontal motions of the solar magnetic bright points (BPs)
  observed in two wavelengths (SST Halpha and Hinode/SOT G-band)
  is studied in detail. With emphasis on SST results: the velocity
  distribution of horizontal motions is found to be a Gaussian. The
  auto-correlations of observed velocities is also obtained. An
  empirical fit to the observed auto-correlation gives us a positional
  uncertainty of 3 km and the error in the velocity measurements to be
  0.87 km s$^{-1}$. Due to the non-Lorentzian, cusp-like nature of the
  auto-correlation, the power spectrum of the BP motions shows enhanced
  power at frequencies exceeding 0.02 Hz. The diffusion of magnetic field
  due to granular evolution at short timescales is found to satisfy a
  power law with a slope of 1.59.

Title: Preliminary Results on Irradiance Measurements from Lyra
    and Swap
Authors: Kumara, S. T.; Kariyappa, R.; Dominique, M.; Berghmans, D.;
   Damé, L.; Hochedez, J. F.; Doddamani, V. H.; Chitta, Lakshmi Pradeep
Bibcode: 2012AdAst2012E...5K
Altcode: 2012AdAst2012E..10K
  No abstract at ADS

Title: Observations of the Interaction of Acoustic Waves and
    Small-scale Magnetic Fields in a Quiet Sun
Authors: Chitta, Lakshmi Pradeep; Jain, Rekha; Kariyappa, R.;
   Jefferies, Stuart M.
Bibcode: 2012ApJ...744...98C
Altcode: 2012ApJ...744...98P
  The effect of the magnetic field on photospheric intensity and
  velocity oscillations at the sites of small-scale magnetic fields
  (SMFs) in a quiet Sun near the solar disk center is studied. We use
  observations made by the G-band filter in the Solar Optical Telescope
  on board Hinode for intensity oscillations; Doppler velocity, magnetic
  field, and continuum intensity are derived from an Ni I photospheric
  absorption line at 6767.8 Å using the Michelson Doppler Imager on
  board the Solar and Heliospheric Observatory. Our analysis shows that
  both the high-resolution intensity observed in the G band and velocity
  oscillations are influenced by the presence of a magnetic field. While
  intensity oscillations are suppressed at all frequencies in strong
  magnetic field regions compared to weak magnetic field regions,
  velocity oscillations show an enhancement of power in the frequency
  band 5.5-7 mHz. We find that there is a drop of 20%-30% in the p-mode
  power of velocity oscillations within the SMFs when compared to the
  regions surrounding them. Our findings indicate that the nature of the
  interaction of acoustic waves with the quiet Sun SMFs is similar to
  that of large-scale magnetic fields in active regions. We also report
  the first results of the center-to-limb variation of such effects
  using the observations of the quiet Sun from the Helioseismic and
  Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). The
  independent verification of these interactions using SDO/HMI suggests
  that the velocity power drop of 20%-30% in p-modes is fairly constant
  across the solar disk.

