Author name code: pevtsov
ADS astronomy entries on 2022-09-14
author:"Pevtsov, A." 
------------------------------------------------------------------------  

Title: Modeling FETCH Observations of 2005 May 13 CME
Authors: Jensen, Elizabeth A.; Manchester, Ward B., IV; Wexler,
   David B.; Kooi, Jason E.; Nieves-Chinchilla, Teresa; Jian, Lan K.;
   Pevtsov, Alexei; Fung, Shing
Bibcode: 2022arXiv220903350J
Altcode:
  This paper evaluates the quality of CME analysis that has been
  undertaken with the rare Faraday rotation observation of an
  eruption. Exploring the capability of the FETCH instrument hosted on
  the MOST mission, a four-satellite Faraday rotation radio sounding
  instrument deployed between the Earth and the Sun, we discuss the
  opportunities and challenges to improving the current analysis
  approaches.

Title: Multi-height Measurements Of The Solar Vector Magnetic Field:
    A White Paper Submitted To The Decadal Survey For Solar And Space
    Physics (Heliophysics) 2024-2033
Authors: Bertello, L.; Arge, N.; De Wijn, A. G.; Gosain, S.; Henney,
   C.; Leka, K. D.; Linker, J.; Liu, Y.; Luhmann, J.; Macniece, P. J.;
   Petrie, G.; Pevtsov, A.; Pevtsov, A. A.
Bibcode: 2022arXiv220904453B
Altcode:
  This white paper advocates the importance of multi-height measurements
  of the vector magnetic field in the solar atmosphere. As briefly
  described in this document, these measurements are critical for
  addressing some of the most fundamental questions in solar and
  heliospheric physics today, including: (1) What is the origin
  of the magnetic field observed in the solar atmosphere? (2) What
  is the coupling between magnetic fields and flows throughout the
  solar atmosphere? Accurate measurements of the photospheric and
  chromospheric three-dimensional magnetic fields are required for
  a precise determination of the emergence and evolution of active
  regions. Newly emerging magnetic flux in pre-existing magnetic regions
  causes an increase in the topological complexity of the magnetic field,
  which leads to flares and coronal mass ejections. Measurements of the
  vector magnetic field constitute also the primary product for space
  weather operations, research, and modeling of the solar atmosphere
  and heliosphere. The proposed next generation Ground-based solar
  Observing Network Group (ngGONG), a coordinated system of multi-platform
  instruments, will address these questions and provide large datasets
  for statistical investigations of solar feature behavior and evolution
  and continuity in monitoring for space-weather focused endeavors
  both research and operational. It will also enable sun-as-a-star
  investigations, crucial as we look toward understanding other
  planet-hosting stars.

Title: Reconstructing solar magnetic fields from historical
    observations. VIII. AIA 1600 Å contrast as a proxy of solar
    magnetic fields
Authors: Tähtinen, I.; Virtanen, I. I.; Pevtsov, A. A.; Mursula, K.
Bibcode: 2022A&A...664A...2T
Altcode: 2022arXiv220413944T
  Context. The bright regions in the solar chromosphere and temperature
  minimum have a good spatial correspondence with regions of intense
  photospheric magnetic field. Bright regions are visible in different
  emission lines and parts of the continuum. Their observation
  started more than a hundred years ago with the invention of the
  spectroheliograph. While the historical spectroheliograms are
  essential for studying the long-term variability of the Sun, the
  modern satellite-borne observations can help us reveal the nature of
  chromospheric brightenings in previously unattainable detail. <BR />
  Aims: Our aim is to improve the understanding of the relation between
  magnetic fields and radiative structures byf studying modern seeing-free
  observations of far-ultraviolet (FUV) radiation around 1600 Å and
  photospheric magnetic fields. <BR /> Methods: We used Helioseismic and
  Magnetic Imager (HMI) observations of photospheric magnetic fields and
  Atmospheric Imaging Assembly (AIA) observations of FUV contrast around
  1600 Å. We developed a robust method to find contrast thresholds
  defining bright and dark AIA 1600 Å pixels, and we combine them to
  bright and dark clusters. We investigate the relation of magnetic
  fields and AIA 1600 Å radiation in bright and dark clusters. <BR />
  Results: We find that the percentage of bright pixels (ranging from 2%
  to 10%) almost entirely explains the observed variability of 1600 Å
  emission. We developed a multilinear regression model based on the
  percentages of bright and dark pixels, which can reliably predict
  the magnitude of the disk-averaged unsigned magnetic field. We find
  that bright and dark clusters closely correspond respectively to
  the populations of moderate (B &gt; 55 G) and strong (B &gt; 1365
  G) magnetic field HMI clusters. The largest bright clusters have a
  constant mean unsigned magnetic field, as found previously for Ca II
  K plages. However, the magnetic field strength of bright clusters is
  254.7 ± 0.1 G, which is roughly 100 G larger than found earlier for
  Ca II K plages.

Title: Long-Term Solar Variability and Solar Cycle Predictions:
    Current State of Understanding
Authors: Nandi, Dibyendu; Usoskin, Ilya; Pevtsov, Alexei
Bibcode: 2022cosp...44.3520N
Altcode:
  The Sun's activity varies over timescales ranging from the very short
  to the very long. Long-term solar activity variations impact the
  Earth's atmosphere and climate and have implications for space mission
  planning and life-time estimates. This variability across different
  scales is driven by solar magnetic fields which originate in the deep
  convection zone, emerge through the surface and evolve. Understanding
  the physical basis for long-term variability over decadal scales and
  longer is important for developing predictive models for sunspot cycle
  which is an outstanding challenge. Concurrently, exploring the basis
  of fluctuations that lead to extreme episodes such as grand maxima in
  solar activity remains an important exercise with no clear indication
  that such episodes are predictable. We shall review here the current
  state of our understanding of long-term solar variability, and identify
  challenges that are expected to spur future developments in this field.

Title: The Solar Activity Monitor Network - SAMNet
Authors: Erdélyi, Robertus; Korsós, Marianna B.; Huang, Xin; Yang,
   Yong; Pizzey, Danielle; Wrathmall, Steven A.; Hughes, Ifan G.;
   Dyer, Martin J.; Dhillon, Vikram S.; Belucz, Bernadett; Brajša,
   Roman; Chatterjee, Piyali; Cheng, Xuewu; Deng, Yuanyong; Domínguez,
   Santiago Vargas; Joya, Raúl; Gömöry, Peter; Gyenge, Norbert G.;
   Hanslmeier, Arnold; Kucera, Ales; Kuridze, David; Li, Faquan; Liu,
   Zhong; Xu, Long; Mathioudakis, Mihalis; Matthews, Sarah; McAteer,
   James R. T.; Pevtsov, Alexei A.; Pötzi, Werner; Romano, Paolo; Shen,
   Jinhua; Temesváry, János; Tlatov, Andrey G.; Triana, Charles; Utz,
   Dominik; Veronig, Astrid M.; Wang, Yuming; Yan, Yihua; Zaqarashvili,
   Teimuraz; Zuccarello, Francesca
Bibcode: 2022JSWSC..12....2E
Altcode:
  The Solar Activity Magnetic Monitor (SAMM) Network (SAMNet) is a
  future UK-led international network of ground-based solar telescope
  stations. SAMNet, at its full capacity, will continuously monitor
  the Sun's intensity, magnetic, and Doppler velocity fields at
  multiple heights in the solar atmosphere (from photosphere to upper
  chromosphere). Each SAMM sentinel will be equipped with a cluster of
  identical telescopes each with a different magneto-optical filter (MOFs)
  to take observations in K I, Na D, and Ca I spectral bands. A subset
  of SAMM stations will have white-light coronagraphs and emission line
  coronal spectropolarimeters. The objectives of SAMNet are to provide
  observational data for space weather research and forecast. The goal
  is to achieve an operationally sufficient lead time of e.g., flare
  warning of 2-8 h and provide many sought-after continuous synoptic
  maps (e.g., LoS magnetic and velocity fields, intensity) of the lower
  solar atmosphere with a spatial resolution limited only by seeing or
  diffraction limit, and with a cadence of 10 min. The individual SAMM
  sentinels will be connected to their master HQ hub where data received
  from all the slave stations will be automatically processed and flare
  warning issued up to 26 h in advance.

Title: Modern Faraday Rotation Studies to Probe the Solar Wind
Authors: Kooi, Jason E.; Wexler, David B.; Jensen, Elizabeth A.;
   Kenny, Megan N.; Nieves-Chinchilla, Teresa; Wilson, Lynn B., III; Wood,
   Brian E.; Jian, Lan K.; Fung, Shing F.; Pevtsov, Alexei; Gopalswamy,
   Nat; Manchester, Ward B.
Bibcode: 2022FrASS...941866K
Altcode:
  For decades, observations of Faraday rotation have provided unique
  insights into the plasma density and magnetic field structure of
  the solar wind. Faraday rotation (FR) is the rotation of the plane
  of polarization when linearly polarized radiation propagates through
  a magnetized plasma, such as the solar corona, coronal mass ejection
  (CME), or stream interaction region. FR measurements are very versatile:
  they provide a deeper understanding of the large-scale coronal magnetic
  field over a range of heliocentric distances (especially ≈1.5 to
  20 R⊙) not typically accessible to in situ spacecraft observations;
  detection of small-timescale variations in FR can provide information
  on magnetic field fluctuations and magnetohydrodynamic wave activity;
  and measurement of differential FR can be used to detect electric
  currents. FR depends on the integrated product of the plasma
  density and the magnetic field component along the line of sight
  to the observer; historically, models have been used to distinguish
  between their contributions to FR. In the last two decades, though,
  new methods have been developed to complement FR observations with
  independent measurements of the plasma density based on the choice
  of background radio source: calculation of the dispersion measure
  (pulsars), measurement of Thomson scattering brightness (radio
  galaxies), and application of radio ranging and apparent-Doppler
  tracking (spacecraft). New methods and new technology now make it
  possible for FR observations of solar wind structures to return not
  only the magnitude of the magnetic field, but also the full vector
  orientation. In the case of a CME, discerning the internal magnetic
  flux rope structure is critical for space weather applications.

Title: On the Application of the Equal-contrast Technique to Ca-K
    Data from Kodaikanal and Other Observatories
Authors: Singh, Jagdev; Priyal, Muthu; Ravindra, Belur; Bertello,
   Luca; Pevtsov, Alexei A.
Bibcode: 2022ApJ...927..154S
Altcode:
  The "equal-contrast technique" (ECT) methodology, developed by Singh et
  al. to generate uniform long time series of Ca-K images obtained during
  the 20th century from the Kodaikanal Observatory (KO), improved the
  correlation between the plage area and sunspot parameters. The same
  methodology can also be used on other observatory data taken with
  different instruments. We can combine such ECT-corrected images to
  reduce the gaps in the observations and make a long uniform data set
  to study short- and long-term variations. We apply this procedure to
  Mount Wilson Observatory (MWO) historical Ca-K data and recent Ca-K
  filtergrams obtained using narrowband filters at KO and the Mauna Loa
  Solar Observatory (MLSO). To determine the success of this method, the
  results of the analysis of the ECT images obtained from KO, MWO, and
  MLSO are compared. A comparison of the plage and active areas derived
  from KO and MWO images before and after the ECT procedure indicates
  an improvement in the correlation coefficients (CCs) between all the
  data sets after the ECT application. The CC for the combined monthly
  mean Ca-K plage area derived from the KO, MWO, and Precision Solar
  Photometric Telescope (at the MLSO) data with sunspot numbers is 0.96
  for the period 1905-2015. The paper demonstrates that the time series
  of Ca-K data obtained from different instruments after applying the
  ECT procedure becomes uniform in contrast. The combined time series
  of KO and MWO spectroheliograms has 12 hr intervals compared to the
  ≍24 hr gap for a time series from a single observatory.

Title: AWSoM Magnetohydrodynamic Simulation of a Solar Active Region
    with Realistic Spectral Synthesis
Authors: Shi, Tong; Manchester, Ward, IV; Landi, Enrico; van der
   Holst, Bart; Szente, Judit; Chen, Yuxi; Tóth, Gábor; Bertello,
   Luca; Pevtsov, Alexander
Bibcode: 2022ApJ...928...34S
Altcode:
  For the first time, we simulate the detailed spectral line emission
  from a solar active region (AR) with the Alfvén Wave Solar Model
  (AWSoM). We select an AR appearing near disk center on 2018 July 13
  and use the National Solar Observatory's Helioseismic and Magnetic
  Imager synoptic magnetogram to specify the magnetic field at the
  model's inner boundary. To resolve small-scale magnetic features, we
  apply adaptive mesh refinement with a horizontal spatial resolution
  of 0°.35 (4.5 Mm), four times higher than the background corona. We
  then apply the SPECTRUM code, using CHIANTI spectral emissivities,
  to calculate spectral lines forming at temperatures ranging from 0.5
  to 3 MK. Comparisons are made between the simulated line intensities
  and those observed by Hinode/Extreme-ultraviolet Imaging Spectrometer
  where we find close agreement across a wide range of loop sizes and
  temperatures (about 20% relative error for both the loop top and
  footpoints at a temperature of about 1.5 MK). We also simulate and
  compare Doppler velocities and find that simulated flow patterns are
  of comparable magnitude to what is observed. Our results demonstrate
  the broad applicability of the low-frequency AWSoM for explaining the
  heating of coronal loops.

Title: Measurements of the Multi-Height Solar Vector Magnetic Field
Authors: Bertello, L.; Pevtsov, A.; Pevtsov, A. A.
Bibcode: 2022heli.conf.4011B
Altcode:
  The development of sophisticated numerical models of the heliosphere
  have made measurements of the solar vector magnetic field extremely
  relevant today. These measurements and a newly proposed ground-based
  global network will be discussed.

Title: Improving the Understanding of Subsurface Structure and
    Dynamics of Solar Active Regions
Authors: Tripathy, S. C.; Jain, K.; Kholikov, S.; Pevtsov, A.
Bibcode: 2022heli.conf.4017T
Altcode:
  NSO and HAO are promoting the design of a new global ground-based
  network. Here we describe additional science goals that could be
  addressed by this new network through multi-height observations of
  the solar atmosphere.

Title: Analysis of Solar Hemispheric Chromosphere Properties using
    the Kodaikanal Observatory Ca-K Index
Authors: Chowdhury, Partha; Belur, Ravindra; Bertello, Luca; Pevtsov,
   Alexei A.
Bibcode: 2022ApJ...925...81C
Altcode:
  The Kodaikanal Observatory has provided long-term synoptic observations
  of chromospheric activities in the Ca II K line (393.34 nm) since
  1907. This article investigates temporal and periodic variations of the
  hemispheric Ca-K-index time series in the low-latitude zone (±40°),
  utilizing the recently digitized photographic plates of Ca-K images from
  the Kodaikanal Observatory for the period of 1907-1980. We find that
  the temporal evolution of the Ca-K index differs from one hemisphere to
  another, with the solar cycle peaking at different times in the opposite
  hemisphere, except for cycles 14, 15, and 21, when the phase difference
  between the two hemispheres was not significant. The monthly averaged
  data show a higher activity in the northern hemisphere during solar
  cycles 15, 16, 18, 19, and 20, and in the southern hemisphere during
  cycles 14, 17, and 21. We notice an exponentially decaying distribution
  for each hemisphere's Ca-K index and the whole solar disk. We explored
  different midterm periodicities of the measured Ca-K index using the
  wavelet technique, including Rieger-type and quasi-biennial oscillations
  on different timescales present in the time series. We find a clear
  manifestation of the Waldmeier effect (stronger cycles rise faster than
  the weaker ones) in both the hemispheres separately and the whole disk
  in the data. Finally, we have found the presence of the Gnevyshev gap
  (time interval between two cycle maxmima) in both the hemispheric data
  during cycles 15 to 20. Possible interpretations of our findings are
  discussed with the help of existing theoretical models and observations.

Title: AWSoM MHD simulation of a solar active region with realistic
    spectral synthesis
Authors: Manchester, Ward; Shi, Tong; Landi, Enrico; Szente, Judit;
   van der Holst, Bart; Chen, Yuxi; Toth, Gabor; Bertello, Luca; Pevtsov,
   Alexander
Bibcode: 2021AGUFMSH12B..02M
Altcode:
  For the first time, we simulate the detailed spectral line emission
  from a solar active region (AR) with the Alfven Wave Solar Model
  (AWSoM). We select an active region appearing near disk center on
  2018 July 13 and use an NSO-HMI synoptic magnetogram to specify the
  magnetic field at the model's inner boundary. To resolve smaller-scale
  magnetic features, we apply adaptive mesh refinement to resolve the
  AR with a spatial resolution of 0.37 degrees, four times higher than
  the background corona. We then apply the SPECTRUM code informed with
  Chianti spectral emissivities to calculate more than a dozen spectral
  lines forming at temperatures ranging from 0.5 to 3+ MK. Comparisons
  are made between these simulated line profiles and those observed by
  the Hinode/EIS instrument where we find close agreement (within a
  20% margin of error of peak intensity) across a wide range of loop
  sizes and temperatures. We also compare the differential emission
  measure calculated from both the simulation and EIS observation to
  further show the model's ability to capture the plasma temperature and
  density. Finally, we simulate and compare Doppler velocities and find
  that simulated flow patterns to be of comparable magnitude to what
  is observed. Our results demonstrate the broad applicability of the
  low-frequency Alfven wave balanced turbulence theory for explaining
  the heating of coronal loops.

Title: Novel Magnetic Field and Electron Density Measurements of
    CMEs (within AU) with the Proposed Multiview Observatory for Solar
    Terrestrial Science (MOST) Mission
Authors: Jensen, P. E., C. S. P., Elizabeth; Manchester, Ward; Fung,
   Shing; Gopalswamy, Nat; Jian, Lan; Kenny, Megan; Kooi, Jason; Lazio,
   Joseph; Li, Lihua; Nieves-Chinchilla, Teresa; Pevtsov, Alexei; Wexler,
   David; Wilson, Lynn; Wood, Brian; Bale, Stuart; Bastian, Tim
Bibcode: 2021AGUFMSH33A..08J
Altcode:
  The Multiview Observatory for Solar Terrestrial Science (MOST) mission
  concept will be the most advanced solar observatory to date (Gopalswamy
  et al, SH0001, 2021). Comprising four spacecraft, two located in the L4
  and ahead of L4 position and two located in the L5 and behind of the L5
  position, the four lines-of-sight (LOSs) form the basis for the unique
  Faraday Effect Tracker of Coronal and Heliospheric Structures (FETCH)
  instrument (Wexler et al, SH0019, 2021). We report on our modeling
  into the expected Faraday rotation (FR) caused by an Earth-directed
  CME crossing the MOST/FETCH radio-sensing paths using a heliospheric
  3-D MHD model to obtain the necessary LOS data on electron density
  and magnetic field components (see example image). Specifically, we
  utilized simulation data of the 2005 May 13 CME (Manchester IV et al.,
  2014, Plasma Phys. Control. Fusion), which erupted from the north-south
  polarity inversion line of AR 10759 at 16:03 UT, reaching speeds around
  2000 km/s in the corona. The trajectory of the CME at an acute angle
  to the Earth-Sun line crosses each FETCH LOS at a different time. Two
  LOSs are at different viewing angles with little overlap between
  the CME sheath and magnetic flux rope core. A blind test fitting of
  the Faraday rotation functions (Figures 6 and 7 in Jensen et al.,
  2010, Sol. Phys.) to the simulated FETCH observations reproduced the
  orientation of the CME for its handedness as well as its associated
  complementary degenerate solution. In conclusion, one of the four
  LOSs will be more sensitive to observing CME flux rope structure of
  Earthward CMEs, depending on their trajectory. We find that two of the
  four LOSs enable analyzing CME evolution, whereas the other two LOSs
  enable analyzing the average magnetic field vector in the corresponding
  high density regions dominating the measurements at that time. For
  example, the average sheath magnetic field vector can be partially
  measured in the plane of the ecliptic due to the angular differences
  between 2 LOSs. We discuss future work as this effort develops.

Title: FETCH Concept: Investigating Quiescent and Transient Magnetic
    Structures in the Inner Heliosphere using Faraday Rotation of
    Spacecraft Radio Signals
Authors: Wexler, David; Jensen, Elizabeth; Gopalswamy, Nat; Wilson,
   Lynn; Fung, Shing; Nieves-Chinchilla, Teresa; Jian, Lan; Bastian,
   Tim; Pevtsov, Alexei; Manchester, Ward; Kenny, Megan; Lazio, Joseph;
   Wood, Brian; Kooi, Jason
Bibcode: 2021AGUFMSH31A..05W
Altcode:
  The Faraday Effect Tracker of Coronal and Heliospheric structures
  (FETCH) is a new instrument concept being developed to probe coronal
  and interplanetary magnetic field structures in the ambient solar wind,
  corotating interaction regions and coronal mass ejections (CMEs) as
  they evolve in the inner heliosphere. FETCH is one of the instruments
  that constitute the Multiview Observatory for Solar Terrestrial (MOST)
  science mission. FETCH will measure Faraday rotation (FR) of linearly
  polarized spacecraft radio signals transmitted along four lines of sight
  provided by the four MOST spacecraft: two large spacecraft deployed
  at Sun-Earth Lagrange points 4 and 5 and two smaller spacecraft, one
  ahead of L4 and the other behind L5. FETCH will transmit and receive at
  selected radio frequencies in the 1-100 MHz range for lines of sight
  with solar impact parameters &lt; 0.5 AU. FR yields the line-of-sight
  (LOS) integrated product of electron number density and LOS-projected
  magnetic field strengths. The FR measurements will be obtained from
  the Stokes polarization parameters while additional plasma parameters,
  such as electron column density, will be extracted from other signal
  diagnostics. The multifrequency FR data and four lines-of-sight
  will be used to constrain the magnetic field topology and dynamics of
  interplanetary plasma structures upstream from Earth. Unique to this FR
  experiment, the FETCH transmitter-receiver instrumentation is positioned
  such that the entire sensing path remains in interplanetary space, thus
  avoiding the complications of trans-ionospheric FR observations. The
  FETCH key science objectives include: (1) characterizing CME magnetic
  field structure and flux rope orientation, (2) tracking CME propagation
  and shock signatures, (3) understanding the magnetic field features
  of corotating interaction regions in the extended corona and inner
  heliosphere, and (4) determination of large-scale MHD wave organization
  in regions of developed ambient solar wind and its evolution during
  perturbed flows. The MOST mission will build upon the achievements of
  the Solar Heliospheric Observatory (SOHO) and the Solar Terrestrial
  Relations Observatory (STEREO) missions during the last couple of
  decades. FETCH will help fill the long-standing measurement gap of
  magnetic field data in the inner heliosphere.

Title: The Multiview Observatory for Solar Terrestrial Science (MOST)
Authors: Gopalswamy, Nat; Kucera, Therese; Leake, James; MacDowall,
   Robert; Wilson, Lynn; Kanekal, Shrikanth; Shih, Albert; Christe,
   Steven; Gong, Qian; Viall, Nicholeen; Tadikonda, Sivakumar; Fung,
   Shing; Yashiro, Seiji; Makela, Pertti; Golub, Leon; DeLuca, Edward;
   Reeves, Katharine; Seaton, Daniel; Savage, Sabrina; Winebarger, Amy;
   DeForest, Craig; Desai, Mihir; Bastian, Tim; Lazio, Joseph; Jensen,
   P. E., C. S. P., Elizabeth; Manchester, Ward; Wood, Brian; Kooi,
   Jason; Wexler, David; Bale, Stuart; Krucker, Sam; Hurlburt, Neal;
   DeRosa, Marc; Pevtsov, Alexei; Tripathy, Sushanta; Jain, Kiran;
   Gosain, Sanjay; Petrie, Gordon; Kholikov, Shukirjon; Zhao, Junwei;
   Scherrer, Philip; Woods, Thomas; Chamberlin, Philip; Kenny, Megan
Bibcode: 2021AGUFMSH12A..07G
Altcode:
  The Multiview Observatory for Solar Terrestrial Science (MOST) is a
  comprehensive mission concept targeting the magnetic coupling between
  the solar interior and the heliosphere. The wide-ranging imagery and
  time series data from MOST will help understand the solar drivers and
  the heliospheric responses as a system, discerning and tracking 3D
  magnetic field structures, both transient and quiescent in the inner
  heliosphere. MOST will have seven remote-sensing and three in-situ
  instruments: (1) Magnetic and Doppler Imager (MaDI) to investigate
  surface and subsurface magnetism by exploiting the combination of
  helioseismic and magnetic-field measurements in the photosphere; (2)
  Inner Coronal Imager in EUV (ICIE) to study large-scale structures
  such as active regions, coronal holes and eruptive structures by
  capturing the magnetic connection between the photosphere and the
  corona to about 3 solar radii; (3) Hard X-ray Imager (HXI) to image
  the non-thermal flare structure; (4) White-light Coronagraph (WCOR) to
  seamlessly study transient and quiescent large-scale coronal structures
  extending from the ICIE field of view (FOV); (5) Faraday Effect
  Tracker of Coronal and Heliospheric structures (FETCH), a novel radio
  package to determine the magnetic field structure and plasma column
  density, and their evolution within 0.5 au; (6) Heliospheric Imager
  with Polarization (HIP) to track solar features beyond the WCOR FOV,
  study their impact on Earth, and provide important context for FETCH;
  (7) Radio and Plasma Wave instrument (M/WAVES) to study electron beams
  and shocks propagating into the heliosphere via passive radio emission;
  (8) Solar High-energy Ion Velocity Analyzer (SHIVA) to determine spectra
  of electrons, and ions from H to Fe at multiple spatial locations
  and use energetic particles as tracers of magnetic connectivity; (9)
  Solar Wind Magnetometer (MAG) to characterize magnetic structures at
  1 au; (10) Solar Wind Plasma Instrument (SWPI) to characterize plasma
  structures at 1 au. MOST will have two large spacecraft with identical
  payloads deployed at L4 and L5 and two smaller spacecraft ahead of L4
  and behind L5 to carry additional FETCH elements. MOST will build upon
  SOHO and STEREO achievements to expand the multiview observational
  approach into the first half of the 21st Century.

Title: The Next Generation GONG (ngGONG) Project: Ground-based
    Synoptic Studies of the Sun
Authors: Pillet, Valentin; Gilbert, Holly; Pevtsov, Alexei; de Wijn,
   Alfred
Bibcode: 2021AGUFMSH45E2406P
Altcode:
  Ground-based synoptic solar observations provide crucial contextual
  data used to model the large-scale state of the heliosphere. Existing
  ground-based synoptic programs are aging rapidly and are used in
  ways that differ from their original objectives. Most prominently,
  GONG was designed for helioseismology but is most demanded today
  as a provider of the magnetic boundary conditions for solar wind
  models. A wealth of theoretical knowledge about the connectivity
  between the Sun and the planets has emerged in recent years. NSO and
  HAO (and other international partners) are collaborating in defining
  a next-generation GONG (ngGONG) network that incorporates this
  knowledge. This contribution describes current and future contextual
  synoptic observations needed to fully exploit our new understanding of
  the underlying microphysics that leads to magnetic linkages between
  the Earth and the Sun. This combination of a better understanding of
  small-scale processes and the appropriate global context will enable
  a physics-based approach to Space Weather comparable to Terrestrial
  Weather forecasting.

Title: A Comparative Study of Measurements of the Suns Axisymmetric
Flows: A COFFIES Effort
Authors: Upton, Lisa; Jain, Kiran; Komm, Rudolf; Mahajan, Sushant;
   Pevtsov, Alexei; Roudier, Thierry; Tripathy, Sushanta; Ulrich, Roger;
   Zhao, Junwei; Basu, Sarbani; Chen, Ruizhu; DeRosa, Marc; Hess Webber,
   Shea; Hoeksema, J.
Bibcode: 2021AGUFMSH55D1871U
Altcode:
  Consequence Of Fields and Flows in the Interior and Exterior of
  the Sun (COFFIES) is a Phase-1 NASA DRIVE Science Center (DSC),
  with the primary objective of developing a data driven model of
  solar activity. One of COFFIES five primary science questions is
  What drives varying large-scale motions in the Sun? To address this
  question, we are developing a comprehensive catalog of the variable
  differential rotation and meridional circulation flow patterns. This
  catalog includes measurements of these flows as obtained by several
  measurement techniques: Doppler imaging, granule tracking, magnetic
  pattern tracking, magnetic feature tracking, as well as both time
  distance and ring diagram helioseismology. We show a comparison of
  these flows across these varied techniques, with a particular focus
  on the MDI/HMI/GONG/Mount Wilson overlap period (May-July 2010). We
  investigate the uncertainties and attempt to reconcile any discrepancies
  (e.g., due to flow depth or systematics associated with the different
  measurement techniques). This analysis will pave the way toward
  accurately determining the global patterns of axisymmetric flows and
  their regular and irregular variations during the cycle.

Title: Continuous Solar Observations from the Ground-Assessing Duty
    Cycle from GONG Observations
Authors: Jain, Kiran; Tripathy, Sushanta C.; Hill, Frank; Pevtsov,
   Alexei A.
Bibcode: 2021PASP..133j5001J
Altcode: 2021arXiv211006319J
  Continuous observations play an important role in the studies of solar
  variability. While such observations can be achieved from space with
  an almost 100% duty cycle, it is difficult to accomplish a very high
  duty cycle from the ground. In this context, we assess the duty cycle
  that has been achieved from the ground by analyzing the observations of
  a six station network of identical instruments, the Global Oscillation
  Network Group (GONG). We provide a detailed analysis of the duty cycle
  using GONG observations spanning over 18 yr. We also discuss the duty
  cycle of individual sites and point out various factors that may impact
  individual site or network duty cycles. The mean duty cycle of the
  network is 93%, however it reduces by about 5% after all images pass
  through the stringent quality-control checks. The standard deviations
  in monthly and yearly duty cycle values are found to be 1.9% and 2.2%,
  respectively. These results provide a baseline that can be used in
  the planning of future ground-based networks.

Title: Reconstructing solar magnetic fields from historical
    observations. VII. Far-side activity in surface flux transport
    simulations
Authors: Virtanen, I. O. I.; Pevtsov, A. A.; Virtanen, I. I.;
   Mursula, K.
Bibcode: 2021A&A...652A..79V
Altcode:
  Context. The evolution of the photospheric magnetic field can be
  simulated with surface flux transport (SFT) simulations, which allow for
  the study of the evolution of the entire field, including polar fields,
  solely using observations of the active regions. However, because
  only one side of the Sun is visible at a time, active regions that
  emerge and decay on the far-side are not observed and not included
  in the simulations. As a result, some flux is missed. <BR /> Aims:
  We construct additional active regions and apply them to the far-side
  of the Sun in an SFT simulation to assess the possible effects and
  the magnitude of error that the missing far-side flux causes. We
  estimate how taking the missing far-side flux into account affects
  long-term SFT simulations. <BR /> Methods: We identified active
  regions from synoptic maps of the photospheric magnetic field between
  1975 and 2019. We divided them into solar cycle wings and determined
  their lifetimes. Using the properties of observed active regions
  with sufficiently short lifetimes, we constructed additional active
  regions and inserted them into an SFT simulation. <BR /> Results: We
  find that adding active regions with short lifetimes to the far-side
  of the Sun results in significantly stronger polar fields in minimum
  times and slightly delayed polarity reversals. These results partly
  remedy the earlier results, which show overly weak polar fields
  and polarity reversals that are slightly too early when far-side
  emergence is not taken into account. The far-side active regions do
  not significantly affect poleward flux surges, which are mostly caused
  by larger long-living active regions. The far-side emergence leads to
  a weak continuous flow of flux, which affects polar fields over long
  periods of time.

Title: The Intensity and Evolution of the Extreme Solar and
    Geomagnetic Storms in 1938 January
Authors: Hayakawa, Hisashi; Hattori, Kentaro; Pevtsov, Alexei A.;
   Ebihara, Yusuke; Shea, Margaret A.; McCracken, Ken G.; Daglis,
   Ioannis A.; Bhaskar, Ankush T.; Ribeiro, Paulo; Knipp, Delores J.
Bibcode: 2021ApJ...909..197H
Altcode: 2020arXiv201015762H
  Major solar eruptions occasionally direct interplanetary coronal mass
  ejections (ICMEs) to Earth and cause significant geomagnetic storms and
  low-latitude aurorae. While individual extreme storms are significant
  threats to modern civilization, storms occasionally appear in sequence,
  acting synergistically, and cause "perfect storms" on Earth. The stormy
  interval in 1938 January was one of such cases. Here, we analyze the
  contemporary records to reveal its time series on their source active
  regions, solar eruptions, ICMEs, geomagnetic storms, low-latitude
  aurorae, and cosmic-ray (CR) variations. Geomagnetic records show
  that three storms occurred successively on January 17/18 (Dcx ≍ -171
  nT), January 21/22 (Dcx ≍ -328 nT), and January 25/26 (Dcx ≍ -336
  nT). The amplitudes of the CR variations and storm sudden commencements
  (SSCs) show the impact of the first ICME as the largest (≍6% decrease
  in CR and 72 nT in SSC) and the ICMEs associated with the storms that
  followed as more moderate (≍3% decrease in CR and 63 nT in SSC; ≍2%
  decrease in CR and 63 nT in SSC). Interestingly, a significant solar
  proton event occurred on January 16/17 and the Cheltenham ionization
  chamber showed a possible ground-level enhancement. During the first
  storm, aurorae were less visible at midlatitudes, whereas, during the
  second and third storms, the equatorward boundaries of the auroral oval
  were extended down to 40.3° and 40.0° in invariant latitude. This
  contrast shows that the initial ICME was probably faster, with a higher
  total magnitude but a smaller southward component.

Title: Tilt angle and lifetime of sunspot groups
Authors: Nagovitsyn, Yury A.; Osipova, Aleksandra A.; Pevtsov,
   Alexei A.
Bibcode: 2021MNRAS.501.2782N
Altcode: 2020MNRAS.tmp.3619N
  We use the Catalogue of Solar Activity (CSA) to study the latitudinal
  variations of tilt of solar active regions. The tilt angles β are
  computed taking into account changes of the heliographic grid with
  latitude φ. We show that when sunspot groups of different sizes
  and lifetimes are included, a classical representation of the Joy's
  law as a linear function of latitude (β ∝ φ) is only the first
  approximation valid within a limited range of latitudes (-25° ≤ φ
  ≤ +25°). Outside this range, the functional dependence β = f(φ)
  becomes non-linear. Separating the data set on large long-living groups
  (LLG) and small short-living groups (SSG) reveals two quite different
  dependencies in β = f(φ): non-linearity in tilt is only present in
  LLGs and the steepness of linear section of β = f(φ) fit is higher
  for LLGs. This suggests a difference in the physical properties of
  two populations of solar groups, which could be hypothesized as an
  indication of different localization of subsurface zones of their
  formation in the framework of a distributed dynamo. However, since CSA
  contains the coordinates of sunspots averaged over the lifetime (or
  disc passage) of each group, one cannot rule out that the difference
  in tilts of SSG and LLG groups may be affected by the evolution of
  tilt angles during the lifetime/disc passage of the groups.

Title: The Sunspot Drawing Collection of the National Solar
    Observatory at Sacramento Peak (1947-2004)
Authors: Carrasco, V. M. S.; Pevtsov, A. A.; Nogales, J. M.; Vaquero,
   J. M.
Bibcode: 2021SoPh..296....3C
Altcode:
  A complete dataset of sunspot drawings recorded at Sacramento Peak
  Observatory (SPO) from late 1947 till mid-2004 has been digitized. We
  present the history of the observations and describe the data included
  in the drawings. We compare the sunspot number index calculated from
  the SPO data and the International Sunspot Number (S<SUB>N</SUB>v2),
  and we find that both series exhibit a similar behavior. The ratio of
  two sunspot numbers is relatively constant at about 1.2 - 1.3 during
  1955-1995, with larger variations present at the beginning of the
  time series. This work represents the first step for the publication
  of the SPO sunspot catalogue in digital format. More information,
  such as positions and areas of sunspots, will be included in the next
  versions in order to provide the space weather and climate community
  a more complete sunspot catalogue with good quality observations.

Title: International Space Weather Action Team (ISWAT) S1: Long-term
    Solar Variability
Authors: Nandi, Dibyendu; Usoskin, Ilya; Pevtsov, Alexei
Bibcode: 2021cosp...43E2412N
Altcode:
  The COSPAR International Space Weather Action Team (ISWAT) is a global
  hub for space weather community efforts toward the realization
  of the International Living With a Star-COSPAR Space Weather
  Roadmap goals. The community provides an inclusive environment for
  collaborative research efforts, information sharing and capacity
  building in the space weather sciences with a specific focus on
  enabling prediction and applications. Action teams within ISWAT
  address a specific focussed topic around which the team expertise
  is built. In this poster presentation we highlight the ISWAT Action
  Team S1 (Long term solar variability). The team is motivated towards
  understanding solar drivers of long term variability in the heliospheric
  space environment, its impact on geospace and other planets. This
  ISWAT Team is moderated by Dibyendu Nandi, Ilya Usoskin and Alexei
  Pevtsov. Interested scientists can join the team through the ISWAT
  website at: https://iswat-cospar.org/.

Title: Activities related to the COSPAR ISWAT Cluster: Ambient Solar
    Magnetic Field, Heating and Spectral Irradiance
Authors: Reiss, Martin; Pevtsov, Alexei; Linker, Jon; Pinto, Rui;
   Arge, Charles; Muglach, Karin; Henney, Carl J.
Bibcode: 2021cosp...43E2413R
Altcode:
  The understanding of the magnetic field configuration in the solar
  atmosphere is of crucial importance for improving the modelling and
  ultimately the prediction of space weather from Sun to Earth. The
  magnetic field provides the energy source that heats the solar corona
  and accelerates the solar wind, and it also defines the structure of
  the heliosphere. However, many related unresolved questions require
  an interdisciplinary strategy and the coordinated cooperation of
  international partners. Here we present the activities of the 'Ambient
  Solar Magnetic Field, Heating and Spectral Irradiance' cluster embedded
  in the COSPAR ISWAT initiative. The action teams in this cluster will
  focus on critical scientific challenges in the space weather community
  related to the construction of global solar magnetic field maps, use
  of vector field synoptic maps for applied space weather modelling and
  research, magnetic connectivity from the surface of the Sun to any
  point in interplanetary space, magnetic topology of open field lines
  along which solar wind flows accelerate to supersonic speeds, and the
  solar spectral irradiance driving ionization and heating in the Earth's
  upper atmosphere. We will outline the objectives of the individual
  action teams and present the current status and roadmaps. To encourage
  the formation of new action teams, we will also discuss additional
  challenges that should be addressed by the space weather community.

Title: On a limitation of Zeeman polarimetry and imperfect
    instrumentation in representing solar magnetic fields with weaker
    polarization signal
Authors: Pevtsov, A. A.; Liu, Y.; Virtanen, I.; Bertello, L.; Mursula,
   K.; Leka, K. D.; Hughes, A. L. H.
Bibcode: 2021JSWSC..11...14P
Altcode: 2021arXiv210107204P
  Full disk vector magnetic fields are used widely for developing better
  understanding of large-scale structure, morphology, and patterns of
  the solar magnetic field. The data are also important for modeling
  various solar phenomena. However, observations of vector magnetic
  fields have one important limitation that may affect the determination
  of the true magnetic field orientation. This limitation stems from
  our ability to interpret the differing character of the Zeeman
  polarization signals which arise from the photospheric line-of-sight
  vs. the transverse components of the solar vector magnetic field,
  and is likely exacerbated by unresolved structure (non-unity fill
  fraction) as well as the disambiguation of the 180° degeneracy in
  the transverse-field azimuth. Here we provide a description of this
  phenomenon, and discuss issues, which require additional investigation.

Title: Bi-lognormal Distribution of Sunspot Group Areas
Authors: Nagovitsyn, Yury A.; Pevtsov, Alexei A.
Bibcode: 2021ApJ...906...27N
Altcode:
  We use daily observations from the Royal Greenwich Observatory and
  Kislovodsk Mountain Astronomical Station of Pulkovo Observatory to
  study the distribution properties of sunspot areas. To mitigate the poor
  statistics in the distribution of small areas, we introduce a "precision
  randomization" approach based on the assumption that all measured
  areas have a random component within the measurement uncertainty of
  1 millionth of the solar hemisphere (M.S.H.). We confirm the presence
  of two distinct components in the distribution of sunspots previously
  reported by several authors, and show that the area distribution is
  described by the sum of two lognormal distributions responsible for
  small and large groups (sunspots), respectively. We also demonstrate
  that the area of the main spots in the groups correlates well with
  the total area of the group, and, thus, the findings derived for the
  total group areas are equally applicable to the main spots of groups.

Title: Long-term studies of photospheric magnetic fields on the Sun
Authors: Pevtsov, Alexei A.; Bertello, Luca; Nagovitsyn, Yury A.;
   Tlatov, Andrey G.; Pipin, Valery V.
Bibcode: 2021JSWSC..11....4P
Altcode:
  We briefly review the history of observations of magnetic fields on the
  Sun, and describe early magnetograps for full disk measurements. Changes
  in instruments and detectors, the cohort of observers, the knowledge
  base etc may result in non-uniformity of the long-term synoptic
  datasets. Still, such data are critical for detecting and understanding
  the long-term trends in solar activity. We demonstrate the value of
  historical data using studies of active region tilt (Joy's law) and the
  evolution of polar field and its reversal. Using the longest dataset
  of sunspot field strength measurements from Mount Wilson Observatory
  (1917-present) supplemented by shorter datasets from Pulkovo (1956-1997)
  and Crimean (1956-present) observatories we demonstrate that the
  magnetic properties of sunspots did not change over the last hundred
  years. We also show that the relationship between the sunspot area and
  its magnetic flux can be used to extend the studies of magnetic field in
  sunspots to periods with no direct magnetic field measurements. Finally,
  we show how more recent full disk observations of the vector magnetic
  field can be used to study the long-term (solar cycle) variations in
  magnetic helicity on the Sun.

Title: COSPAR International Space Weather Action Teams: Addressing
    Challenges Across the Field of Space Weather.
Authors: Kuznetsova, M. M.; Belehaki, A.; Bisi, M. M.; Bruinsma, S.;
   Fung, S. F.; Glover, A.; Grande, M.; Guo, J.; Jun, I.; Linker, J.;
   Mann, I. R.; Masson, A.; Mendoza, A. M. M.; Murray, S. A.; Nandy, D.;
   Opgenoorth, H. J.; Pevtsov, A. A.; Plainaki, C.; Reiss, M.; Sutton,
   E. K.; Temmer, M.; Usoskin, I. G.; Yao, Z.; Yardley, S.; Zheng, Y.
Bibcode: 2020AGUFMSH0030022K
Altcode:
  Advanced predictions of space weather impacts require improved
  understanding and modeling capabilities of coupled chains of space
  environment processes. It is necessary to assemble parts of the
  source-to-impact puzzle by identifying, addressing and solving
  problems focused on specific physical domains, and then to connect
  all validated solutions from space weather origins on the sun to
  impacts on coupled geospace system, humans and technologies. To
  address the need for multi-disciplinary international space weather
  research community connecting experts in space weather phenomena
  across all domains and experts in space environment impact,
  the COSPAR Panel on Space Weather facilitated establishment of
  a network of International Space Weather Action Teams (ISWAT, <A
  href="https://www.iswat-cospar.org">https://www.iswat-cospar.org</A>,
  @IswatCosparOrg). ISWAT serves as a global hub for community coordinated
  topical collaborations focused on different aspects of space weather
  including advancing understanding, assessment and improvement of
  modeling capabilities, transitioning advances in research to operations,
  optimized utilization of available observations, and generating inputs
  to future instrumentation deployment. Action teams are building
  blocks of ISWAT initiative. ISWAT action teams are organized into
  domain-based ISWAT clusters. Action teams are working in coordinated
  effort across physical domain and across borders. The primary ISWAT
  goal is to advance space weather predictive capabilities based on best
  science available. The ISWAT currently includes more than 250 active
  participants and more than 50 action teams. The presentation will
  overview the outcome from the COSPAR ISWAT Inaugural Working Meeting
  in February 2020, highlight recent progress in advancing physics-based
  predictive capabilities and discuss plans for transforming COSPAR space
  weather Roadmap into a living document maintained by the community.

Title: Intensity and time series of extreme solar-terrestrial storm
    in 1946 March
Authors: Hayakawa, Hisashi; Ebihara, Yusuke; Pevtsov, Alexei A.;
   Bhaskar, Ankush; Karachik, Nina; Oliveira, Denny M.
Bibcode: 2020MNRAS.497.5507H
Altcode: 2020MNRAS.tmp.1670H
  Major solar eruptions occasionally cause magnetic superstorms on
  the Earth. Despite their serious consequences, the low frequency of
  their occurrence provides us with only limited cases through modern
  instrumental observations, and the intensities of historical storms
  before the coverage of the Dst index have been only sporadically
  estimated. Herein, we examine a solar-terrestrial storm that occurred
  in 1946 March and quantitatively evaluate its parameters. During the
  ascending phase of Solar Cycle 18, two moderate sunspot groups caused
  a major flare. The H α flaring area was recorded to be ≥600-1200
  millionths of solar hemisphere, suggesting that this was an M- or
  X-class flare in soft X-ray intensity. Upon this eruption, a rapid
  interplanetary coronal mass ejection (ICME) with an average speed
  of ≍1590 km s<SUP>-1</SUP> was launched. Based on measurements
  in four known mid-latitude and relatively complete magnetograms,
  the arrival of this extreme ICME caused a magnetic superstorm, which
  caused an initial phase with the H-component amplitude of ≥80 nT,
  followed by a main phase whose intensity was reconstructed as ≤-512
  nT using most negative Dst* estimates. Meanwhile, the equatorial
  boundary of the auroral oval extended down to ≤41 ${^{\circ}_{.}}$
  8 in invariant latitude and formed a corona aurora in Watheroo,
  Australia. Interestingly, during this magnetic superstorm, larger
  magnetic disturbances were recorded at dusk and near the dip equator
  on the dayside. Its cause may be associated with a strong westward
  equatorial electrojet and field-aligned current, in addition to the
  contribution from the storm-time ring current.

Title: 70 Years of Chromospheric Solar Activity and Dynamics
Authors: Bertello, Luca; Pevtsov, Alexei A.; Ulrich, Roger K.
Bibcode: 2020ApJ...897..181B
Altcode:
  From 1915 to 1985 the monitoring program of the Mount Wilson
  Observatory, one of the Observatories of the Carnegie Institution of
  Washington, has taken over 35,000 daily images (spectroheliograms)
  of the Sun in the chromospheric resonance line of Ca II K. This
  important database constitutes a unique resource for a variety of
  retrospective analyses of the state of solar magnetism on multidecadal
  timescales. These observations may also hold the key for untangling
  some of the mysteries behind the solar dynamo, which in turn could
  result in a better predictive capability for current dynamo models. We
  describe here a procedure to calibrate and rescale these images so
  that homogeneous Carrington synoptic maps can be derived for the
  whole period covered by these observations. Temporal variations
  in full-disk chromospheric activity clearly show the signature of
  the 11 yr solar cycle, but no evidence is found for a statistically
  significant north/south hemispheric asymmetry. Using a feature-tracking
  technique we were also able to obtain the average solar rotation
  profile. We find no indication of any detectable periodicity in the
  temporal behavior of the orthogonalized rotation rate coefficients,
  suggesting the global chromospheric dynamics has not changed during
  the 70 years investigated in this work. We found also no significant
  evidence in our analysis for a hemispheric asymmetry in rotation rates.

Title: Effect of Additional Magnetograph Observations From Different
    Lagrangian Points in Sun-Earth System on Predicted Properties of
    Quasi-Steady Solar Wind at 1 AU
Authors: Pevtsov, A. A.; Petrie, G.; MacNeice, P.; Virtanen, I. I.
Bibcode: 2020SpWea..1802448P
Altcode:
  Modeling the space weather conditions for a near-Earth environment
  depends on a proper representation of magnetic fields on the
  Sun. There are discussions in the community with respect to
  the value of observations taken at several Lagrangian points
  (L<SUB>1</SUB>-L<SUB>5</SUB>) in the Sun-Earth system. Observations from
  a single (e.g., Earth/L<SUB>1</SUB>) vantage point are insufficient to
  characterize rapid changes in magnetic field on the far side of the
  Sun. Nor can they represent well the magnetic fields near the solar
  poles. However, if the changes in sunspot activity were moderate,
  how well would our predictions of the solar wind based on a single
  viewing point work? How much improvement could we see by adding
  magnetograph observations from L<SUB>5</SUB>, L<SUB>4</SUB>, and even
  L<SUB>3</SUB>? Here, we present the results of our recent modeling,
  which shows the level of improvement in forecasting the properties of
  the solar wind at Earth made possible by using additional observations
  from different vantage points during a period of moderate evolution of
  sunspot activity. As an example, we also show the improvements to the
  solar wind forecast from adding a single observation of the southern
  polar area from out-of-ecliptic spacecraft at -30° heliographic
  latitude vantage point.

Title: Formation of Two Homologous Transequatorial Loops
Authors: Chen, Jie; Pevtsov, Alexei A.; Su, Jiangtao; Erdélyi,
   Robertus; Deng, Yuanyong; Yang, Shangbin; Song, Yongliang
Bibcode: 2020SoPh..295...59C
Altcode:
  The formation mechanism of two homologous transequatorial loops (TLs)
  of July 7-8, 1999 (SOL1999-07-07) is studied. The TLs connected active
  region AR 8614 from the northern hemisphere to AR 8626 in the southern
  hemisphere. The first TL appeared as a distinct structure at 12:49 UT
  on July 7, the second TL appeared at 06:21 UT, on July 8. Important
  results are obtained in this analysis: (i) The configuration of the
  two TLs is similar in X-rays. (ii) The sizes of the two active regions
  related to the TLs increased before and during the formation of the
  two TLs, this induced the expansion of their coronal loops. (iii)
  Both TLs formed globally on a time scale shorter than 110 min (time
  resolution of observations). (iv) An X-shaped coronal structure was
  observed. This observational evidence suggests that the two TLs formed
  by the same physical mechanism, magnetic reconnection, between the
  two expanding magnetic configurations of the two ARs.

Title: Monitoring solar activity with PEPSI
Authors: Dineva, Ekaterina; Denker, Carsten; Strassmeier, Klaus G.;
   Ilyin, Ilya; Pevtsov, Alexei A.
Bibcode: 2020IAUGA..30..351D
Altcode:
  Synoptic Sun-as-a-star observations are carried out with the Potsdam
  Echelle Polarimetric and Spectroscopic Instrument (PEPSI), which
  receives light from the Solar Disk-Integration (SDI) telescope. Daily
  spectra are produced with a high signal-to-noise ratio, providing access
  to unprecedented quasi-continuous, long-term, disk-integrated spectra
  of the Sun with high spectral and temporal resolution. We developed
  tools to monitor and study solar activity on different time-scales
  ranging from daily changes, over periods related to solar rotation,
  to annual and decadal trends. Strong chromospheric absorption lines,
  such as the Ca ii H &amp; K λ3934 &amp; 3968 Å lines, are powerful
  diagnostic tools for solar activity studies, since they trace the
  variations of the solar magnetic field. Other lines, such as Hα λ6563
  Å line and the near-infrared (NIR) Ca ii λ8542 Å line, provide
  additional information on the physical properties in this highly
  complex and dynamic atmospheric layer. Currently, we work on a data
  pipeline for extraction, calibration, and analysis of the PEPSI/SDI
  data. We compare the SDI data with daily spectra from the Integrated
  Sunlight Spectrometer (ISS), which is part of the Synoptic Long-Term
  Investigation of the Sun (SOLIS) facility operated by the U.S. National
  Solar Observatory (NSO). This facilitates cross-calibration and
  validation of the SDI data.

Title: Duffing Oscillator Model of Solar Cycles
Authors: Nagovitsyn, Yury A.; Pevtsov, Alexei A.
Bibcode: 2020ApJ...888L..26N
Altcode:
  We propose that the solar cycle variability could be described in the
  framework of an external quasi-sinusoidal influence on an oscillator
  with cubic nonlinearity and linear damping (Duffing oscillator). To
  demonstrate this, we compare the empirical amplitude-frequency
  dependence with the theoretical one obtained by the Krylov-Bogolyubov
  averaging method. The empirical data are a composite time series
  of 2.0 version of sunspot number series, which starts in 1700,
  and the sunspot group number series by Svalgaard &amp; Schatten,
  scaled to sunspot number, for 1610-1699 interval. We find that while
  this interpretation of solar cycle is a mathematical approximation,
  it explains several properties of solar cycle variability.

Title: Solar Cycle-Related Variability of Sun-as-a-Star Spectral
    Line Profiles
Authors: Kalscheur, M.; Criscuoli, S.; Bertello, L.; Pevtsov, A. A.
Bibcode: 2019AGUFMSH11D3390K
Altcode:
  We use daily observations of the Sun-as-a-star by the high resolution
  Integrated Sunlight Spectrometer (ISS), one of three instruments
  which comprise the Synoptic Optical Long-term Investigations of the
  Sun (SOLIS) facility, to investigate solar cycle-related variability
  in line profiles from three ISS spectral bands. Preliminary results
  show an anti-correlation between the magnitude of the CN band-head
  jump and total unsigned magnetic flux through the decline phase of
  Solar Cycle 23 and the rise and fall of Solar Cycle 24, consistent
  with magnetohydrodynamic simulations. Additional line parameters
  (core intensity, full width at half maximum and equivalent width)
  show correlations with the solar cycle, but may require correction
  for stray light. We investigate two such corrections. Varied line
  parameter responses to thermodynamic and magnetic structures in the
  solar atmosphere may provide a means of disentangling thermal and
  magnetic effects in disk-integrated solar spectra. These line parameter
  responses could in turn inform spectroscopic observations of other
  stars. Ultimately, this work is a step towards a better understanding
  of magnetic activity cycles in other Sun-like stars.

Title: Reconstructing solar magnetic fields from historical
    observations. VI. Axial dipole moments of solar active regions in
    cycles 21-24
Authors: Virtanen, I. O. I.; Virtanen, I. I.; Pevtsov, A. A.;
   Mursula, K.
Bibcode: 2019A&A...632A..39V
Altcode:
  Context. The axial dipole moments of emerging active regions control the
  evolution of the axial dipole moment of the whole photospheric magnetic
  field and the strength of polar fields. Hale's and Joy's laws of
  polarity and tilt orientation affect the sign of the axial dipole moment
  of an active region. If both laws are valid (or both violated), the
  sign of the axial moment is normal. However, for some active regions,
  only one of the two laws is violated, and the signs of these axial
  dipole moments are the opposite of normal. Those opposite-sign active
  regions can have a significant effect, for example, on the development
  of polar fields. <BR /> Aims: Our aim is to determine the axial dipole
  moments of active regions identified from magnetographic observations
  and study how the axial dipole moments of normal and opposite signs are
  distributed in time and latitude in solar cycles 21-24. <BR /> Methods:
  We identified active regions in the synoptic maps of the photospheric
  magnetic field measured at the National Solar Observatory (NSO) Kitt
  Peak (KP) observatory, the Synoptic Optical Long term Investigations of
  the Sun (SOLIS) vector spectromagnetograph (VSM), and the Helioseismic
  and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO),
  and determined their axial dipole moments. <BR /> Results: We find that,
  typically, some 30% of active regions have opposite-sign axial moments
  in every cycle, often making more than 20% of the total axial dipole
  moment. Most opposite-signed moments are small, but occasional large
  moments, which can affect the evolution of polar fields on their own,
  are observed. Active regions with such a large opposite-sign moment may
  include only a moderate amount of total magnetic flux. We find that
  in cycles 21-23 the northern hemisphere activates first and shows
  emergence of magnetic flux over a wider latitude range, while the
  southern hemisphere activates later, and emergence is concentrated to
  lower latitudes. Cycle 24 differs from cycles 21-23 in many ways. Cycle
  24 is the only cycle where the northern butterfly wing includes more
  active regions than the southern wing, and where axial dipole moment of
  normal sign emerges on average later than opposite-signed axial dipole
  moment. The total axial dipole moment and even the average axial moment
  of active regions is smaller in cycle 24 than in previous cycles.

Title: The effect of telescope aperture, scattered light and human
    vision on early measurements of sunspot and group numbers
Authors: Karachik, Nina V.; Pevtsov, Alexei A.; Nagovitsyn, Yury A.
Bibcode: 2019MNRAS.488.3804K
Altcode: 2019arXiv190704932K
  Early telescopic observations of sunspots were conducted with
  instruments of relatively small aperture. These instruments also
  suffered from a higher level of scattered light, and the human eye
  served as a `detector'. The eye's ability to resolve small details
  depends on image contrast, and on average intensity variations
  smaller than ≈3 per cent contrast relative to background are not
  detected even if they are resolved by the telescope. Here we study
  the effect of these three parameters (telescope aperture, scattered
  light and detection threshold of human vision) on sunspot number,
  group number and area of sunspots. As an `ideal' dataset, we employ
  white-light (pseudo-continuum) observations from the Helioseismic
  and Magnetic Imager (HMI) on board the Solar Dynamics Observatory,
  and we model the appearance of sunspots by degrading the HMI images
  to corresponding telescope apertures with added scattered light. We
  discuss the effects of different parameters on sunspot counts and derive
  functional dependences, which could be used to normalize historical
  observations of sunspot counts to a common denominator.

Title: Preservation of Our Astronomical Heritage
Authors: Lattis, James; Osborn, Wayne; Bartlett, Jennifer Lynn;
   Griffin, Elizabeth; Hockey, Thomas; McCluskey, Stephen; Oswalt, Terry;
   Pevtsov, Alexei A.; Schechner, Sara; Trimble, Virginia
Bibcode: 2019BAAS...51g..21L
Altcode: 2019arXiv190710686L; 2019astro2020U..21L
  We argue that it is essential that the Astro2020 survey of the present
  state of American astronomy and the recommendations for the next
  decade address the issue of ensuring preservation of, and making more
  discoverable and accessible, the field’s rich legacy materials.

Title: Research scientists in support of facilities and missions:
    Facility support and research as an interlocked pair
Authors: Soderblom, David; Wilkes, Belinda; Saha, Abhijit; Hall,
   Jeff; Chené, André-Nicolas; Pevtsov, Alexei; Peterson, Bradley M.
Bibcode: 2019BAAS...51g.116S
Altcode: 2019astro2020U.116S
  Scientists who themselves depend on the facilities that they support
  have a much deeper knowledge of the facilities, leading to better user
  support, new modes, more efficient telescope use, and overall more
  and better scientific output. This white paper supports the value of
  research scientists at facilities and missions.

Title: Reconstructing solar magnetic fields from historical
    observations. V. Sunspot magnetic field measurements at Mount Wilson
    Observatory
Authors: Pevtsov, Alexei A.; Tlatova, Kseniya A.; Pevtsov, Alexander
   A.; Heikkinen, Elina; Virtanen, Ilpo; Karachik, Nina V.; Bertello,
   Luca; Tlatov, Andrey G.; Ulrich, Roger; Mursula, Kalevi
Bibcode: 2019A&A...628A.103P
Altcode: 2019arXiv190706492P
  Context. Systematic observations of magnetic field strength and polarity
  in sunspots began at Mount Wilson Observatory (MWO), USA in early
  1917. Except for a few brief interruptions, this historical dataset has
  continued until the present. <BR /> Aims: Sunspot field strength and
  polarity observations are critical in our project of reconstructing
  the solar magnetic field over the last hundred years. We provide a
  detailed description of the newly digitized dataset of drawings of
  sunspot magnetic field observations. <BR /> Methods: The digitization
  of MWO drawings is based on a software package that we developed. It
  includes a semiautomatic selection of solar limbs and other features of
  the drawing, and a manual entry of the time of observations, measured
  field strength, and other notes handwritten on each drawing. The data
  are preserved in an MySQL database. <BR /> Results: We provide a brief
  history of the project and describe the results from digitizing this
  historical dataset. We also provide a summary of the final dataset
  and describe its known limitations. Finally, we compare the sunspot
  magnetic field measurements with those from other instruments, and
  demonstrate that, if needed, the dataset could be continued using modern
  observations such as, for example, the Vector Stokes Magnetograph on the
  Synoptic Optical Long-term Investigations of the Sun platform. <P />The
  digitized drawings are also available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A103">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A103</A>

Title: VizieR Online Data Catalog: Mt Wilson sunspot magnetic field
    measurements (Pevtsov+, 2019)
Authors: Pevtsov, A. A.; Tlatova, K. A.; Pevtsov, A. A.; Heikkinen,
   E.; Virtanen, I.; Karachik, N. V.; Bertello, L.; Tlatov, A. G.;
   Ulrich, R.; Mursula, K.
Bibcode: 2019yCat..36280103P
Altcode:
  MOUNT WILSON OBSERVATORY 150-foot solar tower, SUNSPOT DRAWINGS,
  1917-2016. <P />File: figs3_5.dat (Contains data shown in Figures
  3-5). Three columns contain information on date of observations (Years),
  measured Field strength (gauss), and radius of solar disk on drawings
  (pixels). For measurements on drawings that only have information
  about polarity, but not amplitude, we assigned a fixed value of
  "4900". <P />Files: fig6a.dat and fig6b.dat (Contain data shown
  in Figure 6). Entries correspond to same-day measurements of field
  strengths (in units of Hundreds of gauss) at Mount Wilson Observatory
  (MWO, left column) and Crimean Astrophysical Observatory (CrAO, right
  column). In Figure 6, observations from 1994-2003 are shown by black
  squares, and data for 2004-2014 are shown as filled red circles. <P
  />File: fig7.dat (Contain data for Figure 7). Field strengths of 50
  sunspots observed at Mount Wilson Observatory (MWO) and and by Vector
  Stokes Magnetograph (VSM) on Synoptic Optical Long-term Investigations
  of the Sun (SOLIS) platform. SOLIS/VSM data are derived using SOLIS
  Zeemanfit code. <P />(5 data files).

Title: Reconstructing solar magnetic fields from historical
    observations. IV. Testing the reconstruction method
Authors: Virtanen, I. O. I.; Virtanen, I. I.; Pevtsov, A. A.; Bertello,
   L.; Yeates, A.; Mursula, K.
Bibcode: 2019A&A...627A..11V
Altcode:
  <BR /> Aims: The evolution of the photospheric magnetic field has
  only been regularly observed since the 1970s. The absence of earlier
  observations severely limits our ability to understand the long-term
  evolution of solar magnetic fields, especially the polar fields that
  are important drivers of space weather. Here, we test the possibility
  to reconstruct the large-scale solar magnetic fields from Ca II K
  line observations and sunspot magnetic field observations, and to
  create synoptic maps of the photospheric magnetic field for times
  before modern-time magnetographic observations. <BR /> Methods:
  We reconstructed active regions from Ca II K line synoptic maps
  and assigned them magnetic polarities using sunspot magnetic field
  observations. We used the reconstructed active regions as input in
  a surface flux transport simulation to produce synoptic maps of the
  photospheric magnetic field. We compared the simulated field with
  the observed field in 1975-1985 in order to test and validate our
  method. <BR /> Results: The reconstruction very accurately reproduces
  the long-term evolution of the large-scale field, including the poleward
  flux surges and the strength of polar fields. The reconstruction has
  slightly less emerging flux because a few weak active regions are
  missing, but it includes the large active regions that are the most
  important for the large-scale evolution of the field. Although our
  reconstruction method is very robust, individual reconstructed active
  regions may be slightly inaccurate in terms of area, total flux, or
  polarity, which leads to some uncertainty in the simulation. However,
  due to the randomness of these inaccuracies and the lack of long-term
  memory in the simulation, these problems do not significantly affect
  the long-term evolution of the large-scale field.

Title: Evolution of Magnetic Helicity in Solar Cycle 24
Authors: Pipin, Valery V.; Pevtsov, Alexei A.; Liu, Yang; Kosovichev,
   Alexander G.
Bibcode: 2019ApJ...877L..36P
Altcode: 2019arXiv190500772P
  We propose a novel approach to reconstruct the surface magnetic
  helicity density on the Sun or Sun-like stars. The magnetic vector
  potential is determined via decomposition of vector magnetic-field
  measurements into toroidal and poloidal components. The method is
  verified using data from a non-axisymmetric dynamo model. We apply the
  method to vector field synoptic maps from the Helioseismic and Magnetic
  Imager on board the Solar Dynamics Observatory to study the evolution
  of the magnetic helicity density during solar cycle 24. It is found
  that the mean helicity density of the non-axisymmetric magnetic field
  of the Sun evolves in a way similar to that reported for the current
  helicity density of the solar active regions. It predominantly has a
  negative sign in the northern hemisphere, while it is mainly positive
  in the southern hemisphere. Also, the hemispheric helicity rule for
  the non-axisymmetric magnetic field showed the sign inversion at the
  end of cycle 24. The evolution of the magnetic helicity density of
  a large-scale axisymmetric magnetic field is different from what is
  predicted by dynamo theory. On one hand, the mean large- and small-scale
  components of magnetic helicity density display the hemispheric helicity
  rule of opposite signs at the beginning of cycle 24. However, later
  in the cycle, the two helicities exhibit the same sign, in contrast
  with theoretical expectations.

Title: Historical astronomical data: urgent need for preservation,
    digitization enabling scientific exploration
Authors: Pevtsov, Alexei; Griffin, Elizabeth; Grindlay, Jonathan;
   Kafka, Stella; Bartlett, Jennifer; Usoskin, Ilya; Mursula, Kalevi;
   Gibson, Sarah; Pillet, Valentín; Burkepile, Joan; Webb, David; Clette,
   Frédéric; Hesser, James; Stetson, Peter; Muñoz-Jaramillo, Andres;
   Hill, Frank; Bogart, Rick; Osborn, Wayne; Longcope, Dana
Bibcode: 2019BAAS...51c.190P
Altcode: 2019arXiv190304839P; 2019astro2020T.190P
  This white paper emphasizes critical importance of preservation,
  digitization and scientific exploration of historical astronomical
  data. It outlines the rationale, provides examples of new science
  with such data, and reviews the potential losses to science if nothing
  it done.

Title: Reconstructing Extreme Space Weather From Planet Hosting Stars
Authors: Airapetian, Vladimir; Adibekyan, V.; Ansdell, M.; Alexander,
   D.; Barklay, T.; Bastian, T.; Boro Saikia, S.; Cohen, O.; Cuntz,
   M.; Danchi, W.; Davenport, J.; DeNolfo, G.; DeVore, R.; Dong, C. F.;
   Drake, J. J.; France, K.; Fraschetti, F.; Herbst, K.; Garcia-Sage,
   K.; Gillon, M.; Glocer, A.; Grenfell, J. L.; Gronoff, G.; Gopalswamy,
   N.; Guedel, M.; Hartnett, H.; Harutyunyan, H.; Hinkel, N. R.; Jensen,
   A. G.; Jin, M.; Johnstone, C.; Kahler, S.; Kalas, P.; Kane, S. R.;
   Kay, C.; Kitiashvili, I. N.; Kochukhov, O.; Kondrashov, D.; Lazio, J.;
   Leake, J.; Li, G.; Linsky, J.; Lueftinger, T.; Lynch, B.; Lyra, W.;
   Mandell, A. M.; Mandt, K. E.; Maehara, H.; Miesch, M. S.; Mickaelian,
   A. M.; Mouschou, S.; Notsu, Y.; Ofman, L.; Oman, L. D.; Osten, R. A.;
   Oran, R.; Petre, R.; Ramirez, R. M.; Rau, G.; Redfield, S.; Réville,
   V.; Rugheimer, S.; Scheucher, M.; Schlieder, J. E.; Shibata, K.;
   Schnittman, J. D.; Soderblom, David; Strugarek, A.; Turner, J. D.;
   Usmanov, A.; Van Der Holst, B.; Vidotto, A.; Vourlidas, A.; Way, M. J.;
   Wolk, Scott J.; Zank, G. P.; Zarka, P.; Kopparapu, R.; Babakhanova,
   S.; Pevtsov, A. A.; Lee, Y.; Henning, W.; Colón, K. D.; Wolf, E. T.
Bibcode: 2019BAAS...51c.564A
Altcode: 2019astro2020T.564A; 2019arXiv190306853A
  The goal of this white paper is to identify and describe promising key
  research goals to aid the theoretical characterization and observational
  detection of ionizing radiation from quiescent and flaring upper
  atmospheres of planet hosts as well as properties of stellar coronal
  mass ejections (CMEs) and stellar energetic particle (SEP) events.

Title: Structure and evolution of the photospheric magnetic field in
2010-2017: comparison of SOLIS/VSM vector field and B<SUB>LOS</SUB>
    potential field
Authors: Virtanen, Ilpo I.; Pevtsov, Alexei A.; Mursula, Kalevi
Bibcode: 2019A&A...624A..73V
Altcode: 2019arXiv190410740V
  Context. The line-of-sight (LOS) component of the large-scale
  photospheric magnetic field has been observed since the 1950s, but
  the daily full-disk observations of the full vector magnetic field
  started only in 2010 using the SOLIS Vector Stokes Magnetograph (VSM)
  and the SDO helioseismic and magnetic imager (HMI). Traditionally,
  potential field extrapolations are based on the assumption that the
  magnetic field in the photosphere is approximately radial. The validity
  of this assumption has not been tested yet. <BR /> Aims: We investigate
  here the structure and evolution of the three components of the solar
  large-scale magnetic field in 2010-2017, covering the ascending to
  mid-declining phase of solar cycle 24, using SOLIS/VSM vector synoptic
  maps of the photospheric magnetic field. <BR /> Methods: We compare
  the observed VSM vector magnetic field to the potential vector field
  derived using the VSM LOS magnetic field observations as an input. The
  new vector field data allow us to derive the meridional inclination and
  the azimuth angle of the magnetic field and to investigate their solar
  cycle evolution and latitudinal profile of these quantities. <BR />
  Results: SOLIS/VSM vector data show that the photospheric magnetic field
  is in general fairly non-radial. In the meridional plane the field is
  inclined toward the equator, reflecting the dipolar structure of the
  solar magnetic field. Rotationally averaged meridional inclination
  does not have significant solar cycle variation. While the vector
  radial component B<SUB>r</SUB> and the potential radial component
  B<SUP>PFSS</SUP><SUB>r</SUB> are fairly similar, the meridional and
  zonal components do not agree very well. We find that SOLIS/VSM vector
  observations are noisy at high latitudes and suffer from the vantage
  point effect more than LOS observations. This is due to different noise
  properties in the LOS and transverse components of the magnetic field,
  which needs to be addressed in future studies.

Title: Extended statistical analysis of emerging solar active regions
Authors: Kutsenko, Alexander S.; Abramenko, Valentina I.; Pevtsov,
   Alexei A.
Bibcode: 2019MNRAS.484.4393K
Altcode: 2019MNRAS.tmp..310K; 2018arXiv181112089K
  We use observations of line-of-sight magnetograms from Helioseismic and
  Magnetic Imager onboard of Solar Dynamics Observatory to investigate
  polarity separation, magnetic flux, flux emergence rate, twist and
  tilt of solar emerging active regions. Functional dependence of
  polarity separation and maximum magnetic flux of an active region is
  in agreement with a simple model of flux emergence as the result of
  buoyancy forces. Our investigation did not reveal any strong dependence
  of emergence rate on twist properties of active regions.

Title: VizieR Online Data Catalog: 24 years monitoring of Sun and
    Sun-like stars (Radick+, 2018)
Authors: Radick, R. R.; Lockwood, G. W.; Henry, G. W.; Hall, J. C.;
   Pevtsov, A. A.
Bibcode: 2019yCat..18550075R
Altcode:
  Starting in the late 1970s, variations in total solar irradiance (TSI)
  have been monitored by radiometers aboard a series of spacecraft,
  including the Total Irradiance Monitor (TIM) instrument on the
  SOlar Radiation and Climate Experiment (SORCE) satellite from 2003
  to the present. <P />Observations to monitor relatively bright
  stars very similar to the Sun have been made with the Solar-Stellar
  Spectrograph (SSS) at Lowell Observatory and automatic photometric
  telescopes (APTs) at Fairborn Observatory beginning in 1992 and
  1993, respectively. In all, 72 stars, including 18 from the broader
  R98-L07 (1998ApJS..118..239R and 2007ApJS..171..260L) sample, have
  been observed at both locations, some for as long as 24 seasons. These
  joint observations provide the primary stellar data for this paper. <P
  />(3 data files).

Title: Some Features of the Two Sunspot Group Populations' Properties
Authors: Nagovitsyn, Yu. A.; Pevtsov, A. A.; Osipova, A. A.; Ivanov,
   V. G.
Bibcode: 2018Ge&Ae..58.1170N
Altcode:
  This paper describes some features of the two sunspot group populations'
  properties: Large Long-living Groups, LLG, and Small Short-living, SSG
  (each distributed lognormally), divided by the lifetime of the group
  (less and strictly more than 5 days). It is shown that the relative
  fraction of LLG varies with the latitude from 0.45 at mid-latitudes
  to 0.25-0.30 at low and high latitudes. The SSG has a wider spread in
  latitude than the LLG. It is shown that the N-S asymmetry is an overall
  global process to which both the LLG and the SSG are exposed. At the
  same time, the asymmetry of LLG shows a slightly higher dispersion of
  oscillations than the asymmetry of SSG. Other properties of sunspot
  groups in the context of the two separate populations existence are
  considered.

Title: The magnetic field vector of the Sun-as-a-star - II. Evolution
    of the large-scale vector field through activity cycle 24
Authors: Vidotto, A. A.; Lehmann, L. T.; Jardine, M.; Pevtsov, A. A.
Bibcode: 2018MNRAS.480..477V
Altcode: 2018MNRAS.tmp.1832V; 2018arXiv180706334V
  In this work, we investigate how the large-scale magnetic field of the
  Sun, in its three vector components, has evolved during most of cycle
  24, from 2010 January to 2018 April. To filter out the small-scale
  field of the Sun, present in high-resolution synoptic maps, we use a
  spherical harmonic decomposition method, which decomposes the solar
  field in multipoles with different ℓ degrees. By summing together the
  low-ℓ multipoles, we reconstruct the large-scale field at a resolution
  similar to observed stellar magnetic fields, which allows the direct
  comparison between solar and stellar magnetic maps. During cycle 24, the
  `Sun-as-a-star' magnetic field shows a polarity reversal in the radial
  and meridional components, but not in the azimuthal component. The
  large-scale solar field remains mainly poloidal with {≳ } 70{{ per
  cent}} of its energy contained in the poloidal component. During its
  evolution, the large-scale field is more axisymmetric and more poloidal
  when near minima in sunspot numbers, and with a larger intensity near
  maximum. There is a correlation between toroidal energy and sunspot
  number, which indicates that spot fields are major contributors to the
  toroidal large-scale energy of the Sun. The solar large-scale magnetic
  properties fit smoothly with observational trends of stellar magnetism
  reported in See et al. The toroidal (&lt; B_tor^2 &gt;) and poloidal
  (&lt; B_pol^2 &gt;) energies are related as &lt; B_tor^2 &gt; ∝
  &lt; B_pol^2 &gt; ^{1.38± 0.04}. Similar to the stellar sample, the
  large-scale field of the Sun shows a lack of toroidal non-axisymmetric
  field.

Title: Roadmap for Reliable Ensemble Forecasting of the Sun-Earth
    System
Authors: Nita, Gelu; Angryk, Rafal; Aydin, Berkay; Banda, Juan;
   Bastian, Tim; Berger, Tom; Bindi, Veronica; Boucheron, Laura; Cao,
   Wenda; Christian, Eric; de Nolfo, Georgia; DeLuca, Edward; DeRosa,
   Marc; Downs, Cooper; Fleishman, Gregory; Fuentes, Olac; Gary, Dale;
   Hill, Frank; Hoeksema, Todd; Hu, Qiang; Ilie, Raluca; Ireland,
   Jack; Kamalabadi, Farzad; Korreck, Kelly; Kosovichev, Alexander;
   Lin, Jessica; Lugaz, Noe; Mannucci, Anthony; Mansour, Nagi; Martens,
   Petrus; Mays, Leila; McAteer, James; McIntosh, Scott W.; Oria, Vincent;
   Pan, David; Panesi, Marco; Pesnell, W. Dean; Pevtsov, Alexei; Pillet,
   Valentin; Rachmeler, Laurel; Ridley, Aaron; Scherliess, Ludger; Toth,
   Gabor; Velli, Marco; White, Stephen; Zhang, Jie; Zou, Shasha
Bibcode: 2018arXiv181008728N
Altcode:
  The authors of this report met on 28-30 March 2018 at the New Jersey
  Institute of Technology, Newark, New Jersey, for a 3-day workshop
  that brought together a group of data providers, expert modelers, and
  computer and data scientists, in the solar discipline. Their objective
  was to identify challenges in the path towards building an effective
  framework to achieve transformative advances in the understanding
  and forecasting of the Sun-Earth system from the upper convection
  zone of the Sun to the Earth's magnetosphere. The workshop aimed to
  develop a research roadmap that targets the scientific challenge
  of coupling observations and modeling with emerging data-science
  research to extract knowledge from the large volumes of data (observed
  and simulated) while stimulating computer science with new research
  applications. The desire among the attendees was to promote future
  trans-disciplinary collaborations and identify areas of convergence
  across disciplines. The workshop combined a set of plenary sessions
  featuring invited introductory talks and workshop progress reports,
  interleaved with a set of breakout sessions focused on specific topics
  of interest. Each breakout group generated short documents, listing
  the challenges identified during their discussions in addition to
  possible ways of attacking them collectively. These documents were
  combined into this report-wherein a list of prioritized activities
  have been collated, shared and endorsed.

Title: Reconstructing solar magnetic fields from historical
    observations. III. Activity in one hemisphere is sufficient to cause
    polar field reversals in both hemispheres
Authors: Virtanen, I. O. I.; Virtanen, I. I.; Pevtsov, A. A.;
   Mursula, K.
Bibcode: 2018A&A...616A.134V
Altcode:
  <BR /> Aims: Sunspot activity is often hemispherically asymmetric,
  and during the Maunder minimum, activity was almost completely limited
  to one hemisphere. In this work, we use surface flux simulation to
  study how magnetic activity limited only to the southern hemisphere
  affects the long-term evolution of the photospheric magnetic field
  in both hemispheres. The key question is whether sunspot activity
  in one hemisphere is enough to reverse the polarity of polar fields
  in both hemispheres. <BR /> Methods: We simulated the evolution of
  the photospheric magnetic field from 1978 to 2016 using the observed
  active regions of the southern hemisphere as input. We studied the flow
  of magnetic flux across the equator and its subsequent motion towards
  the northern pole. We also tested how the simulated magnetic field is
  changed when the activity of the southern hemisphere is reduced. <BR />
  Results: We find that activity in the southern hemisphere is enough
  to reverse the polarity of polar fields in both hemispheres by the
  cross-equatorial transport of magnetic flux. About 1% of the flux
  emerging in the southern hemisphere is transported across the equator,
  but only 0.1%-0.2% reaches high latitudes to reverse and regenerate a
  weak polar field in the northern hemisphere. The polarity reversals in
  the northern hemisphere are delayed compared to the southern hemisphere,
  leading to a quadrupole Sun lasting for several years.

Title: Tilt of Sunspot Bipoles in Solar Cycles 15 to 24
Authors: Tlatova, Ksenia; Tlatov, Andrey; Pevtsov, Alexei; Mursula,
   Kalevi; Vasil'eva, Valeria; Heikkinen, Elina; Bertello, Luca; Pevtsov,
   Alexander; Virtanen, Ilpo; Karachik, Nina
Bibcode: 2018SoPh..293..118T
Altcode: 2018arXiv180707913T
  We use recently digitized sunspot drawings from Mount Wilson
  Observatory to investigate the latitudinal dependence of tilt angles
  of active regions and its change with solar cycle. The drawings cover
  the period from 1917 to present and contain information as regards
  polarity and strength of magnetic field in sunspots. We identified
  clusters of sunspots of same polarity, and used these clusters to
  form "bipole pairs". The orientation of these bipole pairs was used
  to measure their tilts. We find that the latitudinal profile of tilts
  does not monotonically increase with latitude as most previous studies
  assumed, but instead, it shows a clear maximum at about 25 - 30 degree
  latitudes. Functional dependence of tilt (γ ) on latitude (φ ) was
  found to be γ =(0.20 ±0.08 )sin(2.80 φ )+(−0.00 ±0.06 ). We also
  find that latitudinal dependence of tilts varies from one solar cycle
  to another, but larger tilts do not seem to result in stronger solar
  cycles. Finally, we find the presence of a systematic offset in tilt
  of active regions (non-zero tilts at the equator), with odd cycles
  exhibiting negative offset and even cycles showing the positive offset.

Title: Current Efforts to Preserve Mt. Wilson Historical Observations
Authors: Pevtsov, Alexander; Pevtsov, Alexei; Virtanen, Ilpo;
   Bertello, Luca
Bibcode: 2018cosp...42E2653P
Altcode:
  Mount Wilson Observatory (MWO) located in the San Gabriel Mountains
  near Pasadena, California, has been taking solar observations since the
  early 1900s. The earliest of which include the nearly unbroken record
  of daily sunspot activity from 1917 through the present day. Full disk
  spectroheliograms in Ca K spectral line were taken from 1915 through
  1985. Starting from the 1950s synoptic magnetograms were also taken
  utilizing the full disk longitudinal magnetograph at MWO. However,
  working with historical long-term dataset presents unique challenges. To
  be useful, data from hand drawings needed to be digitized. Furthermore,
  data server failures in the past have led to a loss of public access
  to parts of these historical data sets. Over the past several years
  we have worked to process and extract metadata from MWO sunspot
  drawings. Recently, a new project on the preservation of historical
  magnetograms has been started. These efforts have provided insight
  into dealing with issues presented by historical datasets. In this
  presentation we provide an overview of current efforts in preserving the
  historical (solar) datasets from MWO. We discuss some of these issues
  and the application of these insights with regard to the recovery and
  processing of early magnetogram data along with the current plans to
  transfer this data to the public domain.

Title: Design of a next generation synoptic solar observing network:
    solar physics research integrated network group (SPRING)
Authors: Gosain, Sanjay; Roth, Markus; Hill, Frank; Pevtsov, Alexei;
   Martinez Pillet, Valentin; Thompson, Michael J.
Bibcode: 2018SPIE10702E..4HG
Altcode:
  Long-term synoptic observations of the Sun in different wavelength
  regions are essential to understand its secular behavior. Such
  observations have proven very important for discovery of 11 year
  solar activity cycle, 22 year magnetic cycle, polar field reversals,
  Hale's polarity law, Joy's law, that helped Babcock and Leighton
  to propose famous solar dynamo model. In more recent decades, the
  societal impact of the secular changes in Sun's output has been felt in
  terms of solar inputs to terrestrial climate-change and space-weather
  hazards. Further, it has been realized that to better understand the
  activity phenomena such as flares and coronal mass ejections (CMEs)
  one needs synoptic observations in multiple spectral lines to enable
  tomographic inference of physical parameters. Currently, there are
  both space and ground based synoptic observatories. However, given
  the requirements for the long-term stability and reliability of such
  synoptic datasets, ground-based facilities are more preferable. Also,
  the ground based observatories are easy to maintain or upgrade while
  detailed and frequent calibrations are easily possible. The only
  ground-based facility that currently provides full-disk velocity and
  magnetic field maps of the Sun around the clock and at good cadence,
  is the Global Oscillations Network Group (GONG) network of National
  Solar Observatory (NSO) which is operational since the mid 90s. Due
  to its aging instrumentation, operating for nearly three decades, and
  new requirements to obtain multiwavelength observations, a need is felt
  in the solar community to build a next generation synoptic observatory
  network. A group of international observatories have come together under
  the auspices of SOLARNET program, funded by European Union (EU), to
  carryout a preliminary design study of such a synoptic solar observing
  facility called "SPRING", which stands for Solar Physics Research
  Integrated Network Group. In this article we will present concept of
  SPRING and the optical design concept of its major instruments.ts.

Title: Modeling the Global Coronal Field with Simulated Synoptic
    Magnetograms from Earth and the Lagrange Points L<SUB>3</SUB>,
    L<SUB>4</SUB>, and L<SUB>5</SUB>
Authors: Petrie, Gordon; Pevtsov, Alexei; Schwarz, Andrew; DeRosa, Marc
Bibcode: 2018SoPh..293...88P
Altcode:
  The solar photospheric magnetic flux distribution is key to structuring
  the global solar corona and heliosphere. Regular full-disk photospheric
  magnetogram data are therefore essential to our ability to model
  and forecast heliospheric phenomena such as space weather. However,
  our spatio-temporal coverage of the photospheric field is currently
  limited by our single vantage point at/near Earth. In particular,
  the polar fields play a leading role in structuring the large-scale
  corona and heliosphere, but each pole is unobservable for &gt;6 months
  per year. Here we model the possible effect of full-disk magnetogram
  data from the Lagrange points L<SUB>4</SUB> and L<SUB>5</SUB>, each
  extending longitude coverage by 60<SUP>∘</SUP>. Adding data also from
  the more distant point L<SUB>3</SUB> extends the longitudinal coverage
  much further. The additional vantage points also improve the visibility
  of the globally influential polar fields. Using a flux-transport model
  for the solar photospheric field, we model full-disk observations from
  Earth/L<SUB>1</SUB>, L<SUB>3</SUB>, L<SUB>4</SUB>, and L<SUB>5</SUB>
  over a solar cycle, construct synoptic maps using a novel weighting
  scheme adapted for merging magnetogram data from multiple viewpoints,
  and compute potential-field models for the global coronal field. Each
  additional viewpoint brings the maps and models into closer agreement
  with the reference field from the flux-transport simulation, with
  particular improvement at polar latitudes, the main source of the fast
  solar wind.

Title: Composite Photospheric Synoptic Magnetic Maps
Authors: Bertello, Luca; Pevtsov, Alexei A.; Petrie, Gordon J. D.
Bibcode: 2018tess.conf11502B
Altcode:
  Photospheric synoptic maps of the Sun's magnetic field are the primary
  drivers of both coronal and heliospheric

Title: Modeling the Global Coronal Field with Simulated Synoptic
    Magnetograms from Earth and the Lagrange points L3, L4 and L5
Authors: Petrie, Gordon J. D.; Pevtsov, Alexei A.; Schwarz, Andrew
   Michael; DeRosa, Marc
Bibcode: 2018tess.conf40132P
Altcode:
  The solar photospheric magnetic flux distribution determines the
  global structure of the solar corona and heliosphere. Regular
  full-disk photospheric magnetogram data are therefore essential to
  our ability to model and forecast heliospheric phenomena such as space
  weather. However, our spatio-temporal coverage of the photospheric field
  is currently limited by our single vantage point at/near Earth. In
  particular, the polar fields define the large-scale structure of the
  corona and heliosphere but each pole is unobservable for &gt; 6 months
  per year. Here we model the possible effect of full-disk magnetogram
  data from the Lagrange points L4 and L5, each extending longitude
  coverage by 60 degrees. Adding data also from the more distant point
  L3 extends longitudinal coverage much further. The additional vantage
  points also improve the visibility of the globally influential polar
  fields. Using a flux-transport model for the solar photospheric field
  we model full-disk observations from Earth/L1, L3, L4 and L5 over a
  solar cycle, construct synoptic maps using a novel weighting scheme
  adapted for merging magnetogram data from multiple viewpoints, and
  compute potential-field models for the global coronal field. Each
  additional viewpoint significantly improves the performance of the
  maps and models with particular improvement at polar latitudes, the
  main source of the fast solar wind.

Title: Is activity in one hemisphere enough to maintain the magnetic
    cycle?
Authors: Virtanen, Iiro; Virtanen, Ilpo; Pevtsov, Alexei; Mursula,
   Kalevi
Bibcode: 2018EGUGA..2018095V
Altcode:
  Sunspot activity is often hemispherically asymmetric, and during
  the Maunder minimum activity was almost completely limited to one
  hemisphere. We use surface flux simulations to study how magnetic
  activity limited only to the southern hemisphere affects the long-term
  evolution of the photospheric magnetic field in both hemispheres. The
  key question is whether activity in one hemisphere is enough to maintain
  the magnetic cycle of polar areas in both hemispheres. We simulate the
  evolution of the field from 1978 to 2016 using the observed active
  regions of the southern hemisphere as input. We study the flow of
  magnetic flux across the equator and its subsequent motion towards
  the northern pole. We find that activity in the southern hemisphere
  is enough to maintain the magnetic cycle in both hemispheres by the
  cross-equatorial flow of magnetic flux. About one percent of the flux
  emerging in the southern hemisphere is transported across the equator,
  but only 0.1%-0.2% reaches high latitudes to reverse and regenerate a
  weak polar field in the northern hemisphere. The magnetic cycle of the
  northern hemisphere is delayed compared to the southern hemisphere,
  leading to a quadrupole Sun lasting for several years.

Title: Reconstructing Solar Magnetic Field Evolution Over the
    Past Century
Authors: Pevtsov, Alexei
Bibcode: 2018EGUGA..2013274P
Altcode:
  This talk will review the current state of long- term synoptic programs
  and present results of several recent projects aimed at reconstructing
  a more detailed picture of solar activity over the last century. In
  particular, we will present the recent findings and discuss open
  questions of research projects by international team 420 supported
  by the International Space Science Institute (Bern, Switzerland,
  see http://www.issibern.ch/teams/solheliomagnet/#). The team aims at
  developing methods to study the evolution of magnetic activity prior to
  modern age of solar magnetography using historical data from Kodaikanal
  (India) and Mount Wilson (USA) observatories and modern surface flux
  transport models.

Title: Electric Currents in the Solar Atmosphere
Authors: Fleishman, Gregory D.; Pevtsov, Alexei A.
Bibcode: 2018GMS...235...43F
Altcode:
  No abstract at ADS

Title: Patterns of Variation for the Sun and Sun-like Stars
Authors: Radick, Richard R.; Lockwood, G. Wesley; Henry, Gregory W.;
   Hall, Jeffrey C.; Pevtsov, Alexei A.
Bibcode: 2018ApJ...855...75R
Altcode:
  We compare patterns of variation for the Sun and 72 Sun-like stars by
  combining total and spectral solar irradiance measurements between 2003
  and 2017 from the SORCE satellite, Strömgren b, y stellar photometry
  between 1993 and 2017 from Fairborn Observatory, and solar and stellar
  chromospheric Ca II H+K emission observations between 1992 and 2016
  from Lowell Observatory. The new data and their analysis strengthen
  the relationships found previously between chromospheric and brightness
  variability on the decadal timescale of the solar activity cycle. Both
  chromospheric H+K and photometric b, y variability among Sun-like stars
  are related to average chromospheric activity by power laws on this
  timescale. Young active stars become fainter as their H+K emission
  increases, and older, less active, more Sun-age stars tend to show
  a pattern of direct correlation between photometric and chromospheric
  emission variations. The directly correlated pattern between total solar
  irradiance and chromospheric Ca II emission variations shown by the Sun
  appears to extend also to variations in the Strömgren b, y portion of
  the solar spectrum. Although the Sun does not differ strongly from its
  stellar age and spectral class mates in the activity and variability
  characteristics that we have now studied for over three decades, it
  may be somewhat unusual in two respects: (1) its comparatively smooth,
  regular activity cycle, and (2) its rather low photometric brightness
  variation relative to its chromospheric activity level and variation,
  perhaps indicating that facular emission and sunspot darkening are
  especially well-balanced on the Sun.

Title: Two Populations of Sunspots: Differential Rotation
Authors: Nagovitsyn, Yu. A.; Pevtsov, A. A.; Osipova, A. A.
Bibcode: 2018AstL...44..202N
Altcode:
  To investigate the differential rotation of sunspot groups using the
  Greenwich data, we propose an approach based on a statistical analysis
  of the histograms of particular longitudinal velocities in different
  latitude intervals. The general statistical velocity distributions
  for all such intervals are shown to be described by two rather than
  one normal distribution, so that two fundamental rotation modes exist
  simultaneously: fast and slow. The differentiality of rotation for
  the modes is the same: the coefficient at sin<SUP>2</SUP> in Faye's
  law is 2.87-2.88 deg/day, while the equatorial rotation rates differ
  significantly, 0.27 deg/day. On the other hand, an analysis of the
  longitudinal velocities for the previously revealed two differing
  populations of sunspot groups has shown that small short-lived
  groups (SSGs) are associated with the fast rotation mode, while
  large long-lived groups (LLGs) are associated with both fast and
  slow modes. The results obtained not only suggest a real physical
  difference between the two populations of sunspots but also give new
  empirical data for the development of a dynamo theory, in particular,
  for the theory of a spatially distributed dynamo.

Title: Space Weather Forecasting and Supporting Research in the USA
Authors: Pevtsov, A. A.
Bibcode: 2017Ge&Ae..57..769P
Altcode: 2016arXiv161102652P
  In the United State, scientific research in space weather is funded by
  several Government Agencies including the National Science Foundation
  (NSF) and the National Aeronautics and Space Agency (NASA). For
  civilian and commercial purposes, space weather forecast is done by
  the Space Weather Prediction Center (SWPC) of the National Oceanic
  and Atmospheric Administration (NOAA). Observational data for modeling
  come from the network of groundbased observatories funded via various
  sources, as well as from the instruments on spacecraft. Numerical
  models used in forecast are developed in framework of individual
  research projects. The article provides a brief review of current
  state of space weather-related research and forecasting in the USA.

Title: Search for a Signature of Twist-removal in the Magnetic Field
    of Sunspots in Relation with Major Flares
Authors: Burtseva, Olga; Gosain, Sanjay; Pevtsov, Alexei A.
Bibcode: 2017ApJ...849..103B
Altcode: 2017arXiv171102166B
  We investigate the restructuring of the magnetic field in sunspots
  associated with two flares: the X6.5 flare on 2006 December 6 and the
  X2.2 flare on 2011 February 15. The observed changes were evaluated
  with respect to the so-called twist-removal model, in which helicity
  (twist) is removed from the corona as the result of an eruption. Since
  no vector magnetograms were available for the X6.5 flare, we applied the
  azimuthal symmetry approach to line-of-sight magnetograms to reconstruct
  the pseudo-vector magnetic field and investigate the changes in average
  twist and inclination of magnetic field in the sunspot around the
  time of the flare. For the X2.2 flare, results from the full vector
  magnetograms were compared with the pseudo-vector field data. For
  both flares, the data show changes consistent with the twist-removal
  scenario. We also evaluate the validity of the azimuthal symmetry
  approach on simple isolated round sunspots. In general, the derivations
  based on the azimuthal symmetry approach agree with true-vector field
  data though we find that even for symmetric sunspots the distribution of
  the magnetic field may deviate from an axially symmetric distribution.

Title: On the timing of the next great solar activity minimum
Authors: Tlatov, A. G.; Pevtsov, A. A.
Bibcode: 2017AdSpR..60.1108T
Altcode:
  The long-term variations in solar activity are studied using the
  dataset comprised of sunspot number and 1<SUP>4</SUP>C radioisotope
  timeseries. We use a novel S<SUB>200</SUB> index to identify possible
  past Grand Minima (GM). The Maunder, Oort, Wolf and Spörer Minima fall
  in phase with the minimum of S<SUB>200</SUB> index. We also show GM
  develop in clusters, with a separation of about 400-600 years between
  individual GM. Extending these found similarities to modern solar
  activity, it is predicted that next grand solar minimum may occur in
  about ∼ 2090 ± 20 .

Title: VizieR Online Data Catalog: Calibrated solar S-index time
    series (Egeland+, 2017)
Authors: Egeland, R.; Soon, W.; Baliunas, S.; Hall, J. C.; Pevtsov,
   A. A.; Bertello, L.
Bibcode: 2017yCat..18350025E
Altcode:
  The Mount Wilson HK Program observed the Moon with both the HKP-1
  and HKP-2 instruments. After removing 11 obvious outliers, there
  are 162 HKP-1 observations taken from 1966 September 2 to 1977 June
  4 with the Mount Wilson 100 inch reflector, covering the maximum
  of cycle 20 and the cycle 20-21 minimum. As mentioned in Baliunas+
  (1995ApJ...438..269B), observations of the Moon resumed in 1993 with
  the HKP-2 instrument. After removing 10 obvious outliers, there are 75
  HKP-2 observations taken from 1994 March 27 to 2002 November 23 with
  the Mount Wilson 60 inch reflector, covering the end of cycle 22 and
  the cycle 23 minimum, extending just past the cycle 23 maximum. The
  end of observations coincides with the unfortunate termination of
  the HK Project in 2003. <P />We seek to extend our time series of
  solar variability beyond cycle 23 by establishing a proxy to the
  NSO Sacramento Peak (NSO/SP) observations taken from 1976 to 2016,
  covering cycles 21 to 24. The spectral intensity scale is set by
  integrating a 0.53Å band centered at 3934.869Å in the K-line wing
  and setting it to the fixed value of 0.162. <P />We extend the S-index
  record back to cycle 20 using the composite K time series of Bertello+
  (2016SoPh..291.2967B). See section 3 for further explanations. <P />(1
  data file).

Title: Sub-Pixel Magnetic Field and Plasma Dynamics Derived from
    Photospheric Spectral Data
Authors: Rasca, Anthony P.; Chen, James; Pevtsov, Alexei A.
Bibcode: 2017SPD....4810104R
Altcode:
  Current high-resolution observations of the photosphere show
  small dynamic features at the resolving limit during emerging flux
  events. However, line-of-sight (LOS) magnetogram pixels only contain
  the net uncanceled magnetic flux, which is expected to increase
  for fixed regions as resolution limits improve. Using a new method
  with spectrographic images, we quantify distortions in photospheric
  absorption (or emission) lines caused by sub-pixel magnetic field and
  plasma dynamics in the vicinity of active regions and emerging flux
  events. Absorption lines—quantified by their displacement, width,
  asymmetry, and peakedness—have previously been used with Stokes I
  images from SOLIS/VSM to relate line distortions with sub-pixel plasma
  dynamics driven by solar flares or small-scale flux ropes. The method
  is extended to include the full Stokes parameters and relate inferred
  sub-pixel dynamics with small-scale magnetic fields. Our analysis is
  performed on several sets of spectrographic images taken by SOLIS/VSM
  while observing eruptive and non-eruptive active regions. We discuss
  the results of this application and their relevance for understanding
  magnetic fields signatures and coupled plasma properties on sub-pixel
  scales.

Title: Reconstructing solar magnetic fields from historical
    observations. II. Testing the surface flux transport model
Authors: Virtanen, I. O. I.; Virtanen, I. I.; Pevtsov, A. A.; Yeates,
   A.; Mursula, K.
Bibcode: 2017A&A...604A...8V
Altcode:
  <BR /> Aims: We aim to use the surface flux transport model to
  simulate the long-term evolution of the photospheric magnetic field
  from historical observations. In this work we study the accuracy of
  the model and its sensitivity to uncertainties in its main parameters
  and the input data. <BR /> Methods: We tested the model by running
  simulations with different values of meridional circulation and
  supergranular diffusion parameters, and studied how the flux
  distribution inside active regions and the initial magnetic field
  affected the simulation. We compared the results to assess how sensitive
  the simulation is to uncertainties in meridional circulation speed,
  supergranular diffusion, and input data. We also compared the simulated
  magnetic field with observations. <BR /> Results: We find that there is
  generally good agreement between simulations and observations. Although
  the model is not capable of replicating fine details of the magnetic
  field, the long-term evolution of the polar field is very similar in
  simulations and observations. Simulations typically yield a smoother
  evolution of polar fields than observations, which often include
  artificial variations due to observational limitations. We also find
  that the simulated field is fairly insensitive to uncertainties in
  model parameters or the input data. Due to the decay term included
  in the model the effects of the uncertainties are somewhat minor or
  temporary, lasting typically one solar cycle.

Title: How radial is the photospheric magnetic field?
Authors: Virtanen, Ilpo; Pevtsov, Alexei; Mursula, Kalevi
Bibcode: 2017shin.confE..51V
Altcode:
  The new data set of SOLIS/VSM vector synoptic maps of photospheric
  magnetic fields since 2010 reveals new features about the structure and
  evolution of the solar magnetic field. We investigate the structure
  and evolution of large scale vector magnetic fields in 2010 - 2016,
  covering the ascending to declining phase of solar cycle 24. We compare
  the observed VSM vector magnetic field to the potential vector field
  derived using the VSM line-of-sight magnetic field observations as
  an input. The new data set allows to derive the inclination angle,
  the meridional inclination and the azimuth angle of the magnetic field
  and to investigate solar cycle evolution and latitudinal profile of
  these quantities.

Title: Ca II K 1-A Emission Index Composites
Authors: Bertello, Luca; Marble, Andrew R.; Pevtsov, Alexei A.
Bibcode: 2017arXiv170200838B
Altcode:
  We describe here a procedure to combine measurements in the 393.37 nm Ca
  II K spectral line taken at different observatories. Measurements from
  the National Solar Observatory (NSO) Integrated Sunlight Spectrometer
  (ISS) on the Synoptic Optical Long-term Investigations of the Sun
  (SOLIS) telescope, the NSO/Sac Peak Ca II K-Line Monitoring Program,
  and Ca II K filtergrams from Kodaikanal Solar Observatory (KKL) are
  merged together to create a pair of composites of the Ca II K 1-A
  emission index. These composites are publicly available from the SOLIS
  website at http://solis.nso.edu/0/iss/.

Title: Long-term variations in sunspot magnetic field-area relation
Authors: Nagovitsyn, Y. A.; Pevtsov, A. A.; Osipova, A. A.
Bibcode: 2017AN....338...26N
Altcode: 2016arXiv160801132N
  Using observations of sunspot magnetic field strengths (H) from the
  Crimean Astrophysical Observatory (CrAO) and area (S) of sunspots from
  the Kislovodsk Mountain Astronomical Station of Pulkovo Observatory,
  we investigate the changes in the relation between H and S over the
  period of about two solar cycles (1994-2013). The data were fitted
  by H = A + B log S, where A = (778+/-46) and B = (778+/-25). We show
  that the correlation between H and S varies with the phase of solar
  cycle, and $A$ coefficient decreases significantly after year 2001,
  while B coefficient does not change significantly. Furthermore, our
  data confirm the presence of two distinct populations in distribution
  of sunspots (small sunspots with weaker field strength and large
  sunspots with stronger field). We show that relative contribution of
  each component to the distribution of sunspots by their area changes
  with the phase of solar cycle and on longer-then-cycle periods. We
  interpret these changes as a signature of a long-term (centennial)
  variations in properties of sunspots.

Title: The Mount Wilson Observatory S-index of the Sun
Authors: Egeland, Ricky; Soon, Willie; Baliunas, Sallie; Hall,
   Jeffrey C.; Pevtsov, Alexei A.; Bertello, Luca
Bibcode: 2017ApJ...835...25E
Altcode: 2016arXiv161104540E
  The most commonly used index of stellar magnetic activity is the
  instrumental flux scale of singly ionized calcium H &amp; K line
  core emission, S, developed by the Mount Wilson Observatory (MWO)
  HK Project, or the derivative index {R}<SUB>{HK</SUB>}<SUP>\prime
  </SUP>. Accurately placing the Sun on the S scale is important for
  comparing solar activity to that of the Sun-like stars. We present
  previously unpublished measurements of the reflected sunlight from
  the Moon using the second-generation MWO HK photometer during solar
  cycle 23 and determine cycle minimum {S}<SUB>23,\min </SUB>=0.1634+/-
  0.0008, amplitude {{Δ }}{S}<SUB>23</SUB>=0.0143+/- 0.0012, and mean
  &lt; {S}<SUB>23</SUB>&gt; =0.1701+/- 0.0005. By establishing a proxy
  relationship with the closely related National Solar Observatory
  Sacramento Peak calcium K emission index, itself well correlated with
  the Kodaikanal Observatory plage index, we extend the MWO S time series
  to cover cycles 15-24 and find on average &lt; {S}<SUB>\min </SUB>&gt;
  =0.1621+/- 0.0008, &lt; {{Δ }}{S}<SUB>{cyc</SUB>}&gt; =0.0145+/-
  0.0012, &lt; {S}<SUB>{cyc</SUB>}&gt; =0.1694+/- 0.0005. Our measurements
  represent an improvement over previous estimates that relied on stellar
  measurements or solar proxies with non-overlapping time series. We
  find good agreement from these results with measurements by the
  Solar-Stellar Spectrograph at Lowell Observatory, an independently
  calibrated instrument, which gives us additional confidence that we
  have accurately placed the Sun on the S-index flux scale.

Title: On the Presence of Two Populations of Sunspots
Authors: Nagovitsyn, Yury A.; Pevtsov, Alexei A.
Bibcode: 2016ApJ...833...94N
Altcode:
  Using historical (1894-1976) and more modern (1977-2014) observations,
  we investigate statistical properties of distributions of sunspot
  areas and their long-term variations. We confirm the presence of two
  populations of sunspots with smaller and larger areas, and show that
  sunspot/group lifetime can be used to separate the two populations
  on small short-lived sunspot groups (SSG) and large long-lived groups
  (LLG). The area properties of LLG are nearly constant over the entire
  period of observations, while the SSGs show significant long-term
  variations. Based on the presence of long-term variations in one
  component and the absence of those in the other, we suggest that the
  production of two populations of sunspots (SSG and LLG) may be affected
  by different processes.

Title: Sub-Pixel Magnetic Field Dynamics Derived from Photospheric
    Spectral Line Profiles
Authors: Rasca, A.; Chen, J.; Pevtsov, A. A.; Yurchyshyn, V.;
   Bertello, L.
Bibcode: 2016AGUFMSH13C2308R
Altcode:
  Current high-resolution observations of the photosphere show
  small dynamic features at the resolving limit during emerging flux
  events. However, line-of-sight (LOS) magnetogram pixels only contain
  the net uncanceled magnetic flux, which is expected to increase
  for fixed regions as resolution limits improve. Using a new method
  with spectrographic images, we quantify distortions in photospheric
  absorption (or emission) lines caused by sub-pixel magnetic field and
  plasma dynamics in the vicinity of active regions and emerging flux
  events. Absorption lines—quantified by their displacement, width,
  asymmetry, and peakedness—have previously been used with Stokes
  I images from SOLIS/VSM to relate line distortions with sub-pixel
  plasma dynamics driven by solar flares or small-scale flux ropes. The
  method is extended to include the full Stokes parameters and relate
  inferred sub-pixel dynamics with small-scale magnetic fields. Our
  analysis is performed on several sets of spectrographic images taken
  by SOLIS/VSM and NST/NIRIS while observing eruptive and non-eruptive
  active regions. We discuss the results of this application and their
  relevance for understanding magnetic fields signatures and coupled
  plasma properties on sub-pixel scales.

Title: The vorticity of Solar photospheric flows on the scale of
    granulation
Authors: Pevtsov, A. A.
Bibcode: 2016Ge&Ae..56..972P
Altcode: 2016arXiv160601390P
  We employ time sequences of images observed with a G-band filter
  (λ4305Å) by the Solar Optical Telescope (SOT) on board of Hinode
  spacecraft at different latitude along solar central meridian to study
  vorticity of granular flows in quiet Sun areas during deep minimum of
  solar activity. Using a feature correlation tracking (FCT) technique,
  we calculate the vorticity of granular-scale flows. Assuming the
  known pattern of vertical flows (upward in granules and downward
  in intergranular lanes), we infer the sign of kinetic helicity of
  these flows. We show that the kinetic helicity of granular flows and
  intergranular vortices exhibits a weak hemispheric preference, which
  is in agreement with the action of the Coriolis force. This slight
  hemispheric sign asymmetry, however, is not statistically significant
  given large scatter in the average vorticity. The sign of the current
  helicity density of network magnetic fields computed using full disk
  vector magnetograms from the Synoptic Optical Long-term Investigations
  of the Sun (SOLIS) does not show any hemispheric preference. The
  combination of these two findings suggests that the photospheric dynamo
  operating on the scale of granular flows is non-helical in nature.

Title: Modeling the Global Coronal Field with Simulated Synoptic
    Magnetograms from L1 and L5
Authors: Petrie, G. J. D.; Bertello, L.; Pevtsov, A. A.
Bibcode: 2016AGUFMSH11C2242P
Altcode:
  In solar physics and space weather research, full-disk photospheric
  magnetograms are routinely used to map the full solar surface in
  near-real-time, and coronal field models are extrapolated from these
  data. One major shortcoming of this approach is that, at present,
  the magnetograms can only be taken from the Earth's direction. Thus
  data immediately eastward of the sub-Earth point in synoptic maps
  are around three weeks old, missing much active-region evolution and
  leading to inaccuracies in the models. A new magnetograph at L5 would
  update the synoptic maps at this critical location east of central
  meridian and would provide a more accurate, up-to-date picture of the
  global photospheric and coronal field. We demonstrate the value of L5
  observations by simulating the construction of synoptic magnetograms
  from both L1 and L5 directions using past near-real-time data from two
  observatories: the Synoptic Optical Long-term Investigations of the
  Sun (SOLIS) Vector Spectromagnetograph (VSM) and Global Oscillation
  Network Group (GONG). We extrapolate potential-field source-surface
  (PFSS) coronal field models and compare their open-field and streamer
  distributions to coronal observations from the Solar Terrestrial
  Relations Observatory (STEREO) and the Solar Dynamics Observatory
  (SDO) Atmospheric Imaging Assembly (AIA).

Title: Blending of Ground- and Space-Based Magnetograms: Application
    to L1-L5 Solar Wind and Coronal Hole Predictions
Authors: Berger, T. E.; Pevtsov, A. A.; Martinez-Pillet, V.; Bertello,
   L.; Petrie, G. J. D.; Arge, C. N.; Henney, C. J.; Biesecker, D. A.
Bibcode: 2016AGUFMSH11C2241B
Altcode:
  We examine the effect of blending ground-based Global Oscillations
  Network Group (GONG) line-of-sight solar magnetic flux maps
  ("magnetograms") with space-based magnetograms from the Solar Dynamics
  Observatory (SDO) Helioseismic and Magnetic Imager (HMI) instrument on
  solar wind and coronal hole model-based forecasts. The longitudinally
  blended maps are used to "reforecast" solar wind conditions using the
  Wang-Sheeley-Arge (WSA) solar wind model during historical periods
  of coronal hole High Speed Streams (HSS) and Corotating Interaction
  Regions (CIRs) and compared to Advanced Composition Explorer (ACE)
  data at the L1 Lagrangian point. The same WSA runs are repeated using
  GONG and HMI data alone to determine the effect of data blending. The
  blended maps are also used to create Potential Field Source Surface
  (PFSS) maps of open coronal field lines and compared with historical
  coronal hole data from SDO Atmospheric Imaging Assembly (AIA)
  images. The study addresses the feasibility of combining ground- and
  space-based data from instruments with widely disparate and varying
  spatiotemporal resolution and flux sensitivity levels for use as inputs
  to solar wind and coronal hole forecasting models. The results are
  relevant to mission studies considering blended data inputs from,
  e.g., L5 Lagrangian point satellite instruments with ground-based
  measurements on the Sun-Earth line, as well as to expected magnetogram
  data from the Solar Orbiter Polarimetric and Helioseismic Imager (PHI)
  instrument. This study complements others that examine the utility
  of having multiple viewpoint (e.g. L1 and L5) magnetogram inputs to
  solar wind models by exploring data blending from disparate instruments.

Title: Pixel Analysis of Photospheric Spectral Data. I. Plasma
    Dynamics
Authors: Rasca, Anthony P.; Chen, James; Pevtsov, Alexei A.
Bibcode: 2016ApJ...832...53R
Altcode:
  Recent observations of the photosphere using high spatial and temporal
  resolution show small dynamic features at or below the current
  resolving limits. A new pixel dynamics method has been developed to
  analyze spectral profiles and quantify changes in line displacement,
  width, asymmetry, and peakedness of photospheric absorption lines. The
  algorithm evaluates variations of line profile properties in each pixel
  and determines the statistics of such fluctuations averaged over all
  pixels in a given region. The method has been used to derive statistical
  characteristics of pixel fluctuations in observed quiet-Sun regions,
  an active region with no eruption, and an active region with an ongoing
  eruption. Using Stokes I images from the Vector Spectromagnetograph
  (VSM) of the Synoptic Optical Long-term Investigations of the Sun
  (SOLIS) telescope on 2012 March 13, variations in line width and
  peakedness of Fe I 6301.5 Å are shown to have a distinct spatial and
  temporal relationship with an M7.9 X-ray flare in NOAA 11429. This
  relationship is observed as stationary and contiguous patches of
  pixels adjacent to a sunspot exhibiting intense flattening in the line
  profile and line-center displacement as the X-ray flare approaches
  peak intensity, which is not present in area scans of the non-eruptive
  active region. The analysis of pixel dynamics allows one to extract
  quantitative information on differences in plasma dynamics on sub-pixel
  scales in these photospheric regions. The analysis can be extended to
  include the Stokes parameters and study signatures of vector components
  of magnetic fields and coupled plasma properties.

Title: Correlation Between Sunspot Number and Ca II K Emission Index
Authors: Bertello, Luca; Pevtsov, Alexei; Tlatov, Andrey; Singh, Jagdev
Bibcode: 2016SoPh..291.2967B
Altcode: 2016arXiv160601092B; 2016SoPh..tmp..121B
  Long-term synoptic observations in the resonance line of Ca II K
  constitute a fundamental database for a variety of retrospective
  analyses of the state of the solar magnetism. Synoptic Ca II K
  observations began in late 1904 at the Kodaikanal Observatory in
  India. In the early 1970s, the National Solar Observatory (NSO) at
  Sacramento Peak (USA) started a new program of daily Sun-as-a-star
  observations in the Ca II K line. Today the NSO is continuing these
  observations through its Synoptic Optical Long-term Investigations
  of the Sun (SOLIS) facility. These different data sets can be
  combined into a single disk-integrated Ca II K index time series
  that describes the average properties of the chromospheric emission
  over several solar cycles. We present such a Ca II K composite and
  discuss its correlation with the new entirely revised sunspot number
  data series. For this preliminary investigation, the scaling factor
  between pairs of time series was determined assuming a simple linear
  model for the relationship between the monthly mean values during the
  duration of overlapping observations.

Title: Two populations of sunspots and secular variations of their
    characteristics
Authors: Nagovitsyn, Yu. A.; Pevtsov, A. A.; Osipova, A. A.; Tlatov,
   A. G.; Miletskii, E. V.; Nagovitsyna, E. Yu.
Bibcode: 2016AstL...42..703N
Altcode:
  We investigate the magnetic fields and total areas of mid- and
  low-latitude sunspots based on observations at the Greenwich and
  Kislovodsk (sunspot areas) and Mount Wilson, Crimean, Pulkovo,
  Ural, IMIS, Ussuriysk, IZMIRAN, and Shemakha (magnetic fields)
  observatories. We show that the coefficients in the linear form of the
  dependence of the logarithm of the total sunspot area S on its maximum
  magnetic field H change with time. Two distinct populations of sunspots
  are identified using the twodimensional H-log S occurrence histogram:
  small and large, separated by the boundaries log S = 1.6 ( S = 40 MSH)
  and H = 2050 G. Analysis of the sunspot magnetic flux also reveals
  the existence of two lognormally distributed populations with the mean
  boundary between them Φ = 10<SUP>21</SUP> Mx. At the same time, the
  positions of the flux occurrence maxima for the populations change on
  a secular time scale: by factors of 4.5 and 1.15 for small and large
  sunspots, respectively. We have confirmed that the sunspots form two
  physically distinct populations and show that the properties of these
  populations change noticeably with time. This finding is consistent
  with the hypothesis about the existence of two magnetic field generation
  zones on the Sun within the framework of a spatially distributed dynamo.

Title: Impact of Magnetic Carrington Synoptic and Spatial Variance
    Maps in Modeling of the Corona and Solar Wind
Authors: Bertello, Luca; Pevtsov, Alexie A.; Petrie, Gordon J. D.;
   Hughes, Anna L. H.; Macniece, Peter J.
Bibcode: 2016usc..confE..42B
Altcode:
  Synoptic maps derived from the measured photospheric solar longitudinal
  magnetic field are routinely used to drive coronal and heliospheric
  models. The recent development of spatial variance maps has provided
  an additional resource to better understanding the limitation of these
  models. In addition, measurements of the vector magnetic field are now
  available from different instruments (e.g. SDO/HMI, SOLIS/VSM) and can
  be used to compute synoptic maps of the true radial field. However,
  due to the low sensitivity of these measurements in regions of weak
  magnetic field, the adoption of these maps has been very limited. An
  effort is underway at NSO to merge both longitudinal and vector
  measurements together and derive more reliable synoptic maps of the
  radial field. An even more ambitious project is ongoing to produce also
  the first radial synoptic maps derived from SOLIS/VSM chromospheric
  measurements. Validation and diagnostic capability of these products
  will be discussed.

Title: The Possible Impact of L5 Magnetograms on Non-potential Solar
    Coronal Magnetic Field Simulations
Authors: Weinzierl, Marion; Mackay, Duncan H.; Yeates, Anthony R.;
   Pevtsov, Alexei A.
Bibcode: 2016ApJ...828..102W
Altcode:
  The proposed Carrington-L5 mission would bring instruments to the
  L5 Lagrange point to provide us with crucial data for space weather
  prediction. To assess the importance of including a magnetograph,
  we consider the possible differences in non-potential solar coronal
  magnetic field simulations when magnetograph observations are available
  from the L5 point, compared with an L1-based field of view (FOV). A
  timeseries of synoptic radial magnetic field maps is constructed to
  capture the emergence of two active regions from the L5 FOV. These
  regions are initially absent in the L1 magnetic field maps, but are
  included once they rotate into the L1 FOV. Non-potential simulations
  for these two sets of input data are compared in detail. Within the
  bipolar active regions themselves, differences in the magnetic field
  structure can exist between the two simulations once the active regions
  are included in both. These differences tend to reduce within 5 days
  of the active region being included in L1. The delayed emergence in L1
  can, however, lead to significant persistent differences in long-range
  connectivity between the active regions and the surrounding fields, and
  also in the global magnetic energy. In particular, the open magnetic
  flux and the location of open magnetic footpoints, are sensitive to
  capturing the real-time of emergence. These results suggest that a
  magnetograph at L5 could significantly improve predictions of the
  non-potential corona, the interplanetary magnetic field, and of solar
  wind source regions on the Sun.

Title: Dynamo Sensitivity In Solar Analogs With 50 Years Of Ca II
    H &amp; K Activity
Authors: Egeland, Ricky; Soon, Willie; Baliunas, Sallie; Hall,
   Jeffrey C.; Pevtsov, Alexei A.; Henry, Gregory W.
Bibcode: 2016csss.confE...6E
Altcode: 2016csss.confE..73E; 2016arXiv160904756E
  The Sun has a steady 11-year cycle in magnetic activity most well-known
  by the rising and falling in the occurrence of dark sunspots on the
  solar disk in visible bandpasses. The 11-year cycle is also manifest
  in the variations of emission in the Ca II H &amp; K line cores, due to
  non-thermal (i.e. magnetic) heating in the lower chromosphere. The large
  variation in Ca II H &amp; K emission allows for study of the patterns
  of long-term variability in other stars thanks to synoptic monitoring
  with the Mount Wilson Observatory HK photometers (1966-2003) and Lowell
  Observatory Solar-Stellar Spectrograph (1994-present). Overlapping
  measurements for a set of 27 nearby solar-analog (spectral types G0-G5)
  stars were used to calibrate the two instruments and construct time
  series of magnetic activity up to 50 years in length. Precise properties
  of fundamental importance to the dynamo are available from Hipparcos,
  the Geneva-Copenhagen Survey, and CHARA interferometry. Using these
  long time series and measurements of fundamental properties, we do
  a comparative study of stellar "twins" to explore the sensitivity
  of the stellar dynamo to small changes to structure, rotation, and
  composition. We also compare this sample to the Sun and find hints
  that the regular periodic variability of the solar cycle may be rare
  among its nearest neighbors in parameter space.

Title: Solar Ca II K Observations
Authors: Bertello, Luca; Pevtsov, Alexei A.; Tlatov, Andrey; Singh,
   Jagdev
Bibcode: 2016AsJPh..25..295B
Altcode:
  Some of the most important archives of past and current long-term solar
  synoptic observations in the resonance line of Ca II K are described
  here. These observations are very important for understanding the
  state of the solar magnetism on time scales up to several decades. The
  first observations of this kind began in 1904 at the Kodaikanal
  Observatory (India), followed by similar programs at different other
  locations. Regular full-disk Ca II K monitoring programs started in 1915
  at the Mount Wilson Observatory (USA) and in 1917 at the National Solar
  Observatory of Japan. Beginning in 1919 and in 1926 regular observations
  were taken also at the Paris-Meudon Observatory (France) and at the
  "Donati solar tower telescope of the Arcetri Astrophysical Observatory
  in Italy, respectively. In 1926 the the Astronomical Observatory of
  the Coimbra University in Portugal started its own program of Ca II
  K observations. Although some of these programs have been terminated
  over the years, their data archives constitute a unique resource for
  studies of solar variability. In the early 1970s, the National Solar
  Observatory (NSO) at Sacramento Peak (USA) started a new program of
  daily Sun-as-a-star observations in the Ca II K line. Today the NSO is
  continuing these observations through its Synoptic Optical Long-term
  Investigations of the Sun (SOLIS) facility.

Title: Pixel Analysis and Plasma Dynamics Characterized by
    Photospheric Spectral Data
Authors: Rasca, Anthony P.; Chen, James; Pevtsov, Alexei A.
Bibcode: 2016shin.confE.142R
Altcode:
  Recent observations of the photosphere using high spatial and temporal
  resolution show small dynamic features at or below the current
  resolving limits. A new pixel dynamics method has been developed to
  analyze spectral profiles and quantify changes in line displacement,
  width, asymmetry, and peakedness of photospheric absorption lines. The
  algorithm evaluates variations of line profile properties in each
  pixel and determines the statistics of such fluctuations averaged
  over all pixels in a given region of interest. The method has been
  used to derive the statistical characteristics of pixel fluctuations
  in observed quiet-sun regions, an active region with no eruption, and
  an active region with an ongoing eruption. Using Stokes I images from
  SOLIS/VSM on 2012 March 13, variations in line width and peakedness
  of Fe I 630.15 nm, are shown to have a distinct spatial and temporal
  relationship with an M7.9 X-ray flare in NOAA 11429. This relationship
  is observed as contiguous patches of pixels near a sunspot exhibiting
  intense flattening in the line profile and line-center displacement
  as the X-ray flare approaches peak intensity, which is not present
  in area scans of the non-eruptive active region. Furthermore, these
  patches of pixels remain stationary relative to the sunspot. These
  results connote that the analysis of pixel dynamics allows one to
  extract quantitative information on differences in plasma dynamics on
  sub-pixel scales in these regions in the photosphere. The analysis can
  be extended to include the Stokes parameters and study signatures of
  vector components of magnetic fields and coupled plasma properties.

Title: The Solar Dynamo Zoo
Authors: Egeland, Ricky; Soon, Willie; Baliunas, Sallie; Hall,
   Jeffrey C.; Pevtsov, Alexei A.; Henry, Gregory W.
Bibcode: 2016csss.confE..72E
Altcode:
  We present composite time series of Ca II H &amp; K line core emission
  indices of up to 50 years in length for a set of 27 solar-analog stars
  (spectral types G0-G5; within 10% of the solar mass) and the Sun. These
  unique data are available thanks to the long-term dedicated efforts
  of the Mount Wilson Observatory HK project, the Lowell Observatory
  Solar-Stellar Spectrograph, and the National Solar Observatory/Air Force
  Research Laboratory/Sacramento Peak K-line monitoring program. The Ca II
  H &amp; K emission originates in the lower chromosphere and is strongly
  correlated with the presence of magnetic plage regions in the Sun. These
  synoptic observations allow us to trace the patterns long-term magnetic
  variability and explore dynamo behavior over a wide range of rotation
  regimes and stellar evolution timescales.In this poster, the Ca HK
  observations are expressed using the Mount Wilson S-index. Each time
  series is accompanied by a Lomb-Scargle periodogram, fundemental stellar
  parameters derived from the Geneva-Copenhagen Survey, and statistics
  derived from the time series including the median S-index value and
  seasonal and long-term amplitudes. Statistically significant periodogram
  peaks are ranked according to a new cycle quality metric. We find that
  clear, simple, Sun-like cycles are the minority in this sample.

Title: Solar Physics Research in the Russian Subcontinent - Current
    Status and Future
Authors: Pevtsov, A. A.; Nagovitsyn, Yu. A.; Tlatov, A. G.; Demidov,
   M. L.
Bibcode: 2016AsJPh..25..477P
Altcode: 2016arXiv160601331P
  Modern research in solar physics in Russia is a multifaceted endeavor,
  which includes multi-wavelength observations from the ground-
  and space-based instruments, extensive theoretical and numerical
  modeling studies, new instrument development, and cross-disciplinary
  and international research. The research is conducted at the research
  organizations under the auspices of the Russian Academy of Sciences
  and to a lesser extent, by the research groups at Universities. Here,
  we review the history of solar physics research in Russia, and provide
  an update on recent developments.

Title: Pixel Analysis and Plasma Dynamics Characterized by
    Photospheric Spectral Data
Authors: Rasca, Anthony P.; Chen, James; Pevtsov, Alexei A.
Bibcode: 2016SPD....47.1206R
Altcode:
  Recent observations of the photosphere using high spatial and temporal
  resolutions show small dynamic features at the resolving limit during
  emerging flux events. However, line-of-sight (LOS) magnetogram pixels
  only contain the net uncanceled magnetic flux, which is expected to
  increase for fixed regions as resolution limits improve. A new pixel
  dynamics method uses spectrographic images to characterize photospheric
  absorption line profiles by variations in line displacement, width,
  asymmetry, and peakedness and is applied to quiet-sun regions,
  active regions with no eruption, and an active region with an ongoing
  eruption. Using Stokes I images from SOLIS/VSM on 2012 March 13,
  variations in line width and peakedness of Fe I 6301.5 Å are shown
  to have a strong spatial and temporal relationship with an M7.9 X-ray
  flare originating from NOAA 11429. This relationship is observed as
  a flattening in the line profile as the X-ray flare approaches peak
  intensity and was not present in area scans of a non-eruptive active
  region on 2011 April 14. These results are used to estimate dynamic
  plasma properties on sub-pixel scales and provide both spatial and
  temporal information of sub-pixel activity at the photosphere. The
  analysis can be extended to include the full Stokes parameters and
  study signatures of magnetic fields and coupled plasma properties.

Title: Full-Disk Chromospheric Vector Magnetograms with Ca II 854.2
nm line: Some Promising Applications
Authors: Gosain, Sanjay; Harvey, J. W.; Harker, Brian; Pillet, V. M.;
   Pevtsov, Alexei A.; Marble, Andrew R.; Bertello, Luca; + SOLIS-Team
Bibcode: 2016SPD....47.0103G
Altcode:
  Over the last decade, the focus of solar magnetometry has shifted
  outward from the photosphere to the chromospheric layers. The reasons
  for this are many. With regards to instrumentation faster detectors
  with more sensitivity have become available, as have fast electro-optic
  modulators. Also, there are several potential benefits of observing
  vector fields in the chromospheric layer as the magnetic field is
  more force-free in this layer as compared to the photosphere. Coronal
  force-free field extrapolations are more reliable using chromospheric
  fields as the lower boundary condition and free magnetic energy is
  readily computed using the magnetic virial theorem. Recently, a full
  Stokes polarimeter for the chromospheric Ca II 854.2 nm spectral line
  was developed and installed in the Vector Spectromagnetograph (VSM)
  instrument on the Synoptic Optical Long-term Investigations of the
  Sun (SOLIS) telescope. We present details of this new polarimeter,
  full disk spectropolarimetric observations and vector magnetograms
  of the chromosphere, and examples of some promising applications
  (e.g., maps of normal component of electric current density in the
  chromosphere, free magnetic energy estimated using virial theorem,
  and non-potentiality parameter magnetic shear angle).This work
  utilizes SOLIS data obtained by the NSO Integrated Synoptic Program
  (NISP), managed by the National Solar Observatory, which is operated
  by the Association of Universities for Research in Astronomy (AURA),
  Inc. under a cooperative agreement with the National Science Foundation

Title: HMI Synoptic Maps Produced by NSO/NISP
Authors: Hughes, Anna L. H.; Bertello, Luca; Marble, Andrew R.; Oien,
   Niles A.; Petrie, Gordon; Pevtsov, Alexei A.
Bibcode: 2016arXiv160503500H
Altcode:
  Recently, the National Solar Observatory (NSO) Solar-atmosphere Pipeline
  Working Group has undertaken the production of synoptic maps from
  Helioseismic and Magnetic Imager (HMI) magnetograms. A set of maps has
  been processed spanning the data available for 2010-2015 using twice
  daily images (taken at UT midnight and noon) and running them through
  the same algorithms used to produce SOLIS/VSM 6302l mean-magnetic
  and spatial-variance maps. The contents of this document provide an
  overview of what these maps look like, and the processing steps used
  to generate them from the original HMI input data.

Title: Dynamo Sensitivity in Solar Analogs with 50 Years of Ca II
    H &amp; K Activity
Authors: Egeland, Ricky; Soon, Willie H.; Baliunas, Sallie L.; Hall,
   Jeffrey C.; Pevtsov, Alexei A.; Henry, Gregory W.
Bibcode: 2016SPD....4720307E
Altcode:
  The Sun has a steady 11-year cycle in magnetic activity most well-known
  by the rising and falling in the occurrence of dark sunspots on the
  solar disk in visible bandpasses. The 11-year cycle is also manifest
  in the variations of emission in the Ca II H &amp; K line cores, due to
  non-thermal (i.e. magnetic) heating in the lower chromosphere. The large
  variation in Ca II H &amp; K emission allows for study of the patterns
  of long-term variability in other stars thanks to synoptic monitoring
  with the Mount Wilson Observatory HK photometers (1966-2003) and Lowell
  Observatory Solar-Stellar Spectrograph (1994-present). Overlapping
  measurements for a set of 27 nearby solar-analog (spectral types G0-G5)
  stars were used to calibrate the two instruments and construct time
  series of magnetic activity up to 50 years in length. Precise properties
  of fundamental importance to the dynamo are available from Hipparcos,
  the Geneva-Copenhagen Survey, and CHARA interferometry. Using these
  long time series and measurements of fundamental properties, we do
  a comparative study of stellar "twins" to explore the sensitivity
  of the stellar dynamo to small changes to structure, rotation, and
  composition. We also compare this sample to the Sun and find hints
  that the regular periodic variability of the solar cycle may be rare
  among its nearest neighbors in parameter space.

Title: Current Status of the SOLIS Program: Improved and New Data
    Products
Authors: Bertello, Luca; Britanik, John; Callahan, Lorraine; Gosain,
   Sanjay; Harker, Brian; Harvey, J. W.; Hughes, A.; Marble, A.; Pevtsov,
   Alexei A.; Wentzel, Thomas
Bibcode: 2016SPD....47.1002B
Altcode:
  Over the past year the Synoptic Optical Long-term Investigations of the
  Sun (SOLIS) team has made significant improvements to the data products
  provided to the solar and heliospheric community. In particular,
  a considerable effort has been dedicated to reprocess the archive of
  vector and longitudinal photospheric magnetograms, from 2003 to present,
  using the latest production code. This endeavor is now near completion
  and will assure that all derived magnetic products, such as synoptic
  maps and flux time series, are consistently calibrated. In addition,
  new products have been recently developed. For example, time series of
  daily averages of the Sun's polar magnetic field derived from full-disk
  photospheric FeI 630.15 nm longitudinal magnetograms are now available
  from the SOLIS website at solis.nso.edu/0/vsm/vsm_plrfield.html.New
  intensity calibration of high resolution Ca II K &amp; H spectra from
  the Integrated Sunlight Spectrometer (ISS) led to significant reduction
  in daily variations of parameters derived from these spectra. The
  SOLIS team will soon also release Carrington synoptic maps for the
  three components of the photospheric magnetic field derived from vector
  measurements taken in the Fe I 630.15 nm spectral line. The addition,
  in late 2015, of a new Ca II 854.2 nm full-Stokes polarimeter into
  the SOLIS core program of magnetic observations will make available to
  the community daily chromospheric measurements of the complete Stokes
  polarization vector.We present here a summary of these improvements,
  with particular emphasis on the new products that can be accessed
  from the SOLIS data page at solis.nso.edu/0/solis_data.html. For
  a description of the diagnostic capability of the new Ca II 854.2
  nm spectro-polarimeter and preliminary results we refer to other
  presentations by SOLIS team members at this meeting.This work utilizes
  SOLIS data obtained by the NSO Integrated Synoptic Program (NISP),
  managed by the National Solar Observatory, which is operated by
  the Association of Universities for Research in Astronomy (AURA),
  Inc. under a cooperative agreement with the National Science Foundation.

Title: Ca II 854.2 nm Spectromagnetograms: A Powerful Chromospheric
    Diagnostic
Authors: Harvey, J. W.; Bertello, Luca; Branston, D.; Britanik, J.;
   Bulau, S.; Cole, L.; Gosain, Sanjay; Harker, Brian; Jones, Harrison P.;
   Marble, A.; Martinez Pillet, V.; Pevtsov, A.; Schramm, K.; Streander,
   Kim; Villegas, H.
Bibcode: 2016SPD....4710106H
Altcode:
  The transition from physical dominance by plasma flows in the
  photosphere to magnetic pressure in the solar chromosphere motivates
  as many diagnostic observations as possible across this important
  region. Among the few ground-accessible spectral lines formed within
  the chromosphere, the Ca II 854.2 nm line has the desirable properties
  of presence everywhere on the solar disk, Zeeman sensitivity, and
  narrow line width. Mapped observations of circular polarization within
  this line (spectromagnetograms) have been made at NSO infrequently
  since 1974, with regular daily full-disk observations starting in
  August 1996. Full-disk spectral observations of the complete Stokes
  polarization vector are now being made regularly since November
  2015. It is not easy to estimate chromospheric magnetic field
  properties from the 854.2 nm line profile polarization. To provide
  rough quick-look vector field maps we found that the weak-field
  approximation provides a fair first estimate of the line-of-sight
  component but appears to be too simple to interpret the transverse
  magnetic field from frequently asymmetric, linearly-polarized line
  profiles. More realistic estimates of the chromospheric vector field,
  short of extremely lengthy, full 3D, non-local radiative transfer
  inversions, are being investigated. We briefly introduce recent
  instrumental modifications and observational characteristics, sample
  observations, and results concerning the expansion of the chromospheric
  field with increasing height, the presence of large areas of weak,
  nearly horizontal fields, and field estimates in plages, sunspots,
  flares, filaments, and filament channels. The Stokes spectra will be
  freely available to the community.This work utilizes SOLIS data obtained
  by the NSO Integrated Synoptic Program (NISP), managed by the National
  Solar Observatory, which is operated by the Association of Universities
  for Research in Astronomy (AURA), Inc. under a cooperative agreement
  with the National Science Foundation.

Title: HMI Vector and Uncertainty Carrington Synoptic Maps
Authors: Bertello, Luca; Hughes, A.; Gosain, Sanjay; Harker, Brian;
   Harvey, J. W.; Marble, Andrew R.; Pevtsov, Alexei A.
Bibcode: 2016SPD....47.1001B
Altcode:
  Based on our experience with data from the Vector Spectromagnetograph
  (VSM) instrument, which is part of the Synoptic Optical Long-term
  Investigations of the Sun (SOLIS)facility, we have produced HMI vector
  and uncertainty synoptic maps for all Carrington rotations from May
  2010 through December 2015. HMI observations provide 12-minute cadence
  magnetograms, both for longitudinal and full-Stokes measurements. For
  this investigation we have used only two magnetograms per day, 12
  hours apart, which are sufficient to produce accurate maps in the
  longitude-sine(latitude) projection with 1x1 square-degree resolution
  at the equator. From both the HMI longitudinal and vector magnetograms
  we have computed radial-flux and spatial-variance synoptic maps. For
  the longitudinal data, we have included pole-filled radial-flux maps,
  and for the vector data, we have included synoptic maps of the poloidal
  and toroidal magnetic flux.We describe here the procedure to generate
  those maps and discuss some of their main features. A comparison with
  similar products from the SOLIS/VSM is also addressed. The HMI data
  used are courtesy of NASA/SDO and HMI science teams.

Title: Flare-related changes in pseudo-vector magnetic field derived
    from line-of-sight magnetograms
Authors: Burtseva, Olga; Gosain, Sanjay; Pevtsov, Alexei A.
Bibcode: 2016SPD....47.0637B
Altcode:
  Longitudinal field is a projection of full vector field to
  the line-of-sight direction. Thus, it is possible to derive some
  information about the vector field from line-of-sight data in round
  sunspots, assuming that average properties of vector magnetic field in
  these sunspots depend mostly on distance from center of sunspot. Under
  this assumption, one can reconstruct vertical, radial, and tangential
  components of vector magnetic field using azimuthal averaging. This
  technique can be useful for investigation of twist and inclination in
  magnetic field in particular in flaring regions when vector data are
  not available. In this study we validate the cylindrical symmetry
  technique on example of a simple round sunspot. Then we attempt
  to study changes in (pseudo-vector) magnetic fields in isolated and
  round sunspots associated with flare events using SDO/HMI longitudinal
  magnetograms. We compare the pseudo-vector results with vector data.

Title: The Solar Dynamo Zoo
Authors: Egeland, Ricky; Soon, Willie H.; Baliunas, Sallie L.; Hall,
   Jeffrey C.; Pevtsov, Alexei A.; Henry, Gregory W.
Bibcode: 2016SPD....47.1103E
Altcode:
  We present composite time series of Ca II H &amp; K line core emission
  indices of up to 50 years in length for a set of 27 solar-analog stars
  (spectral types G0-G5; within ~10% of the solar mass) and the Sun. These
  unique data are available thanks to the long-term dedicated efforts
  of the Mount Wilson Observatory HK project, the Lowell Observatory
  Solar-Stellar Spectrograph, and the National Solar Observatory/Air Force
  Research Laboratory/Sacremento Peak K-line monitoring program. The
  Ca II H &amp; K emission originates in the lower chromosphere and is
  strongly correlated with the presence of magnetic plage regions in
  the Sun. These synoptic observations allow us to trace the patterns
  long-term magnetic variability and explore dynamo behavior over a wide
  range of rotation regimes and stellar evolution timescales.

Title: The Need for Synoptic Solar Observations from the Ground
Authors: Pevtsov, A. A.
Bibcode: 2016ASPC..504...71P
Altcode:
  Synoptic observations are indispensable in studies of long-term effects
  pertinent to variation in solar radiative output, space weather and
  space climate, as well as for understanding the physics of global
  processes taking place on our nearest star. Synoptic data also allow
  putting the Sun in the context of stellar evolution. Historically,
  the main-stay of such observations has been groundbased although the
  improving longevity of space-borne instruments puts some space missions
  into the category of synoptic facilities. Space- and groundbased
  (synoptic) observations are complementary to each other; neither is
  inferior or superior to the other. Groundbased facilities can have
  a long-term (50 years+) operations horizon, and in comparison with
  their spacebased counterparts, they are less expensive to operate
  and have fewer restrictions on international collaboration and data
  access. The instruments can be serviced, upgraded, and cross-calibrated
  to ensure the continuity and uniformity of long-term data series. New
  measurements could be added in response to changes in understanding
  the solar phenomena. Some drawbacks such as day-night cycle and
  the variable atmospheric seeing can be mitigated e.g., by creating
  global networks and by employing adaptive optics. Furthermore, the
  groundbased synoptic observations can serve as a backbone and a back-up
  to spacebased observations. Here I review some existing groundbased
  synoptic facilities, describe plans for future networks, and outline
  the current efforts in strengthening the international collaboration
  in synoptic solar observations from the ground.

Title: Long-term Synoptic Observations of Ca II K and Magnetic Flux
Authors: Bertello, L.; Pevtsov, A. A.; Tlatov, A. G.
Bibcode: 2016ASPC..504..213B
Altcode:
  Long-term synoptic observations in the resonance line of Ca II K and
  measurements of the solar magnetic flux over several decades constitute
  a fundamental database for a variety of retrospective analyzes of the
  state of the solar magnetism. These data archives may also hold the key
  for untangling some of the mysteries behind the solar dynamo, which in
  turn could result in a better predictive capability of current dynamo
  models. Synoptic Ca II K observations began in 1907 at the Kodaikanal
  observatory (India) and in 1915 at the Mt Wilson (California, USA)
  solar observatory. By the early 1970s a number of synoptic programs
  for solar magnetic observations were established that provided full
  disk magnetograms. These programs include measurements carried out at
  the Mt Wilson 150-foot solar tower, Wilcox observatory (California,
  USA), and by the National Solar Observatory (NSO, USA). Today the
  NSO is continuing these observations through its Synoptic Optical
  Long-term Investigations of the Sun (SOLIS) facility. We will review
  some of these historical observations, their properties, and their
  importance for understanding the behavior of the solar magnetic field
  over multidecadal time scales. We will also show recent results about
  using Ca II K spectroheliograms and sunspot magnetic field measurements
  to reconstruct homogeneous series of pseudo-magnetograms prior the
  magnetograph era.

Title: The reversal of the Sun's magnetic field in cycle 24
Authors: Mordvinov, Aleksandr; Pevtsov, Alexei; Bertello, Luka;
   Petri, Gordon
Bibcode: 2016STP.....2a...3M
Altcode: 2016arXiv160202460M; 2016SZF.....2a...3M
  Analysis of synoptic data from the Vector Spectromagnetograph (VSM)
  of the Synoptic Optical Long-term Investigations of the Sun (SOLIS)
  and the NASA/NSO Spectromagnetograph (SPM) at the NSO/Kitt Peak Vacuum
  Telescope facility shows that the reversals of solar polar magnetic
  fields exhibit elements of a stochastic process, which may include the
  development of specific patterns of emerging magnetic flux, and the
  asymmetry in activity between northern and southern hemispheres. The
  presence of such irregularities makes the modeling and prediction
  of polar field reversals extremely hard if possible. In a classical
  model of solar activity cycle, the unipolar magnetic regions (UMRs)
  of predominantly following polarity fields are transported polewards
  due to meridional flows and diffusion. The UMRs gradually cancel out
  the polar magnetic field of the previous cycle, and rebuild the polar
  field of opposite polarity setting the stage for the next cycle. We
  show, however, that this deterministic picture can be easily altered
  by the developing of a strong center of activity, or by the emergence
  of an extremely large active region, or by a "strategically placed"
  coronal hole. We demonstrate that the activity occurring during the
  current cycle 24 may be the result of this randomness in the evolution
  of the solar surface magnetic field.

Title: Reconstructing solar magnetic fields from historical
    observations. I. Renormalized Ca K spectroheliograms and
    pseudo-magnetograms
Authors: Pevtsov, Alexei A.; Virtanen, Ilpo; Mursula, Kalevi; Tlatov,
   Andrey; Bertello, Luca
Bibcode: 2016A&A...585A..40P
Altcode:
  <BR /> Aims: The present work is the first in a series of articles that
  develop a new proxy to represent the evolution of magnetic activity in
  past solar cycles by combining the information from historical Ca II
  K line spectroheliograms and sunspot magnetic field measurements. <BR
  /> Methods: We use synoptic (Carrington) maps from 1915-1985 that
  were derived from daily Ca K line observations at Mount Wilson
  Observatory to identify the chromospheric plages and to create synoptic
  pseudo-magnetograms. We use historical observations of sunspot magnetic
  fields from 1917 to the present to assign polarity to pixels situated
  within plages. The original Ca K spectroheliograms are nonuniform in
  their brightness, and we develop a novel approach to re-normalize their
  intensities. <BR /> Results: We show that a homogeneous long-term series
  of pseudo-magnetograms can be successfully constructed by combining
  sunspot field measurements and plages with renormalized intensities. In
  our tests, about 80% of pixels situated within plages showed the same
  magnetic polarity as the synoptic magnetograms taken with the Kitt
  Peak Vacuum Telescope. Finally, we discuss possible approaches to
  further improve the agreement between observed and pseudo-magnetograms.

Title: Pixel Analysis and Plasma Dynamics Characterized by
    Photospheric Spectral Data
Authors: Rasca, A.; Chen, J.; Pevtsov, A. A.
Bibcode: 2015AGUFMSH21C..07R
Altcode:
  Continued advances in solar observations have led to higher-resolution
  magnetograms and surface (photospheric) images, revealing bipolar
  magnetic features operating near the resolution limit during emerging
  flux events and other phenomena used to predict solar eruptions
  responsible for geomagnetic plasma disturbances. However, line of sight
  (LOS) magnetogram pixels only contain the net uncanceled magnetic flux,
  which is expected to increase for fixed regions as resolution limits
  improve. A pixel dynamics model utilizing Stokes I spectral profiles
  was previously-used to quantify changes in the Doppler shift, width,
  asymmetry, and tail flatness of Fe I lines at 6301.5 and 6302.5 Å and
  used pixel-by-pixel line profile fluctuations to characterize quiet
  and active regions on the Sun. We use this pixel dynamics model with
  circularly polarized photospheric data (e.g., SOLIS data) to estimate
  plasma dynamic properties at a sub-pixel level. The analysis can be
  extended to include the full Stokes parameters and study signatures
  of magnetic fields and coupled plasma properties on sub-pixel scales.

Title: Effect of uncertainties in solar synoptic magnetic flux maps
    in modeling of solar wind
Authors: Pevtsov, Alexei A.; Bertello, Luca; MacNeice, Peter
Bibcode: 2015AdSpR..56.2719P
Altcode:
  Recently, the NSO/SOLIS team developed variance (error) maps that
  represent uncertainties in magnetic flux synoptic charts. These
  uncertainties are determined by the spatial variances of the magnetic
  flux distribution from full disk magnetograms that contribute to each
  bin in the synoptic chart. Here we present a study of the effects of
  variances on solar wind parameters (wind speed, density, magnetic field,
  and temperature) derived using the WSA-ENLIL model and ensemble modeling
  approach. We compare the results of the modeling with near-Earth solar
  wind magnetic field and plasma data as extracted from NASA/GSFC's OMNI
  data set. We show that analysis of uncertainties may be useful for
  understanding the sensitivity of the model predictions to short-term
  evolution of magnetic field and noise in the synoptic magnetograms.

Title: H-alpha Off-limb Carrington Synoptic Charts
Authors: Bertello, L.; Hughes, A.; Pevtsov, A. A.
Bibcode: 2015AGUFMSH43B2459B
Altcode:
  Observations of solar prominences (and filaments) reveal complex
  structures, with significant differences inmorphology, lifetime, and
  complexity of their magnetic field environment. Studies of thesesolar
  features are important for understanding not only their origin but
  also their role as precursors ofenergetic events such as flares and
  coronal mass ejections.The Global Oscillation Network Group (GONG)
  operated by the National Solar Observatory provides, amongother
  products, high-cadence broad-band intensity measurements centered
  in the core ofH-alpha spectral line. Prominences and filaments are
  quite visible in these 2Kx2K images, that havebeen obtained since
  2010. We recentlystarted a project to produce an archive of H-alpha
  off-limb Carrington Synoptic Charts to be usedfor systematic studies
  of prominences. Charts are produced to show both spatial and temporal
  variabilityof these features in four radius zones covering 1.01 to
  1.11 solar radii. Here we describe the applied methodologyand show
  some preliminary results. Potential applications of interest to the
  solar and heliospheric communitiesare also briefly discussed.

Title: Long-term variations in the sunspot magnetic fields and bipole
    properties from 1918 to 2014
Authors: Tlatova, K. A.; Vasil'eva, V. V.; Pevtsov, A. A.
Bibcode: 2015Ge&Ae..55..896T
Altcode:
  Long-term measurements of sunspot magnetic fields have been analyzed
  with the use of the Mount Wilson Observatory (MWO) data based on
  the digitization of the magnetic field strength ( B) and umbra and
  pore areas from 1918 to 2014. Time variations in the magnetic field
  strength and the possible causes of such variations (which are related
  to variations in the solar activity level and instrumental effects) were
  considered. It was shown that artifacts related to instrumental effects
  exist in the measurements for small sunspots and pores. The magnetic
  bipole characteristics were determined for the sunspot groups. It was
  established that the tilt angle of the magnetic bipole axis of even
  cycles is larger than the tilt angle in the next odd cycles (except
  for the cycle pair 22-23).

Title: How a Magnetograph at L5 would Improve the Space Weather
    Prediction Capabilities.
Authors: Pevtsov, A. A.
Bibcode: 2015AGUFMSH14A..01P
Altcode:
  Knowledge of the magnetic field in solar photosphere plays essential
  role in modern space weather forecast. Thus, for example, synoptic
  maps of radial magnetic field are used as input for WSA-ENLIL model
  to derive the properties of solar wind near the Earth and elsewhere
  in the solar system. Topology of magnetic field around the source
  regions of flares and CMEs offers important clues about orientation of
  magnetic field in interplanetary magnetic clouds and ejecta, and the
  information about large-scale magnetic connectivity in solar corona
  brings better understanding of remote triggering solar eruptions. Here
  I will review the benefits of having magnetograph observations from
  two viewing points: one at Earth-viewing angle and the other at L5,
  and I will discuss the type of magnetographs that can be considered
  for a future L5 mission.

Title: Cylindrical Symmetry of Sunspots as a Proxy for Flare-Related
    Changes in Pseudo-Vector Magnetic Field Derived from Line-of-Sight
    Magnetograms
Authors: Pevtsov, A. A.; Burtseva, O.; Gosain, S.
Bibcode: 2015AGUFMSH43B2450P
Altcode:
  Large-scale changes in the magnetic field twist and inclination in
  flaring regions are often observed in vector magnetograms. When
  vector data are not available, such changes can be investigated
  using proxies for vector magnetic fields derived from line-of-sight
  magnetograms. Longitudinal field is a projection of full vector field
  to the line-of-sight direction. Thus, it is possible to derive some
  information about the vector field from line-of-sight data in round
  sunspots, assuming that average properties of vector magnetic field in
  these sunspots depend mostly on distance from center of sunspot. Under
  this assumption, one can reconstruct vertical, radial, and tangential
  components of vector magnetic field using azimuthal averaging. We
  study changes in (pseudo-vector) magnetic fields, including twist
  and inclination of the magnetic fields, in sunspots associated with
  flare events using SDO/HMI longitudinal magnetograms. We compare the
  pseudo-vector results with SDO/HMI vector data.

Title: Digital tabulation of historical sunspot field strength
    measurements from the Mount Wilson Observatory
Authors: Pevtsov, Alexei; Tlatov, Andrey; Bertello, Luca; Ulrich, Roger
Bibcode: 2015IAUGA..2236421P
Altcode:
  Direct (manual) measurements of magnetic fields in sunspots based on
  Zeeman effect represent the longest magnetic observations pertinent to
  the Sun. Regular (daily) observations started in about 1917 and continue
  till present. The data consist of daily drawings of sunspots with the
  corresponding field strength and polarity measurements handwritten on
  these drawings. All the drawings are now scanned to digital images
  (JPEG format). However, the lack of tabulated data severely limits
  the use of this unique data set. Here we report on status and the
  results of several recent projects aimed at the digital tabulation of
  MWO drawings. While the individual projects are funded independently by
  the American (USA) and Russian funding agencies, the groups collaborate
  closely with each other to achieve a common goal - creation of tabulated
  data set contacting magnetic field and sunspot position information
  derived from the drawings. The collaboration is coordinated by the
  IAU working group on Coordination of Synoptic Observations of the Sun.

Title: Long-term Synoptic Observations of the Sun.
Authors: Pevtsov, Alexei
Bibcode: 2015IAUGA..2246721P
Altcode:
  Does the solar activity vary with time, or it remains constant? How
  systematic or intermittent solar cycles are? Long-term observations of
  the Sun are the reason we know answers to these questions. Ultimately,
  the development of a better understanding of stellar cycles will
  require similar long-term observations of other sun-like stars. To
  facilitate international collaboration on synoptic long-term solar
  observations, IAU created a working group on “Coordination of Synoptic
  Observations of the Sun.” The working group provides a forum for
  discussion of all issues relevant to past, current, and future synoptic
  programs, preservation, calibration, and access to synoptic solar data
  products. This talk will provide a summary of recent activity by this
  IAU WG. It will also present a brief overview of recent research on
  sun-as-a-star conducted at the US National Solar Observatory.

Title: Comparisons of Photospheric Pixel Dynamics Between Eruptive
    and Non-Eruptive Regions
Authors: Rasca, Anthony P.; Chen, James; Pevtsov, Alexei A.
Bibcode: 2015shin.confE..34R
Altcode:
  Recent advances in solar observations have led to higher-resolution
  surface (photosphere) images that reveal bipolar magnetic features
  operating near the resolution limit during emerging flux events. Further
  improvements in resolution are expected to reveal even smaller dynamic
  features. Such photospheric features provide observable indications of
  what is happening before, during, and after flux emergence, eruptions
  in the corona, and other phenomena. Visible changes in photospheric
  active regions also play a major role in predicting eruptions that are
  responsible for geomagnetic plasma disturbances. A new method has been
  developed to extract physical information from photospheric data (e.g.,
  SOLIS Stokes parameters) based on the statistics of pixel-by-pixel
  variations in spectral (absorption or emission) line quantities such
  as line profile Doppler shift, width, asymmetry, and flatness. Such
  properties are determined by the last interaction between detected
  photons and optically thick photospheric plasmas, and may contain
  extractable information on local plasma properties at sub-pixel
  scales. Applying the method to photospheric data with high spectral
  resolution, our pixel-by-pixel analysis is performed for various
  regions on the solar disk, ranging from quiet-Sun regions to active
  regions exhibiting eruptions, characterizing photospheric dynamics
  using spectral profiles. In particular, the method quantitatively
  characterizes the time profile of changes in spectral properties in
  photospheric features and provides improved physical constraints on
  observed quantities.

Title: Hard X-Ray Emission During Flares and Photospheric Field
    Changes
Authors: Burtseva, O.; Martínez-Oliveros, J. C.; Petrie, G. J. D.;
   Pevtsov, A. A.
Bibcode: 2015ApJ...806..173B
Altcode: 2015arXiv150500509B
  We study the correlation between abrupt permanent changes of magnetic
  field during X-class flares observed by the Global Oscillation Network
  Group and Helioseismic and Magnetic Imager instruments, and the hard
  X-ray (HXR) emission observed by RHESSI, to relate the photospheric
  field changes to the coronal restructuring and investigate the origin
  of the field changes. We find that spatially the early RHESSI emission
  corresponds well to locations of the strong field changes. The field
  changes occur predominantly in the regions of strong magnetic field
  near the polarity inversion line (PIL). The later RHESSI emission does
  not correspond to significant field changes as the flare footpoints
  are moving away from the PIL. Most of the field changes start before
  or around the start time of the detectable HXR signal, and they
  end at about the same time or later than the detectable HXR flare
  emission. Some of the field changes propagate with speed close to that
  of the HXR footpoint at a later phase of the flare. The propagation of
  the field changes often takes place after the strongest peak in the
  HXR signal when the footpoints start moving away from the PIL, i.e.,
  the field changes follow the same trajectory as the HXR footpoint,
  but at an earlier time. Thus, the field changes and HXR emission are
  spatio-temporally related but not co-spatial nor simultaneous. We also
  find that in the strongest X-class flares the amplitudes of the field
  changes peak a few minutes earlier than the peak of the HXR signal. We
  briefly discuss this observed time delay in terms of the formation of
  current sheets during eruptions.

Title: The Minimum of Solar Cycle 23: As Deep as It Could Be?
Authors: Muñoz-Jaramillo, Andrés; Senkpeil, Ryan R.; Longcope,
   Dana W.; Tlatov, Andrey G.; Pevtsov, Alexei A.; Balmaceda, Laura A.;
   DeLuca, Edward E.; Martens, Petrus C. H.
Bibcode: 2015ApJ...804...68M
Altcode: 2015arXiv150801222M
  In this work we introduce a new way of binning sunspot group data
  with the purpose of better understanding the impact of the solar
  cycle on sunspot properties and how this defined the characteristics
  of the extended minimum of cycle 23. Our approach assumes that
  the statistical properties of sunspots are completely determined
  by the strength of the underlying large-scale field and have no
  additional time dependencies. We use the amplitude of the cycle
  at any given moment (something we refer to as activity level) as a
  proxy for the strength of this deep-seated magnetic field. We find
  that the sunspot size distribution is composed of two populations:
  one population of groups and active regions and a second population
  of pores and ephemeral regions. When fits are performed at periods
  of different activity level, only the statistical properties of the
  former population, the active regions, are found to vary. Finally,
  we study the relative contribution of each component (small-scale
  versus large-scale) to solar magnetism. We find that when hemispheres
  are treated separately, almost every one of the past 12 solar minima
  reaches a point where the main contribution to magnetism comes from
  the small-scale component. However, due to asymmetries in cycle phase,
  this state is very rarely reached by both hemispheres at the same
  time. From this we infer that even though each hemisphere did reach
  the magnetic baseline, from a heliospheric point of view the minimum
  of cycle 23 was not as deep as it could have been.

Title: The Diagnostic Value of Photospheric Fraunhofer Lines in
    Sun-as-a-Star Observations
Authors: Bertello, Luca; Pevtsov, Alexei A.; Marble, Andrew R.
Bibcode: 2015TESS....140311B
Altcode:
  The distinctive sensitivity of photospheric Fraunhofer lines to
  variations in the thermodynamic and magnetic structures of the solar
  atmosphere provides an excellent tool to investigate these variations
  at different time scales.We used daily Sun-as-a-star spectra taken
  with the Integrated Sunlight Spectrometer (ISS) and longitudinal
  magnetograms from the Vector SpectroMagnetograph (VSM) to study the
  correlation between the global magnetic flux and changes in the line
  shape of several photospheric spectral lines during different phases of
  the solar cycle. ISS and VSM are two of three instruments comprising
  the Synoptic Optical Long-term Investigations of the Sun (SOLIS)
  facility operated by the National Solar Observatory. We find a clear
  signature of temporal global magnetic flux variations in several of
  these photospheric spectral lines, suggesting that the results of our
  analysis can be used to develop a better understanding of the magnetic
  cycle of activity in other solar-type stars.

Title: The Minimum of Solar Cycle 23: As Deep as It Could Be?
Authors: Munoz-Jaramillo, Andres; Senkpeil, Ryan; Longcope, Dana;
   Tlatov, Andrey; Pevtsov, Alexei A.; Balmaceda, Laura; DeLuca, Edward
   E.; Martens, Petrus C.
Bibcode: 2015TESS....130803M
Altcode:
  After a lull lasting more than 60 years of seemly uniform solar minima,
  the solar minimum of solar cycle 23 came as a great surprise due to its
  depth, duration, and record lows in a wide variety of solar activity
  indices and solar wind properties. One of the consequence of such an
  event is the revival of the interest in extreme minima, grand minima,
  and the identification of a solar basal state of minimum magnetic
  activity.In this presentation we will discuss a new way of binning
  sunspot group data, with the purpose of better understanding the impact
  of the solar cycle on sunspot properties, and how this defined the
  characteristics of the extended minimum of cycle 23. Our main result
  is centered around the fact that the sunspot size distribution is
  composed of two populations, a population of groups and active regions,
  and second of pores and ephemeral regions. We find that only the
  properties of the former population, the active regions, is found to
  vary with the solar cycle, while the propeties of pores and ephemeral
  regions does not.Taking advantage of our statistical characterization
  we probe the question of the solar baseline magnetism. We find that,
  when hemispheres are treated separately, almost every one of the past
  12 solar minima reaches such a point. However, due to asymmetries in
  cycle phase, the basal state is very rarely reached by both hemispheres
  at the same time. From this we infer that, even though each hemisphere
  did reach the magnetic baseline, from a heliospheric point of view
  the minimum of cycle 23 was not as deep as it could have been.

Title: Pixel Dynamics Analysis of Photospheric Spectral Data
Authors: Rasca, Anthony P.; Chen, James; Pevtsov, Alexei A.
Bibcode: 2015TESS....110301R
Altcode:
  Recent advances in solar observations have led to higher-resolution
  surface (photosphere) images that reveal bipolar magnetic features
  operating near the resolution limit during emerging flux events. Further
  improvements in resolution are expected to reveal even smaller dynamic
  features. Such photospheric features provide observable indications of
  what is happening before, during, and after flux emergence, eruptions
  in the corona, and other phenomena. Visible changes in photospheric
  active regions also play a major role in predicting eruptions that are
  responsible for geomagnetic plasma disturbances. A new method has been
  developed to extract physical information from photospheric data (e.g.,
  SOLIS Stokes parameters) based on the statistics of pixel-by-pixel
  variations in spectral (absorption or emission) line quantities such
  as line profile Doppler shift, width, asymmetry, and flatness. Such
  properties are determined by the last interaction between detected
  photons and optically thick photospheric plasmas, and may contain
  extractable information on local plasma properties at sub-pixel
  scales. Applying the method to photospheric data with high spectral
  resolution, our pixel-by-pixel analysis is performed for various
  regions on the solar disk, ranging from quiet-Sun regions to active
  regions exhibiting eruptions, characterizing photospheric dynamics
  using spectral profiles. In particular, the method quantitatively
  characterizes the time profile of changes in spectral properties in
  photospheric features and provides improved physical constraints on
  observed quantities.

Title: Uncertainties Associated to Near Real-Time Synoptic Magnetic
    maps and Implications for Solar Wind Models
Authors: Bertello, Luca; Pevtsov, Alexei A.; Macniece, Peter
Bibcode: 2015TESS....111003B
Altcode:
  Beginning with May 2006 data, the National Solar Observatory is
  providing uncertainty (spatial-variance) maps to accompany its
  database of magnetic flux synoptic charts. Early studies using few
  selected integral Carrington rotation maps have shown the impact of
  these uncertainty maps on the outcome numerical models of the coronal
  magnetic field and the solar wind (e.g., Bertello et al. 2014, Solar
  Physics, 289 (7), 2419). Here we discuss the evolution of solar
  wind parameters at Earth computed from the WSA-ENLIL model using
  the more suitable near real-time magnetic flux synoptic charts and
  their corresponding uncertainty maps. We investigated the short-term
  variations in these parameters during periods of low and high levels
  of solar activity to determine the predictive capabilities of these
  maps at different phases of the solar cycle. Our preliminary analysis
  based on integral synoptic maps suggests that during the period of
  low solar activity the short-term variations in solar wind parameters
  are within the scatter of the ensemble modeling. When the activity is
  high, the short-term variations in the observed parameters are larger
  than the scatter from the modeling. The results of this investigation
  will help to get a better understanding about some aspects of existing
  models of the solar wind that may require further improvements.

Title: Application of Mutual Information Methods in Time-Distance
    Helioseismology
Authors: Keys, Dustin; Kholikov, Shukur; Pevtsov, Alexei A.
Bibcode: 2015SoPh..290..659K
Altcode: 2015SoPh..tmp...15K; 2015arXiv150105597K
  We apply a new technique, the mutual information (MI) from information
  theory, to time-distance helioseismology, and demonstrate that it can
  successfully reproduce several classic results based on the widely
  used cross-covariance method. MI quantifies the deviation of two
  random variables from complete independence and represents a more
  general method for detecting dependencies in time series than the
  cross-covariance function, which only detects linear relationships. We
  briefly describe the MI-based technique and discuss the results of
  applying MI to derive the solar differential profile, a travel-time
  deviation map for a sunspot, and a time-distance diagram from quiet-Sun
  measurements.

Title: Small-scale and Global Dynamos and the Area and Flux
Distributions of Active Regions, Sunspot Groups, and Sunspots:
    A Multi-database Study
Authors: Muñoz-Jaramillo, Andrés; Senkpeil, Ryan R.; Windmueller,
   John C.; Amouzou, Ernest C.; Longcope, Dana W.; Tlatov, Andrey G.;
   Nagovitsyn, Yury A.; Pevtsov, Alexei A.; Chapman, Gary A.; Cookson,
   Angela M.; Yeates, Anthony R.; Watson, Fraser T.; Balmaceda, Laura A.;
   DeLuca, Edward E.; Martens, Petrus C. H.
Bibcode: 2015ApJ...800...48M
Altcode: 2014arXiv1410.6281M
  In this work, we take advantage of 11 different sunspot group,
  sunspot, and active region databases to characterize the area
  and flux distributions of photospheric magnetic structures. We
  find that, when taken separately, different databases are better
  fitted by different distributions (as has been reported previously
  in the literature). However, we find that all our databases can be
  reconciled by the simple application of a proportionality constant,
  and that, in reality, different databases are sampling different
  parts of a composite distribution. This composite distribution
  is made up by linear combination of Weibull and log-normal
  distributions—where a pure Weibull (log-normal) characterizes the
  distribution of structures with fluxes below (above) 10<SUP>21</SUP>Mx
  (10<SUP>22</SUP>Mx). Additionally, we demonstrate that the Weibull
  distribution shows the expected linear behavior of a power-law
  distribution (when extended to smaller fluxes), making our results
  compatible with the results of Parnell et al. We propose that this is
  evidence of two separate mechanisms giving rise to visible structures
  on the photosphere: one directly connected to the global component of
  the dynamo (and the generation of bipolar active regions), and the other
  with the small-scale component of the dynamo (and the fragmentation of
  magnetic structures due to their interaction with turbulent convection).

Title: Properties of sunspot umbrae of leading and trailing polarity
    in 1917-2013
Authors: Tlatov, Andrey Georgievich; Tlatova, K. A.; Vasil'eva, V. V.;
   Pevtsov, A. A.; Mursula, K.
Bibcode: 2015AdSpR..55..835T
Altcode:
  Using the software developed by us, we produced a digitized
  (tabulated) database of sunspot umbrae and pores observed at Mount
  Wilson Observatory (MWO) in 1917-2013. The database includes the
  heliographic coordinates, areas and the polarity and strength of
  magnetic fields of umbrae and pores in the MWO sunspot drawings. Using
  this database we study here the properties and long-term variation
  of sunspot umbrae and pores, separately for leading and trailing
  polarity spots. We find that the leading sunspots have tendency for
  larger umbrae and stronger magnetic field strength than the trailing
  spots. The average field strength and area of sunspot umbrae vary
  with sunspot cycle. Furthermore, the mean magnetic field strength
  in sunspot umbrae exhibits a gradual increase from early 1960s to
  1990s. The nature of this increase is discussed.

Title: SOLIS: Reconciling Disk-integrated and Disk-resolved Spectra
    from the Sun
Authors: Pevtsov, Alexei A.; Bertello, Luca; Harker, Brian; Giampapa,
   Mark; Marble, Andrew R.
Bibcode: 2015csss...18..887P
Altcode: 2014arXiv1411.7266P
  Unlike other stars, the surface of the Sun can be spatially resolved
  to a high degree of detail . But the Sun can also be observed as if
  it was a distant star. The availability of solar disk-resolved and
  disk-integrated spectra offers an opportunity to devise methods to
  derive information about the spatial distribution of solar features
  from Sun-as-a-star measurements. Here, we present an update on work
  done at the National Solar Observatory to reconcile disk-integrated
  and disk-resolved solar spectra from the Synoptic Optical Long-term
  Investigation of the Sun (SOLIS) station. The results of this work
  will lead to a new approach to infer the information about the spatial
  distribution of features on other stars, from the overall filling factor
  of active regions to, possibly, the latitude/longitude distribution
  of features.

Title: Solar Cycle Dependency of Sun-as-a-Star Photospheric Spectral
    Line Profiles
Authors: Bertello, Luca; Pevtsov, Alexei A.; Giampapa, Mark S.;
   Marble, Andrew R.
Bibcode: 2015csss...18..693B
Altcode:
  We investigate solar-cycle related changes in the profile of several
  photospheric spectral lines taken with the Integrated Sunlight
  Spectrometer (ISS) operating at the National Solar Observatory at Kitt
  Peak (Arizona). ISS, which is one of three instruments comprising
  the Synoptic Optical Long-term Investigations of the Sun (SOLIS)
  facility, is designed to obtain high spectral resolution (R = 300,000)
  observations of the Sun-as-a-star in a broad range of wavelengths
  (350 nm - 1100 nm). Daily measurements were obtained since December
  2006, covering the decline of solar cycle 23 and the rising phase of
  cycle 24. We present time series of line parameters and discuss their
  correlation to indices of solar magnetic activity. Because of their
  different response to variations in the thermodynamic and magnetic
  structures of the solar atmosphere, the measured line shape parameters
  provide an excellent tool for disentangling thermal and magnetic effects
  occurring during different phases of the solar cycle. The results of
  this analysis may also help with developing a better understanding of
  magnetic cycles of activity in other solar-like stars.

Title: Magnetic Helicity, Tilt, and Twist
Authors: Pevtsov, Alexei A.; Berger, Mitchell A.; Nindos, Alexander;
   Norton, Aimee A.; van Driel-Gesztelyi, Lidia
Bibcode: 2015sac..book..285P
Altcode:
  No abstract at ADS

Title: Quantifying Photospheric Processes Using a New Pixel Dynamics
    Model
Authors: Rasca, A.; Chen, J.; Pevtsov, A. A.
Bibcode: 2014AGUFMSH41B4130R
Altcode:
  Recent advances in solar observations have led to higher-resolution
  surface (photosphere) images that reveal bipolar magnetic features
  operating near the resolution limit during emerging flux events. Further
  improvements in resolution are expected to reveal even smaller dynamic
  features. Such photospheric features provide observable indications of
  what is happening before, during, and after flux emergence, eruptions
  in the corona, and other phenomena. Visible changes in photospheric
  active regions also play a major role in predicting eruptions that
  are responsible for geomagnetic plasma disturbances. We present a new
  method to extract physical information from photospheric data (e.g.,
  SOLIS Stokes parameters) based on the statistics of pixel-by-pixel
  variations in spectral (absorption) line quantities such as line
  profile width, asymmetry, and flatness. Such properties are determined
  by the last interaction between detected photons and optically thick
  photospheric plasmas, and may contain extractable information on
  local plasma properties at sub-pixel scales. Applying the method to
  photospheric data with high spectral resolution, our pixel-by-pixel
  analysis is performed for various regions on the solar disk, ranging
  from quiet-Sun regions to active regions exhibiting eruptions,
  characterizing photospheric dynamics using spectral profiles. In
  particular, the method quantitatively characterizes the time profile
  of changes in spectral properties in photospheric features and provides
  improved physical constraints on observed quantities.

Title: What SDO tells us about structure and evolution of coronal
    bright points
Authors: Karachik, N. V.; Minenko, E.; Sattarov, I.; Pevtsov, A. A.;
   Sherdonov, C. T.
Bibcode: 2014AN....335.1037K
Altcode:
  Using magnetograms and coronal images from two instruments on board
  the Solar Dynamics Observatory (SDO), we study structure and evolution
  of a limited number of coronal bright points (CBPs). Our results show
  that the relation between CBPs and their magnetic footpoints is not
  simple. In some cases, CBP may appear as a bright portion of a larger
  loop (with clearly identifiable footpoints), and in some cases, an
  isolated CBP may develop between magnetic poles, which might not be
  the closest ones to each other or which might not be involved in the
  magnetic flux cancellation. We suggest that the magnetic connectivity
  responsible for formation of isolated coronal bright points is governed
  by the orientation of the large-scale magnetic field.

Title: Magnetic Helicity, Tilt, and Twist
Authors: Pevtsov, Alexei A.; Berger, Mitchell A.; Nindos, Alexander;
   Norton, Aimee A.; van Driel-Gesztelyi, Lidia
Bibcode: 2014SSRv..186..285P
Altcode:
  Since its introduction to astro- and solar physics, the concept of
  helicity has proven to be useful in providing critical insights into
  physics of various processes from astrophysical dynamos, to magnetic
  reconnection and eruptive phenomena. Signature of helicity was also
  detected in many solar features, including orientation of solar active
  regions, or Joy's law. Here we provide a summary of both solar phenomena
  and consider mutual relationship and its importance for the evolution
  of solar magnetic fields.

Title: Global Solar Free Magnetic Energy and Electric Current Density
    Distribution of Carrington Rotation 2124
Authors: Tadesse, Tilaye; Pevtsov, Alexei A.; Wiegelmann, T.; MacNeice,
   P. J.; Gosain, S.
Bibcode: 2014SoPh..289.4031T
Altcode: 2013arXiv1310.5790T
  Solar eruptive phenomena, like flares and coronal mass ejections
  (CMEs), are governed by magnetic fields. To describe the structure of
  these phenomena one needs information on the magnetic flux density and
  the electric current density vector components in three dimensions
  throughout the atmosphere. However, current spectro-polarimetric
  measurements typically limit the determination of the vector magnetic
  field to only the photosphere. Therefore, there is considerable
  interest in accurate modeling of the solar coronal magnetic field
  using photospheric vector magnetograms as boundary data. In this work,
  we model the coronal magnetic field for global solar atmosphere using
  nonlinear force-free field (NLFFF) extrapolation codes implemented to
  a synoptic maps of photospheric vector magnetic field synthesized from
  the Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term
  Investigations of the Sun (SOLIS) as boundary condition. Using
  the resulting three-dimensional magnetic field, we calculate the
  three-dimensional electric current density and magnetic energy
  throughout the solar atmosphere for Carrington rotation 2124 using our
  global extrapolation code. We found that spatially, the low-lying,
  current-carrying core field demonstrates a strong concentration of
  free energy in the active-region core, from the photosphere to the
  lower corona (about 70 Mm). The free energy density appears largely
  co-spatial with the electric current distribution.

Title: Investigation of the evolution of coronal bright points and
    magnetic field topology
Authors: Minenko, E. P.; Karachik, N. V.; Sattarov, I.; Pevtsov, A. A.
Bibcode: 2014AstL...40..510M
Altcode:
  Our investigation has been carried using the instruments onboard
  the Solar Dynamics Observatory (SDO) providing a high resolution of
  images (AIA photographs and HMI magnetograms). We have investigated
  the structure and magnetic evolution of several coronal bright points
  and small scale N-S polarity magnetic fluxes closely associated with
  them. We also compare the evolution of the magnetic polarities of
  elementary isolated sources of positive and negative fluxes (magnetic
  bipoles) and coronal bright points. Tiny ("elementary") coronal bright
  points have been detected. A standard coronal bright point is shown
  to be a group of "elementary" coronal bright points that flare up
  sequentially. Our investigation shows that a change in the magnetic
  fluxes of opposite polarities is observed before the flare of a coronal
  bright point. We show that not all cases of the formation of coronal
  bright points are described by the magnetic reconnection model. This
  result has not been considered previously and has not been pointed
  out by other authors.

Title: Uncertainties in Solar Synoptic Magnetic Flux Maps
Authors: Bertello, L.; Pevtsov, A. A.; Petrie, G. J. D.; Keys, D.
Bibcode: 2014SoPh..289.2419B
Altcode: 2013arXiv1312.0509B; 2014SoPh..tmp....7B
  Magnetic flux synoptic charts are critical for a reliable modeling
  of the corona and heliosphere. Until now, however, these charts were
  provided without uncertainty estimates. The uncertainties are due to
  instrumental noise in the measurements and to the spatial variance
  of the magnetic flux distribution that contributes to each bin in the
  synoptic chart. We describe here a simple method to compute synoptic
  magnetic flux maps and their corresponding magnetic flux spatial
  variance charts that can be used to estimate the uncertainty in the
  results of coronal models. We have tested this approach by computing a
  potential-field source-surface model of the coronal field for a Monte
  Carlo simulation of Carrington synoptic magnetic flux maps generated
  from the variance map. We show that these uncertainties affect both
  the locations of source-surface neutral lines and the distributions
  of coronal holes in the models.

Title: Hemispheric Distribution of Subsurface Kinetic Helicity and
    Its Variation with Magnetic Activity
Authors: Komm, R.; Gosain, S.; Pevtsov, A. A.
Bibcode: 2014SoPh..289.2399K
Altcode: 2014SoPh..tmp...25K
  We study the hemispheric distribution of the kinetic helicity of
  subsurface flows in the near-surface layers of the solar convection
  zone and its variation with magnetic activity. We determine subsurface
  flows with a ring-diagram analysis applied to Global Oscillation Network
  Group (GONG) Dopplergrams and Dynamics Program data from the Michelson
  Doppler Imager (MDI) instrument onboard the Solar and Heliospheric
  Observatory (SOHO). We determine the average kinetic helicity density
  as a function of Carrington rotation and latitude. The average kinetic
  helicity density at all depths and the kinetic helicity, integrated
  over 2 - 7 Mm, follow the same hemispheric rule as the current/magnetic
  helicity proxies with predominantly positive values in the southern
  and negative ones in the northern hemisphere. This holds true for all
  levels of magnetic activity from quiet to active regions. However,
  this is a statistical result; only about 55 % of all locations follow
  the hemispheric rule. But these locations have larger helicity values
  than those that do not follow the rule. The average values of helicity
  density increase with depth for all levels of activity, which might
  reflect an increase of the characteristic size of convective motions
  with greater depth. The average helicity of subsets of high magnetic
  activity is about five times larger than that of subsets of low
  activity. The solar-cycle variation of helicity is thus mainly due to
  the presence or absence of active regions. During the rising phase of
  cycle 24, locations of high magnetic activity at low latitudes show
  a weaker hemispheric behavior compared to the rising phase of cycle 23.

Title: Magnetic Helicity of the Global Field in Solar Cycles 23 and 24
Authors: Pipin, V. V.; Pevtsov, A. A.
Bibcode: 2014ApJ...789...21P
Altcode: 2014arXiv1402.2386P
  For the first time we reconstruct the magnetic helicity density of
  the global axisymmetric field of the Sun using the method proposed by
  Brandenburg et al. and Pipin et al. To determine the components of
  the vector potential, we apply a gauge which is typically employed
  in mean-field dynamo models. This allows for a direct comparison of
  the reconstructed helicity with the predictions from the mean-field
  dynamo models. We apply this method to two different data sets:
  the synoptic maps of the line-of-sight magnetic field from the
  Michelson Doppler Imager (MDI) on board the Solar and Heliospheric
  Observatory (SOHO) and vector magnetic field measurements from the
  Vector Spectromagnetograph (VSM) on the Synoptic Optical Long-term
  Investigations of the Sun (SOLIS) system. Based on the analysis of
  the MDI/SOHO data, we find that in solar cycle 23 the global magnetic
  field had positive (negative) magnetic helicity in the northern
  (southern) hemisphere. This hemispheric sign asymmetry is opposite to
  the helicity of the solar active regions, but it is in agreement with
  the predictions of mean-field dynamo models. The data also suggest
  that the hemispheric helicity rule may have reversed its sign during
  the early and late phases of cycle 23. Furthermore, the data indicate
  an imbalance in magnetic helicity between the northern and southern
  hemispheres. This imbalance seems to correlate with the total level
  of activity in each hemisphere in cycle 23. The magnetic helicity
  for the rising phase of cycle 24 is derived from SOLIS/VSM data,
  and qualitatively its latitudinal pattern is similar to the pattern
  derived from SOHO/MDI data for cycle 23.

Title: Effect of Uncertainties in Solar Synoptic Magnetic Flux Maps
    in Modeling of Solar Wind
Authors: Pevtsov, Alexei; Bertello, Luca; Petrie, Gordon
Bibcode: 2014shin.confE.159P
Altcode:
  Recently, the NSO/SOLIS team developed variance (error) maps that
  represent uncertainties in magnetic flux synoptic charts. These
  uncertainties are determined by the spatial variance of the magnetic
  flux distribution from full disk magnetograms that contributes to each
  bin in the synoptic chart. The algorithm for the creation of these
  maps is described in Bertello, L., Pevtsov, A.A., Petrie, G.J.D.,
  Keys, D.: 2014

Title: Temporal Variation of Photospheric Spectral Lines Profiles
    with the Solar Cycle of Activity
Authors: Bertello, Luca; Pevtsov, Alexei A.; Marble, Andrew R.
Bibcode: 2014AAS...22421831B
Altcode:
  We investigated the variations in the sun-as-a-star profiles of
  several photospheric spectral lines observed during the decline of
  solar cycle 23 and the rising phase of cycle 24. Daily measurements
  were taken with the Integrated Sunlight Spectrometer (ISS) operating at
  the National Solar Observatory at Kitt Peak (Arizona) since December
  2006. ISS, which is one of three instruments comprising the Solar
  Optical Investigations of the Sun (SOLIS) facility, is designed to
  obtain high spectral resolution (R = 300,000) observations of the Sun
  as a star in a broad range of wavelengths (350 nm - 1100 nm). Recent
  improvements in the spectral calibration of SOLIS/ISS measurements have
  significantly enhanced the diagnostic capabilities of these data.We will
  present time series of line parameters and discuss their correlation
  to the global magnetic flux. Because of their different response to
  variations in the thermodynamic and magnetic structures of the solar
  atmosphere, the measured line shape parameters provide an excellent
  tool to disentangle thermal and magnetic effects occurring during
  different phases of the solar cycle. The results of this analysis may
  also help with developing a better understanding of magnetic cycle of
  activity in other solar-like stars.

Title: First use of synoptic vector magnetograms for global nonlinear,
    force-free coronal magnetic field models
Authors: Asfaw, Tilaye Tadesse; Pevtsov, Alexei A.; Macneice, Peter J.
Bibcode: 2014AAS...22432326A
Altcode:
  The magnetic field permeating the solar atmosphere is generally
  thought to provide the energy for much of the activity seen in the
  solar corona, such as flares, coronal mass ejections (CMEs), etc. To
  overcome the unavailability of coronal magnetic field measurements,
  photospheric magnetic field vector data can be used to reconstruct
  the coronal field. Currently, there are several modelling techniques
  being used to calculate three dimensional field lines into the solar
  atmosphere. For the first time, synoptic maps of a photospheric vector
  magnetic field synthesized from the vector spectromagnetograph (VSM) on
  Synoptic Optical Long term Investigations of the Sun (SOLIS) are used
  to model the coronal magnetic field and estimate free magnetic energy
  in the global scale. The free energy (i.e., the energy in excess of
  the potential field energy) is one of the main indicators used in space
  weather forecasts to predict the eruptivity of active regions. We solve
  the nonlinear force free field equations using an optimization principle
  in spherical geometry. The resulting three-dimensional magnetic fields
  are used to estimate the magnetic free energy content, which is the
  difference of the magnetic energies between the nonpotential field
  and the potential field in the global solar corona. For comparison,
  we overlay the extrapolated magnetic field lines with the extreme
  ultraviolet (EUV) observations by the atmospheric imaging assembly (AIA)
  on board the Solar Dynamics Observatory (SDO).For a single Carrington
  rotation 2121, we find that the global nonlinear force free field
  (NLFFF) magnetic energy density is 10.3% higher than the potential
  one. Most of this free energy is located in active regions.

Title: Case Study of a Magnetic Transient in NOAA 11429 Observed by
    SDO/HMI During the M7.9 Flare on 13 March 2012
Authors: Harker, Brian; Pevtsov, Alexei A.
Bibcode: 2014AAS...22412334H
Altcode:
  NOAA 11429 was the source of an M7.9 X-ray flare at the western solar
  limb (N18° W63°) on 2012 March 13 at 17:12 UT. Observations of the
  line-of-sight magnetic flux and the Stokes I and V profiles from which
  it is derived were carried out by the Solar Dynamics Observatory
  Helioseismic and Magnetic Imager (SDO/HMI) with a 45 s cadence
  over the full disk, at a spatial sampling of 0.''5. During flare
  onset, a transient patch of negative flux can be observed in SDO/HMI
  magnetograms to rapidly appear within the positive polarity penumbra of
  NOAA 11429. We present here a detailed study of this magnetic transient
  and offer interpretations as to whether this highly debated phenomenon
  represents a "real" change in the structure of the magnetic field at the
  site of the flare, or is instead a product of instrumental/algorithmic
  artifacts related to particular SDO/HMI data reduction techniques.

Title: Relating photospheric magnetic field changes and hard X-ray
    emission during flares
Authors: Petrie, Gordon; Burtseva, Olga; Martinez Oliveros, Juan
   Carlos; Pevtsov, Alexei A.
Bibcode: 2014AAS...22412328P
Altcode:
  We study the correlation between abrupt permanent changes of magnetic
  field during strong flares observed by GONG and HMI instruments,
  and the location of hard X-ray (HXR) emission observed by RHESSI to
  relate the field changes to the reconnection processes in the corona
  and investigate the origin of the field changes. The chromospheric
  HXR emission in solar flares is generally regarded as the footprints
  of magnetic field lines newly reconnected in the corona. Also, the
  footpoint motions traveling away from the neutral lines are considered
  to be indicative of the reconnection occurring in arcade magnetic
  fields of increasing heights. Our analysis of six flares shows that the
  early HXR emission corresponds well to locations of the strong field
  changes. The later HXR emission does not correspond to significant
  field changes as the footpoint is moving away from the neutral line
  in later stages of the flare. The field changes and HXR emission are
  spatio-temporally related, but not simultaneous. The field changes start
  earlier and end later than the detectable HXR signal. The strongest
  X-class flares in our analysis show a well-defined peak in the field
  changes a few minutes earlier than the peak in the HXR emission. The
  timing relationship between the HXR and the largest photospheric field
  changes may indicate an indirect physical relationship between these
  phenomena. Tracing of the field changes at the footpoints’ locations
  shows that in most of the flares the field changes propagated at a
  speed similar to that of the HXR footpoint moving away from the neutral
  line. However most of the field changes occurred earlier in time. O.B.,
  G.P. and A.P. are partially supported by NASA grant NNX14AE05G.

Title: Bimodal Distribution of Magnetic Fields and Areas of Sunspots
Authors: Tlatov, Andrey G.; Pevtsov, Alexei A.
Bibcode: 2014SoPh..289.1143T
Altcode: 2013arXiv1308.0535T
  We applied automatic identification of sunspot umbrae and penumbrae
  to daily observations from the Helioseismic Magnetic Imager (HMI)
  on board the Solar Dynamics Observatory (SDO) to study their magnetic
  flux density (B) and area (A). The results confirm an already known
  logarithmic relationship between the area of sunspots and their maximum
  flux density. In addition, we find that the relation between average
  magnetic flux density () and sunspot area shows a bimodal distribution:
  for small sunspots and pores (A≤20 millionth of solar hemisphere,
  MSH), (gauss), and for large sunspots (A≥100 MSH), is about 600
  G. For intermediate sunspots, average flux density linearly decreases
  from about 800 G to 600 G. A similar bimodal distribution was found
  in several other integral parameters of sunspots. We show that this
  bimodality can be related to different stages of sunspot penumbra
  formation and can be explained by the difference in average inclination
  of magnetic fields at the periphery of small and large sunspots.

Title: A Comparison Between Nonlinear Force-Free Field and Potential
    Field Models Using Full-Disk SDO/HMI Magnetogram
Authors: Tadesse, Tilaye; Wiegelmann, T.; MacNeice, P. J.; Inhester,
   B.; Olson, K.; Pevtsov, A.
Bibcode: 2014SoPh..289..831T
Altcode: 2012arXiv1212.5639T
  Measurements of magnetic fields and electric currents in the
  pre-eruptive corona are crucial to the study of solar eruptive
  phenomena, like flares and coronal mass ejections (CMEs). However,
  spectro-polarimetric measurements of certain photospheric lines
  permit a determination of the vector magnetic field only at the
  photosphere. Therefore, there is considerable interest in accurate
  modeling of the solar coronal magnetic field using photospheric vector
  magnetograms as boundary data. In this work, we model the coronal
  magnetic field above multiple active regions with the help of a
  potential field and a nonlinear force-free field (NLFFF) extrapolation
  code over the full solar disk using Helioseismic and Magnetic Imager
  (SDO/HMI) data as boundary conditions. We compare projections of the
  resulting magnetic field lines with full-disk coronal images from
  the Atmospheric Imaging Assembly (SDO/AIA) for both models. This
  study has found that the NLFFF model reconstructs the magnetic
  configuration closer to observation than the potential field model
  for full-disk magnetic field extrapolation. We conclude that many
  of the trans-equatorial loops connecting the two solar hemispheres
  are current-free.

Title: Properties of Magnetic Neutral Line Gradients and Formation
    of Filaments
Authors: Karachik, Nina V.; Pevtsov, Alexei A.
Bibcode: 2014SoPh..289..821K
Altcode: 2013arXiv1307.3317K
  We investigate the gradients of magnetic fields across neutral
  lines (NLs) and compare their properties for NLs with and without
  chromospheric filaments. Our results show that there is a range of
  preferred magnetic field gradients where the filament formation is
  enhanced. On the other hand, a horizontal gradient of the magnetic
  field across an NL alone does not appear to be a single factor that
  determines if a filament will form (or not) in a given location.

Title: First use of synoptic vector magnetograms for global nonlinear,
    force-free coronal magnetic field models
Authors: Tadesse, T.; Wiegelmann, T.; Gosain, S.; MacNeice, P.;
   Pevtsov, A. A.
Bibcode: 2014A&A...562A.105T
Altcode: 2013arXiv1309.5853T
  Context. The magnetic field permeating the solar atmosphere is
  generally thought to provide the energy for much of the activity
  seen in the solar corona, such as flares, coronal mass ejections
  (CMEs), etc. To overcome the unavailability of coronal magnetic field
  measurements, photospheric magnetic field vector data can be used to
  reconstruct the coronal field. Currently, there are several modelling
  techniques being used to calculate three-dimensional field lines into
  the solar atmosphere. <BR /> Aims: For the first time, synoptic maps
  of a photospheric-vector magnetic field synthesized from the vector
  spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations
  of the Sun (SOLIS) are used to model the coronal magnetic field and
  estimate free magnetic energy in the global scale. The free energy
  (i.e., the energy in excess of the potential field energy) is one of
  the main indicators used in space weather forecasts to predict the
  eruptivity of active regions. <BR /> Methods: We solve the nonlinear
  force-free field equations using an optimization principle in spherical
  geometry. The resulting three-dimensional magnetic fields are used
  to estimate the magnetic free energy content E<SUB>free</SUB> =
  E<SUB>nlfff</SUB> - E<SUB>pot</SUB>, which is the difference of the
  magnetic energies between the nonpotential field and the potential field
  in the global solar corona. For comparison, we overlay the extrapolated
  magnetic field lines with the extreme ultraviolet (EUV) observations
  by the atmospheric imaging assembly (AIA) on board the Solar Dynamics
  Observatory (SDO). <BR /> Results: For a single Carrington rotation
  2121, we find that the global nonlinear force-free field (NLFFF)
  magnetic energy density is 10.3% higher than the potential one. Most
  of this free energy is located in active regions.

Title: Cyclic and Long-Term Variation of Sunspot Magnetic Fields
Authors: Pevtsov, Alexei A.; Bertello, Luca; Tlatov, Andrey G.;
   Kilcik, Ali; Nagovitsyn, Yury A.; Cliver, Edward W.
Bibcode: 2014SoPh..289..593P
Altcode: 2013arXiv1301.5935P
  Measurements from the Mount Wilson Observatory (MWO) were used to
  study the long-term variations of sunspot field strengths from 1920 to
  1958. Following a modified approach similar to that presented in Pevtsov
  et al. (Astrophys. J. Lett.742, L36, 2011), we selected the sunspot
  with the strongest measured field strength for each observing week and
  computed monthly averages of these weekly maximum field strengths. The
  data show the solar cycle variation of the peak field strengths
  with an amplitude of about 500 - 700 gauss (G), but no statistically
  significant long-term trends. Next, we used the sunspot observations
  from the Royal Greenwich Observatory (RGO) to establish a relationship
  between the sunspot areas and the sunspot field strengths for cycles 15
  - 19. This relationship was used to create a proxy of the peak magnetic
  field strength based on sunspot areas from the RGO and the USAF/NOAA
  network for the period from 1874 to early 2012. Over this interval,
  the magnetic field proxy shows a clear solar cycle variation with an
  amplitude of 500 - 700 G and a weaker long-term trend. From 1874 to
  around 1920, the mean value of magnetic field proxy increases by about
  300 - 350 G, and, following a broad maximum in 1920 - 1960, it decreases
  by about 300 G. Using the proxy for the magnetic field strength as the
  reference, we scaled the MWO field measurements to the measurements of
  the magnetic fields in Pevtsov et al. (2011) to construct a combined
  data set of maximum sunspot field strengths extending from 1920 to early
  2012. This combined data set shows strong solar cycle variations and no
  significant long-term trend (the linear fit to the data yields a slope
  of − 0.2±0.8 G year<SUP>−1</SUP>). On the other hand, the peak
  sunspot field strengths observed at the minimum of the solar cycle show
  a gradual decline over the last three minima (corresponding to cycles
  21 - 23) with a mean downward trend of ≈ 15 G year<SUP>−1</SUP>.

Title: Active Regions with Superpenumbral Whirls and Their Subsurface
    Kinetic Helicity
Authors: Komm, R.; Gosain, S.; Pevtsov, A.
Bibcode: 2014SoPh..289..475K
Altcode:
  We search for a signature of helicity flow from the solar interior
  to the photosphere and chromosphere. For this purpose, we study two
  active regions, NOAA 11084 and 11092, that show a regular pattern of
  superpenumbral whirls in chromospheric and coronal images. These two
  regions are good candidates for comparing magnetic/current helicity with
  subsurface kinetic helicity because the patterns persist throughout the
  disk passage of both regions. We use photospheric vector magnetograms
  from SOLIS/VSM and SDO/HMI to determine a magnetic helicity proxy, the
  spatially averaged signed shear angle (SASSA). The SASSA parameter
  produces consistent results leading to positive values for NOAA
  11084 and negative ones for NOAA 11092 consistent with the clockwise
  and counter-clockwise orientation of the whirls. We then derive
  the properties of the subsurface flows associated with these active
  regions. We measure subsurface flows using a ring-diagram analysis of
  GONG high-resolution Doppler data and derive their kinetic helicity,
  h<SUB>z</SUB>. Since the patterns persist throughout the disk passage,
  we analyze synoptic maps of the subsurface kinetic helicity density. The
  sign of the subsurface kinetic helicity is negative for NOAA 11084
  and positive for NOAA 11092; the sign of the kinetic helicity is
  thus anticorrelated with that of the SASSA parameter. As a control
  experiment, we study the subsurface flows of six active regions without
  a persistent whirl pattern. Four of the six regions show a mixture
  of positive and negative kinetic helicity resulting in small average
  values, while two regions are clearly dominated by kinetic helicity
  of one sign or the other, as in the case of regions with whirls. The
  regions without whirls follow overall the same hemispheric rule in
  their kinetic helicity as in their current helicity with positive
  values in the southern and negative values in the northern hemisphere.

Title: Solar Origins of Space Weather and Space Climate: Preface
Authors: González Hernández, I.; Komm, R.; Pevtsov, A.; Leibacher,
   J. W.
Bibcode: 2014SoPh..289..437G
Altcode:
  No abstract at ADS

Title: The sun-as-a-star solar spectrum
Authors: Pevtsov, A. A.; Bertello, L.; Marble, A. R.
Bibcode: 2014AN....335...21P
Altcode:
  The Sun is the only star for which individual surface features can
  be observed directly. For other stars, the properties of starspots,
  stellar rotation, stellar flares, etc, are derived indirectly via
  variation of star-integrated spectral line profiles or their luminosity
  measurements. Solar disk-integrated and disk-resolved observations allow
  for investigations of the contribution of individual solar disk features
  to sun-as-a-star spectra. Here, we provide a brief overview of three
  sun-as-a-star programs, currently in operation, and describe recent
  improvements in observations and data reduction for the Integrated
  Sunlight Spectrometer (ISS), one of three instruments comprising
  the Synoptic Optical Long-term Investigations of the Sun (SOLIS)
  system. Next, we discuss studies employing sun-as-a-star observations
  (including Ca II K line as proxy for total unsigned magnetic flux
  and 2800 MHz radio flux) as well as the effects of flares on solar
  disk-integrated spectra.

Title: Long-term variation of statistical properties of sunspot field
    strengths and their relation to the characteristics of solar cycles
    in 1917-2013
Authors: Tlatova, Kseniya; Pevtsov, Alexei; Tlatov, Andrey; Vasilieva,
   Valeria; Kalevi Mursula
Bibcode: 2014cosp...40E3357T
Altcode:
  We use the digitized sunspot daily drawings from the Mount Wilson
  Observatory (MWO) from 1917—2013 to study the long-term variations
  in sunspot magnetic fields. Tabulated data include the heliographic
  coordinates, area and magnetic field strength of all sunspots on
  solar disk. Sunspot field strengths exhibit several trends, which
  can be of instrumental or truly solar in its nature. For example,
  in depth analysis of the sunspot field strength and the sunspot areas
  indicates long-term changes in the observing system at the MWO, when
  systematically larger number of measurements in sub-features (e.g.,
  multiple umbrae) was made beginning mid-20th century. The final data
  set is used to study several properties of active regions including
  difference (in field strength and area) between leading and following
  sunspots, the orientation of magnetic field in sunspots and pores
  relative to vertical direction, and the Hale (polarity) and Joy (tilt)
  laws over the period of about ten solar cycles.

Title: Uncertainties in Solar Synoptic Maps and their Effect on
    Models.
Authors: Pevtsov, Alexei; Petrie, Gordon; Bertello, Luca
Bibcode: 2014cosp...40E2524P
Altcode:
  Synoptic charts of the photospheric magnetic flux are used widely in
  modeling the magnetic field in the corona and the heliosphere. Recently,
  the National Solar Observatory began producing accompanying maps of
  uncertainties (http://solis.nso.edu/0/ermaps/6302l_er2138-2157.html),
  which can be used to estimate the uncertainty in the results of coronal
  models. These maps of uncertainties represent the spatial variance
  of the magnetic flux distribution that contributes to each bin in
  the synoptic chart. We will describe a method to compute synoptic
  magnetic flux spatial variance charts, and discuss the effects of
  these uncertainties on models of the coronal magnetic field and the
  solar wind speed.

Title: Case Study of a Magnetic Transient in NOAA 11429 Observed by
    SDO/HMI during the M7.9 Flare on 2012 March 13
Authors: Harker, Brian J.; Pevtsov, Alexei A.
Bibcode: 2013ApJ...778..175H
Altcode:
  NOAA 11429 was the source of an M7.9 X-ray flare at the western solar
  limb (N18° W63°) on 2012 March 13 at 17:12 UT. Observations of the
  line-of-sight magnetic flux and the Stokes I and V profiles from which
  it is derived were carried out by the Solar Dynamics Observatory
  Helioseismic and Magnetic Imager (SDO/HMI) with a 45 s cadence
  over the full disk, at a spatial sampling of 0.''5. During flare
  onset, a transient patch of negative flux can be observed in SDO/HMI
  magnetograms to rapidly appear within the positive polarity penumbra of
  NOAA 11429. We present here a detailed study of this magnetic transient
  and offer interpretations as to whether this highly debated phenomenon
  represents a "real" change in the structure of the magnetic field at the
  site of the flare, or is instead a product of instrumental/algorithmic
  artifacts related to particular SDO/HMI data reduction techniques.

Title: The Effect of Latitudinal Averaging of Surface Tracers on
    Patterns of Torsional Oscillations
Authors: Tlatov, A. G.; Pevtsov, A. A.
Bibcode: 2013ASPC..478..297T
Altcode:
  Latitudinal bands of faster/slower (relative to average) rate
  of rotation of solar plasma (also known as the solar torsional
  oscillations) are considered to be a fundamental property of the Sun,
  and are believed to be important for the solar dynamo. Torsional
  oscillations are derived from the measurements of the solar rotation
  at the photosphere and (indirectly, via methods of helioseismology)
  in the convection zone. Here, we raise a possibility that the pattern
  of the torsional oscillations could be a spurious feature resulting
  from a combination of the differential rotation, the latitudinal
  drift of tracers, and a weighted contribution of tracers within an
  averaging window used to determine the solar rotation rate. This
  spurious feature may have an effect on both the observations taken in
  the solar photosphere (e.g., Doppler velocity measurements and feature
  tracking techniques) and the helioseismic measurements.

Title: Fifty Years of Seismology of the Sun and Stars
Authors: Jain, K.; Tripathy, S. C.; Hill, F.; Leibacher, J. W.;
   Pevtsov, A. A.
Bibcode: 2013ASPC..478.....J
Altcode:
  No abstract at ADS

Title: Zeemanfit: Use and Development of the solis_vms_zeemanfit code
Authors: Hughes, Anna L. H.; Harvey, Jack; Marble, Andrew R.; Pevtsov,
   Alexei A.
Bibcode: 2013arXiv1312.2026H
Altcode:
  The purpose of the SOLIS Zeemanfit Code is to provide a
  straight-forward, easily checked measure of the total magnetic-field
  strength in the high-strength umbral regions of the solar disk. In the
  highest-strength regions, the Zeeman splitting of the 6302-angstrom
  Fe line becomes wide enough for the triplet nature of the line to be
  visible by eye in non-polarized light. Therefore, a three-line fit
  to the spectra should, in principle, provide a fairly robust measure
  of the total magnetic-field strength. The code uses the Level-1.5
  spec-cube data of the SOLIS VSM 6302-vector observations (specifically
  the Stokes-I and Stokes-V components) to fit the line profiles at each
  appropriate pixel and calculate the magnetic-field-strength from the
  line-center separation of the two fit 6302.5 sigma-components. The
  6301.5-angstrom Fe line is also present and fit in the VSM 6302-vector
  data, but it is an anomalous-Zeeman line with a weaker response to
  magnetic fields. Therefore, no magnetic- field measure is derived from
  this portion of the spectral fit.

Title: Camera Gap Removal in SOLIS/VSM Images
Authors: Marble, Andrew R.; Callahan, Lorraine; Pevtsov, Alexei A.
Bibcode: 2013arXiv1312.2909M
Altcode:
  The Vector Spectromagnetograph (VSM) instrument on the Synoptic Optical
  Longterm Investigations of the Sun (SOLIS) telescope is capable of
  obtaining spectropolarimetry for the full Sun (or a select latitudinal
  range) with one arcsecond spatial resolution and 0.05 Angstrom spectral
  resolution. This is achieved by scanning the Sun in declination and
  building up spectral cubes for multiple polarization states, utilizing
  a beamsplitter and two separate 2k x 2k CCD cameras. As a result, the
  eastern and western hemispheres of the Sun are separated in preliminary
  VSM images by a vertical gap with soft edges and variable position and
  width. Prior to the comprehensive analysis presented in this document,
  a trial-and-error approach to removing the gap had yielded an algorithm
  that was inconsistent, undocumented, and responsible for incorrectly
  eliminating too many image columns. Here we describe, in detail, the
  basis for a new, streamlined, and properly calibrated prescription for
  locating and removing the gap that is correct to within approximately
  one arcsecond (one column).

Title: Synoptic Magnetic Variance Maps and Their Effects on
    Field-extrapolation Coronal Models
Authors: Bertello, Luca; Pevtsov, A. A.; Keys, D.; Petrie, G.
Bibcode: 2013SPD....44..113B
Altcode:
  The prediction and description of the conditions throughout the
  heliosphere relies today mostly on potential field source surface (PFSS)
  and magnetohydrodynamics coronal and heliospheric models. Carrington
  synoptic maps are produced from individual magnetograms and used as
  the primary drivers for these models. However, the uncertainties on the
  flux distribution across synoptic maps have never been included in the
  models. As the measure of uncertainties, we produced synoptic spatial
  variance (1-sigma standard deviation) maps derived from the distribution
  of pixel values in the sky magnetograms that contribute the to average
  flux in each bin of the final Carrington map. Each variance-map is then
  used to generate a series of Carrington maps where the value of each
  bin differs, randomly, from the original value by up to 3-sigma. We
  discuss here how the uncertainty in the Carrington map affects the
  location of neutral lines and the footpoint locations of the open-field,
  the model coronal holes, determined from a standard PFSS model. In this
  preliminary investigation we studied two distinct periods, corresponding
  to minimum and maximum of solar activity. We show that the variance
  in the derived synoptic maps does not affect significantly the shape
  of neutral line or general location of coronal holes. The position of
  neutral lines and boundaries of coronal holes can be shifted by as much
  as 5 degrees in some locations.Abstract (2,250 Maximum Characters):
  The prediction and description of the conditions throughout the
  heliosphere relies today mostly on potential field source surface (PFSS)
  and magnetohydrodynamics coronal and heliospheric models. Carrington
  synoptic maps are produced from individual magnetograms and used as
  the primary drivers for these models. However, the uncertainties on
  the flux distribution across synoptic maps have never been included
  in the models. As the measure of uncertainties, we produced synoptic
  spatial variance (1-sigma standard deviation) maps derived from the
  distribution of pixel values in the sky magnetograms that contribute
  the to average flux in each bin of the final Carrington map. Each
  variance-map is then used to generate a series of Carrington maps where
  the value of each bin differs, randomly, from the original value by
  up to 3-sigma. We discuss here how the uncertainty in the Carrington
  map affects the location of neutral lines and the footpoint locations
  of the open-field, the model coronal holes, determined from a standard
  PFSS model. In this preliminary investigation we studied two distinct
  periods, corresponding to minimum and maximum of solar activity. We
  show that the variance in the derived synoptic maps does not affect
  significantly the shape of neutral line or general location of coronal
  holes. The position of neutral lines and boundaries of coronal holes
  can be shifted by as much as 5 degrees in some locations.

Title: Improvements to the SOLIS Program: New Data Products and
    Recent Science Results
Authors: Bertello, Luca; Callahan, L.; Gusain, S.; Harker, B.; Harvey,
   J. W.; Hughes, A.; Marble, A. R.; Pevtsov, A. A.; SOLIS
Bibcode: 2013SPD....44..135B
Altcode:
  Over the past two years the Synoptic Optical Long-term Investigations
  of the Sun (SOLIS) Team made significant improvements to the data
  products provided to the solar and heliospheric community. Longitudinal
  photospheric magnetograms were consistently calibrated to form a
  uniform magnetic flux series from 2003-present. Improvements in image
  geometric corrections enabled providing the high spectral resolution
  spectra for each pixel in full disk images taken in Fe 630.2 nm and
  Ca II 854.2 nm. New treatment of high resolution spectra from the
  Integrated Sunlight Spectrometer (ISS), led to significant reduction
  in daily variations of parameters derived from these spectra. Several
  new data products were developed, including simultaneous photospheric
  and chromospheric longitudinal magnetograms derived from the core and
  wings of Ca II 854.2 nm line, and derivation of field strength via
  fitting non-polarized I-profiles of Fe I 630.2 nm. We present here a
  summary of recent changes to this database, with particular emphasis on
  chromospheric longitudinal magnetic field and intensity measurements,
  photospheric vector magnetic field products, and high-spectral
  resolution integrated sunlight data. Some significant results derived
  from the analysis of these data sets will also be presented, while
  more detailed descriptions about specific topics will be referred to
  other SOLIS-related studies presented at this meeting.

Title: Hard X-ray emission during flares and changes in the
    photospheric magnetic field
Authors: Burtseva, Olga; Martinez Oliveros, J.; Petrie, G.; Pevtsov, A.
Bibcode: 2013SPD....44...73B
Altcode:
  We study the relationship between the field changes in the photosphere
  and the reconnection processes in the corona by comparing the locations
  of abrupt permanent changes of the magnetic field during strong flares
  observed by the GONG and HMI instruments and hard X-ray (HXR) emission
  observed by RHESSI. The chromospheric HXR emission in solar flares
  is generally believed to mark the footprints of magnetic field lines
  newly reconnected in the corona. Also, the footpoint motions away from
  the neutral line are considered to be indicative of the reconnection
  occurring in arcade magnetic fields at different heights. Our analysis
  of six flares that occurred during the declining phase of cycle 23
  shows that the strongest field changes are well correlated in space,
  with the HXR footprints moving away from the neutral line in later
  stages of the flare. The majority of field changes and HXR footpoints
  are spatio-temporally related but not simultaneous. We will also compare
  changes in the HMI vector magnetic field measurements with the location
  and evolution of flare footpoints and discuss possible implications for
  topology of the magnetic field at the reconnection site in the corona.

Title: First Synoptic Maps of Photospheric Vector Magnetic Field
from SOLIS/VSM: Non-radial Magnetic Fields and Hemispheric Pattern
    of Helicity
Authors: Gusain, Sanjay; Pevtsov, A. A.; Rudenko, G. V.; Anfinogentov,
   S. A.; Pevtsov, A. A.; Rudenko, G. V.; Anfinogentov, S. A.
Bibcode: 2013SPD....44..111G
Altcode:
  We use daily full-disk vector magnetograms from Vector
  Spectromagnetograph (VSM) on Solar Optical Long-term Investigations
  of the Sun (SOLIS) system to synthesize the first Carrington maps
  of the photospheric vector magnetic field. We describe these maps
  and make a comparison of observed radial field with the radial field
  estimate from LOS magnetograms. Further, we employ these maps to study
  the hemispheric pattern of current helicity density, Hc, during the
  rising phase of the solar cycle 24. Longitudinal average over the 23
  consecutive solar rotations shows a clear signature of the hemispheric
  helicity rule, i.e. Hc is predominantly negative in the North and
  positive in South. The hemispheric pattern for individual Carrington
  rotations is statistically weak, consistent with previous studies of
  active regions’ helicity. Although our data include the early phase
  of cycle 24, there appears no evidence for a possible (systematic)
  reversal of the hemispheric helicity rule at the beginning of cycle as
  predicted by some dynamo models. Further, we compute the hemispheric
  pattern in active region latitudes (-30 ≤ θ ≤ 30) separately
  for weak (100&lt; |Br| &lt;500 G)and strong (|Br| &gt;1000 G) radial
  magnetic fields. We find that while the current helicity of strong
  fields follows the well-known hemispheric rule (i.e., θ.Hc &lt; 0),
  Hc of weak fields exhibits an inverse hemispheric behavior (i.e., θ.Hc
  &gt; 0) albeit with large statistical scatter.Abstract (2,250 Maximum
  Characters): We use daily full-disk vector magnetograms from Vector
  Spectromagnetograph (VSM) on Solar Optical Long-term Investigations
  of the Sun (SOLIS) system to synthesize the first Carrington maps
  of the photospheric vector magnetic field. We describe these maps
  and make a comparison of observed radial field with the radial field
  estimate from LOS magnetograms. Further, we employ these maps to study
  the hemispheric pattern of current helicity density, Hc, during the
  rising phase of the solar cycle 24. Longitudinal average over the 23
  consecutive solar rotations shows a clear signature of the hemispheric
  helicity rule, i.e. Hc is predominantly negative in the North and
  positive in South. The hemispheric pattern for individual Carrington
  rotations is statistically weak, consistent with previous studies of
  active regions’ helicity. Although our data include the early phase
  of cycle 24, there appears no evidence for a possible (systematic)
  reversal of the hemispheric helicity rule at the beginning of cycle as
  predicted by some dynamo models. Further, we compute the hemispheric
  pattern in active region latitudes (-30 ≤ θ ≤ 30) separately
  for weak (100&lt; |Br| &lt;500 G)and strong (|Br| &gt;1000 G) radial
  magnetic fields. We find that while the current helicity of strong
  fields follows the well-known hemispheric rule (i.e., θ.Hc &lt; 0),
  Hc of weak fields exhibits an inverse hemispheric behavior (i.e.,
  θ.Hc &gt; 0) albeit with large statistical scatter.

Title: First Synoptic Maps of Photospheric Vector Magnetic Field
from SOLIS/VSM: Non-radial Magnetic Fields and Hemispheric Pattern
    of Helicity
Authors: Gosain, S.; Pevtsov, A. A.; Rudenko, G. V.; Anfinogentov,
   S. A.
Bibcode: 2013ApJ...772...52G
Altcode: 2013arXiv1305.3294G
  We use daily full-disk vector magnetograms from Vector
  Spectromagnetograph on Synoptic Optical Long-term Investigations of the
  Sun system to synthesize the first Carrington maps of the photospheric
  vector magnetic field. We describe these maps and make a comparison
  of the observed radial field with the radial field estimate from
  line-of-sight magnetograms. Furthermore, we employ these maps to study
  the hemispheric pattern of current helicity density, H<SUB>c</SUB> ,
  during the rising phase of solar cycle 24. The longitudinal average
  over the 23 consecutive solar rotations shows a clear signature of
  the hemispheric helicity rule, i.e., H<SUB>c</SUB> is predominantly
  negative in the north and positive in the south. Although our data
  include the early phase of cycle 24, there appears to be no evidence for
  a possible (systematic) reversal of the hemispheric helicity rule at the
  beginning of the cycle as predicted by some dynamo models. Furthermore,
  we compute the hemispheric pattern in active region latitudes (-30°
  &lt;= θ &lt;= 30°) separately for weak (100 G &lt; |B<SUB>r</SUB> |
  &lt; 500 G) and strong (|B<SUB>r</SUB> | &gt; 1000 G) radial magnetic
  fields. We find that while the current helicity of strong fields follows
  the well-known hemispheric rule (i.e., θ · H<SUB>c</SUB> &lt; 0),
  H<SUB>c</SUB> of weak fields exhibits an inverse hemispheric behavior
  (i.e., θ · H<SUB>c</SUB> &gt; 0), albeit with large statistical
  scatter. We discuss two plausible scenarios to explain the opposite
  hemispheric trend of helicity in weak and strong field regions.

Title: New Synoptic Measurements of Umbral Magnetic Fields
Authors: Harvey, J. W.; Hughes, A.; Marble, A.; Livingston, W. C.;
   Pevtsov, A.; SOLIS Team
Bibcode: 2013SPD....44..112H
Altcode:
  Apparent cyclic and secular changes of sunspot umbral intensities and
  magnetic field strengths have been reported for many decades. Monthly
  measurements since 1998 show changes that have been interpreted
  as a decline in sunspot vigor that, if continued, may lead to very
  few visible sunspots in forthcoming cycles (Livingston, Penn, and
  Svalgaard 2012, ApJ 757, L8). This dramatic notion is controversial
  (e.g. Nagovitsyn, Pevtsov, and Livingston 2012, ApJ 758, L20),
  and additional observational evidence is needed based on a minimum
  of interpretational steps and selection effects. The SOLIS vector
  spectromagnetograph has recorded photospheric spectra around 630.2
  nm over the full solar disk daily since late 2003 with spatial and
  spectral pixel dimensions of about one arc second and 2.3 pm. We fit
  the unpolarized intensity spectra in sunspots with a simple Zeeman
  triplet model using a single field strength, which works well if
  the field strength exceeds about 2 kG and the individual spectra
  are not affected by strong Doppler and/or Zeeman variations. The
  derived total field strengths may be compared with independent
  spectrograph-based measurements from NSO, Hinode/SP, and Mt. Wilson
  and with filter-based measurements from SoHO/MDI and SDO/HMI. NSO
  plans to implement this reduction for all suitable archived SOLIS
  spectra and to continue daily umbral field strength measurements
  as cycle 24 proceeds. Preliminary comparisons show good agreements
  in some cases and systematic differences in others.Abstract (2,250
  Maximum Characters): Apparent cyclic and secular changes of sunspot
  umbral intensities and magnetic field strengths have been reported for
  many decades. Monthly measurements since 1998 show changes that have
  been interpreted as a decline in sunspot vigor that, if continued, may
  lead to very few visible sunspots in forthcoming cycles (Livingston,
  Penn, and Svalgaard 2012, ApJ 757, L8). This dramatic notion is
  controversial (e.g. Nagovitsyn, Pevtsov, and Livingston 2012, ApJ
  758, L20), and additional observational evidence is needed based on
  a minimum of interpretational steps and selection effects. The SOLIS
  vector spectromagnetograph has recorded photospheric spectra around
  630.2 nm over the full solar disk daily since late 2003 with spatial
  and spectral pixel dimensions of about one arc second and 2.3 pm. We
  fit the unpolarized intensity spectra in sunspots with a simple Zeeman
  triplet model using a single field strength, which works well if the
  field strength exceeds about 2 kG and the individual spectra are not
  affected by strong Doppler and/or Zeeman variations. The derived total
  field strengths may be compared with independent spectrograph-based
  measurements from NSO, Hinode/SP, and Mt. Wilson and with filter-based
  measurements from SoHO/MDI and SDO/HMI. NSO plans to implement this
  reduction for all suitable archived SOLIS spectra and to continue daily
  umbral field strength measurements as cycle 24 proceeds. Preliminary
  comparisons show good agreements in some cases and systematic
  differences in others.

Title: Helicity of Subsurface Flows and Magnetic Activity in the
    Photosphere
Authors: Komm, Rudolf; Gosain, S.; Pevtsov, A. A.
Bibcode: 2013shin.confE..43K
Altcode:
  Subsurface flows associated with active regions show generally
  large values of kinetic helicity density. The vertical component
  of kinetic helicity is defined as the product of the curl of the
  horizontal velocities and the vertical velocity component. It is
  thus the equivalent of current helicity determined from vector
  magnetograms. The vertical component of kinetic helicity follows
  on average the hemispheric rule established for current helicity
  with negative values in the northern hemisphere and positive values
  in the southern one. We analyze 11 years of GONG Dopplergrams and
  derive subsurface flows from the surface to a depth of 16 Mm with
  the ring-diagram technique. From these velocities, we calculate
  the kinetic helicity density and integrate it over selected depth
  ranges. We will study the kinetic helicity as a function of time and
  latitude for different levels of magnetic activity, such as active
  and quiet regions. We will present the latest results.

Title: First Synoptic Maps of Photospheric Vector Magnetic Field
from SOLIS/VSM: Non-Radial Magnetic Fields and Hemispheric Pattern
    of Helicity
Authors: Gusain, Sanjay; Pevtsov, A. A.; Rudenko, G. V.; Anfinogentov,
   S. A.
Bibcode: 2013shin.confE....G
Altcode:
  We use daily full-disk vector magnetograms from Vector
  Spectromagnetograph (VSM) on Solar Optical Long-term Investigations
  of the Sun (SOLIS) system to synthesize the first Carrington maps
  of the photospheric vector magnetic field. We describe these maps
  and make a comparison of observed radial field with the radial field
  estimate from LOS magnetograms. Further, we employ these maps to study
  the hemispheric pattern of current helicity density, Hc, during the
  rising phase of the solar cycle 24. Longitudinal average over the 23
  consecutive solar rotations shows a clear signature of the hemispheric
  helicity rule, i.e. Hc is predominantly negative in the North and
  positive in South. The hemispheric pattern for individual Carrington
  rotations is statistically weak, consistent with previous studies of
  active regions' helicity. Although our data include the early phase
  of cycle 24, there appears no evidence for a possible (systematic)
  reversal of the hemispheric helicity rule at the beginning of cycle as
  predicted by some dynamo models. Further, we compute the hemispheric
  pattern in active region latitudes (-30°-+30°) separately for weak
  (100&lt; |Br| &lt;500 G)and strong (|Br| &gt;1000 G) radial magnetic
  fields. We find that while the current helicity of strong fields
  follows the well-known hemispheric rule (i.e., ? &lt; 0), Hc of weak
  fields exhibits an inverse hemispheric behavior (i.e., ? &gt; 0)
  albeit with large statistical scatter.

Title: On Possible Variations of Basal Ca II K Chromospheric Line
    Profiles with the Solar Cycle
Authors: Pevtsov, Alexei A.; Bertello, Luca; Uitenbroek, Han
Bibcode: 2013ApJ...767...56P
Altcode:
  We use daily observations of the Ca II K line profiles of the
  Sun-as-a-star taken with the Integrated Sunlight Spectrometer from 2006
  December through 2011 July to deconvolve the contributions from the
  quiet (basal) chromosphere and with magnetic network/plage areas. The
  0.5 Å emission index computed from basal profiles shows a significantly
  reduced modulation (as compared with one derived from the observed
  profiles) corresponding to the Sun's rotation. For basal contribution
  of the Ca II K line, the peak in power spectrum corresponding to solar
  rotation is broad and not well defined. Power spectra for the plage
  contribution show two narrow well-defined peaks corresponding to solar
  rotation at two distinct latitudes, in agreement with the latitudinal
  distribution of activity on the Sun at the end of Cycle 23 and beginning
  of Cycle 24. We use the lack of a signature of solar rotation in the
  basal (quiet Sun) component as an indication of a successful removal
  of the active Sun (plage) component. Even though the contribution
  from solar activity is removed from the basal line profiles, we find
  a weak dependency of intensity in the line core (K3) of basal profiles
  with the phase of the solar cycle. Such dependency could be the result
  of changes in thermal properties of basal chromosphere with the solar
  cycle. As an alternative explanation, we also discuss a possibility that
  the basal component does not change with the phase of the solar cycle.

Title: Resolving Azimuth Ambiguity Using Vertical Nature of Solar
    Quiet-Sun Magnetic Fields
Authors: Gosain, S.; Pevtsov, A. A.
Bibcode: 2013SoPh..283..195G
Altcode: 2012arXiv1210.6691G; 2012SoPh..tmp..243G
  The measurement of solar magnetic fields using the Zeeman effect
  diagnostics has a fundamental 180° ambiguity in the determination
  of the azimuth angle of the transverse field component. There are
  several methods that are used in the community and each one has its
  merits and demerits. Here we present a disambiguation idea that is
  based on the assumption that most of the magnetic field on the sun
  is predominantly vertical. While the method is not applicable to
  penumbra or other features harboring predominantly horizontal fields
  like the sheared neutral lines, it is useful for regions where fields
  are predominantly vertical like network and plage areas. The method is
  tested with the full-disk solar vector magnetograms observed by the
  SOLIS/VSM instrument. We find that statistically about 60 - 85 % of
  the pixels in a typical full-disk magnetogram has a field inclination
  in the range of 0 - 30° with respect to the local solar normal, and
  thus can be successfully disambiguated by the proposed method. Due to
  its non-iterative nature, the present method is extremely fast and
  therefore can be used as a good initial guess for iterative schemes
  like the non-potential field computation (NPFC) method. Furthermore,
  the method is insensitive to noisy pixels as it does not depend upon
  the neighboring pixels or derivatives.

Title: Full-disk nonlinear force-free field extrapolation of SDO/HMI
    and SOLIS/VSM magnetograms
Authors: Tadesse, T.; Wiegelmann, T.; Inhester, B.; MacNeice, P.;
   Pevtsov, A.; Sun, X.
Bibcode: 2013A&A...550A..14T
Altcode: 2012arXiv1210.3668T
  Context. The magnetic field configuration is essential for understanding
  solar explosive phenomena, such as flares and coronal mass ejections. To
  overcome the unavailability of coronal magnetic field measurements,
  photospheric magnetic field vector data can be used to reconstruct the
  coronal field. Two complications of this approach are that the measured
  photospheric magnetic field is not force-free and that one has to apply
  a preprocessing routine to achieve boundary conditions suitable for the
  force-free modeling. Furthermore the nonlinear force-free extrapolation
  code should take uncertainties into account in the photospheric field
  data. They occur due to noise, incomplete inversions, or azimuth
  ambiguity-removing techniques. <BR /> Aims: Extrapolation codes in
  Cartesian geometry for modeling the magnetic field in the corona do not
  take the curvature of the Sun's surface into account and can only be
  applied to relatively small areas, e.g., a single active region. Here
  we apply a method for nonlinear force-free coronal magnetic field
  modeling and preprocessing of photospheric vector magnetograms in
  spherical geometry using the optimization procedure to full disk vector
  magnetograms. We compare the analysis of the photospheric magnetic
  field and subsequent force-free modeling based on full-disk vector
  maps from Helioseismic and Magnetic Imager (HMI) onboard the solar
  dynamics observatory (SDO) and Vector Spectromagnetograph (VSM) of the
  Synoptic Optical Long-term Investigations of the Sun (SOLIS). <BR />
  Methods: We used HMI and VSM photospheric magnetic field measurements
  to model the force-free coronal field above multiple solar active
  regions, assuming magnetic forces to dominate. We solved the nonlinear
  force-free field equations by minimizing a functional in spherical
  coordinates over a full disk and excluding the poles. After searching
  for the optimum modeling parameters for the particular data sets, we
  compared the resulting nonlinear force-free model fields. We compared
  quantities, such as the total magnetic energy content, free magnetic
  energy, the longitudinal distribution of the magnetic pressure,
  and surface electric current density, using our spherical geometry
  extrapolation code. <BR /> Results: The magnetic field lines obtained
  from nonlinear force-free extrapolation based on HMI and VSM data show
  good agreement. However, the nonlinear force-free extrapolation based
  on HMI data contain more total magnetic energy, free magnetic energy,
  the longitudinal distribution of the magnetic pressure, and surface
  electric current density than do the VSM data.

Title: Comparison of Ground-Based and Space-Based Longitudinal
    Magnetograms
Authors: Pietarila, A.; Bertello, L.; Harvey, J. W.; Pevtsov, A. A.
Bibcode: 2013SoPh..282...91P
Altcode: 2012arXiv1209.6390P; 2012SoPh..tmp..274P
  We compare photospheric line-of-sight magnetograms from the
  Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector
  Spectro-Magnetograph (VSM) instrument with observations from the
  150-foot Solar Tower at Mt. Wilson Observatory (MWO), the Helioseismic
  and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO),
  and the Michelson Doppler Imager (MDI) on the Solar and Heliospheric
  Observatory (SOHO). We find very good agreement between VSM and
  the other data sources for both disk-averaged flux densities and
  pixel-by-pixel measurements. We show that the VSM mean flux density
  time series is of consistently high signal-to-noise ratio with no
  significant zero offsets. We discuss in detail some of the factors -
  spatial resolution, flux dependence, and position on the solar disk -
  affecting the determination of scaling between VSM and SOHO/MDI or
  SDO/HMI magnetograms. The VSM flux densities agree well with spatially
  smoothed data from MDI and HMI, although the scaling factors show a
  clear dependence on flux density. The factor to convert VSM to HMI
  increases with increasing flux density (from ≈1 to ≈1.5). The
  nonlinearity is smaller for the VSM vs. SOHO/MDI scaling factor (from
  ≈1 to ≈1.2).

Title: Signature of Differential Rotation in Sun-as-a-star Ca II
    K Measurements
Authors: Bertello, L.; Pevtsov, A. A.; Pietarila, A.
Bibcode: 2012ApJ...761...11B
Altcode: 2012arXiv1210.5556B
  The characterization of solar surface differential rotation (SDR) from
  disk-integrated chromospheric measurements has important implications
  for the study of differential rotation and dynamo processes in other
  stars. Some chromospheric lines, such as Ca II K, are very sensitive
  to the presence of activity on the disk and are an ideal choice
  for investigating SDR in Sun-as-a-star observations. Past studies
  indicate that when the activity is low, the determination of Sun's
  differential rotation from integrated-sunlight measurements becomes
  uncertain. However, our study shows that using the proper technique, SDR
  can be detected from these type of measurements even during periods of
  extended solar minima. This paper describes results from the analysis
  of the temporal variations of Ca II K line profiles observed by the
  Integrated Sunlight Spectrometer during the declining phase of Cycle 23
  and the rising phase of Cycle 24, and discusses the signature of SDR
  in the power spectra computed from time series of parameters derived
  from these profiles. The methodology described is quite general, and
  could be applied to photometric time series of other main-sequence
  stars for detecting differential rotation.

Title: The Build-Up to Eruptive Solar Events Viewed as the Development
    of Chiral Systems
Authors: Martin, S. F.; Panasenco, O.; Berger, M. A.; Engvold, O.;
   Lin, Y.; Pevtsov, A. A.; Srivastava, N.
Bibcode: 2012ASPC..463..157M
Altcode: 2012arXiv1212.3646M
  When we examine the chirality or observed handedness of the
  chromospheric and coronal structures involved in the long-term build-up
  to eruptive events, we find that they evolve in very specific ways to
  form two and only two sets of large-scale chiral systems. Each system
  contains spatially separated components with both signs of chirality,
  the upper portion having negative (positive) chirality and the lower
  part possessing positive (negative) chirality. The components within
  a system are a filament channel (represented partially by sets of
  chromospheric fibrils), a filament (if present), a filament cavity,
  sometimes a sigmoid, and always an overlying arcade of coronal
  loops. When we view these components as parts of large-scale chiral
  systems, we more clearly see that it is not the individual components
  of chiral systems that erupt but rather it is the approximate upper
  parts of an entire evolving chiral system that erupts. We illustrate the
  typical pattern of build-up to eruptive solar events first without and
  then including the chirality in each stage of the build-up. We argue
  that a complete chiral system has one sign of handedness above the
  filament spine and the opposite handedness in the barbs and filament
  channel below the filament spine. If the spine has handedness, the
  observations favor its having the handedness of the filament cavity and
  coronal loops above. As the separate components of a chiral system form,
  we show that the system appears to maintain a balance of right-handed
  and left-handed features, thus preserving an initial near-zero net
  helicity. We further argue that the chiral systems allow us to identify
  key sites of energy transformation and stored energy later dissipated in
  the form of concurrent CMEs, erupting filaments and solar flares. Each
  individual chiral system may produce many successive eruptive events
  above a single filament channel. Because major eruptive events
  apparently do not occur independent of, or outside of, these unique
  chiral systems, we hypothesize that the development of chiral systems:
  (1) are fundamental to the occurrence of eruptive solar events and (2)
  preserve an approximate balance between positive and negative helicity
  (right and left-handed chirality) while preparing to release energy
  in the form of CMEs, erupting filaments, and flares.

Title: Coronal Magnetic Field Structure and Evolution for Flaring
    AR 11117 and Its Surroundings
Authors: Tadesse, Tilaye; Wiegelmann, T.; Inhester, B.; Pevtsov, A.
Bibcode: 2012SoPh..281...53T
Altcode: 2011arXiv1109.4889T; 2012SoPh..tmp...60T
  In this study, photospheric vector magnetograms obtained with the
  Synoptic Optical Long-term Investigations of the Sun (SOLIS) survey
  are used as boundary conditions to model three-dimensional nonlinear
  force-free (NLFF) coronal magnetic fields as a sequence of NLFF
  equilibria in spherical geometry. We study the coronal magnetic field
  structure inside an active region and its temporal evolution. We compare
  the magnetic field configuration obtained from NLFF extrapolation
  before and after the flaring event in active region (AR) 11117 and
  its surroundings observed on 27 October 2010, and we also compare
  the magnetic field topologies and the magnetic energy densities and
  study the connectivities between AR 11117 and its surroundings. During
  the investigated time period, we estimate the change in free magnetic
  energy from before to after the flare to be 1.74×10<SUP>32</SUP> erg,
  which represents about 13.5 % of the NLFF magnetic energy before the
  flare. In this study, we find that electric currents from AR 11117 to
  its surroundings were disrupted after the flare.

Title: On a Possible Explanation of the Long-term Decrease in Sunspot
    Field Strength
Authors: Nagovitsyn, Yury A.; Pevtsov, Alexei A.; Livingston,
   William C.
Bibcode: 2012ApJ...758L..20N
Altcode:
  Recent studies revealed a controversy in long-term variations in sunspot
  field strengths. On one hand, the sunspot field strengths computed
  by averaging both large and small sunspots and pores show a gradual
  decrease over the declining phase of solar Cycle 23 and the rising
  phase of Cycle 24. On the other hand, the strongest sunspot field
  strengths demonstrate only solar cycle variations with no long-term
  decline. Here, we investigate the field strength and area properties of
  sunspots in an attempt to reconcile the presence of both tendencies in
  recent sunspot field strength measurements. First, we analyze the data
  set from Penn &amp; Livingston, and we show that in addition to the
  previously reported long-term decline, the data show the solar cycle
  variation when only sunspots with the strongest magnetic fields are
  included. Next, we investigate the variations in the number of sunspots
  of different sizes, and we find a negative correlation between the
  numbers of small and large sunspots. Finally, we show that during the
  period of 1998-2011, the number of large sunspots gradually decreased,
  while the number of small sunspots steadily increased. We suggest that
  this change in the fraction of small and large sunspots (perhaps,
  due to changes in the solar dynamo) can explain the gradual decline
  in average sunspot field strength as observed by Penn &amp; Livingston.

Title: The role of helicity in solar magnetic fields
Authors: Pevtsov, Alexei
Bibcode: 2012cosp...39.1493P
Altcode: 2012cosp.meet.1493P
  As the famous saying goes, the sun would be a boring star without
  its magnetic field. Equally, the sun's magnetic field would be
  a boring "element" without its topological complexity (helicity)
  and dynamics. Helicity protagonist would argue that it plays an
  important role in many processes that define creation, evolution,
  and disappearance of magnetic fields. The productivity of solar
  dynamo depends on kinetic helicity of its flows, but significant
  accumulation of magnetic helicity may also have a negative feed-back
  on dynamo. Propagation of magnetic fields through the convection
  zone and solar atmosphere may be affected by their helicity. Pre-
  and post-flare dynamics of magnetic fields and flows may reflect
  the transport of helicity through solar atmosphere, while helicity
  scale-separation could reveal itself in the interplanetary fields. But
  the skeptic might say that, perhaps, the role of helicity is overrated,
  and one does not need it at all? In this talk, I will provide a critical
  review of helicity concept and its role in solar magnetic fields.

Title: Helicity patterns on the sun.
Authors: Pevtsov, Alexei
Bibcode: 2012cosp...39.1494P
Altcode: 2012cosp.meet.1494P
  Helicity on the Sun comes on different scales and patterns. But what
  do these patterns tell us about the interior and outer layers of
  our home star? Why do we see a global (hemispheric) and large scale
  (complexes of activity) organization of helicity and what does it
  mean? Are there non-random patterns of helicity on small (granulation)
  scale? How helicity changes with solar cycle, and how it flows through
  solar atmosphere and heliosphere? In this talk, I will review the
  current state of helicity studies and discuss the questions posed above.

Title: Active regions with superpenumbral whirls and their subsurface
    flow vorticity
Authors: Komm, Rudolf W.; Gosain, S.; Pevtsov, A.
Bibcode: 2012shin.confE.119K
Altcode:
  We search for a signature of helicity flow from the solar interior to
  the photosphere and chromosphere. We study two active regions NOAA
  11084 and NOAA 11092 that show a regular pattern of superpenumbral
  whirls in H-alpha. The pattern persists throughout the disk passage
  of both regions. We use photospheric vector magnetograms from
  SOLIS/VSM to determine two helicity proxies: vertical component of
  the current helicity density (Hc_z=Jz.Bz) and the mean twist parameter
  (alpha_z=&lt;Jz/Bz&gt;), and to study their evolution. We compare the
  two proxies of magnetic helicity with the properties of the subsurface
  flows below the active regions. For this purpose, we analyze subsurface
  flows measured with a ring-diagram analysis of GONG high-resolution
  Doppler data and derive their vorticity. As a control experiment,
  we study the subsurface flows of six active regions that do not show
  a regular whirl pattern in the chromosphere.

Title: Magnetic Helicity of Solar Active Regions as Revealed by
    Vector Magnetograms and Coronal X-Ray Images
Authors: Xu, Haiqing; Gao, Yu; Zhang, Hongqi; Sakurai, Takashi;
   Hagino, Masaoki; Sokoloff, Dmitry; Pevtsov, Alexei A.
Bibcode: 2012PASJ...64...54X
Altcode:
  We have used photospheric vector magnetograms of 15 different
  solar active regions to calculate the current helicity parameter,
  α<SUB>av</SUB>, and the linear force-free field (LFFF) parameter,
  α<SUB>best</SUB>, that fits best the observed transverse field. The
  data were obtained with the Solar Magnetic Field Telescope at the
  Huairou Solar Observing Station, the National Astronomical Observatories
  of China, the Solar Flare Telescope of the National Astronomical
  Observatory of Japan, and the Haleakala Stokes Polarimeter at the
  Mees Solar Observatory, University of Hawaii, from 1997 to 2000. The
  agreement in sign of α<SUB>av</SUB> between three vector magnetographs
  is better than 90%. For α<SUB>best</SUB>, the agreement is 80%-90%. The
  line-of-sight magnetograms observed with the Michelson-Doppler Imager
  (MDI) on SOHO and coronal X-ray images observed with the Soft X-ray
  Telescope (SXT) on Yohkoh have been used to determine the constant
  α<SUB>c</SUB> of the LFFF in the corona. The value of α<SUB>c</SUB>
  corresponds to the extrapolated coronal field whose field lines best
  match, by visual inspection, the structure of coronal loops in X-ray
  images. It is found that the sign agreement between photospheric
  α<SUB>av</SUB> or α<SUB>best</SUB> and coronal α<SUB>c</SUB>
  is lower (60%-85%). We consider the differences in measurements,
  observing conditions, data reduction methods, and limitation in LFFF
  extrapolation, and discuss their contributions to the dispersions in
  the hemispheric sign rule of helicity.

Title: Detection of Solar Differential Rotation in Disk-Integrated
    Ca II K Measurements
Authors: Bertello, Luca; Pietarila, A.; Pevtsov, A. A.
Bibcode: 2012AAS...22020311B
Altcode:
  The characterization of solar differential rotation (SDR) from
  disk-integrated chromospheric measurements has important implications
  for the study of differential rotation and dynamo processes in other
  stars. Chromospheric lines, such as Ca II K, are very sensitive
  to the presence of activity on the disk and are an ideal choice for
  investigating SDR in Sun-as-a star <P />obervations. Here we use daily
  observations from the SOLIS Integrated Sunlight Spectrometer (ISS) to
  study the temporal variations of the Ca II K line profiles from 2006
  to 2012. <P />We discuss the signature of SDR in the power spectra
  computed from time series of parameters derived from these profiles,
  and the implications for detecting differential rotation in other
  Main-Sequence stars.

Title: Abrupt Changes in the Photospheric Magnetic Structures and
    H-alpha Chromosphere during the 2006 December 6 X6.5 Flare
Authors: Petrie, Gordon; Balasubramaniam, K. S.; Burtseva, O.; Pevtsov,
   A. A.
Bibcode: 2012AAS...22020406P
Altcode:
  The active region NOAA 10930 produced the last X-class flares of Solar
  Cycle 23, including one at 18:29 UT on December 6. Here we investigate
  the relationships between signatures of this flare observed in the
  photospheric magnetic field and white light and continuum intensity, and
  H-alpha chromosphere. We use GONG 1-minute magnetograms and continuum
  intensity images and (1-minute?) ISOON white-light and H-alpha images to
  show that (1) the sunspot penumbral area and mean intensity decreased
  abruptly during the flare; (2) the magnetic field changed in large,
  contiguous patterns inside and immediately outside the southern and
  western penumbra resulting in a more vertical penumbral field on
  average; (3) H-alpha brightenings were observed to begin around the
  GOES start time, the earliest occurring near the sunspot and later
  brightenings progressively further north and south of the center of
  the active region near magnetic neutral lines; and (4) most of the
  detected photospheric flux cancellation (75%) occurred during the flare,
  and the remainder before the flare.

Title: Ca II K And H Spectral Line Profiles From “Basal” And
    “Magnetic” Chromospheres
Authors: Pevtsov, Alexei A.; Bertello, L.
Bibcode: 2012AAS...22020309P
Altcode:
  We use observations from the SOLIS’ Integrated Sunlight Spectrometer
  (ISS) to investigate properties of the Ca II K and H disk-integrated
  spectral line profiles. The ISS daily observations cover the period
  of declining phase of cycle 23 and rising phase of cycle 24. The
  contributions of “basal” and “magnetic” chromospheres were
  extracted from the observed profiles in order to investigate their
  solar-cycle dependency. Our analysis suggests that the properties of
  the "basal" chromosphere may change slightly with the cycle of activity.

Title: Solar Cycle Variations of Sunspot Magnetic Field Strengths
    from the Mount Wilson Observatory
Authors: Pevtsov, Alexei A.; Bertello, L.; Tlatov, A.; Nagovitsyn,
   Y.; Kilcik, A.
Bibcode: 2012AAS...22011003P
Altcode:
  We used historical synoptic data the Mount Wilson Observatory (MWO)
  to study long-term changes in sunspot magnetic field strengths over
  the period of 1920-1959. By selecting sunspots with the strongest
  field strength for each observing day, we find that the average
  field strengths in sunspots vary with solar cycle with amplitude
  of a few hundred Gauss. The data show no statistically significant
  long-term trend over the period of about 40 years covered by these
  observations. We also find that the fractional distribution of sunspots
  changes from cycle to cycle. From Cycle 15 to Cycle 19, MWO data show
  a steady increase in fraction of sunspots with weaker field strengths
  (&lt;1000 G), while the fraction of sunspots with strongest field
  strengths (&gt;3000 G) steadily decreases. The fraction of sunspots
  with field strengths between 1000-3000 Gauss does not change in any
  systematic way. In contract, the fractional distribution of sunspots
  by their area (i.e., small, intermediate, and large) taken from the
  Greenwich observatory data set does not change during the same period
  of time. The different behavior in these two fractional distributions
  might indicate some physical changes in the properties of sunspots
  (e.g., sunspots of about the same area show progressively smaller
  field strengths), or it could be the result of some systematic
  instrumental/observational effects. We discuss our findings in the
  framework of these two possible explanations.

Title: Coronal Mass Ejections from Magnetic Systems Encompassing
    Filament Channels Without Filaments
Authors: Pevtsov, Alexei A.; Panasenco, Olga; Martin, Sara F.
Bibcode: 2012SoPh..277..185P
Altcode:
  Well-developed filament channels may be present in the solar atmosphere
  even when there is no trace of filament material inside them. Such
  magnetic systems with filament channels without filaments can result
  in coronal mass ejections that might appear to have no corresponding
  solar surface source regions. In this case study, we analyze CMEs on
  9 August 2001 and 3 March 2011 and trace their origins to magnetic
  systems with filament channels containing no obvious filament material
  on the days around the eruptions.

Title: Magnetic Connectivity Between Active Regions 10987, 10988,
    and 10989 by Means of Nonlinear Force-Free Field Extrapolation
Authors: Tadesse, Tilaye; Wiegelmann, T.; Inhester, B.; Pevtsov, A.
Bibcode: 2012SoPh..277..119T
Altcode: 2011SoPh..tmp..167T; 2011arXiv1104.2246T; 2011SoPh..tmp..236T;
   2011SoPh..tmp..102T
  Extrapolation codes for modelling the magnetic field in the corona
  in Cartesian geometry do not take the curvature of the Sun's surface
  into account and can only be applied to relatively small areas, e.g.,
  a single active region. We apply a method for nonlinear force-free
  coronal magnetic field modelling of photospheric vector magnetograms in
  spherical geometry which allows us to study the connectivity between
  multi-active regions. We use Vector Spectromagnetograph (VSM) data
  from the Synoptic Optical Long-term Investigations of the Sun (SOLIS)
  survey to model the coronal magnetic field, where we study three
  neighbouring magnetically connected active regions (ARs 10987, 10988,
  10989) observed on 28, 29, and 30 March 2008, respectively. We compare
  the magnetic field topologies and the magnetic energy densities and
  study the connectivities between the active regions. We have studied
  the time evolution of the magnetic field over the period of three
  days and found no major changes in topologies, as there was no major
  eruption event. From this study we have concluded that active regions
  are much more connected magnetically than the electric current.

Title: Complex Magnetic Evolution and Magnetic Helicity in the
    Solar Atmosphere
Authors: Pevtsov, Alexei A.
Bibcode: 2012ASSP...30...83P
Altcode: 2012snc..book...83P; 2013arXiv1302.0414P
  Solar atmosphere is a single system unified by the presence of
  large-scale magnetic fields. Topological changes in magnetic fields
  that occur in one place may have consequences for coronal heating
  and eruptions for other, even remote locations. Coronal magnetic
  fields also play role in transport of magnetic helicity from Sun's
  subphotosphere/upper convection zone to the interplanetary space. We
  discuss observational evidence pertinent to some aspects of the solar
  corona being a global interconnected system, i.e., large-scale coronal
  heating due to new flux emergence, eruption of chromospheric filament
  resulting from changes in magnetic topology triggered by new flux
  emergence, sunspots rotation as manifestation of transport of helicity
  through the photosphere, and potential consequences of re-distribution
  of energy from solar luminosity to the dynamo for solar cycle variations
  of solar irradiance.

Title: Pulsating B Stars observed by Kepler
Authors: McNamara, Bernard J.; Jackiewicz, J.; McKeever, J.; McAteer,
   J.; Boucheron, L.; Cao, H.; Voelz, D.; DeGrave, K.; Kirk, M.; Taylor,
   G.; Al-Ghraibah, A.; Pevtsov, A.; Calabro, B.; Hao, Y.
Bibcode: 2012AAS...21934804M
Altcode:
  In a prior investigation using Kepler data, Balona et al. (2011) found
  that the measured frequency spectra of B stars differed from those
  obtained using ground-based observations. The low amplitude frequency
  spectra of these stars are quantified by analyzing a larger number
  of B stars. These stars are then categorized using the designations
  adopted in the Balona et al. study. Most of the frequencies we
  measure are too small to be detected using conventional ground-based
  observations. They are probably associated with higher order spherical
  harmonic modes. A plan to determine the l values of the larger amplitude
  B star frequencies is presented.

Title: Detecting Flows, Waves and Nanoflares in the Solar Corona
Authors: Calabro, Brandon; McAteer, J.; Pevtsov, A.
Bibcode: 2012AAS...21914403C
Altcode:
  Oscillations in the solar corona are studied using data from Solar
  Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA). We use
  a wavelet analysis to search for oscillatory signals in 3D datacubes
  (two spatial dimensions, one temporal dimension) in each of the extreme
  ultraviolet passbands of AIA, and apply a pixel-grouping algorithm to
  enable us to study coherent patches of the solar corona. By looking
  at regions of the corona that oscillate at similar periodicity we
  are able to interpret the spatial behavior of oscillations through
  different heights in the solar corona and transversely across the
  corona. We propose a method to identify and extract flows, waves,
  and nanoflares and determine the contribution of each of these in
  heating coronal plasma.

Title: Automated Classification of Flaring Behavior in Solar Active
Regions: Preliminary Results
Authors: Al-Ghraibah, Amani; Boucheron, L. E.; McAteer, R.; Cao, H.;
   Jackiewicz, J.; McNamara, B.; Voelz, D.; Calabro, B.; DeGrave, K.;
   Hao, Y.; Kirk, M.; Pevtsov, A.; Mckeever, J.; Taylor, G.
Bibcode: 2012AAS...21914516A
Altcode:
  Solar active events are the source of many energetic and
  geo-effective events such as solar flares and coronal mass ejections
  (CMEs). Understanding how these complex source regions evolve and
  produce these events is of fundamental importance, not only to solar
  physics but also the demands of space weather forecasting. In this
  poster, we present preliminary results from our analysis of the physical
  properties of active region magnetic fields using fractal-, gradient-,
  neutral line-, emerging flux-, and wavelet-based techniques. These
  analyses look to use the defined physical measures to form a predictive
  model for flaring behavior in active regions.

Title: Intelligent Search of Solar Data
Authors: Hao, Yifan; Cao, H.; McNamara, B.; Jackiewicz, J.; McAteer,
   J.; Boucheron, L.; Voelz, D.; Kirk, M.; Taylor, G.; DeGrave, K.;
   Al-Ghraibah, A.; Pevtsov, A.; Calabro, B.
Bibcode: 2012AAS...21914411H
Altcode:
  The enormous amount of solar data, a result of new observational
  missions, needs to be stored and retrieved intelligently. Existing
  systems (e.g., the Science Archive of the Solar and Heliospheric
  Observatory (SOHO); soho.nascom.nasa.go) manage solar data merely
  using metadata (e.g., the time that images were taken) or other
  programmatic information. As a result, such systems can only support
  very primitive queries (e.g., images taken in June 2009). From such
  search results, scientists have to manually select their needed data
  for further analysis. <P />On the other hand, solar data, either raw or
  processed, are often associated with semantic information such as the
  active regions in an image and corresponding text annotations. Such
  semantic knowledge can provide much more insights to the data and
  can help scientists quickly find data that are related to a specific
  research goal or topic (e.g., solar flares or coronal mass ejections). A
  solar data management system should be able to intelligently utilize
  such relevant semantic data to facilitate solar data retrieval,
  and ultimately saves investigators valuable time. <P />In this work,
  we build a prototype for the intelligent retrieval and exploration
  of solar data by utilizing the semantic knowledge associated with
  raw or processed solar data. The core of this prototype is a "query
  processing” component that utilizes a unified index technique to
  support both simple basic queries (e.g., images taken in 2007) over the
  metadata and intelligent topic queries (e.g., image regions related
  to "Solar Flares” in 2007) over other complicated contents. This
  component leverages different types of data (especially semantic
  information) to improve the search accuracy. Besides this, another
  facilitating component is designed to provide user-friendly result
  exploration functionalities when the "query processing” component
  returns a set of query results.

Title: Coronal Loop Detection and Seismology
Authors: Pevtsov, Alexander; McAteer, R. T. J.; Jackiewicz, J.;
   McNamara, B.; Kirk, M.; Degrave, K.; Boucheron, L.; Calabro, B.
Bibcode: 2012AAS...21914405P
Altcode:
  We study the spatial distribution and temporal evolution of coronal
  loops using data from the Solar Dynamics Observatory (SDO) Atmospheric
  Imaging Assembly (AIA). We apply an automated coronal loop detection
  algorithm that is maximized for accuracy and completeness, and
  reconnects orphaned segments of coronal loops, to extreme ultraviolet
  images of the solar corona. We quantify the loop size distribution
  with a scaling index in each of the SDO AIA passbands, and show how
  this changes with time. This provides new insights into the physical
  mechanisms that create coronal structure.

Title: Detecting the Large Scale Magnetic Helicity Patterns on the
    Sun using SOLIS/VSM and SDO/HMI fulldisk vector magnetograms
Authors: Gosain, S.; Pevtsov, A. A.; Rudenko, G. V.; Anfinogentov,
   S. A.
Bibcode: 2011AGUFMSH31A1996G
Altcode:
  We use fulldisk vector magnetograms observed by the SOLIS/VSM and
  SDO/HMI instruments to compute the large scale helicity patterns on the
  sun. Such studies have been carried out in the past using the vector
  magnetograms reconstructed from temporal sequence of longitudinal
  magnetograms. These earlier results suggest the presence of a
  hemispheric pattern in the sign of current helicity density. However,
  these reconstructed vector magnetograms have their limitations as
  they are based on certain assumptions. On the other hand, the direct
  observations of the vector magnetic field of the full sun by SOLIS/VSM
  and SDO/HMI instruments can now be used to investigate the large scale
  magnetic helicity patterns on the sun. We use the newly developed
  SFQ azimuth disambiguation method in our study. This method has been
  shown to work better for fulldisk vector magnetograms as it solves
  the ambiguity in spherical geometry. We present the first results of
  current helicity computation for full disk vector magnetograms and
  compare the inferences from two instruments.

Title: Long-term Trends in Sunspot Magnetic Fields
Authors: Pevtsov, Alexei A.; Nagovitsyn, Yury A.; Tlatov, Andrey G.;
   Rybak, Alexey L.
Bibcode: 2011ApJ...742L..36P
Altcode:
  Recent studies indicate that a maximum field strength in sunspots
  shows a gradual decrease over the last several years. By extrapolating
  this trend, Penn &amp; Livingston proposed that sunspots may completely
  disappear in the not-so-distant future. To verify these recent findings,
  we employ historic synoptic data sets from seven observatories in the
  former USSR covering the period from 1957 to 2011 (from 1998 to 2011,
  observations were taken at only one observatory). Our results indicate
  that while sunspot field strengths rise and wane with solar cycle,
  there is not a long-term trend that would suggest a gradual decrease in
  sunspot magnetic fields over the four and a half solar cycles covered
  by these observations.

Title: Investigation of Some Problems of Moreton Wave
Authors: Pevtsov, A. A.; Mamedov, S. H.; Dzhalilov, N. S.; Gulu-zadeh,
   J. M.; Mustafa, F. R.
Bibcode: 2011AzAJ....6d..22P
Altcode:
  In the paper we study the some problems of physics of Moreton
  wave. Two possible variants of explanation were considered for the
  observed pattern of this wave in the Hα-line: 1) using cloud model
  of the wave front, located in the upper chromosphere and committing a
  radial movement up and down, and 2) by shifting the entire absorption
  line Hα. It was shown that for any values of the optical parameters
  of clouds, namely: S- source function, τ-optical thickness, Δλd
  - Doppler width and Doppler shift Δλsh of the cloud within the
  absorption line of Hα (due to radial motions of the cloud- the wave
  front) is not possible to obtain the observed brightness curve of
  the front within Hα- absorption line. It is shown that the observed
  wave-front image can only and only by shifting the entire absorption
  line Nα. Based on this, it was concluded that the Moreton waves
  propagate in the formation lay of absorption lines Hα, in other words,
  in the photosphere and lower chromosphere. It is shown that the Moreton
  wave is not observed in the upper chromosphere, which also confirms
  the conclusion given above. Further, it is shown that this wave can
  not propagate in the corona, since the cooling time of the coronal
  gas to a temperature of 10 000 K an order of magnitude greater than
  the wave period.

Title: The Disappearing Solar Filament of 2003 June 11: A Three-body
    Problem
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Cliver, E. W.;
   Martin, S. F.; Panasenco, O.
Bibcode: 2011ApJ...743..202B
Altcode:
  The eruption of a large quiescent filament on 2003 June 11 was preceded
  by the birth of a nearby active region—a common scenario. In this
  case, however, the filament lay near a pre-existing active region
  and the new active region did not destabilize the filament by direct
  magnetic connection. Instead it appears to have done so indirectly
  via magnetic coupling with the established region. Restructuring
  between the perturbed fields of the old region and the filament
  then weakened the arcade overlying the midpoint of filament, where
  the eruption originated. The inferred rate (~11° day<SUP>-1</SUP>)
  at which the magnetic disturbance propagates from the mature region
  to destabilize the filament is larger than the mean speed (~5º-6°
  day<SUP>-1</SUP>) but still within the scatter obtained for Bruzek's
  empirical relationship between the distance from a newly formed
  active region to a quiescent filament and the time from active region
  appearance to filament disappearance. The higher propagation speed in
  the 2003 June 11 case may be due to the "broadside" (versus ''end-on")
  angle of attack of the (effective) new flux to the coronal magnetic
  fields overlying a central section of the axis of the filament.

Title: The Longitudinal Solar Magnetic Field Measured by SOLIS
Authors: Bertello, L.; Pevtsov, A. A.; Harvey, J. W.; Pietarila, A.
Bibcode: 2011AGUFMSH13B1934B
Altcode:
  The SOLIS (Synoptic Optical Long-term Investigations of the Sun) Vector
  SpectroMagnetograph (VSM) instrument operating at the National Solar
  Observatory at Kitt Peak (Arizona) measures the full-disk longitudinal
  solar magnetic field in the neutral iron spectral lines at 630.15-630.25
  nm (photosphere) and ionized calcium spectral line at 854.2 nm (lower
  chromosphere). Over the past two years a considerable effort has
  been made to improve the quality of the computed magnetograms. Major
  modifications to the original data reduction pipeline include a
  new determination of the instrumental magnetic bias and an improved
  calibration of the computed solar magnetic flux density. As a result of
  these changes SOLIS-VSM is now able to provide, among other products,
  more reliable values of the full-disk mean magnetic flux measured in
  the photosphere and low chromosphere. In addition, these improvements
  open the possibility to extend the current SOLIS-VSM catalog with new
  products of interest to the solar and heliospheric communities. We
  describe some of these changes and their impact on the quality of the
  derived SOLIS-VSM data.

Title: Properties of Magnetic Neutral Lines and Filament Formation
Authors: Karachik, N.; Pevtsov, A. A.
Bibcode: 2011AGUFMSH31A1988K
Altcode:
  Chromospheric filaments form at a boundary (neutral lines) between
  opposite polarity magnetic fields. Both the total length of neutral
  lines and the number of filaments vary with solar cycle. However, our
  analysis indicates that the cycle variation in filaments is not due
  to the change in the total length of neutral lines. It is likely due
  to change in magnetic properties of neutral lines. In present study
  we compare properties of magnetic polarity inversion lines that have
  filaments above them and those without filaments, and investigate how
  these properties change during the solar cycle. Our results show that
  the gradient as well as convergence/divergence of magnetic field across
  neutral lines are not the major factors that determine the places of
  filament formation. We discuss the role of canceling magnetic features
  in filament formation and their cycle variation.

Title: 3- and 5- Minute Oscillatory Behavior in the Solar Corona
Authors: Calabro, Brandon; McAteer, James; Pevtsov, Alexander
Bibcode: 2011APS..4CF.F1020C
Altcode:
  We study the spatially- and temporally-localized oscillatory behavior
  of the solar corona using a 6-hour sequence of narrowband 171A (extreme
  ultraviolet) image from the SWAP instrument onboard Proba2. We use a
  Morlet wavelet transform to extract oscillation parameters from the
  temporal evolution of emission in each pixel and study the variation
  in space and time of oscillatory power in the 3- and 5-minute band. We
  extract and compare these parameters between active Sun, quiet Sun and
  coronal hole regions. In each region of the corona studied the 5-minute
  periodicity is more prevalent than the 3-minute periodicity by a factor
  of 2--3. All areas of the corona exhibit a similar temporal behavior in
  the 5-minute band, suggesting a global driving mechanism. However, the
  dominance of the 5-minute periodicity is stronger in active regions than
  in other areas of the Sun. The 3-minute periodicity in active regions
  tends to be localized in the sunspot umbra, whereas the 5-minute is
  more prevalent in the penumbra.

Title: Ground-based synoptic instrumentation for solar observations
Authors: Balasubramaniam, K. S.; Pevtsov, Alexei
Bibcode: 2011SPIE.8148E..09B
Altcode: 2011SPIE.8148E...8B
  We will describe the status of current ground-based solar spectroscopic
  and imaging instruments used in solar observations. We will describe the
  advantages and disadvantages of using these two classes of instruments
  with examples drawn from the Improved Solar Optical Observing Network
  (ISOON) and Synoptic Long Term Investigations of the Sun (SOLIS)
  Network. Besides instrumental requirements and lessons learned
  from existing ground-based instruments, this talk will also focus
  on the future needs and requirements of ground-based solar optical
  observations.

Title: Subsurface kinetic helicity of flows near active regions
Authors: Komm, R.; Jain, K.; Petrie, G.; Pevtsov, A.; González
   Hernández I.; Hill, F.
Bibcode: 2011sdmi.confE..68K
Altcode:
  We study the temporal variation of subsurface flows associated with
  emerging and decaying active regions on the Sun. We measure the
  subsurface flows analyzing GONG high-resolution Doppler data with
  ring-diagram analysis. We can detect the emergence of magnetic flux
  in these flows when averaging over a sufficiently large sample. In a
  previous study, we have found that emerging flux has a faster rotation
  than the ambient fluid and pushes it up, as indicated by enhanced
  vertical velocity and faster-than-average zonal flow. Here, we show
  that the kinetic helicity density of subsurface flows increases when
  new flux emerges and decreases when flux decays.

Title: Improvements in the Determination of ISS Ca II K Parameters
Authors: Bertello, L.; Pevtsov, A. A.; Harvey, J. W.; Toussaint, R. M.
Bibcode: 2011SoPh..272..229B
Altcode: 2011arXiv1106.4265B
  Measurements of the ionized Ca II K line are one of the major resources
  for long-term studies of solar and stellar activity. They also play a
  critical role in many studies related to solar irradiance variability,
  particularly as a ground-based proxy to model the solar ultraviolet flux
  variation that may influence the Earth's climate. Full disk images of
  the Sun in Ca II K have been available from various observatories for
  more than 100 years and latter synoptic Sun-as-a-star observations
  in Ca II K began in the early 1970s. One of these instruments, the
  Integrated Sunlight Spectrometer (ISS) has been in operation at Kitt
  Peak (Arizona) since late 2006. The ISS takes daily observations
  of solar spectra in nine spectra bands, including the Ca II K and H
  lines. We describe recent improvements in data reduction of Ca II K
  observations, and present time variations of nine parameters derived
  from the profile of this spectral line.

Title: Two types of coronal bright points their characteristics,
    and evolution
Authors: Sattarov, Isroil; Karachik, Nina V.; Sherdanov, Chori T.;
   Tillaboev, Azlarxon M.; Pevtsov, Alexei A.
Bibcode: 2011IAUS..273..343S
Altcode:
  Using maximum brightness of coronal bright point's (CBP) as a criterion,
  we separate them on two categories: dim CBPs, associated with areas of
  a quiet Sun, and bright CBPs, associated with an active Sun. This study
  reports on characteristics of two types of CBPs and their evolution.

Title: Solar Wind and Coronal Bright Points inside Coronal Holes
Authors: Karachik, Nina V.; Pevtsov, Alexei A.
Bibcode: 2011ApJ...735...47K
Altcode:
  Observations of 108 coronal holes (CHs) from 1998-2008 were used to
  investigate the correlation between fast solar wind (SW) and several
  parameters of CHs. Our main goal was to establish the association
  between coronal bright points (CBPs; as sites of magnetic reconnection)
  and fast SW. Using in situ measurements of the SW, we have connected
  streams of the fast SW at 1 AU with their source regions, CHs. We
  studied a correlation between the SW speed and selected parameters of
  CHs: total area of the CH, total intensity inside the CH, fraction of
  area of the CH associated with CBPs, and their integrated brightness
  inside each CH. In agreement with previous studies, we found that the
  SW speed most strongly correlates with the total area of the CHs. The
  correlation is stronger for the non (de)projected areas of CHs (which
  are measured in image plane) suggesting that the near-equatorial parts
  of CHs make a larger contribution to the SW measured at near Earth
  orbit. This correlation varies with solar activity. It peaks for periods
  of moderate activity, but decreases slightly for higher or lower levels
  of activity. A weaker correlation between the SW speed and other studied
  parameters was found, but it can be explained by correlating these
  parameters with the CH's area. We also studied the spatial distribution
  of CBPs inside 10 CHs. We found that the density of CBPs is higher
  in the inner part of CHs. As such, results suggest that although the
  reconnection processes occurring in CBPs may contribute to the fast SW,
  they do not serve as the main mechanism of wind acceleration.

Title: Subsurface kinetic helicity of flows near active regions
Authors: Komm, Rudolf; Jain, K.; Petrie, G.; Pevtsov, A.; González
   Hernández, I.; Hill, F.
Bibcode: 2011shin.confE.142K
Altcode:
  We study the flows in the upper solar convection zone determined from
  GONG data using the standard dense-pack ring-diagram analysis and derive
  daily and synoptic maps of the velocity components. We also calculate
  the vorticity and the kinetic helicity density of the flows. Previous
  studies have shown that the vorticity is enhanced near locations
  of active regions and that the kinetic helicity density associated
  with active regions correlates well with the X-ray flare intensity of
  active regions. These fluid dynamics descriptors are thus promising
  indicators for investigating the relation between active regions and
  associated subsurface flows. Here, we focus on the temporal evolution of
  subsurface kinetic helicity density during flux emergence and decay. We
  will present the latest results.

Title: The Pulsation Spectra of Kepler B Stars
Authors: McNamara, Bernard J.; Jackiewicz, J.; McAteer, J.; Boucheron,
   L.; Cao, H.; Voelz, D.; Kirk, M.; Taylor, G.; DeGrave, K.; Al-Ghraibah,
   A.; Lovekin, C.; Pevtsov, A.
Bibcode: 2011AAS...21840717M
Altcode: 2011BAAS..43G40717M
  The Kepler satellite was designed to obtain single filter light curves
  of over 100,000 stars during its 3.5 year mission lifetime. The accuracy
  of each 30 minute measurement is about 20 parts per million for a 12
  magnitude star. Although the primary objective of Kepler is to search
  for the signal of Earth-like planets in these light curves, dozens of
  pulsating B stars are included among its targets. This poster presents
  and overview of the techniques used to obtain the frequency spectra of
  these stars. A data base of stellar pulsation spectra is also presented
  along with the positions of these stars in the HR diagram. A major
  limitation of the Kepler data is that it does not allow the spherical
  harmonic l and m values of the pulsation modes to be determined. To
  do this multi-color observations or spectra are needed. A list of
  candidate stars for this type of effort is provided. This work is
  supported by the NASA Kepler Guest Observer program.

Title: Ca Ii K and H Measurements from the SOLIS ISS Instrument
Authors: Bertello, Luca; Pevtsov, A. A.; Pietarila, A.; Harvey, J. W.;
   Toussaint, R. M.; SOLIS Team
Bibcode: 2011SPD....42.1744B
Altcode: 2011BAAS..43S.1744B
  Precise measurements of the disk integrated solar Ca II K and H
  lines play a critical role in many investigations of solar activity
  and studies related to solar irradiance variability. The Integrated
  Sunlight Spectrometer(ISS) operating at the National Solar Observatory
  at Kitt Peak (Arizona) since December 2006 is designed to obtain
  high spectral resolution (R = 300,000) observations of the Sun as
  a star in a broad range of wavelengths (350 nm -1100 nm). The ISS
  is one of three instruments, with the Vector Spectro-Magnetograph
  (VSM) and the Full Disk Patrol (FDP), comprising the Synoptic Optical
  Long-term Investigations of the Sun (SOLIS) - a synoptic facility for
  solar observations operating at NSO/Kitt Peak. The ISS takes daily
  observations of solar spectra in nine spectral bands, including the Ca
  II K and H lines. We describe recent improvements in data reduction of
  Ca II K and H observations, and present time variations of parameters
  derived from the profiles of these spectral lines. Some properties of
  these time series are also discussed.

Title: The Influence of Rotation on the Pulsation Spectra of B-stars
Authors: McNamara, Bernard J.; Jackiewicz, J.; Lovekin, C.; McAteer,
   J.; Boucheron, L.; Cao, H.; Voelz, D.; Kirk, M.; Taylor, G.; DeGrave,
   K.; Al-Ghraibah, A.; Pevtsov, A.
Bibcode: 2011SPD....42.1619M
Altcode: 2011BAAS..43S.1619M
  B stars are known to oscillate in both radial and non-radial
  modes. Rotation may play an important role in altering the pulsation
  spectra of these stars. The results of 2D model calculations of rapidly
  rotating early B-stars is presented and used to examine the frequency
  spectrum of a Kepler B-star. The reduction procedure to determine
  stellar pulsation spectra using Kepler data sets is discussed, as well
  as the need for additional observational data to constrain the B-star
  spherical harmonic l and m values. This work is supported by the NASA
  Kepler Guest Observer program.

Title: Properties of Magnetic Neutral Lines and Chromospheric
    Filaments Formation
Authors: Karachik, Nina; Pevtsov, A. A.
Bibcode: 2011SPD....42.1728K
Altcode: 2011BAAS..43S.1728K
  Chromospheric filaments - large concentrations of dense and cool
  material held in place by magnetic fields - form at a boundary (neutral
  lines) between opposite polarity magnetic fields. However, not all
  magnetic neutral lines have filaments above them. In present research
  we compare properties of magnetic polarity inversion lines that have
  filaments above them and those without filaments, and investigate how
  these properties change during solar cycle. The results are used to
  establish the conditions at neutral lines that may lead to formation
  of the chromospheric filaments.

Title: Solar Magnetic Fields As Observed By Solis
Authors: Pietarila, Anna; Bertello, L.; Callahan, L.; Harker, B.;
   Harvey, J.; Marble, A.; Pevtsov, A.; Toussaint, R.
Bibcode: 2011SPD....42.1746P
Altcode: 2011BAAS..43S.1746P
  The Vector Spectromagnetograph (VSM), part of the Synoptic Optical
  Long-term Investigations of the Sun (SOLIS), makes spectropolarimetric
  observations of the full-disk of the Sun in the photospheric Fe I lines
  around 630 nm (Stokes I, Q, U and V) and the chromospheric Ca II 854.2
  nm (Stokes I and V) line. We present some of the updated SOLIS VSM data
  products and show how they compare with data from other instruments,
  e.g., SDO/HMI and Hinode/SP. We also illustrate some of the differences
  between the photospheric and chromospheric magnetograms, and how they
  can be used to study the height variation of the magnetic field.

Title: Hemispheric Trends In The Current Helicity Of The Large Scale
Solar Magnetic Fields : Vsm/solis And Hmi/sdo Observations
Authors: Gosain, Sanjay; Pevtsov, A.
Bibcode: 2011SPD....42.1719G
Altcode: 2011BAAS..43S.1719G
  The current helicity of the large-scale magnetic fields has been
  computed in the past by using reconstructed vector magnetograms. Such
  magnetograms were derived by using sequence of line-of-sight
  magnetograms (Pevtsov A. A., and Latushko S. M.: 2000) and were
  used for studying the helicity patterns during cycle 22 and 23. The
  reconstruction method, however, made several assumptions, and the
  meridional component could not be reconstructed very well. Full disk
  vector magnetograms, which are now available from VSM/SOLIS and HMI/SDO
  are very promising to make a systematic study of large scale helicity
  patterns during cycle 24, which is in its onset phase. In this paper
  we shall discuss our preliminary results about the large-scale helicity
  patterns in the beginning of cycle 24 using VSM/SOLIS and HMI/SDO full
  disk vector magnetograms.

Title: Coronal Loop detection and seismology
Authors: Pevtsov, Alexander; McAteer, R. T. J.; Jackiewicz, J.; Kirk,
   M.; McNamara, B.; DeGrave, K.; Amani Al-Ghraibah, A.; Boucheron, L.;
   Voelz, D.; Cao, H.; Taylor, G.
Bibcode: 2011SPD....42.1822P
Altcode: 2011BAAS..43S.1822P
  Using a TRACE image with a bipolar active region and over one hundred
  distinguishable loops, we examine several current methods for automated
  coronal loop detection. Using the same TRACE image, several new
  approaches are also taken in an attempt to increase accuracy and
  completeness rates for the automated detection process. By means
  of these new methods the expectation is to achieve a higher degree
  of completeness while maintaining a high level of accuracy in the
  detection process. To increase completeness, an automated attempt for
  the reconnection between orphaned loop segments will also be tested. In
  the future, an approach to reconstruction of three-dimensional images
  from several two-dimensional images can be devised by using the detected
  coronal loops and a known 3D offset of each image. However this process
  heavily depends on the ability to accurately and completely detect
  the coronal loops.

Title: Status of Synoptic Optical Long-term Investigation of the Sun
    (SOLIS) Project.
Authors: Pevtsov, Alexei A.; Streander, K.; Harvey, J.; Bertello,
   L.; Branston, D.; Britanik, J.; Callahan, L.; Cole, L.; Galayda, E.;
   Harker, B.; Hauth, D.; Jaksha, D.; Leiker, C.; Marble, A.; Mills, N.;
   Pietarila, A.; Schramm, K.; Stover, E.; Suarez-Sola, I.; Toussaint, R.
Bibcode: 2011SPD....42.1747P
Altcode: 2011BAAS..43S.1747P
  Synoptic Optical Long-term Investigation of the Sun (SOLIS) is a suite
  of three instruments to study various aspects of solar activity. Vector
  Stokes Magnetograph (VSM) takes full disk longitudinal magnetograms in
  the photosphere (Fe I 6301 A-6302 A) and the chromosphere (Ca II 8542A),
  the photospheric vector magnetograms, and the full disk images of
  equivalent width for He I 10830A. The VSM data set goes back to August
  2003. Integrated Sunlight Spectrometer (ISS) provides high-resolution
  spectra of sun-as-a-star for nine selected spectral bands (starting
  from December 2006). Full-Disk Patrol (FDP) observes full disk images
  of the Sun at high cadence in several selected wavelengths (starting
  from June 2011). In the last two years, the SOLIS instruments and
  data reduction went through a series of upgrades resulting in higher
  data quality. These upgrades include new CCD cameras, photo guider,
  and improved magbias calculations and data reduction of ISS line
  profiles. We will present the current status of SOLIS, and show the
  comparison between SOLIS observations with other instruments.

Title: Nonlinear force-free field extrapolation in spherical
geometry: improved boundary data treatment applied to a SOLIS/VSM
    vector magnetogram
Authors: Tadesse, T.; Wiegelmann, T.; Inhester, B.; Pevtsov, A.
Bibcode: 2011A&A...527A..30T
Altcode: 2010arXiv1011.6285T
  Context. Understanding the 3D structure of coronal magnetic field
  is important to understanding: the onset of flares and coronal mass
  ejections, and the stability of active regions, and to monitoring
  the magnetic helicity and free magnetic energy and other phenomena
  in the solar atmosphere. Routine measurements of the solar magnetic
  field are mainly carried out in the photosphere. Therefore, one has to
  infer the field strength in the upper layers of the solar atmosphere
  from the measured photospheric field based on the assumption that the
  corona is force-free. Meanwhile, those measured data are inconsistent
  with the above force-free assumption. Therefore, one has to apply some
  transformations to these data before nonlinear force-free extrapolation
  codes can be applied. <BR /> Aims: Extrapolation codes in Cartesian
  geometry for modelling the magnetic field in the corona do not take the
  curvature of the Sun's surface into account and can only be applied to
  relatively small areas, e.g., a single active region. Here we apply
  a method for nonlinear force-free coronal magnetic field modelling
  and preprocessing of photospheric vector magnetograms in spherical
  geometry using the optimization procedure. <BR /> Methods: We solve
  the nonlinear force-free field equations by minimizing a functional
  in spherical coordinates over a restricted area of the Sun. We extend
  the functional by an additional term, which allows us to incorporate
  measurement errors and treat regions lacking observational data. We
  use vector magnetograph data from the Synoptic Optical Long-term
  Investigations of the Sun survey (SOLIS) to model the coronal magnetic
  field. We study two neighbouring magnetically connected active regions
  observed on May 15 2009. <BR /> Results: For vector magnetograms with
  variable measurement precision and randomly scattered data gaps (e.g.,
  SOLIS/VSM), the new code yields field models that satisfy the solenoidal
  and force-free condition significantly better as it allows deviations
  between the extrapolated boundary field and observed boundary data
  within the measurement errors. Data gaps are assigned an infinite
  error. We extend this new scheme to spherical geometry and apply it
  for the first time to real data.

Title: Temperature variability in X-ray bright points observed
    with Hinode/XRT
Authors: Kariyappa, R.; Deluca, E. E.; Saar, S. H.; Golub, L.; Damé,
   L.; Pevtsov, A. A.; Varghese, B. A.
Bibcode: 2011A&A...526A..78K
Altcode:
  <BR /> Aims: We investigate the variability in temperature as
  a function of time among a sample of coronal X-ray bright points
  (XBPs). <BR /> Methods: We analysed a 7-h (17:00-24:00 UT) long time
  sequence of soft X-ray images observed almost simultaneously in two
  filters (Ti_poly and Al_mesh) on April 14, 2007 with X-ray telescope
  (XRT) onboard the Hinode mission. We identified and selected 14 XBPs
  for a detailed analysis. The light curves of XBPs were derived using
  the SolarSoft library in IDL. The temperature of XBPs was determined
  using the calibrated temperature response curves of the two filters
  by means of the intensity ratio method. <BR /> Results: We find that
  the XBPs show a high variability in their temperature and that the
  average temperature ranges from 1.1 MK to 3.4 MK. The variations
  in temperature are often correlated with changes in average X-ray
  emission. It is evident from the results of time series that the XBP
  heating rate can be highly variable on short timescales, suggesting
  that it has a reconnection origin.

Title: Non-polar Coronal Holes and Solar Wind
Authors: Karachik, N.; Pevtsov, A. A.
Bibcode: 2010AGUFMSH11B1668K
Altcode:
  We investigate properties of non-polar coronal holes (CHs) and their
  correlation with fast solar wind at 1 AU. Using EIT/SOHO observations
  taken from 1998-2008 in 195A and 284A wavelength bands, we identify
  boundaries of coronal holes, and compute their area, total brightness
  of corona integrated over the CH, as well as the area and total
  brightness of pixels inside the CH associated with coronal bright points
  (CBPs). We investigate the effect of each parameter on solar wind speed,
  the mutual dependency of the parameters, and their changes with the
  sunspot activity. Our findings suggest that the reconnection events
  associated with coronal bright points situated in CHs do not play a
  major role in acceleration of the fast solar wind.

Title: Coronal Mass Ejections from Empty Filament Channels
Authors: Pevtsov, A. A.; Panasenco, O.
Bibcode: 2010AGUFMSH51A1659P
Altcode:
  Well-developed filament channels may be present in solar atmosphere
  even when there is no trace of filament material inside it. When
  erupted, such “empty” filament channels could result in coronal
  mass ejections that might appear having no corresponding solar surface
  source region. We analyze the magnetic field configuration and eruption
  of a complete filament system, which includes magnetic neutral line,
  filament channel, and the coronal cavity, but without the filament
  material inside. We show that the presence of filament visible in
  H-alpha or He II 304 A is not necessary for the eruption of magnetic
  systems with "empty" filament channels.

Title: Solar Dynamo: Comparing Models with Observations. (Invited)
Authors: Pevtsov, A. A.
Bibcode: 2010AGUFMSH42A..01P
Altcode:
  Dynamo, an elusive process taking place in solar interior, is bedrock
  of solar activity. It converts thermal to magnetic energy and supplies
  magnetic fields to processes that ultimately define the solar upper
  atmosphere and the Sun’s short and long-term activity. The solar
  dynamo is twofold: strong magnetic fields are generated by a deep-seated
  dynamo, while surface fields are the results of a near-surface turbulent
  dynamo. Several theoretical and numerical models of these dynamo
  processes have been developed. Recent improvements in high resolution
  solar observations, helioseismology, and vector magnetography allow for
  in depth comparison with model predictions, and eventually, may serve
  as an ultimate test for or against a specific model. In this talk, I
  will review the existing dynamo models and contrast their predictions
  with the observations.

Title: On the Origin of the Solar Moreton Wave of 2006 December 6
Authors: Balasubramaniam, K. S.; Cliver, E. W.; Pevtsov, A.; Temmer,
   M.; Henry, T. W.; Hudson, H. S.; Imada, S.; Ling, A. G.; Moore, R. L.;
   Muhr, N.; Neidig, D. F.; Petrie, G. J. D.; Veronig, A. M.; Vršnak,
   B.; White, S. M.
Bibcode: 2010ApJ...723..587B
Altcode:
  We analyzed ground- and space-based observations of the eruptive flare
  (3B/X6.5) and associated Moreton wave (~850 km s<SUP>-1</SUP> ~270°
  azimuthal span) of 2006 December 6 to determine the wave driver—either
  flare pressure pulse (blast) or coronal mass ejection (CME). Kinematic
  analysis favors a CME driver of the wave, despite key gaps in coronal
  data. The CME scenario has a less constrained/smoother velocity versus
  time profile than is the case for the flare hypothesis and requires an
  acceleration rate more in accord with observations. The CME picture is
  based, in part, on the assumption that a strong and impulsive magnetic
  field change observed by a GONG magnetograph during the rapid rise phase
  of the flare corresponds to the main acceleration phase of the CME. The
  Moreton wave evolution tracks the inferred eruption of an extended
  coronal arcade, overlying a region of weak magnetic field to the west
  of the principal flare in NOAA active region 10930. Observations of
  Hα foot point brightenings, disturbance contours in off-band Hα
  images, and He I 10830 Å flare ribbons trace the eruption from 18:42
  to 18:44 UT as it progressed southwest along the arcade. Hinode EIS
  observations show strong blueshifts at foot points of this arcade
  during the post-eruption phase, indicating mass outflow. At 18:45
  UT, the Moreton wave exhibited two separate arcs (one off each flank
  of the tip of the arcade) that merged and coalesced by 18:47 UT to
  form a single smooth wave front, having its maximum amplitude in
  the southwest direction. We suggest that the erupting arcade (i.e.,
  CME) expanded laterally to drive a coronal shock responsible for the
  Moreton wave. We attribute a darkening in Hα from a region underlying
  the arcade to absorption by faint unresolved post-eruption loops.

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

Title: Distribution of Magnetic Bipoles on the Sun over Three
    Solar Cycles
Authors: Tlatov, Andrey G.; Vasil'eva, Valerya V.; Pevtsov, Alexei A.
Bibcode: 2010ApJ...717..357T
Altcode:
  We employ synoptic full disk longitudinal magnetograms to study
  latitudinal distribution and orientation (tilt) of magnetic bipoles in
  the course of sunspot activity during cycles 21, 22, and 23. The data
  set includes daily observations from the National Solar Observatory
  at Kitt Peak (1975-2002) and Michelson Doppler Imager on board the
  Solar and Heliospheric Observatory (MDI/SOHO, 1996-2009). Bipole
  pairs were selected on the basis of proximity and flux balance of two
  neighboring flux elements of opposite polarity. Using the area of the
  bipoles, we have separated them into small quiet-Sun bipoles (QSBs),
  ephemeral regions (ERs), and active regions (ARs). We find that in
  their orientation, ERs and ARs follow Hale-Nicholson polarity rule. As
  expected, AR tilts follow Joy's law. ERs, however, show significantly
  larger tilts of opposite sign for a given hemisphere. QSBs are randomly
  oriented. Unlike ARs, ERs also show a preference in their orientation
  depending on the polarity of the large-scale magnetic field. These
  orientation properties may indicate that some ERs may form at or near
  the photosphere via the random encounter of opposite polarity elements,
  while others may originate in the convection zone at about the same
  location as ARs. The combined latitudinal distribution of ERs and ARs
  exhibits a clear presence of Spörer's butterfly diagram (equatorward
  drift in the course of a solar cycle). ERs extend the ARs' "wing" of the
  butterfly diagram to higher latitudes. This high latitude extension of
  ERs suggests an extended solar cycle with the first magnetic elements
  of the next cycle developing shortly after the maximum of the previous
  cycle. The polarity orientation and tilt of ERs may suggest the presence
  of poloidal fields of two configurations (new cycle and old cycle)
  in the convection zone at the declining phase of the sunspot cycle.

Title: Magnetic Fields and Hα Filament Formation during Solar Minimum
Authors: Panasenco, O.; Pevtsov, A.
Bibcode: 2010ASPC..428..123P
Altcode:
  We use multi-instrument observations taken during April 2007, a period
  of extremely low sunspot activity, to investigate the properties
  of magnetic fields and their potential role in the formation of
  chromospheric filaments. For one studied case, we found that some of the
  necessary conditions for forming a filament are: (1) a well-developed
  filament channel exists, and (2) an overlying arcade is present, but
  apparently there is insufficient material in Hα in the chromosphere to
  form an enduring filament. Furthermore, when plasma observed in He II
  304 Å is injected into the filament channel, we do see an Hα filament
  appearing for a short period of time. Therefore, we conclude that the
  main reason for the absence of filaments in Hα is that a mechanism
  supplying material for a filament into the filament channel does not
  work as efficiently as in other periods of the solar activity cycle.

Title: Formation of Coronal Holes on the Ashes of Active Regions
Authors: Karachik, Nina V.; Pevtsov, Alexei A.; Abramenko, Valentyna I.
Bibcode: 2010ApJ...714.1672K
Altcode:
  We investigate the formation of isolated non-polar coronal holes (CHs)
  on the remnants of decaying active regions (ARs) at the minimum/early
  ascending phase of sunspot activity. We follow the evolution of
  four bipolar ARs and measure several parameters of their magnetic
  fields including total flux, imbalance, and compactness. As regions
  decay, their leading and following polarities exhibit different
  dissipation rates: loose polarity tends to dissipate faster than
  compact polarity. As a consequence, we see a gradual increase in flux
  imbalance inside a dissipating bipolar region, and later a formation
  of a CH in place of more compact magnetic flux. Out of four cases
  studied in detail, two CHs had formed at the following polarity of
  the decaying bipolar AR, and two CHs had developed in place of the
  leading polarity field. All four CHs contain a significant fraction
  of magnetic field of their corresponding AR. Using potential field
  extrapolation, we show that the magnetic field lines of these CHs
  were closed on the polar CH at the North, which at the time of the
  events was in imbalance with the polar CH at the South. This topology
  suggests that the observed phenomenon may play an important role in
  transformation of toroidal magnetic field to poloidal field, which is
  a key step in transitioning from an old solar cycle to a new one. The
  timing of this observed transition may indicate the end of solar cycle
  23 and the beginning of cycle 24.

Title: Formation of Coronal Holes on the Ashes of Active Regions
Authors: Karachik, Nina; Pevtsov, A.; Abramenko, V.
Bibcode: 2010AAS...21640104K
Altcode: 2010BAAS...41..857K
  We investigate the formation of isolated non-polar coronal holes
  (CHs) on the remnants of decaying active regions. We follow the
  evolution of four bipolar active regions, and measure their total
  magnetic flux, imbalance, and compactness. As regions decay, their
  leading and following polarities exhibit different dissipation
  rate. As a consequence, we see a gradual increase in flux imbalance
  inside dissipating bipolar magnetic field, and later, a formation of
  a coronal hole at the site of more compact magnetic flux. Out of four
  cases studied in detail, two CHs had formed at the following polarity
  of decaying active regions, and two coronal holes had developed at the
  leading polarity field. Using potential field extrapolation we show
  that the magnetic field lines of these coronal holes were closed on
  polar coronal hole at the North, which at the time of the events was
  in imbalance with the polar coronal hole at the South. This topology
  suggests that the observed phenomenon may play an important role in
  transformation of toroidal magnetic field to poloidal field, which is
  a key step in transitioning from the old solar cycle to a new cycle.

Title: On Detection of Cosmic Ray Streaks in MDI Magnetograms
Authors: Keys, Dustin M.; Pevtsov, A.
Bibcode: 2010AAS...21641511K
Altcode:
  Any attempt to automatically identify the small-scale features of
  magnetograms taken from space-borne instruments will be encumbered
  by the inevitable cosmic ray streak which can be comparable in size
  with physical features of interest. If the size of the data set is
  very large it becomes necessary for any method of removing cosmic ray
  streaks to be automated. In this work, we concentrate on geometric
  properties of cosmic ray streaks (long narrow linear features) as the
  main criteria for their identification. <P />Three automated methods
  were investigated as possible candidates. The first one is based on
  the linear fit of isolated contours. The standard deviation from the
  fit was used to identify linear features; however, this proved to
  be an insufficient filter since on such a small scale the contours of
  possible solar features may have a good linear fit and thus return false
  positives. The second method is an unsuccessful attempt to identify
  cosmic rays based on their values in the accumulator space of a Radon
  transformation of the image. After removing features in a binary
  scaled version of the image which were too large to be cosmic rays,
  the peaks in the accumulator space of that image were used to identify
  the rays. More sophisticated techniques for identifying the signature
  of the ray in accumulator space might have greater success. The final
  attempt uses morphological dilation and erosion operations of the binary
  scaled version of the image to bring out the shape of the cosmic ray
  by using linear structuring elements. The results of the last approach
  are the most promising and deserve further investigation.

Title: Solar Cycle 23 in Coronal Bright Points
Authors: Sattarov, Isroil; Pevtsov, Alexei A.; Karachik, Nina V.;
   Sherdanov, Chori T.; Tillaboev, A. M.
Bibcode: 2010SoPh..262..321S
Altcode: 2010SoPh..tmp...32S
  We describe an automatic routine to identify coronal bright points
  (CBPs) and apply this routine to SOHO/EIT observations taken in the 195
  Å spectral range during solar cycle 23. We examine the total number
  of CBPs and its change in the course of this solar cycle. Unlike some
  other recent studies, we do find a modest ≈30% decrease in the number
  of CBPs associated with maximum of sunspot activity. Using the maximum
  brightness of CBPs as a criterion, we separate them on two categories:
  dim CBPs, associated with areas of a quiet Sun, and bright CBPs,
  associated with an active Sun. We find that the number of dim coronal
  bright points decreases at the maximum of sunspot cycle, while the
  number of bright CBPs increases. The latitudinal distributions suggest
  that dim CBPs are distributed uniformly over the solar disk. Active Sun
  CBPs exhibit a well-defined two-hump latitudinal profile suggestive
  of enhanced production of this type of CBPs in sunspot activity
  belts. Finally, we investigate the relative role of two mechanisms in
  cycle variations of CBP number, and conclude that a change in fraction
  of solar surface occupied by the quiet Sun's magnetic field is the
  primary cause, with the visibility effect playing a secondary role.

Title: A new dynamo pattern revealed by solar helical magnetic fields
Authors: Zhang, Hongqi; Sakurai, T.; Pevtsov, A.; Gao, Yu; Xu, Haiqing;
   Sokoloff, D. D.; Kuzanyan, K.
Bibcode: 2010MNRAS.402L..30Z
Altcode: 2009arXiv0911.5713Z; 2010MNRAS.tmpL...1Z
  A previously unobservable mirror asymmetry of the solar magnetic field -
  a key ingredient of the dynamo mechanism which is believed to drive the
  11-year activity cycle - has recently been measured. This was achieved
  through systematic monitoring of solar active regions carried out for
  more than 20 years at observatories in Mees, Huairou and Mitaka. In this
  Letter we report on detailed analysis of vector magnetic field data,
  obtained at Huairou Solar Observing Station in China. Electric current
  helicity (the product of current and magnetic field components in the
  same direction) was estimated from the data and a latitude-time plot of
  solar helicity during the last two solar cycles has been produced. We
  find that like sunspots helicity patterns propagate equatorwards,
  but unlike sunspot polarity helicity in each solar hemisphere does not
  change sign from cycle to cycle, thus confirming the theory. There are,
  however, two significant time-latitudinal domains in each cycle when
  the sign briefly inverts. Our findings shed new light on stellar and
  planetary dynamos and are yet to be included in the theory.

Title: Transport of open magnetic flux between solar polar regions
Authors: Pevtsov, Alexei A.; Abramenko, Valentyna I.
Bibcode: 2010IAUS..264..210P
Altcode:
  We present the observations of coronal hole that has originated at
  the polar hole in one hemisphere, extended to equatorial region,
  got disconnected and transported across the equator to polar region
  of opposite hemisphere.

Title: Groundbased synoptic observations of the Sun by SOLIS and
    their role in irradiance measurements.
Authors: Pevtsov, Alexei
Bibcode: 2010cosp...38.1688P
Altcode: 2010cosp.meet.1688P
  Synoptic Optical Long-term Investigations of the Sun (SOLIS) is a
  synoptic facility for solar observations operated by the National
  Solar Observatory (NSO) at its Kitt Peak site. Current SOLIS data
  products include: daily full disk vector magnetograms in Fe I 630.2 nm,
  chromo-spheric line-of-sight magnetograms in Ca II 854.2 nm, full disk
  images in HeI 1083.0 nm and derived maps of coronal holes, as well
  as the spectral observations of the Sun as a star in the wavelength
  range of 350 nm -1100 nm. SOLIS magnetic and helium observations
  continue the historic synoptic data set from the NSO's Kitt Peak
  Vacuum Telescope facility from 1975-2002 to present, and can be used
  to construct proxies characterizing solar irradiance, and to bridge
  and calibrate the data sets taken with the space-borne instruments
  such as SOHO/MDI and SDO/HMI. In my talk, I review the current state
  of the SOLIS project, and discuss the irra-diance proxies based on the
  magnetic field and other traditional groundbased observations to better
  understand the origin of irradiance variations for climate studies.

Title: The latitude of ephemeral regions as an indicator for
    solar-cycle strength .
Authors: Tlatov, A. G.; Pevtsov, A. A.
Bibcode: 2010MmSAI..81..814T
Altcode: 2010arXiv1008.0185T
  Digitized images of full-disk Ca K spectroheliograms from two solar
  observatories were used to study the cycle variation of ephemeral
  regions (ERs) over the ten solar cycles 14-23. We calculate the
  monthly averaged unsigned latitude of ERs and compare it with the
  annual sunspot number. We find that the average latitude of ERs
  can be used as a predictor for the strength of a solar cycle. For
  a short-term prediction (dT ∼ 1-2 years), the maximum latitude of
  ERs (in the current cycle) defines the amplitude of that cycle (the
  higher the latitude of ERs, the larger the amplitudes of the sunspot
  cycle). For a long-term prediction (dT ∼ 1.5 solar cycles), the
  latitude of ERs during the declining phase of the nth cycle determines
  the amplitude of the (n+2)th cycle (the lower the latitude of ERs,
  the stronger is the sunspot cycle). Using this latter dependency,
  we forecast the amplitude of sunspot cycle 24 to be at W = 92 ± 13
  (in units of annual sunspot number).

Title: Formation of Coronal Holes on Ashes of Decaying Active Regions
Authors: Karachik, N.; Pevtsov, A. A.
Bibcode: 2010neme.confP..24K
Altcode:
  No abstract at ADS

Title: Vorticity of Granular Flows from NST Observations
Authors: Pevtsov, A. A.; Abramenko, V.; Yurchyshyn, V.; Goode, P. R.
Bibcode: 2009AGUFMSH53B..04P
Altcode:
  We use observations taken with the New Solar Telescope (NST) at Big
  Bear Solar Observatory, the world largest solar optical telescope with
  diffraction limited spatial resolution of 0.06 arc seconds, to study
  vorticity of granular flows in quiet Sun areas. We employ sequence
  of images observed with TiO (705.7 nm) filter with time cadence
  of 30 seconds. The atmospheric seeing effects were removed using
  speckle-reconstruction technique. We compare vorticity of granular
  flows and intergranular vortices, and discuss our findings in the
  framework of surface (turbulent) dynamo.

Title: Solar Magnetic Helicity Injected into the Heliosphere:
    Magnitude, Balance, and Periodicities Over Solar Cycle 23
Authors: Georgoulis, M. K.; Rust, D. M.; Pevtsov, A. A.; Bernasconi,
   P. N.; Kuzanyan, K. M.
Bibcode: 2009ApJ...705L..48G
Altcode:
  Relying purely on solar photospheric magnetic field measurements that
  cover most of solar cycle 23 (1996-2005), we calculate the total
  relative magnetic helicity injected into the solar atmosphere, and
  eventually shed into the heliosphere, over the latest cycle. Large
  active regions dominate the helicity injection process with ~5.7
  × 10<SUP>45</SUP> Mx<SUP>2</SUP> of total injected helicity. The
  net helicity injected is lsim1% of the above output. Peculiar
  active-region plasma flows account for ~80% of this helicity; the
  remaining ~20% is due to solar differential rotation. The typical
  helicity per active-region CME ranges between (1.8-7) × 10<SUP>42</SUP>
  Mx<SUP>2</SUP> depending on the CME velocity. Accounting for various
  minor underestimation factors, we estimate a maximum helicity injection
  of ~6.6 × 10<SUP>45</SUP> Mx<SUP>2</SUP> for solar cycle 23. Although
  no significant net helicity exists over both solar hemispheres,
  we recover the well-known hemispheric helicity preference, which is
  significantly enhanced by the solar differential rotation. We also
  find that helicity injection in the solar atmosphere is an inherently
  disorganized, impulsive, and aperiodic process.

Title: Just how much Helicity did the Sun Shed in Solar Cycle
    23? Magnitude, Balance, Periodicities, and Further Implications
Authors: Georgoulis, Manolis K.; Rust, D. M.; Pevtsov, A. A.;
   Bernasconi, P. N.; Kuzanyan, K. M.
Bibcode: 2009SPD....40.0606G
Altcode:
  Using solar magnetic field measurements, we calculate the total
  relative magnetic helicity injected in the solar atmosphere and
  eventually <P />transported to the heliosphere in the course of the
  latest solar cycle. We report on (i) the magnitude of the heliospheric
  helicity over cycle 23, (ii) the net helicity and its significance,
  and (iii) the possible <P />periodicities of helicity injection in
  the solar atmosphere. Our simple calculations raise several questions
  regarding the fundamental nature of solar magnetism. The lack of
  significant net helicity may place the solar dynamo in the category
  of <P />astrophysical dynamos without a net helicity effect over an
  average time scale. The strong enhancement of the hemispheric helicity
  preference by solar differential rotation - although the latter has a
  much weaker effect than intrinsic active-region plasma flows - warrants
  further investigation. Finally, the absence of any credible periodicity
  of helicity injection, in spite of numerous reported periodicities in
  solar activity, perhaps prompts the re-evaluation of the notion that the
  Sun works through a sequence of internal cycles: active-region emergence
  and evolution appears as an inherently disorganized, aperiodic process.

Title: The Solar Moreton Wave Of 6 December 2006: Evidence For A
    CME Driver
Authors: Pevtsov, Alexei A.; Balasubramaniam, K. S.; Cliver, E. W.;
   Neidig, D. F.; Petrie, G. J. D.
Bibcode: 2009SPD....40.3703P
Altcode:
  We analyze ground- and space-based observations of the major eruptive
  flare and associated Moreton wave of 6 December 2006. The Moreton wave
  spanned 270° in azimuth and exhibited a variable speed time profile
  as it propagated away from the source region. The Hα wave traveled
  1.2 Rsun from the S06E63 site of the eruption and white-light flare
  toward the southwest in 15 minutes where it disrupted a large quiescent
  filament. A preceding coronal wave was observed in a single He 10830 Å
  image. Potential field analysis of the active region magnetic fields
  and a comparison of ISOON images of the eruptive flare in line-center
  Hα with off-band images of the wave indicate that the Moreton wave
  was driven by a coronal mass ejection.

Title: Erupting Chromospheric Filaments
Authors: Balasubramaniam, K. S.; Cliver, E.; Pevtsov, A.; Martin,
   S.; Panasenco, O.
Bibcode: 2009SPD....40.1010B
Altcode:
  Erupting filaments are commonly associated with coronal mass
  ejections. They represent the chromospheric structures most closely tied
  to the underlying photospheric magnetic fields. We present an analysis
  of the eruption of an unusually large filament on the SE quadrant of the
  solar disc on 2003 June 11. The data are drawn from USAF/NSO Improved
  Solar Observing Optical Network, Solar and Heliospheric Observatory,
  and ground-based telescopes at NSO. The filament rises with an initial
  slow speed of 6-7 km/s over a period of 2 hours and later erupts by
  rapidly accelerating to 170 km/s second in the following 30 minutes. The
  filament eruption is accompanied by a flare in a neighboring active
  region. We trace morphological and topological changes in the filament
  and overlying arcade before and during its eruption, and interpret
  these changes in terms of physical structure of the filament and whole
  filament system. The destabilization of the filament and its overlying
  coronal arcade are related to interactions with a new emerging active
  region, and adjacent active region.

Title: A Century of Solar Ca II Measurements and Their Implication
    for Solar UV Driving of Climate
Authors: Foukal, Peter; Bertello, Luca; Livingston, William C.;
   Pevtsov, Alexei A.; Singh, Jagdev; Tlatov, Andrey G.; Ulrich, Roger K.
Bibcode: 2009SoPh..255..229F
Altcode:
  Spectroheliograms and disk-integrated flux monitoring in the strong
  resonance line of Ca II (K line) provide the longest record of
  chromospheric magnetic plages. We compare recent reductions of the Ca II
  K spectroheliograms obtained since 1907 at the Kodaikanal, Mt. Wilson,
  and US National Solar Observatories. Certain differences between the
  individual plage indices appear to be caused mainly by differences
  in the spectral passbands used. Our main finding is that the indices
  show remarkably consistent behavior on the multidecadal time scales of
  greatest interest to global warming studies. The reconstruction of solar
  ultraviolet flux variation from these indices differs significantly
  from the 20th-century global temperature record. This difference is
  consistent with other findings that, although solar UV irradiance
  variation may affect climate through influence on precipitation and
  storm tracks, its significance in global temperature remains elusive.

Title: A New Method of Calibration of Photographic Plates from Three
    Historic Data Sets
Authors: Tlatov, Andrey G.; Pevtsov, Alexei A.; Singh, Jagdev
Bibcode: 2009SoPh..255..239T
Altcode:
  We analyze the synoptic data taken in the Ca II K spectral line with
  spectroheliographs at Kodaikanal Observatory from 1907 to 1999, at
  Mount Wilson Observatory from 1915 to 1985, and at the National Solar
  Observatory at Sacramento Peak from 1963 to 2002. Photographic data
  were digitized and calibrated following the same set of procedures
  developed by the authors of this paper. Using calibrated data, we have
  outlined bright plages and have calculated a plage index defined as
  the fraction of solar hemisphere occupied by the chromospheric plages
  and enhanced network. We present a detailed description of our method
  and provide a brief comparison of Ca II K plage indices derived using
  data from these three historic data sets.

Title: Causes of Solar Activity
Authors: Giampapa, Mark S.; Gibson, Sarah; Harvey, J. W.; Hill, Frank;
   Norton, Aimee A.; Pevtsov, A.
Bibcode: 2009astro2010S..92G
Altcode:
  No abstract at ADS

Title: Dynamos and magnetic fields of the Sun and other cool stars,
    and their role in the formation and evolution of stars and in the
    habitability of planets
Authors: Schrijver, Karel; Carpenter, Ken; Karovska, Margarita; Ayres,
   Tom; Basri, Gibor; Brown, Benjamin; Christensen-Dalsgaard, Joergen;
   Dupree, Andrea; Guinan, Ed; Jardine, Moira; Miesch, Mark; Pevtsov,
   Alexei; Rempel, Matthias; Scherrer, Phil; Solanki, Sami; Strassmeier,
   Klaus; Walter, Fred
Bibcode: 2009astro2010S.262S
Altcode:
  No abstract at ADS

Title: The Role of Heavy Ions as Coronal Diagnostics: Recent Results
    from Total Solar Eclipse Observations
Authors: Habbal, S. R.; Daw, A. N.; Morgan, H.; Johnson, J.;
   Druckmuller, M.; Druckmullerova, H.; Scholl, I.; Arndt, M. B.;
   Pevtsov, A.
Bibcode: 2008AGUFMSH11A..04H
Altcode:
  Recent advancements in coronal imaging capabilities and image processing
  techniques, have led to new diagnostic capabilities for the exploration
  of the solar atmosphere during total solar eclipses. In particular,
  the suite of Fe spectral lines in the visible and near infrared, namely
  Fe XIV 5303, Fe X 6374 A, Fe XI 7892 A, and Fe XIII 10747 A, together
  with continuum white light emission, continue to unveil the intricate
  topology of the magnetic field, density structures and temperature
  distribution in the solar corona. These spectral lines are also yielding
  new insights into the role of heavy ions as diagnostics of the coronal
  plasma. This presentation will focus on the surprising results from the
  eclipse observations of 2006 and 2008. With observations in H alpha,
  neutral helium He I 5876 A and the Fe lines representing four different
  ionization states of iron, it is shown how the distribution of neutrals
  and heavy ions in select magnetic structures in the corona, provides
  new insights into the physics of the coronal plasma, with implications
  for models of coronal heating processes. These observations also shed
  light on the source regions and properties of neutrals and minor ions
  measured in interplanetary space.

Title: Variations of solar activity over the last 100 years using
    observations in CaII-K spectral line.
Authors: Tlatov, A.; Pevtsov, A. A.; Singh, J.
Bibcode: 2008AGUSMSP53B..02T
Altcode:
  We present analysis of synoptic full disk daily spectroheliograms taken
  in CaII-K spectral line at Kodaikanal Observatory (1907-1999), Mount
  Wilson Observatory (1915-1985), and the National Solar Observatory at
  Sacramento Peak (1963-2002). All Photographic images were digitized and
  reduced in a systematic way. The data reduction included circularization
  of solar disk images and correction for scattered light and response
  function of photographic film. Using localized intensity threshold,
  we identified three types of chromospheric features: plage, network
  element, and ephemeral bright point; and we study coordinates, total
  area, and irradiance of these features. In this talk, we discuss
  relationship between sunspot activity, total area of chromospheric
  plages and their irradiance over a period of several solar cycles.

Title: On the Solar Cycle Variation of the Hemispheric Helicity Rule
Authors: Pevtsov, A. A.; Canfield, Richard C.; Sakurai, T.; Hagino, M.
Bibcode: 2008ApJ...677..719P
Altcode:
  We study the statistical significance of observed temporal variations
  of the solar active-region hemispheric helicity rule, as measured by the
  latitudinal gradient of the best-fit linear force-free-field parameter,
  dα/dvarphi . Using data from four different vector magnetographs,
  we compute and compare average annual dα/dvarphi values for these
  instruments for 19 years from solar cycles 21, 22, and 23. We find that
  although every instrument shows the "wrong" sign for the hemispheric
  rule in some years, there is no agreement among the instruments on
  which years are abnormal. None of the four data sets shows annual
  values of dα/dvarphi departing from the hemispheric helicity rule
  by more than 3 σ. We conclude that because the hemispheric helicity
  rule is a weak tendency with significant scatter, an annual subset of
  active regions is likely to produce statistically unreliable results.

Title: What helicity can tell us about solar magnetic fields
Authors: Pevtsov, Alexei A.
Bibcode: 2008JApA...29...49P
Altcode:
  No abstract at ADS

Title: Solar helicity as a constraint on the solar dynamo
Authors: Pevtsov, Alexei
Bibcode: 2008cosp...37.2424P
Altcode: 2008cosp.meet.2424P
  Solar magnetic fields are the result of two types of dynamo. Strong
  magnetic fields of active regions are likely to be generated by
  helical dynamo operating at the base/low portion of the convective
  zone. Weak (intra-) network field is probably generated by a separate,
  non-helical dynamo. In this talk I will review the role of helicity in
  these two types of dynamo, and present observational tests allowing
  to characterize helical and non-helical dynamos by their helicity
  signature.

Title: Evershed Flow, Oscillations, and Sunspot Structure
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Olmschenk, S.
Bibcode: 2008ASPC..383..279B
Altcode:
  Using high resolution spectroscopy at high cadence, we probe
  oscillatory properties of the Evershed effect flows. We employ
  Doppler measurements in several spectral lines to show that the
  Evershed flow is modulated at periods lasting a few tens of minutes,
  at the photosphere and chromosphere. The phase of this modulation is
  always outward propagating irrespective of whether the spectral line
  originates in the photosphere or chromosphere. From a power-spectrum
  analysis, we show that periods of peak power shift to longer periods as
  magnetic field strength increases (going from the umbra to the outer
  penumbra), at photospheric levels. At the chromosphere the periods
  shift to longer periods as the magnetic field shifts from stronger to
  weaker fields. An analysis of these phenomena and their influence on
  the sunspot structure will be presented.

Title: Chromospheric observations of erupting filaments with the
    Optical Solar Patrol Network (OSPaN) telescope
Authors: Cliver, Edward; Balasubramaniam, K. S.; Cliver, E. W.;
   Engvold, O.; Pevtsov, A.; Martin, S.; Panasenco, O.
Bibcode: 2008cosp...37..562C
Altcode: 2008cosp.meet..562C
  Using AFRL/NSO OSPaN telescope chromospheric images, we present movies
  and analyses of the eruption of a quiescent filament (11 June 2003)
  and an active region filament (13 May 2005). In both cases, widely
  separated regions of the solar surface were affected by the eruptions,
  either via the Moreton waves they generated (inferred from winking
  filaments) or through direct magnetic connection (manifested by
  sequential chromospheric brightenings). We investigate the topology
  of the magnetic fields in which these eruptions occur and use Doppler
  measurements to understand the dynamics of the eruptions.

Title: Helicity as the Ultimate Test to the Surface Dynamo Problem
Authors: Pevtsov, A. A.; Longcope, D. W.
Bibcode: 2007ASPC..369...99P
Altcode:
  It has become widely accepted that large-scale magnetic structures
  on the Sun, such as active regions, are the product of a dynamo of
  periodicity approximately 22 years situated at or near the base of
  the convection zone. There has been speculation that the intermixed,
  small-scale photospheric magnetic field is generated by a second
  dynamo operating at or near the solar surface. Numerical simulations
  have shown that such dynamo could work, although it would not be as
  effective in generating flux as the more conventional deep-seated
  dynamo. Since they are driven by flows of different sizes operating
  on different time scales, the magnetic fields generated by these two
  dynamos should be quantitatively different. In particular, there are
  well-studied helical trends in the large-scale magnetic which could be
  imprinted on them by the deep, slow flows of the dynamo which generates
  them; these helical trends would be absent from a field generated by
  a surface dynamo. We propose that observations of magnetic/current
  helicity at very small scales can be used to establish the role of
  the second, surface dynamo on the Sun.

Title: Statistical Correlations between Parameters of Photospheric
    Magnetic Fields and Coronal Soft X-Ray Brightness
Authors: Tan, Changyi; Jing, Ju; Abramenko, V. I.; Pevtsov, A. A.;
   Song, Hui; Park, Sung-Hong; Wang, Haimin
Bibcode: 2007ApJ...665.1460T
Altcode:
  Using observations of more than 160 active regions, we investigate
  the relationship between the coronal X-ray brightness, L<SUB>B</SUB>,
  and five parameters derived from the photospheric magnetic fields. The
  coronal X-ray brightness and the magnetic measures were obtained
  from co-aligned SFD composite images from the Yohkoh SXT and
  full-disk magnetograms from the SOHO MDI, respectively. The
  magnetic parameters are (1) the length of strong-gradient
  magnetic neutral lines, L<SUB>GNL</SUB>, (2) the magnetic energy
  dissipation, ɛ, (3) the unsigned line-of-sight magnetic flux, Φ,
  (4) the horizontal velocities, V<SUB>h</SUB>, of random footpoint
  motions in the photosphere, and (5) a proxy for the Poynting flux,
  E=(1/4π)V<SUB>h</SUB>B<SUB>z</SUB><SUP>2</SUP>, which characterizes
  the energy flux from the photosphere into the corona due to random
  footpoint motions. All measures except V<SUB>h</SUB> were analyzed
  in both the extensive (total) and intensive (average over an area)
  forms. In addition, we used the area-averaged strong gradient
  (&gt;50 G) of the magnetic field, ∇B<SUB>z</SUB>, as an intensive
  form of L<SUB>GNL</SUB>. We found that the Pearson correlation
  coefficient between the total X-ray brightness and the total magnetic
  measures decreases as 0.97, 0.88, 0.86, and 0.47 for Φ, E, ɛ, and
  L<SUB>GNL</SUB>, respectively. The correlation coefficient between
  the averaged X-ray brightness and the averaged magnetic measures
  varied as 0.67, 0.71, 0.57, and 0.49 for &lt;Φ&gt;, , &lt;ɛ&gt;,
  and &lt;∇B<SUB>z</SUB>&gt;, respectively. We also found that the
  velocities of the footpoint motions have no dependencies with Φ and
  L<SUB>B</SUB>. We concluded that the observed high correlation between
  L<SUB>B</SUB> and E is mainly due to the magnetic field. The energy
  of the Poynting flux is in the range 10<SUP>6.7</SUP>-10<SUP>7.6</SUP>
  ergs cm<SUP>-2</SUP> s<SUP>-1</SUP> for the majority of active regions,
  which is sufficient to heat the corona due to footpoint random motions
  of magnetic flux tubes.

Title: Magnetic Dissipation in the Photosphere and Heating of
    the Corona
Authors: Abramenko, Valentyna; Pevtsov, A.
Bibcode: 2007AAS...210.5301A
Altcode: 2007BAAS...39..164A
  Spatio-temporal dynamics of magnetic flux tubes in the photosphere of
  the Sun is thought to be a driving mechanism for energy built-up and
  energy release phenomena in the solar atmosphere. Turbulent flows in
  the photosphere braid and intertwine magnetic flux tubes creating a
  complex topology of the magnetic field in the corona which might result
  in the coronal heating. Intensity of turbulent flows of the magnetic
  flux tubes in the photosphere can be quantified by the magnetic energy
  dissipation rate, which is proportional to the averaged transverse
  velocity increment of the magnetic flux tubes. We suggest to derive 2D
  magnetic energy dissipation structures from line-of-sight magnetograms
  of active regions and then calculate the dissipation rate. Our findings
  for 104 active regions showed that the magnetic dissipation rate is in
  a good agreement (with the correlation coefficient close to 0.7) with
  the temperature and the emission measure of the corona. Our results
  present strong observational support for those coronal heating models
  that rely on random footpoint motions as an energy source to heat the
  corona above active regions.

Title: Online Catalog for Filament-Sigmoid Correlation
Authors: Merriot, Ivy; Pevtsov, A.; Martens, P.
Bibcode: 2007AAS...210.9504M
Altcode: 2007BAAS...39..224M
  A new online catalog correlating H-alpha filaments with SXT sigmoids
  gives researchers, teachers and pre-college students the ability to
  access digital H-alpha images online that were previously available
  only at the physical location of the NSO at Sunspot, NM. This web-based
  catalog correlates SOHO's SXT sigmoids from 1993-1998 as described
  in a non-online catalog created by Zach Blehm under the direction
  of Richard Canfield, MSU-Bozeman, with H-alpha filament activity as
  described by Ivy Merriot under the direction of Alexei Pevtsov, NSO,
  and Petrus Martens, MSU-Bozeman. The H-alpha images were digitized
  from film archives of the Flare Patrol Telescope at Sunspot, NM. Use
  of the online catalog will be demonstrated at the poster site with
  critical comments encouraged.

Title: Moreton Waves And Filament Eruptions
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Neidig, D. F.
Bibcode: 2007AAS...210.2504B
Altcode: 2007BAAS...39..131B
  Moreton waves are traditionally associated with large flares, and may
  accompany filament and mass ejections. Using high-spectral resolution
  and high cadence full-disk chromospheric imaging measurements, we track
  two exceptional Moreton waves (2003 October 29 and 2006 December 6)
  to understand the nature of this phenomenon. We employ arguments drawn
  from spectroscopy, wave propagation and formation of prominences to
  affirm the location of Moreton waves to coronal heights. We present
  evidence that the Moreton wave sweeps filamentary material and thus
  became visible in chromospheric spectral line. We suggest a model to
  describe the associations between the flares, filament eruptions and
  coronal mass ejections.

Title: Are Moreton Waves Coronal Phenomena?
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Neidig, D. F.
Bibcode: 2007ApJ...658.1372B
Altcode:
  We report on permeability characteristics of the upper solar atmosphere
  due to the progression of a Moreton wave. An exceptional Moreton wave
  is tracked to cover most of the Sun, following an unusually large
  solar X-ray flare observed on 2003 October 29. Using Hα intensity
  and Doppler measurements, the Moreton wave is tracked for as long as
  12 minutes. Moving outward, the wave circumnavigates strong-field
  active regions. The wave sweeps through solar magnetic neutral
  lines, disrupting material from filament and filament channels,
  thereby accentuating the visibility of the wave. We establish that
  the requirement for the visibility of a Moreton wave is the necessary
  presence of higher density material in the layers of the corona, besides
  reaffirming that Moreton waves are observed only when the speed of the
  disturbance exceeds Mach 2. We suggest that the cause can be a removal
  of significant amount of material from the solar upper atmosphere due
  to a coronal mass ejection.

Title: On anticorrelation of number of coronal bright points with
    sunspot number during 1996 2006
Authors: Sattarov, Isroil; Pevtsov, Alexei A.; Karachik, Nina V.;
   Sherdanov, Chori T.
Bibcode: 2007AdSpR..39.1777S
Altcode:
  Using full disk SOHO/EIT data from 1996 to 2006, we study latitudinal
  distribution of coronal bright points over a period of one solar
  cycle. We confirm the presence of two distinct types of coronal bright
  points, and find that the number of "quiet corona" bright points in
  high latitudes anticorrelate with solar activity as defined by the
  sunspot numbers.

Title: Helicity comparison among three magnetographs
Authors: Xu, Haiqing; Gao, Yu; Zhang, Hongqi; Sakurai, T.; Pevtsov,
   A. A.; Sokoloff, D.
Bibcode: 2007AdSpR..39.1715X
Altcode:
  We compare vector magnetograms of 228 active regions observed by Solar
  Magnetic Field Telescope (SMFT) at Huairou (HR) Solar Observing Station
  and the Solar Flare Telescope (SFT) at Mitaka (MTK) of the National
  Astronomical Observatory of Japan from 1992 to 2005 and 55 active
  regions observed by SFT and Haleakala Stokes Polarimeter (HSP) at Mees
  Solar Observatory, University of Hawaii from 1997 to 2000. Two helicity
  parameters, current helicity density h<SUB>c</SUB> and α<SUB>ff</SUB>
  coefficient of linear force free field are calculated. From this
  comparison we conclude: (1) the mean azimuthal angle differences of
  transverse fields between HR and MTK data are systematic smaller than
  that between MTK and Mees data; (2) there are 83.8% of h<SUB>c</SUB>
  and 78.1% of α<SUB>ff</SUB> for 228 active regions observed at HR and
  MTK agree in sign, and the Pearson linear correlation coefficient
  between these two data sets is 0.72 for h<SUB>c</SUB> and 0.56
  for α<SUB>ff</SUB>. There are 61.8% of h<SUB>c</SUB> and 58.2% of
  α<SUB>ff</SUB> for 55 active regions observed at MTK and Mees agree
  in sign, and the Pearson linear correlation coefficient between these
  two data sets is 0.34 for h<SUB>c</SUB> and 0.31 for α<SUB>ff</SUB>;
  (3) there is a basic agreement on time variation of helicity parameters
  in active regions observed at HR, Mees, and MTK.

Title: Sequential chromospheric brightenings: The case for
    chromospheric evaporation
Authors: Pevtsov, Alexei A.; Balasubramaniam, K. S.; Hock, Rachel A.
Bibcode: 2007AdSpR..39.1781P
Altcode:
  Sequential chromospheric brightenings (SCBs [Balasubramaniam, K.S.,
  Pevtsov, A.A., Neidig, D.F., Cliver, E.W., Thompson, B.J., Young, C.A.,
  Martin, S.F., &amp; Kiplinger, A., 630, 1160, 2005.]) are observed
  in conjunction with some strong solar flares, prominence eruptions,
  and coronal mass ejections (CMEs). SCBs are seen as wave-like trains
  of increased brightening of chromospheric network elements propagating
  away from the site of eruption (flare/CME). Although network magnetic
  field usually has mixed polarity, only areas of one (prevailing)
  polarity are seen as SCBs; there are no brightenings in neighboring
  network elements of opposite (minor) polarity. This polarity rule,
  relative timing of SCBs in respect to beginning of flare/CME, and
  their location in respect to overall topology of each event suggest
  that SCBs may be caused by high energy particles precipitating from
  the reconnection site at the tail of CME. In this work we use TRACE,
  SOHO/MDI, and H-alpha OSPAN data to demonstrate that indeed SCB
  locations exhibit several properties of chromospheric evaporation.

Title: Multi-Line Spectro-Polarimetry of Filaments
Authors: Balasubramaniam, K. S.; Sankarasubramanian, K.; Pevtsov, A. A.
Bibcode: 2006ASPC..358...68B
Altcode:
  We present a spectro-polarimetric study of magnetic structures
  supporting quiescent filaments, using the combined DLSP-Phase I and
  ASP observations. The observations were performed using the spectral
  lines Fe I 6301.5 Å and 6302.5 Å at the photospheric level, and Mg
  I 5172.7 Å and H I 6562.8 Å (Hα) at the chromospheric level. Using
  non-linear, force-free extrapolation models, we reconstruct the 3D
  magnetic loop-geometry and compare the viability of the magnetic
  filament dips as the true location of the filaments, as seen in Hα.

Title: Plasma Flows in Emerging Sunspots in Pictures
Authors: Pevtsov, A.; Lamb, J. B.
Bibcode: 2006ASPC..354..249P
Altcode:
  We present pictorial results of our study of plasma flows in fifteen
  emerging active regions using Dopplergrams, magnetograms, and white
  light observations from SOHO-MDI. The study focuses on: (1) asymmetric
  flows between two opposite polarities of the same active region, (2)
  search for systematic flows in area of active region development prior
  to flux emergence, and (3) timing between development of the Evershed
  flow and sunspot penumbra. Asymmetric flows are found in three active
  regions. In two regions, flows are directed from following to preceding
  polarity, and in one region material flows from the preceding to the
  following polarity. We observed no consistent plasma flows at the future
  location of an active region before its emergence. We describe one case
  when sunspot penumbra developed before establishing the Evershed flow.

Title: An Overview of Existing Algorithms for Resolving the
180<SUP>°</SUP> Ambiguity in Vector Magnetic Fields: Quantitative
    Tests with Synthetic Data
Authors: Metcalf, Thomas R.; Leka, K. D.; Barnes, Graham; Lites,
   Bruce W.; Georgoulis, Manolis K.; Pevtsov, A. A.; Balasubramaniam,
   K. S.; Gary, G. Allen; Jing, Ju; Li, Jing; Liu, Y.; Wang, H. N.;
   Abramenko, Valentyna; Yurchyshyn, Vasyl; Moon, Y. -J.
Bibcode: 2006SoPh..237..267M
Altcode: 2006SoPh..tmp...14M
  We report here on the present state-of-the-art in algorithms used
  for resolving the 180° ambiguity in solar vector magnetic field
  measurements. With present observations and techniques, some assumption
  must be made about the solar magnetic field in order to resolve
  this ambiguity. Our focus is the application of numerous existing
  algorithms to test data for which the correct answer is known. In
  this context, we compare the algorithms quantitatively and seek to
  understand where each succeeds, where it fails, and why. We have
  considered five basic approaches: comparing the observed field to a
  reference field or direction, minimizing the vertical gradient of the
  magnetic pressure, minimizing the vertical current density, minimizing
  some approximation to the total current density, and minimizing some
  approximation to the field's divergence. Of the automated methods
  requiring no human intervention, those which minimize the square of
  the vertical current density in conjunction with an approximation for
  the vanishing divergence of the magnetic field show the most promise.

Title: Number of Bright Points at Low Temperature EIT Data over
    Entire Soho Mission
Authors: Sattarov, I.; Pevtsov, A. A.; Karachik, N. V.; Sherdanov,
   C. T.
Bibcode: 2006ESASP.617E.158S
Altcode: 2006soho...17E.158S
  No abstract at ADS

Title: Coronal Heating and Photospheric Turbulence Parameters:
    Observational Aspects
Authors: Abramenko, V. I.; Pevtsov, A. A.; Romano, P.
Bibcode: 2006ApJ...646L..81A
Altcode:
  In this study, the soft X-ray luminosity of the solar corona, measured
  by the Yohkoh spacecraft for 104 well-developed and decaying active
  regions, is compared to the magnetic field parameters determined from
  SOHO MDI high-resolution magnetograms. We calculate and compare (1)
  two area-independent characteristics of the magnetic field: the index
  (α) of the magnetic power spectrum, E(k)~k<SUP>-α</SUP>, and the
  magnetic energy dissipation rate (ɛ¯/η), which is a proxy for the
  energy of random footpoint motions induced by turbulent convection in
  the photosphere and below; and (2) four area-independent parameters
  of the soft X-ray emission: the area-normalized flux in Yohkoh Al.1
  and AlMgMn channels, and the emission measure and temperature of the
  coronal plasma. Here we report that the area-normalized soft X-ray flux
  correlates with both the power index α (Pearson correlation coefficient
  ρ=0.72/Al.1 and 0.73/AlMgMn) and the magnetic energy dissipation rate
  ɛ¯/η (ρ=0.68/Al.1 and 0.70/AlMgMn). Also, both magnetic parameters
  are well-correlated with the logarithm of the emission measure (ρ=0.72)
  and the logarithm of temperature [ρ=0.59/α and 0.63/(ɛ¯/η)]. Our
  results present strong observational support to those coronal heating
  models that rely on random footpoint motions as an energy source to
  heat the corona above active regions.

Title: Does Visibility Effect Fully Explain Cycle Variations of
    Coronal Bright Points?
Authors: Sattarov, I.; Pevtsov, A. A.; Karachik, N. V.; Sherdanov,
   C. T.
Bibcode: 2006ESASP.617E...6S
Altcode: 2006soho...17E...6S
  No abstract at ADS

Title: Rotation of Solar Corona from Tracking of Coronal Bright Points
Authors: Karachik, Nina; Pevtsov, Alexei A.; Sattarov, Isroil
Bibcode: 2006ApJ...642..562K
Altcode:
  An automated procedure for identification of coronal bright points
  is applied to selected EIT images observed at various phases of the
  solar cycle. The procedure finds about 400 bright points on a single
  EIT image observed at 195 Å. The positions of the bright points are
  tracked to study the profile of solar rotation in the solar corona. It
  is shown that the rotation of coronal bright points closely follows the
  latitudinal rotation profile of the underlying photospheric magnetic
  field. It is also demonstrated that coronal features situated at the
  same heliographic coordinates but different heights in the corona may
  exhibit different rotation rates.

Title: Helicity Measurements from Two Magnetographs
Authors: Pevtsov, A. A.; Dun, J. P.; Zhang, H.
Bibcode: 2006SoPh..234..203P
Altcode:
  We use 270 pairs of vector magnetograms observed by Haleakala Stokes
  Polarimeter (HSP) and Solar Magnetic Field Telescope (SMFT) of Huairou
  Solar Observing Station from 1997 to 2000 to compare current helicity
  derived by these two instruments. We apply the same approach to both
  data sets to resolve 180<SUP>∘</SUP> azimuth ambiguity and compute
  α coefficient of linear force-free field. After careful consideration
  of various aspects of both data sets, we find that in ≈80% of cases
  SMFT and HSP data result in the same sign of α, and the Pearson linear
  correlation coefficient between two data sets is r<SUB>p</SUB> = 0.64.

Title: On anticorrelation of number Coronal Bright Points with
    sunspot number during 1996-2005
Authors: Sattarov, I.; Pevtsov, A. A.; Karachik, N. V.; Sherdanov,
   Ch. T.
Bibcode: 2006cosp...36..906S
Altcode: 2006cosp.meet..906S
  X-ray Bright Points XBP were extensively studied using observations
  from various instruments Golub at all 1979 have found that number of
  XBP anticorralates with sunspot number and suggested that solar cycle
  may be characterized as an oscillator in wavenumber space This finding
  prompted Yoshmura 1983 to suggest the existence of a secondary cycle of
  magnetic activity running in opposite phase to the sunspot cycle Later
  however Nakakubo Hara 2000 concluded that the variation in number of
  XBPs might be visibility effect as the enhanced brightness of corona
  may hinder identification of dimmer XBPs Sattarov at al 2002 found
  that while the number of bright points exhibit cyclic variation the
  number of magnetic bipoles with a particular flux and separation does
  not change from solar minimum to the aximum Hence Sattarov et al 2002
  concluded that cyclic variation in XBP number may be a visibility effect
  In this work we continue study the properties of coronal bright points
  CBPs features that include XBPs and bright points in other wavelengths
  using SOHO EIT 195A data from 1996-2005 We identify CBPs using automatic
  procedure developed by us Sattarov et al 2005 Karachik et al 2006 and
  calculate various parameters including heliographic position intensity
  area and background intensity around each CBP Analyzing this data we
  found that the latitude-cycle variations of CBPs cannot be completely
  explained by the visibility effect These irregularities suggest that
  in addition to the visibility effect the actual number of coronal
  bright points

Title: Sequential Chromospheric Brightenings as Signature of
    Chromospheric Evaporation
Authors: Pevtsov, A.; Balasubramaniam, K.
Bibcode: 2006cosp...36.3671P
Altcode: 2006cosp.meet.3671P
  Sequential Chromospheric Brightennings SCBs Balasubramaniam et
  al 2005 are observed in conjunction with some strong solar flares
  prominence eruptions and coronal mass ejections CMEs SCBs are seen as
  wave-like trains of increased brightenning of chromospheric network
  elements propagating away from the site of eruption flare CME Although
  network magnetic field usually has mixed polarity only areas of one
  polarity prevailing or unbalanced field are seen as SCBs there are no
  brightenings in neighboring network elements of opposite minor polarity
  This polarity rule relative timing of SCBs in respect to beginning
  of flare CME and their location in respect to overall topology of
  each event suggest that SCBs may be caused by high energy particles
  precipitating from the reconnection site at the tail of CME In this work
  we use TRACE SOHO MDI and H-alpha ISOON data to demonstrate that indeed
  SCB locations exhibit several properties of chromospheric evaporation

Title: Large scale solar chromospheric eruptive activity - a signature
    of magnetic reconnection
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Neidig, D. F.; Hock,
   R. A.
Bibcode: 2006ilws.conf...65B
Altcode:
  A new class of large-scale solar chromospheric eruptive activity,
  sequential chromospheric brightenings (SCBs), has been reported by
  Balasubramaniam et al. (2005). SCBs are chromospheric network points
  (outside of active regions) that sequentially brighten over a narrow
  path of chromospheric network points. SCBs appear as single or multiple
  trains of brightenings, the underlying magnetic poles of each train
  having the same (negative or positive) polarity. SCBs may be associated
  with the following phenomena: solar flares, filament eruptions, CMEs,
  disappearing transequatorial loops, Moreton and EIT waves. We present
  an understanding of SCBs and their place in respect to these related
  eruptive phenomena.

Title: On anticorrelation of number Coronal Bright Points with
    sunspot number during 1996-2005
Authors: Sattarov, I.; Pevtsov, A. A.; Karachik, N. V.; Sherdanov,
   Ch. T.
Bibcode: 2006cosp...36..974S
Altcode: 2006cosp.meet..974S
  X-ray Bright Points XBP were extensively studied using observations
  from various instruments Golub at all 1979 have found that number of
  XBP anticorralates with sunspot number and suggested that solar cycle
  may be characterized as an oscillator in wavenumber space This finding
  prompted Yoshmura 1983 to suggest the existence of a secondary cycle of
  magnetic activity running in opposite phase to the sunspot cycle Later
  however Nakakubo Hara 2000 concluded that the variation in number of
  XBPs might be visibility effect as the enhanced brightness of corona
  may hinder identification of dimmer XBPs Sattarov at al 2002 found
  that while the number of bright points exhibit cyclic variation the
  number of magnetic bipoles with a particular flux and separation does
  not change from solar minimum to the maximum Hence Sattarov et al 2002
  concluded that cyclic variation in XBP number may be a visibility effect
  In this work we continue study the properties of coronal bright points
  CBPs features that include XBPs and bright points in other wavelengths
  using SOHO EIT 195A data from 1996-2005 We identify CBPs using automatic
  procedure developed by us Sattarov et al 2005 Karachik et al 2006 and
  calculate various parameters including heliographic position intensity
  area and background intensity around each CBP Analyzing this data we
  found that the latitude-cycle variations of CBPs cannot be completely
  explained by the visibility effect These irregularities suggest that
  in addition to the isibility effect the actual number of coronal bright
  points on

Title: Case Study of A Sequential Chromospheric Brightening (SCB)
    Associated with May 6, 2005 event.
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Hock, R. A.
Bibcode: 2005AGUFMSH13A0282B
Altcode:
  A sequential chromospheric brightening on May 6, 2005 was accompanied
  by a long duration event, LDE (16:00 - 23:00 UT). Using data from ISOON/
  H-alpha, TRACE (171 A), and SOHO/MDI (magnetograms) missions, we present
  a detailed sequential study of this event that includes large scale
  coronal disturbances(TRACE 171 A), ribbon-ed flares (ISOON &amp; TRACE),
  large scale motion of magnetic fields (SOHO/MDI), filament eruptions
  (ISOON &amp; TRACE), flares, and post-flare loops (TRACE). This event
  presents a good example in the continuity of events comprising an
  LDE. We present a model of the eruptive process of this LDE.

Title: Large-scale Structures in Distribution of Coronal Bright Points
Authors: Sattarov, I.; Pevtsov, A. A.; Karachik, N. V.; Sattarova,
   B. J.
Bibcode: 2005ASPC..346..363S
Altcode:
  We use an automatic procedure to find coronal bright points (BPs)
  in EIT 195 Å data observed in 1996 near deep minimum of solar
  activity. Identifying BPs as bright features with EUV flux above
  80 DN/s and diameter about 2.6 -- 58 arcsec, we find about 463±22
  BPs per solar disk. We also uncover small intensity fluctuations
  of diffuse corona surrounding the BPs. The intensity fluctuations
  correlate with BP brightness: increase in brightness of BPs causes
  increase in coronal intensity.

Title: Large-scale Structures and their Role in Solar Activity
Authors: Sankarasubramanian, K.; Penn, M.; Pevtsov, A.
Bibcode: 2005ASPC..346.....S
Altcode:
  No abstract at ADS

Title: Accumulation of Filament Material at the Boundaries of
    Supergranular Cells
Authors: Pevtsov, A. A.; Neidig, D.
Bibcode: 2005ASPC..346..219P
Altcode:
  We use ISOON full disk H-alpha images to study fragmented filaments
  whose main body consists of non-connected clumps of dark filament
  material. Fragmented filaments may represent the early evolution
  of filament development. We describe two filaments that began
  their formation with a few individual clumps, which later grew and
  interconnected to form a continuous body typical of a chromospheric
  filament. We show that there is a tendency for clumps to be situated
  over the boundaries of supergranular cells. We suggest that this
  preference in initial accumulation of material above areas of persistent
  downflows (boundary of supergranules) may be a key in understanding why
  the chromospheric filaments show a much stronger hemispheric helicity
  rule than other solar features (e.g. sunspots).

Title: Active Longitudes in Distribution of EIT Bright Points
Authors: Sattarov, I.; Pevtsov, A. A.; Karachik, N. V.; Sattarova,
   B. J.
Bibcode: 2005ASPC..346..395S
Altcode:
  Using a previously developed automatic procedure for identifying coronal
  bright points (BPs) in EIT/Fe XII 195 Å data, we study longitudinal
  and latitudinal distributions of EUV BPs on solar surface near the
  minimum of the solar activity cycle in 1996. Longitudinal distributions
  indicate the presence of active longitudes in coronal BPs. Latitudinal
  distributions are asymmetric relative to the equator and show secondary
  peaks associated with areas of emerging flux. We find this result as
  suggestive of two types of coronal BPs: quiet Sun and active Sun BPs.

Title: ISOON H-alpha Survey of Sequential Chromospheric Brightenings
Authors: Hock, R.; Balasubramaniam, K. S.; Pevtsov, A. A.
Bibcode: 2005AGUFMSH13A0281H
Altcode:
  Balasubramaniam et al. (2005, ApJ, 630, 1160) described a new
  phenomenon, sequential chromospheric brightenings (SCBs), associated
  with eruption of a transequatorial loop and a CME. We present a
  detailed survey of SCBs between December 2000 - May 2005. From this
  survey we have identified 17 SCBs observed during this period. We
  discuss the relationship between SCBs, and filament eruptions,
  coronal mass ejections (CME), and flares. The data were drawn from
  ISOON, SOHO/LASCO, and NOAA/GOES, and RHESSI missions. We find that:
  all SCBs are accompanied by filament eruption or disappearance; SCBs
  are more likely to be longer in duration than associated flares;
  and a large fraction of the SCBs (71%) are associated with CMEs.

Title: Sequential Chromospheric Brightenings beneath a Transequatorial
    Halo Coronal Mass Ejection
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Neidig, D. F.; Cliver,
   E. W.; Thompson, B. J.; Young, C. A.; Martin, S. F.; Kiplinger, A.
Bibcode: 2005ApJ...630.1160B
Altcode:
  Analyses of multiwavelength data sets for a solar eruption at ~21:30
  UT on 2002 December 19 show evidence for the disappearance of a
  large-scale, transequatorial coronal loop (TL). In addition, coronal
  manifestations of the eruption (based on SOHO EIT and LASCO images)
  include large-scale coronal dimming, flares in each associated active
  region in the northern and southern hemispheres, and a halo CME. We
  present detailed observations of the chromospheric aspects of this
  event based on Hα images obtained with the ISOON telescope. The
  ISOON images reveal distant flare precursor brightenings, sympathetic
  flares, and, of most interest herein, four nearly cospatial propagating
  chromospheric brightenings. The speeds of the propagating disturbances
  causing these brightenings are 600-800 km s<SUP>-1</SUP>. The inferred
  propagating disturbances have some of the characteristics of Hα
  and EIT flare waves (e.g., speed, apparent emanation from the flare
  site, subsequent filament activation). However, they differ from
  typical Hα chromospheric flare waves (also known as Moreton waves)
  because of their absence in off-band Hα images, small angular
  arc of propagation (&lt;30°), and their multiplicity. Three of
  the four propagating disturbances consist of a series of sequential
  chromospheric brightenings of network points that suddenly brighten in
  the area beneath the TL that disappeared earlier. SOHO MDI magnetograms
  show that the successively brightened points that define the inferred
  propagating disturbances were exclusively of one polarity, corresponding
  to the dominant polarity of the affected region. We speculate that
  the sequential chromospheric brightenings represent footpoints of
  field lines that extend into the corona, where they are energized in
  sequence by magnetic reconnection as coronal fields tear away from
  the chromosphere during the eruption of the transequatorial CME. We
  report briefly on three other events with similar narrow propagating
  disturbances that were confined to a single hemisphere.

Title: Coronal Bright Points at the Minimum of Sunspot Activity
Authors: Pevtsov, A. A.; Sattarov, I.; Karachik, N. V.; Sattarova,
   B. J.
Bibcode: 2005AGUSMSP22A..02P
Altcode:
  We will present the results of our study of coronal bright points (XBPs)
  using SoHO/EIT data and an automatic identification procedure. The
  latitude-longitude distribution of XBPs obtained during the period of
  very low sunspot activity (March-April 1996) is not uniform; it exhibits
  structures reminiscent of active longitudes and large-scale circular
  patterns. We also find XBPs associated with emergence of ephemeral
  active regions. We interpret these results as a possible indication
  of two types of XBPs that are characteristic of quiet and active Sun.

Title: First-Light Instrumentation for the Advanced Technology
    Solar Telescope
Authors: Rimmele, T.; Balasubramaniam, K.; Berger, T.; Elmore, D.;
   Gary, A.; Keller, C.; Kuhn, J.; Lin, H.; Mickey, D.; Pevtsov, A.;
   Robinson, B.; Sigwarth, M.; Soccas-Navarro, H.
Bibcode: 2005AGUSMSP34A..03R
Altcode:
  The 4m Advanced Technology Solar Telescope (ATST) is the next
  generation ground based solar telescope. In this paper we provide
  an overview of the ATST post-focus instrumentation. The majority of
  ATST instrumentation is located in an instrument Coude lab facility,
  where a rotating platform provides image de-rotation. A high order
  adaptive optics system delivers a corrected beam to the Coude lab
  facility. Alternatively, instruments can be mounted at the Nasmyth
  focus. For example, instruments for observing the faint corona
  preferably will be mounted at Nasmyth where maximum throughput
  is achieved. In addition, the Nasmyth focus has minimum telescope
  polarization and minimum stray light. We give an overview of the
  initial set of first generation instruments: the Visible-Light
  Broadband Imager (VLBI), the Visible Spectro-Polarimeter (ViSP),
  the Near-IR Spectro-Polarimeter (NIRSP), which includes a coronal
  module, and the Visible Tunable Filter. We also discuss the unique and
  efficient approach to the ATST instrumentation, which builds on the use
  of common components such as detector systems, polarimetry packages
  and various opto-mechanical components. For example, the science
  requirement for polarimetric sensitivity (10-5 relative to intensity)
  and accuracy (5'10-4 relative to intensity) place strong constraints
  on the polarization analysis and calibration units. Consequently,
  these systems are provided at the facility level, rather than making
  it part of the requirement for each instrument.

Title: Coronal Magnetography of Solar Active Region 8365 with the
    SSRT and NoRH Radio Heliographs
Authors: Ryabov, B. I.; Maksimov, V. P.; Lesovoi, S. V.; Shibasaki,
   K.; Nindos, A.; Pevtsov, A.
Bibcode: 2005SoPh..226..223R
Altcode:
  Microwave maps of solar active region NOAA 8365 are used to derive
  the coronal magnetograms of this region. The technique is based on
  the fact that the circular polarization of a radio source is modified
  when microwaves pass through the coronal magnetic field transverse to
  the line of sight. The observations were taken with the Siberian Solar
  Radio Telescope (SSRT) on October 21 - 23 and with the Nobeyama Radio
  Heliograph (NoRH) on October 22 - 24, 1998. The known theory of wave
  mode coupling in quasi-transverse (QT) region is employed to evaluate
  the coronal magnetograms in the range of 10 - 30 G at the wavelength 5.2
  cm and 50 - 110 G at 1.76 cm, taking the product of electron density and
  the scale of coronal field divergence to be constant of 10<SUP>18</SUP>
  cm<SUP>−2</SUP>. The height of the QT-region is estimated from the
  force-free field extrapolations as 6.2 × 10<SUP>9</SUP> cm for the
  20 G and 2.3 × 10<SUP>9</SUP> cm for 85 G levels. We find that on
  large spatial scale, the coronal magnetograms derived from the radio
  observations show similarity with the magnetic fields extrapolated
  from the photosphere.

Title: On a Cyclic Variation of the Hemispheric Helicity Rule
Authors: Pevtsov, A. A.; Hagyard, M. J.; Blehm, Z.; Smith, J. E.;
   Canfield, R. C.; Sakurai, T.; Hagino, M.
Bibcode: 2005HiA....13..140P
Altcode:
  We report the result of a study of magnetic helicity in solar active
  regions during 1980-2000 (cycles 21-23). Using the vector magnetograms
  from four different instruments (Haleakala Stokes Polarimeter Marshall
  Space Flight Center Mitaka Solar Flare Telescope and Okayama Observatory
  Solar Telescope) we calculated the force-free parameter alpha as
  in Pevtsov et al. (1995). We use alpha as the proxy for current
  helicity. For each instrument we computed a gradient dalpha/dL as
  the linear fit of alpha versus latitude L using annual subsets of
  data. The hemispheric helicity rule (negative/positive helicity in
  northern/southern hemisphere) can be expressed in terms of this gradient
  as dalpha/dL &lt; 0. We find that each instrument exhibits change in
  sign of this gradient for some years which implies that the hemispheric
  helicity rule may not hold in some phases of a solar cycle (see Hagino
  and Sakurai 2002). However we do not see consistency between different
  instruments in regards to years disobeying the rule. The disagreement
  may be due to difference in observations and/or insufficient number of
  magnetograms in some years. We conclude that the present data sets do
  not allow to make statistically significant inference about possible
  cyclic variation of the hemispheric helicity rule.

Title: Helicity of Magnetic Clouds and Their Associated Active Regions
Authors: Leamon, Robert J.; Canfield, Richard C.; Jones, Sarah L.;
   Lambkin, Keith; Lundberg, Brian J.; Pevtsov, Alexei A.
Bibcode: 2005HiA....13..132L
Altcode:
  Magnetic clouds are closely associated with Coronal Mass Ejections
  (CMEs). Most CMEs are associated with active regions. What is the
  relationship between the topology of these clouds and the associated
  active region? For our purposes magnetic clouds can be modeled
  adequately by a cylindrical force-free magnetic configuration (Lepping
  1990). We have modeled the magnetic field topology of 14 magnetic
  clouds and their associated active regions to determine values of
  the force-free field parameter for both as well as total currents and
  fluxes. We find that the number of turns of the magnetic field in the
  full length of the cloud is typically an order of magnitude greater
  than the same quantity in the associated active region. This finding
  compels us to reject models of flux rope formation that do not invoke
  magnetic reconnection and helicity conservation.

Title: Helicity Generation and Signature in Solar Atmosphere
Authors: Pevtsov, A. A.
Bibcode: 2005HiA....13...89P
Altcode:
  To fully understand the origin evolution and topology of solar magnetic
  fields one should comprehend their magnetic helicity. Observationally
  non-zero helicity reveals itself in the patterns of electric currents
  inside active regions super-/penumbral sunspot whirls the shape of
  coronal loops and the fine structure of chromospheric filaments. The
  patterns of helicity are present on different spatial scales ranging
  from several arc seconds to a size of a solar hemisphere. Some
  patterns may bear information about deep subphotospheric processes
  (e.g. dynamo turbulent convection). Other may originate at or near the
  photosphere. In this talk we will review the observations of magnetic
  and current helicity on the Sun. We will also discuss the different
  mechanisms operating at or near the photosphere and we will compare
  them with the observations.

Title: Of Tilt and Twist
Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca
   A.; Howard, Robert F.; Pevtsov, Alexei A.
Bibcode: 2005HiA....13..135H
Altcode:
  Using Haleakala Stokes Polarimeter active-region vector magnetograms
  and Mt. Wilson Observatory full-disk longitudinal magnetograms we
  measure both the overall twist (using the force-free-field parameter
  alpha) and tilt of 368 active regions. This dataset clearly shows two
  well-known phenomena Joy's law and the hemispheric helicity rule as well
  as a lesser-known twist-tilt relationship which is the point of this
  work. Those regions that closely follow Joy's law show no twist-tilt
  relationship as expected if the twist originates from convective
  buffeting of initially untwisted and unwrithed flux tubes within the
  convection zone through the Sigma effect. Those regions that strongly
  depart from Joy's law show significantly larger than average twist
  and a very strong twist-tilt relationship. These properties suggest
  that the twist-tilt relationship in these regions is due to kinking of
  flux tubes that are initially highly twisted but not strongly writhed
  perhaps as a result of dynamo action.

Title: Plasma Flows in Emerging Sunspots
Authors: Lamb, J. B.; Pevtsov, A. A.
Bibcode: 2004AAS...205.1001L
Altcode: 2004BAAS...36.1349L
  We study plasma flows in emerging active regions on the sun using
  data from the Michelson Doppler Imager (MDI) instrument on the
  SOHO satellite. Dopplergrams are compared with magnetograms and full
  continuum images in an effort to study the relationship between plasma
  flows and the magnetic and white light development of fifteen emerging
  active regions. This study focuses on the following flows in emerging
  active regions: (1) asymmetric flows from one polarity of the active
  region to the opposite polarity, (2) general trends of plasma flows
  before and immediately after emergence and (3) the emergence of Evershed
  flow with respect to penumbral development. The results of this study
  include (1) asymmetric flows found in three active regions. Two active
  regions exhibited flows from following to preceding polarity, confirming
  other observations of asymmetric flows. One active region exhibited
  a flow from the preceding to following polarity which has previously
  been theoretically predicted but until now has not been observed. (2)
  No consistent plasma flows were found before emergence. (3) After
  emergence, active regions exhibited overall downflows in the area around
  the region. (4) There also is some possible evidence for penumbra
  development before Evershed flow. This work was carried out through
  the National Solar Observatory Research Experiences for Undergraduates
  (REU) site program, which is co-funded by the Department of Defense
  in partnership with the National Science Foundation REU Program.

Title: On the Role of the Large-Scale Magnetic Reconnection in the
    Coronal Heating
Authors: Pevtsov, A. A.; Kazachenko, M.
Bibcode: 2004ESASP.575..241P
Altcode: 2004soho...15..241P
  No abstract at ADS

Title: Temporal Variations in the Evershed Flow
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Olmschenk, S.
Bibcode: 2004AGUFMSH13A1141B
Altcode:
  We present results of an analysis of the temporal Doppler variations
  of the Evershed flow. Doppler shifts were calculated from a unique,
  high-resolution data set of 39 spectral lines, spanning the photosphere
  and chromosphere. Our results indicate a quasi-periodic structure
  of the Evershed flow with a typical period between 18-24 minutes in
  the photosphere and 12-18 minutes in the chromosphere. We discuss the
  implications of these results for both the siphon flow and the moving
  flux tube models.

Title: On the Tilt and Twist of Solar Active Regions
Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca
   A.; Nandy, Dibyendu; Howard, Robert F.; Pevtsov, Alexei A.
Bibcode: 2004ApJ...611.1149H
Altcode:
  Tilt and twist are two measurable characteristics of solar active
  regions that can give us information about subsurface physical
  processes associated with the creation and subsequent evolution of
  magnetic flux tubes inside the Sun. Using Mees Solar Observatory active
  region vector magnetograms and Mount Wilson Observatory full-disk
  longitudinal magnetograms, we measure the magnetic twist and tilt
  angles of 368 bipolar active regions. In addition to two well-known
  phenomena, Joy's law and the hemispheric helicity rule, this data set
  also shows a lesser known twist-tilt relationship, which is the focus
  of this study. We find that those regions that closely follow Joy's
  law do not show any twist-tilt dependence. The dispersion in tilt
  angles and the dispersion in twist are also found to be uncorrelated
  with each other. Both of these results are predicted consequences of
  convective buffeting of initially untwisted and unwrithed flux tubes
  through the Σ-effect. However, we find that regions that strongly
  depart from Joy's law show significantly larger than average twist
  and very strong twist-tilt dependence-suggesting that the twist-tilt
  relationship in these regions is due to the kinking of flux tubes that
  are initially highly twisted, but not strongly writhed. This implies
  that some mechanism other than the Σ-effect (e.g., the solar dynamo
  itself or the process of buoyancy instability and flux tube formation)
  is responsible for imparting the initial twist (at the base of the
  solar convection zone) to the flux tubes that subsequently become
  kink-unstable.

Title: Statistical Properties of Superpenumbral Whorls around Sunspots
Authors: Balasubramaniam, K. S.; Pevtsov, Alexei; Rogers, Joey
Bibcode: 2004ApJ...608.1148B
Altcode:
  We study properties of 897 superpenumbral fibrils using Hα Big Bear
  Solar Observatory (BBSO) and photospheric magnetic field National
  Solar Observatory/Kitt Peak (NSO/KP) data of 139 sunspots between
  2000 July and 2001 April. From this low-resolution data, we find
  that about one-third of all superpenumbral fibrils begin inside the
  penumbra. The typical length of fibrils is 2.7 times the sunspot
  white-light penumbral radius. A majority of the fibrils are curved,
  i.e., 67% of them exhibit bow-extent/footpoint separation greater than
  0.1. Both clockwise and counterclockwise fibrils are typically present
  within the same superpenumbra. We show that the topology of fibrils is
  clearly affected by distribution of magnetic fields around the sunspot.

Title: Photospheric and Coronal Currents in Solar Active Regions
Authors: Burnette, Andrew B.; Canfield, Richard C.; Pevtsov, Alexei A.
Bibcode: 2004ApJ...606..565B
Altcode:
  Using photospheric line-of-sight magnetograms from the National
  Solar Observatory (NSO) Kitt Peak and coronal X-ray images from the
  Yohkoh Soft X-Ray Telescope (SXT), we have determined the value of
  the constant α of the linear force-free field model (∇XB=αB)
  that gives the best visual fit to the overall coronal X-ray structure
  (α<SUB>c</SUB>) of 34 flare-productive active regions of relatively
  simple bipolar morphology. Vector magnetograms for 24 of these
  active regions are available from the Haleakala Stokes Polarimeter
  at Mees Solar Observatory. For each of them, we determine the single
  best-fit value of α in the photosphere (α<SUB>p</SUB>) by three
  quite different methods and show that these methods give statistically
  consistent values. By combining this data set with that of the NSO
  and SXT, we are able to compare for the first time quantitatively and
  statistically the observed values of α in the photosphere and corona
  of these regions. We find that the distribution of α<SUB>p</SUB> and
  α<SUB>c</SUB> values is fully consistent with the hypothesis that
  the overall twist density of the magnetic fields of active regions,
  as measured by the linear force-free field parameter α, is the same
  in the photosphere and the corona. We therefore conclude that the
  electric currents that create the nonpotential structure of such solar
  coronal active regions are of subphotospheric origin and pass without
  significant modification through the photosphere.

Title: Helicity of magnetic clouds and their associated active regions
Authors: Leamon, Robert J.; Canfield, Richard C.; Jones, Sarah L.;
   Lambkin, Keith; Lundberg, Brian J.; Pevtsov, Alexei A.
Bibcode: 2004JGRA..109.5106L
Altcode:
  In this work we relate the magnetic and topological parameters of twelve
  interplanetary magnetic clouds to associated solar active regions. We
  use a cylindrically symmetric constant-α force-free model to derive
  field line twist, total current, and total magnetic flux from in situ
  observations of magnetic clouds. We compare these properties with those
  of the associated solar active regions, which we infer from solar
  vector magnetograms. Our comparison of fluxes and currents reveals:
  (1) the total flux ratios Φ<SUB>MC</SUB>/Φ<SUB>AR</SUB> tend to be of
  order unity, (2) the total current ratios I<SUB>MC</SUB>/I<SUB>AR</SUB>
  are orders of magnitude smaller, and (3) there is a statistically
  significant proportionality between them. Our key findings in comparing
  total twists αL are that (1) the values of (αL)<SUB>MC</SUB> are
  typically an order of magnitude greater than those of (αL)<SUB>AR</SUB>
  and (2) there is no statistically significant sign or amplitude
  relationship between them. These findings compel us to believe that
  magnetic clouds associated with active region eruptions are formed by
  magnetic reconnection between these regions and their larger-scale
  surroundings, rather than simple eruption of preexisting structures
  in the corona or chromosphere.

Title: Observational Evidence of Magnetic Flux Submergence in Flux
    Cancellation Sites
Authors: Chae, Jongchul; Moon, Yong-Jae; Pevtsov, Alexei A.
Bibcode: 2004ApJ...602L..65C
Altcode:
  Using high-resolution vector magnetograms of NOAA Active Region
  10043, observed on 2002 July 26 with the Advanced Stokes Polarimeter
  and low-order adaptive optics system, we studied the magnetic field
  topology and line-of-sight velocities in two flux cancellation sites. We
  found that the magnetic field is near horizontal at the place where
  two opposite polarities cancel each other. In addition, we observed
  significant downflows of about 1 km s<SUP>-1</SUP> near the polarity
  reversal line, where the field is horizontal. We interpret these
  observations as the direct evidence of the magnetic flux submergence of
  concave-down (Ω-shaped) magnetic loop at the flux cancellation sites.

Title: Latitudinal Distribution of the Coronal Bright Points at
    Solar Minimum and the Rising Phase of Solar Activity Cycle 23
Authors: Sattarov, I.; Pevtsov, A. A.; Karachek, N. V.; Tillaboev,
   A. M.
Bibcode: 2004IAUS..223..665S
Altcode: 2005IAUS..223..665S
  We show that the latitudinal distribution of the coronal bright points
  (BPs) changes between the solar minimum and the rising phase of solar
  cycle 23. In 1996, the number of BPs peaks near the disk center. In
  1998, the distribution shows bands of increased BPs' number at the
  ARs' latitudes.

Title: Number of Coronal Bright Points in Different Phases of the
    Solar Activity Cycle
Authors: Sattarov, I.; Pevtsov, A. A.; Karachek, N. V.; Sherdanov,
   Ch. T.
Bibcode: 2004IAUS..223..667S
Altcode: 2005IAUS..223..667S
  We present the preliminary results of study of the coronal bright points
  (BPs) using the SoHO/EIT data. We show that the number of BPs in quiet
  Sun areas does not vary between 1996 and 1997. In 1998 the BPs number
  (full disk) increases by about 20%.

Title: Transequatorial Connections: Loops or Magnetic Separators?
Authors: Pevtsov, Alexei A.
Bibcode: 2004IAUS..223..521P
Altcode: 2005IAUS..223..521P
  We provide a brief overview of properties of transequatorial loops
  (TLs) and show that some TLs fit the description of a loop, while
  others appear to be the magnetic separators.

Title: The Relationship Between X-Ray Radiance and Magnetic Flux
Authors: Pevtsov, Alexei A.; Fisher, George H.; Acton, Loren W.;
   Longcope, Dana W.; Johns-Krull, Christopher M.; Kankelborg, Charles
   C.; Metcalf, Thomas R.
Bibcode: 2003ApJ...598.1387P
Altcode:
  We use soft X-ray and magnetic field observations of the Sun (quiet
  Sun, X-ray bright points, active regions, and integrated solar disk)
  and active stars (dwarf and pre-main-sequence) to study the relationship
  between total unsigned magnetic flux, Φ, and X-ray spectral radiance,
  L<SUB>X</SUB>. We find that Φ and L<SUB>X</SUB> exhibit a very nearly
  linear relationship over 12 orders of magnitude, albeit with significant
  levels of scatter. This suggests a universal relationship between
  magnetic flux and the power dissipated through coronal heating. If the
  relationship can be assumed linear, it is consistent with an average
  volumetric heating rate Q~B/L, where B is the average field strength
  along a closed field line and L is its length between footpoints. The
  Φ-L<SUB>X</SUB> relationship also indicates that X-rays provide a
  useful proxy for the magnetic flux on stars when magnetic measurements
  are unavailable.

Title: Coronal Radio Magnetography of Solar Active Region 8365
Authors: Ryabov, B. I.; Nindos, A.; Shibasaki, K.; Maksimov, V. P.;
   Lesovoi, S. V.; Pevtsov, A. A.
Bibcode: 2003AGUFMSH42B0508R
Altcode:
  Using the microwave radio observations with Siberian Solar Radio
  Telescope (SSRT) and Nobeyama Radio Heliograph (NoRH) we derived 2D
  coronal magnetograms of solar active region NOAA 8365. The circular
  polarization (CP) of radio source is modified, when the radiation
  passes through the overlying magnetic field transverse to the
  line-of-sight. This change in CP was used to study the properties of
  coronal fields. We employed the known theory of wave-mode coupling
  in quasi-transverse (QT) region to evaluate the distribution of the
  field strength at the level of transformation of 5.2 cm radiation
  (SSRT, field strength ∼ 10-30 G) and 1.76 cm (NoRH, ∼ 50-110
  G). The magnetic field strength was derived under the assumption
  N~L<SUB>α </SUB> = 10<SUP>18</SUP> cm<SUP>-2</SUP>, where N is electron
  density and L<SUB>α </SUB> is the scale of coronal field divergence
  along line-of-sight. The height of QT-region (H<SUB>QTR</SUB>) was
  estimated using force free field extrapolations, H<SUB>QRT</SUB> =
  6.3 x 10<SUP>9</SUP> cm (2.3 x 10<SUP>9</SUP> cm) for 20 G (85 G). We
  then compared the coronal radio magnetograms and the force free field
  extrapolation of photospheric magnetic field, and we found close
  similarity between them on large spatial scale.

Title: What Is the Role of the Kink Instability in Solar Coronal
    Eruptions?
Authors: Leamon, Robert J.; Canfield, Richard C.; Blehm, Zachary;
   Pevtsov, Alexei A.
Bibcode: 2003ApJ...596L.255L
Altcode:
  We report the results of two simple studies that seek observational
  evidence that solar coronal loops are unstable to the MHD kink
  instability above a certain critical value of the total twist. First,
  we have used Yohkoh soft X-ray telescope image sequences to measure the
  shapes of 191 X-ray sigmoids and to determine the histories of eruption
  (evidenced by cusp and arcade signatures) of their associated active
  regions. We find that the distribution of sigmoid shapes is quite
  narrow and the frequency of eruption does not depend significantly on
  shape. Second, we have used Mees Solar Observatory vector magnetograms
  to estimate the large-scale total twist of active regions in which
  flare-related signatures of eruption are observed. We find no evidence
  of eruption for values of large-scale total twist remotely approaching
  the threshold for the kink instability.

Title: Chirality of Chromospheric Filaments
Authors: Pevtsov, Alexei A.; Balasubramaniam, K. S.; Rogers, Joey W.
Bibcode: 2003ApJ...595..500P
Altcode:
  We use the chromospheric full-disk Hα observations to study the
  chirality of 2310 filaments from 2000-2001. For each filament,
  we identify the spine and its barbs and determine the filament
  chirality as fraction of dextral/sinistral barbs of the total number
  of barbs. We find that 80.2% (558 out of 696) of quiescent filaments
  in the northern hemisphere are dextral and 85.5% (633 out of 740)
  of filaments in southern hemisphere are sinistral, in agreement with
  the well-known hemispheric helicity rule. Our data also show that the
  active-region filaments follow the same rule, though the hemispheric
  dependence is weaker: 74.9% (338 out of 451) of active-region filaments
  in the northern hemisphere are dextral, and 76.7% (297 out of 387)
  of filaments in the southern hemisphere are sinistral. We show that
  quiescent filaments formed on leading and returning arms of the same
  switchback exhibit the same chirality. We also investigate a possible
  change in the hemispheric rule with polarity reversal of the polar
  field, and we find no such change.

Title: Helicity Evolution in Emerging Active Regions
Authors: Pevtsov, Alexei A.; Maleev, Vasily M.; Longcope, Dana W.
Bibcode: 2003ApJ...593.1217P
Altcode:
  We study the evolution of twist and magnetic helicity in the coronal
  fields of active regions as they emerge. We use multiday sequences of
  Solar and Heliospheric Observatory Michelson Doppler Interferometer
  magnetograms to characterize the region's emergence. We quantify
  the overall twist in the coronal field, α, by matching a linear
  force-free field to bright coronal structures in EUV images. At the
  beginning of emergence, all regions studied have α~=0. As the active
  region grows, α increases and reaches a plateau within approximately
  1 day of emergence. The inferred helicity transport rate is larger
  than differential rotation could produce. Following the 2000 work of
  Longcope &amp; Welsch, we develop a model for the injection of helicity
  into the corona by the emergence of a twisted flux tube. This model
  predicts a ramp-up period of approximately 1 day. The observed time
  history α(t) is fitted by this model assuming reasonable values for
  the subphotospheric Alfvén speed. The implication is that helicity
  is carried by twisted flux tubes rising from the convection zone and
  transported across the photosphere by spinning of the poles driven by
  magnetic torque.

Title: Chirality of Chromospheric Filaments
Authors: Pevtsov, A. A.; Balasubramaniam, K. S.; Rogers, J. W.
Bibcode: 2003SPD....34.0709P
Altcode: 2003BAAS...35..821P
  We use the chromospheric full disk H<SUB>α </SUB> observations
  to study chirality (sign of helicity) of 2310 filaments from
  2000--2001. For each filament we identify the spine and its barbs
  and determine the filament chirality as fraction of dextral/sinistral
  barbs of total number of barbs. We find that 80.2% (558 out of 696)
  of quiescent filaments in northern hemisphere are dextral, and 85.5%
  (633 out of 740) of filaments in southern hemisphere are sinistral,
  in agreement with well known hemispheric helicity rule. Our data
  also show that the active region filaments follow the same rule,
  though the hemispheric dependence is weaker; 74.9% (338 out of 451) of
  active region filaments in northern hemisphere are dextral, and 76.7%
  (297 out of 387) of filaments in southern hemisphere are sinistral. We
  show that quiescent filaments formed on leading and returning arms of
  the same switch-back exhibit the same chirality. We also investigate
  a possible change in the hemispheric rule with polarity reversal of
  the polar field and we find no such change.

Title: Transequatorial Loops: General Statistics
Authors: Pevtsov, A. A.
Bibcode: 2003SPD....34.1001P
Altcode: 2003BAAS...35..825P
  Transequatorial loops (TLs), -- coronal structures connecting magnetic
  fields in opposite hemispheres -- were first found in Skylab data,
  and later studied in more detail using Yohkoh SXT images. TLs are most
  clearly found in a few MK data (including EIT284Å). They may connect
  quiet Sun and/or active region areas in opposite hemispheres. In some
  cases, TLs may develop shortly after emergence of active region magnetic
  field, but sometimes they may be present before the active region
  emerges. Helicity of connected areas appears to play more important role
  that the separation between the regions. In most cases interconnected
  regions have the same sign of helicity. Often, TLs may connect two
  regions of same helicity that are far apart (&gt; 70<SUP>o</SUP>)
  and be absent between much closer regions (&lt; 20<SUP>o</SUP>) whose
  helicity is opposite to each other. There is an indication that TLs
  may repeatedly appear at certain longitudes, similar to complexes of
  activity. <P />In this talk, I will review the observational properties
  of transequatorial loops and the corresponding magnetic fields.

Title: Building an On-line Catalog of Correlated Solar Prominence
    and Sigmoid Images
Authors: Merriot, I. V.; Pevtsov, A.; Martens, P.
Bibcode: 2003SPD....34.0310M
Altcode: 2003BAAS...35R.809M
  Predicting solar weather is a complex endeavor. The correlation between
  X-ray sigmoids and coronal mass ejections (CMEs) has been established
  but the role of the prominence in association with the sigmoid and
  CME is not yet clear. This study catalogs the correlation between
  prominence behavior and sigmoids --before and after-- a CME takes
  place. <P />CME, sigmoid and filament occurrences were correlated
  using the Yashiro online catalog (SOHO), the SXT sigmoid catalog
  (Yohkoh) and archived H-alpha film from the flare patrol telscope
  (NSO/SP). When correlations were found, the H-alpha full disk images
  were digitized to embrace time periods of 2-3 hours before and after
  the CME event where changes in filament were notable. The culmination
  of this study will be a web-based catalog of data in text and images
  showing filament dynamics when sigmoids leading to CMEs are present. <P
  />This study is funded, in part, by the Murdoch Foundation.

Title: Signatures of Large Scale Coronal Eruptive Activity, Associated
    Flares, and Propagating Chromospheric Disturbances
Authors: Balasubramaniam, K. S.; Pevtsov, A. A.; Neidig, D. F.;
   Cliver, E. W.; Young, C. A.; Martin, S. F.; Kiplinger, A. L.
Bibcode: 2003SPD....34.0505B
Altcode: 2003BAAS...35..814B
  Analyses of multi-wavelength data sets on 2002 December 19 at
  approximately 2150 UT show evidence of a large-scale, transequatorial
  coronal eruption associated with simultaneous flares in active
  regions in both hemispheres. The coronal manifestations (based on
  EIT, LASCO, and TRACE images) include a large coronal dimming,
  an opening/restructuring of magnetic fields, the formation of a
  transient coronal hole, and a halo CME. In the chromosphere, ISOON
  H-alpha images show distant flare precursor brightenings and several
  sympathetic flares. Originating near the main flare is a rapidly
  propagating (800 km/s), narrowly channeled disturbance detectable as a
  sequential brightening of numerous pre-existing points in the H-alpha
  chromospheric network. This disturbance is not a chromospheric Moreton
  wave, but it does produce a temporary activation of a transequatorial
  filament. This filament does not erupt nor do any other filaments
  in the vicinity. MDI magnetograms show that the brightened network
  points are all of the same polarity (the dominant polarity among the
  points in the disturbance's path), suggesting that the affected field
  lines extend into the corona where they are energized in sequence
  as the eruption tears away. <P />Three other similar eruptive events
  (non-transequatorial) that we studied, while they are less impressive,
  show most of the same phenomena including distant sympathetic flares and
  a propagating disturbance showing close adherence to the monopolarity
  rule. Two of these events do include filament eruptions near the main
  flare. We conclude that the observations of these four events are
  consistent with large scale coronal eruptive activity that triggers
  nearly simultaneous surface activity of various forms separated by
  distances on the same scale as the coronal structures themselves. A
  filament eruption at the main flare site appears not to be a necessity
  in this type of eruptive activity.

Title: On a Cyclic Variation of the Hemispheric Helicity Rule.
Authors: Pevtsov, Alexei A.; Hagyard, Mona J.; Blehm, Zachary; Smith,
   James E.; Canfield, Richard C.; Sakurai, Takashi; Hagino, Masaoki
Bibcode: 2003IAUJD...3E..35P
Altcode:
  We report the result of a study of magnetic helicity in solar active
  regions during 1980-2000 (cycles 21-23). Using the vector magnetograms
  from four different instruments (Haleakala Stokes Polarimeter Marshall
  Space Flight Center Mitaka Solar Flare Telescope and Okayama Observatory
  Solar Telescope) we calculated the force-free parameter alpha as
  in Pevtsov et al. (1995). We use alpha as the proxy for current
  helicity. For each instrument we computed a gradient dalpha/dL as
  the linear fit of alpha versus latitude L using annual subsets of
  data. The hemispheric helicity rule (negative/positive helicity in
  northern/southern hemisphere) can be expressed in terms of this gradient
  as dalpha/dL &lt; 0. We find that each instrument exhibits change in
  sign of this gradient for some years which implies that the hemispheric
  helicity rule may not hold in some phases of a solar cycle (see Hagino
  and Sakurai 2002). However we do not see consistency between different
  instruments in regards to years disobeying the rule. The disagreement
  may be due to difference in observations and/or insufficient number of
  magnetograms in some years. We conclude that the present data sets do
  not allow to make statistically significant inference about possible
  cyclic variation of the hemispheric helicity rule.

Title: Vorticity patterns of sunspot H <SUB>α</SUB> whirls
Authors: Pevtsov, A. A.; Balasubramaniam, K. S.; Rogers, W. J.
Bibcode: 2003AdSpR..32.1905P
Altcode:
  We study vorticity patterns of 897 superpenumbral filaments (in 142
  sunspots) using Hα (BBSO) observations from July 2000 - April 2001. A
  majority (69%) of filaments show noticeable curvature in clockwise (CW)
  or counter-clockwise (CCW) sense. 73% of all sunspots contain both CW
  and CCW filaments in their superpenumbrae. Only 27% of sunspots have
  all their superpenumbral filaments twisted in the same direction. The
  average sunspot vorticity exhibits well-known hemispheric preference,
  but the dependence is weaker than in the previous studies: 34 (54%)
  out of 63 sunspots in the northern hemisphere exhibit CCW pattern of
  superpenumbral whirls, and 45 (69%) of 65 sunspots in the southern
  hemisphere show CW pattern. We suggest that the local distribution
  of magnetic fields around sunspots may affect the curvature of
  superpenumbral filaments and, hence, weaken the hemispheric helicity
  rule.

Title: Statistical Study of X-ray and EUV Bright Points
Authors: Sattarov, I.; Pevtsov, A. A.; Begimqulov, U.; Sherdonov,
   C.; Saidalieva, M.
Bibcode: 2003ASPC..286..393S
Altcode: 2003ctmf.conf..393S
  No abstract at ADS

Title: Preface
Authors: Buechner, Joerg; Pevtsov, Alexei A.
Bibcode: 2003AdSpR..32.1817B
Altcode:
  No abstract at ADS

Title: Helicity transport and generation in the solar convection zone
Authors: Longcope, D. W.; Pevtsov, A. A.
Bibcode: 2003AdSpR..32.1845L
Altcode:
  Magnetic helicity provides a theoretical tool for characterizing the
  solar dynamo and the evolution of the coronal field. The magnetic
  helicity may be inferred from several types of observation including
  vector magnetograms of the photospheric magnetic fields. The
  helicity of an active region reflects, to some degree, the twist
  in the magnetic field below it. Photospheric observations reveal
  a tendency for left-handed chirality in the Northern hemisphere,
  although one-quarter to one-third of the active regions twist in the
  opposite sense. This means that coronal magnetic field has negative
  helicity in the North. Sub-photospheric fields will have left-handed
  twist in the North, although the net helicity also depends on the
  writhe of the flux tube axes. We show that buffeting by turbulence; the
  so-called Σ-effect, can explain the handedness and level of intrinsic
  variation of observed twist. This mechanism does not generate helicity,
  rather it produces twist and writhe of opposite signs. In this scenario,
  helicity of one sign propagates into the corona, while opposing helicity
  propagates downward in the form of torsional Alfvén waves.

Title: Magnetic twist and writhe of active regions. On the origin
    of deformed flux tubes
Authors: López Fuentes, M. C.; Démoulin, P.; Mandrini, C. H.;
   Pevtsov, A. A.; van Driel-Gesztelyi, L.
Bibcode: 2003A&A...397..305L
Altcode: 2014arXiv1411.5626L
  We study the long term evolution of a set of 22 bipolar active regions
  (ARs) in which the main photospheric polarities are seen to rotate one
  around the other during several solar rotations. We first show that
  differential rotation is not at the origin of this large change in
  the tilt angle. A possible origin of this distortion is the nonlinear
  development of a kink-instability at the base of the convective zone;
  this would imply the formation of a non-planar flux tube which,
  while emerging across the photosphere, would show a rotation of its
  photospheric polarities as observed. A characteristic of the flux tubes
  deformed by this mechanism is that their magnetic twist and writhe
  should have the same sign. From the observed evolution of the tilt of
  the bipoles, we derive the sign of the writhe of the flux tube forming
  each AR; while we compute the sign of the twist from transverse field
  measurements. Comparing the handedness of the magnetic twist and writhe,
  we find that the presence of kink-unstable flux tubes is coherent with
  no more than 35% of the 20 cases for which the sign of the twist can
  be unambiguously determined. Since at most only a fraction of the tilt
  evolution can be explained by this process, we discuss the role that
  other mechanisms may play in the inferred deformation. We find that
  36% of the 22 cases may result from the action of the Coriolis force
  as the flux tube travels through the convection zone. Furthermore,
  because several bipoles overpass in their rotation the mean toroidal
  (East-West) direction or rotate away from it, we propose that a
  possible explanation for the deformation of all these flux tubes
  may lie in the interaction with large-scale vortical motions of the
  plasma in the convection zone, including also photospheric or shallow
  sub-photospheric large scale flows.

Title: Current Theoretical Models and Future High Resolution Solar
Observations: Preparing for ATST
Authors: Pevtsov, Alexei A.; Uitenbroek, Han
Bibcode: 2003ASPC..286.....P
Altcode: 2003ctmf.conf.....P
  No abstract at ADS

Title: Helicity Generation and Signature in the Solar Atmosphere
Authors: Pevtsov, Alexei A.
Bibcode: 2003IAUJD...3E..21P
Altcode:
  To fully understand the origin evolution and topology of solar magnetic
  fields one should comprehend their magnetic helicity. Observationally
  non-zero helicity reveals itself in the patterns of electric currents
  inside active regions super-/penumbral sunspot whirls the shape of
  coronal loops and the fine structure of chromospheric filaments. The
  patterns of helicity are present on different spatial scales ranging
  from several arc seconds to a size of a solar hemisphere. Some
  patterns may bear information about deep subphotospheric processes
  (e.g. dynamo turbulent convection). Other may originate at or near the
  photosphere. In this talk we will review the observations of magnetic
  and current helicity on the Sun. We will also discuss the different
  mechanisms operating at or near the photosphere and we will compare
  them with the observations.

Title: Of Twist and Tilt
Authors: Holder, Zachary A.; Canfield, Richard C.; McMullen, Rebecca
   A.; Howard, Robert F.; Pevtsov, Alexei A.
Bibcode: 2003IAUJD...3E..27H
Altcode:
  Using Haleakala Stokes Polarimeter active-region vector magnetograms
  and Mt. Wilson Observatory full-disk longitudinal magnetograms we
  measure both the overall twist (using the force-free-field parameter
  alpha) and tilt of 368 active regions. This dataset clearly shows two
  well-known phenomena Joy's law and the hemispheric helicity rule as well
  as a lesser-known twist-tilt relationship which is the point of this
  work. Those regions that closely follow Joy's law show no twist-tilt
  relationship as expected if the twist originates from convective
  buffeting of initially untwisted and unwrithed flux tubes within the
  convection zone through the Sigma effect. Those regions that strongly
  depart from Joy's law show significantly larger than average twist
  and a very strong twist-tilt relationship. These properties suggest
  that the twist-tilt relationship in these regions is due to kinking of
  flux tubes that are initially highly twisted but not strongly writhed
  perhaps as a result of dynamo action

Title: Helicity patterns on the sun
Authors: Pevtsov, A. A.; Balasubramaniam, K. S.
Bibcode: 2003AdSpR..32.1867P
Altcode:
  Solar magnetic fields exhibit hemispheric preference for negative
  (positive) helicity in northern (southern) hemisphere. The hemispheric
  helicity rule, however, is not very strong, — the patterns of
  mixed helicity were observed at different spatial scales in each
  hemisphere. Helicity patterns on scales larger than the size of typical
  active region were observed in distribution of active regions with
  abnormal (for a give hemisphere) helicity, in large-scale photospheric
  magnetic fields and coronal flux systems. We review the observations
  of large-scale patterns of helicity in solar atmosphere and their
  possible relationship with (sub-)photospheric processes.

Title: Calculation of Magnetic Helicity in a Force-Free Field
Authors: Updike, A. C.; Pevtsov, A.
Bibcode: 2002AAS...201.8306U
Altcode: 2002BAAS...34.1242U
  One of the goals of solar physics is to understand the solar dynamo. By
  studying the nature of magnetic helicity, we are studying the solar
  magnetic field driven by the dynamo. We used chromospheric magnetographs
  in the Ca II line (8543 Å) and EIT images in the Fe XII line (195
  Å) to determine the degree of twist in the magnetic field. Using
  this degree of twist and the known magnetic energy for a given active
  region, we were able to measure the magnetic helicity in this force-free
  region. Over a period of three years (August 1996 to November 1998),
  we studied 37 active regions. We found a 73% hemispheric preference in
  the sign of the helicity - in the northern hemisphere, 70% of the active
  regions exhibited negative helicity; in the southern hemisphere, 76.5%
  of the active regions exhibited positive helicity. Our observations
  agree with earlier studies of the sign of helicity, which used vector
  magnetograms and studies of quiescent filiments.

Title: Current Helicity of Emerging Active Regions
Authors: Pevtsov, A.
Bibcode: 2002AGUFMSH52A0466P
Altcode:
  We employ the SOHO MDI magnetograms and EIT images to study evolution
  of current helicity of solar active regions during early stages of
  their emergence. Using longitudinal magnetograms we compute linear
  force-free fields ∇ x B = α B and compare extrapolated field lines
  with bright coronal structures to constrain the value of α. At the
  beginning of emergence all studied regions have small α ~eq 0. As
  active region grows, α gradually increases and reaches a "plateau"
  within approximately one day of emergence. Using change in separation
  between negative and positive fluxes, we divide regions on "slow" and
  "rapid" emergence. Three regions show "slow" (&gt; 1 day) emergence. For
  these regions α increases faster than the separation. In two "rapid"
  (&lt; 1 day) emerging regions α grows slower that the separation. This
  observed evolution of current helicity is in agreement with Longcope
  and Welsch (2000) model of emergence of subphotospheric twisted flux
  rope into the corona. škip 0.5 truecm V. Maleev is NSO 2002 Summer
  Research Assistant from St. Petersburg State University, Russia

Title: On the origin of peculiar active regions
Authors: Mandrini, C. H.; López Fuentes, M. C.; Démoulin, P.;
   van Driel-Gesztelyi, L.; Pevtsov, A. A.
Bibcode: 2002ESASP.505..121M
Altcode: 2002solm.conf..121M; 2002IAUCo.188..121M
  We study the long term evolution of a set of bipolar active regions
  (ARs) in which the main photospheric polarities are seen to rotate one
  around the other during several solar rotations. After showing that
  differential rotation cannot produce this large change in the tilt
  angle, we interpret this peculiar evolution as being the result of the
  emergence of magnetic flux tubes that are distorted with respect to
  the classical Ω-loop shape. A possible origin of this distortion is
  the nonlinear development of a kink-instability. Flux tubes deformed
  by this mechanism must have the same sign of twist and writhe. From
  the observed evolution of the tilt of the bipoles, we derive the
  sign of the writhe of the tube forming each AR; while we compute
  the sign of the twist from transverse field measurements. Comparing
  the handedness of the magnetic twist and writhe, we find that the
  presence of kink-unstable flux tubes is coherent with no more than 32%
  of the studied cases; so, a small fraction of these peculiar ARs can
  be explained by this process. Then we discuss the role that other
  mechanisms may play inducing the inferred deformation, such as the
  Coriolis force or external rotational motions of the plasma as the
  tube ascends in the convection zone.

Title: Properties of magnetic clouds and geomagnetic storms associated
    with eruption of coronal sigmoids
Authors: Leamon, Robert J.; Canfield, Richard C.; Pevtsov, Alexei A.
Bibcode: 2002JGRA..107.1234L
Altcode:
  We study 46 solar coronal eruptions associated with sigmoids seen
  in images from the Yohkoh Soft X-ray Telescope (SXT). We relate
  the properties of the sigmoids to in situ measurements at 1 AU and
  geomagnetic storms. Our primary result is that erupting sigmoids tend
  to produce geoeffective magnetic clouds (MCs): 85% of the erupting
  sigmoidal structures studied spawned at least a "moderate" (|Dst| ≥
  50 nT) geomagnetic storm. A collateral result is that MCs associated
  with sigmoids do not show the same solar-terrestrial correlations
  as those associated with filaments and, as such, form a distinct
  class of events. First, rather than reversing with the global solar
  dipole (at solar maximum), the leading field in MCs weakly (2:1)
  shows a solar cycle (Hale polarity) based correlation (reversing at
  solar minimum). Second, whereas the handedness of MCs associated with
  filament eruptions is strongly (95%) related to their launch hemisphere,
  that of MCs associated with sigmoid eruptions is only weakly (∼70%)
  so related. Finally, we are unaware of any model of the magnetic fields
  of sigmoids and their eruption that gives a useful prediction of the
  leading field orientation of their associated MC.

Title: The Twist and Tilt of Active Region Magnetic Fields
Authors: Holder, Z. A.; McMullen, R. A.; Canfield, R. C.; Howard,
   R. F.; Pevtsov, A. A.
Bibcode: 2002AAS...200.0305H
Altcode: 2002BAAS...34..642H
  We combine two large datasets to study the formation of the overall
  twist that is present in solar active-region magnetic fields. For
  purposes of discussion, we adopt a simplified model in which the
  magnetic flux bundles that form active regions start as axisymmetric
  toroids, without intrinsic twist, and rise in Ω -shaped loops through
  the convection zone, acquiring writhe through the effect of the Coriolis
  force on the large-scale flows within them, as well as buffeting by
  turbulent convection. The tilt of active regions with respect to the
  equator is an observable manifestation of such writhe, at photospheric
  levels. Owing to magnetic helicity conservation, we expect this tilt
  to be related to twist of the fields of these same regions. Using Mees
  Solar Observatory active-region vector magnetograms and Mount Wilson
  Observatory full-disk longitudinal magnetograms, we have measured both
  the tilt (Mount Wilson) and twist (Mees) of their magnetic fields,
  on active-region scales. This dataset clearly shows two well-known
  phenomena, Joy's law and the hemispheric handedness rule. In this
  paper we present the relationship between twist and tilt and estimate
  the extent to which that relationship is due to a mutual dependence of
  tilt and twist on latitude. We then compare our observational results
  to the simplified model.

Title: Comparison of Helicity Proxies Observed by Haleakala Stokes
    Polarimeter and Huairou Solar Observing Station Vector Magnetograph
Authors: Pevtsov, A. A.; Dun, J. P.; Zhang, H.
Bibcode: 2002AAS...200.0304P
Altcode: 2002BAAS...34..642P
  We conduct detailed comparison of the current helicity measurements
  derived using observations from two vector magnetographs - Haleakala
  Stokes Polarimeter (HSP) at Mees Solar Observatory, and Solar Magnetic
  Fields Telescope (SMFT) at Huairou Solar Observing Station. The data,
  -- 269 pairs of vector magnetograms -- are observed in 1997-2000. We
  apply the same procedure to both data sets to resolve 180\arcdeg\
  azimuth ambiguity and compute the linear force free field coefficient
  α . We find that in 205 (76%) of 269 cases the SMFT and HSP data show
  the same sign of α . Remaining 64 (24%) magnetograms that exhibit
  opposite sign of helicity in two instruments can be explained by several
  reasons including incorrect heliographic coordinates of active regions,
  lack of persistence in polarimetric measurements, spatial misalignment
  between Stokes I, Q, U and V maps, difference in noise level and active
  regions evolution.

Title: Active-Region Filaments and X-ray Sigmoids
Authors: Pevtsov, Alexei A.
Bibcode: 2002SoPh..207..111P
Altcode:
  We use Yohkoh soft X-ray telescope data and Hα full-disk observations
  to study the evolution of chromospheric filaments and coronal
  sigmoids in 6 active regions in association with coronal mass
  ejections (CMEs). In two cases, CMEs are directly observed by the
  SOHO/LASCO C2 coronagraph. In four cases, other observations (magnetic
  clouds, geomagnetic storms, sigmoid-arcade evolution) are used as CME
  indicators. Prior to eruption, each active region shows a bright coronal
  sigmoidal loop and underlying Hα filament. The sigmoid activates,
  erupts and gets replaced by a cusp, or an arcade. In contrast, the
  Hα filament shows no significant changes in association with sigmoid
  eruption and CME. We explain these observations in a framework of the
  classical two-ribbon flare model.

Title: What is the role of the kink instability in eruption of
    X-ray sigmoids?
Authors: Leamon, R.; Canfield, R.; Blehm, Z.; Pevtsov, A.
Bibcode: 2002AGUSMSH32D..03L
Altcode:
  Observers see ample evidence of helical structures in erupting solar
  filaments, X-ray sigmoids and CMEs. It has been argued that the total
  amount of twist in a given loop is a factor in its MHD stability
  [Priest, 1984]. A simple model illustrates this point. Consider a
  cylindrical force-free magnetic field with constant α = T /L, where
  L is the length of the tube and T is the total twist contained within
  it. The tube is stable to the MHD kink instability for total twist
  below a critical value T<SUB>c</SUB> ~ 2 π . Rust and Kumar [1996]
  compared the shape of 49 transient, bright sigmoid structures to the
  signature of a helically kinked flux rope. From a study of the aspect
  ratios of these transient sigmoid brightenings, they inferred that the
  cause of CMEs is the eruption of an unstable, kinked magnetic field. We
  have analyzed 155 X-ray sigmoids in the the Yohkoh SXT data, measuring
  the angle γ at which the sigmoid crosses its central axis and the
  length of the sigmoid along that axis (which is not identical to L,
  but is closely related to it). In a simple 2D force-free analysis,
  Pevtsov et al. [1997] showed that α = ( π / L ) sin γ , implying
  that sin γ is a measure of the total twist T. By simple visual
  inspection of the Yohkoh SXT movies, we have identified well-known
  signatures of eruption, i.e., X-ray cusps and arcades. We find no
  relationship between the frequency of occurrence of such signatures
  of eruption and sin γ . {Pevtsov, A. A.}, {Canfield, R. C.}, and
  {McClymont, A. N.}, Astrophys J., 481, 973, 1997. {Priest, E. R.},
  {Solar Magneto-Hydrodynamics}, {Reidel: Dordrecht}, {1984}. {Rust,
  D. M.}, and {Kumar, A.}, Astrophys J., 464, L199, 1996.

Title: What is the role of the kink instability in eruption of
    X-ray sigmoids?
Authors: Canfield, R. C.; Leamon, R. J.; Blehm, Z.; Pevtsov, A. A.
Bibcode: 2002AAS...200.2001C
Altcode: 2002BAAS...34..672C
  Observers see ample evidence of helical structures in erupting solar
  filaments, X-ray sigmoids and CMEs. It has been argued that the total
  amount of twist in a given loop is a factor in its MHD stability
  [Priest, 1984]. A simple model illustrates this point. Consider
  a cylindrical force-free magnetic field with constant α = T /L,
  where L is the length of the tube and T is the total twist contained
  within it. The tube is stable to the MHD kink instability for total
  twist below a critical value T<SUB>c</SUB> ~ 2 π . Rust and Kumar
  [1996] compared the shape of 49 transient, bright sigmoid structures
  to the signature of a helically kinked flux rope. From a study of the
  aspect ratios of these transient sigmoid brightenings, they inferred
  that the cause of CMEs is the eruption of an unstable, kinked magnetic
  field. We have analyzed 191 X-ray sigmoids in the the Yohkoh SXT data,
  measuring the angle γ at which the sigmoid crosses its central axis
  and the length of the sigmoid along that axis (which is not identical
  to L, but is closely related to it). In a simple 2D force-free analysis,
  Pevtsov et al. [1997] showed that α = ( π / L ) sin γ , implying that
  sin γ is a measure of the total twist T. By simple visual inspection
  of the Yohkoh SXT movies, we have identified well-known signatures of
  eruption, i.e., X-ray cusps and arcades. We find no relationship between
  the frequency of occurrence of such signatures of eruption and sin γ .

Title: X-Ray Bright Points and Photospheric Bipoles during Cycles
    22 and 23
Authors: Sattarov, Isroil; Pevtsov, Alexei A.; Hojaev, Alisher S.;
   Sherdonov, C. T.
Bibcode: 2002ApJ...564.1042S
Altcode:
  We use the Yohkoh soft X-ray telescope (SXT) full-disk images made from
  1993 to 2000 to manually identify X-ray bright points (XBPs). We also
  employ the National Solar Observatory (Kitt Peak) full-disk longitudinal
  magnetograms made between 1992 April and 2001 April and an automatic
  procedure to identify photospheric bipoles whose magnetic field strength
  is above 20 G, with a pole size (cross-section) between 5" and 55.2",
  and with pole separation between 5.5" and 48.3". We use these data to
  study statistical properties of XBPs and photospheric bipoles during
  the declining phase of solar cycle 22 and the rising phase of cycle
  23. The XBP number follows well-known anti-cycle variation. The average
  number of XBPs (~10 per disk image) remained approximately the same
  from 1993 to 1994. Beginning in 1995 it grew, reached a maximum around
  1996 December (~50 XBPs per image), and then dropped back to pre-1995
  levels in 1998. By contrast, the average number of photospheric bipoles
  remained approximately the same (~250 per disk image) between 1992
  and 2001, despite sunspot activity changes from high (1992, cycle 22)
  to low (1996, solar minimum) and a return to high activity again in
  2000 (solar maximum, cycle 23). Since we expect that a fraction of
  photospheric bipoles associated with X-ray bright points is independent
  of solar activity, we see this as a clear indication that the anti-cycle
  variation of XBP numbers is not real. Most likely, the variation in
  XBP numbers is the result of a change in the background brightness
  of the quiet-Sun corona, which is affected by the presence of active
  regions. On the other hand, annual latitudinal histograms of XBPs show
  an excess of coronal bright points at active region latitudes, contrary
  to the effect of changing background brightness. Photospheric bipoles
  show no enhancement of their distribution at active latitudes. We
  consider two alternative explanations for this inconsistency.

Title: Solar Cycle Dependency of X-Ray Bright Points and Photospheric
    Bipoles
Authors: Sattarov, I.; Pevtsov, A. A.; Hojaev, A. S.; Sherdonov, C. T.
Bibcode: 2002mwoc.conf...97S
Altcode:
  Using daily Yohkoh SXT full disk images from 1991-2000 we manually
  identify X-ray bright points (XBPs). We also employ the NSO/KP full disk
  magnetograms from April 1992-June 2000 to identify the photospheric
  bipoles with particular magnetic flux and poles separation. We use
  these data to study statistical properties of XBPs and photospheric
  bipoles during declining phase of cycle 22 and rising phase of cycle
  23. The number of X-ray bright points follows well-known anti-cycle
  variation, reaching a maximum in 1996 (sunspot minimum). By contrast,
  the averaged number of photospheric bipoles remains approximately the
  same during 1992-2000. We see this inconsistency between X-ray and
  magnetic data as clear indication that anti-cycle variation of XBP
  number is apparent, not real effect.

Title: Inferring the Writhe of Emerging Flux Tubes from the Evolution
    of the Orientation of Bipole Axes
Authors: López Fuentes, M. C.; Mandrini, C. H.; Démoulin, P.;
   van Driel-Gesztelyi, L.; Pevtsov, A.
Bibcode: 2002RMxAC..14..108L
Altcode:
  No abstract at ADS

Title: Property and evolution of EUV and X-ray bright points in
    coronal holes
Authors: Sattarov, I.; Pevtsov, A.; Hojaev, A.; Begimqulov, U.;
   Saydalieva, M.; Sherdonov, C.
Bibcode: 2002cosp...34E1156S
Altcode: 2002cosp.meetE1156S
  It has been shown (e.g, Sattarov et al. 2002, ApJ, 564, 1042) that
  solar cycle variation of number of X-ray bright points (XBPs) can be
  caused by the visibility effect, i.e., the increased background due to
  scattering from bright active regions alters identification of XBPs. In
  contrast, the number of photospheric bipoles - the magnetic counterparts
  of bright points - is independent of solar cycle. On the other hand,
  several previous studies reported enhanced density of XBPs in coronal
  holes. It has also been shown that there is positive correlation
  between XBP number in coronal holes and solar wind density. We study
  magnetic and thermal properties, location and motions of EUV and X-ray
  bright points in coronal holes using observations from SOHO EIT (171,
  195 and 304 A) and Yohkoh SXT.

Title: Helicity patterns on the Sun
Authors: Pevtsov, A.
Bibcode: 2002cosp...34E3178P
Altcode: 2002cosp.meetE3178P
  Solar magnetic fields exhibit hemispheric preference for negative (pos-
  itive) helicity in northern (southern) hemisphere. The hemispheric he-
  licity rule, however, is not very strong, - the patterns of opposite
  sign helicity were observed on different spatial scales in each
  hemisphere. For instance, many individual sunspots exhibit patches
  of opposite he- licity inside the single polarity field. There are
  also helicity patterns on scales larger than the size of typical
  active region. Such patterns were observed in distribution of active
  regions with abnormal (for a give hemisphere) helicity, in large-scale
  photospheric magnetic fields and coronal flux systems. We will review
  the observations of large-scale pat- terns of helicity in solar
  atmosphere and their possible relationship with (sub-)photospheric
  processes. The emphasis will be on large-scale pho- tospheric magnetic
  field and solar corona.

Title: Property and evolution of EUV and X-ray bright points in
    coronal holes
Authors: Sattarov, I.; Pevtsov, A.; Hojaev, A.; Begimqulov, U.;
   Saydalieva, M.; Sherdonov, C.
Bibcode: 2002cosp...34E1152S
Altcode: 2002cosp.meetE1152S
  It has been shown (e.g, Sattarov et al. 2002, ApJ, 564, 1042) that
  solar cycle variation of number of X-ray bright points (XBPs) can be
  caused by the visibility effect, i.e., the increased background due to
  scattering from bright active regions alters identification of XBPs. In
  contrast, the number of photospheric bipoles - the magnetic counterparts
  of bright points - is independent of solar cycle. On the other hand,
  several previous studies reported enhanced density of XBPs in coronal
  holes. It has also been shown that there is positive correlation
  between XBP number in coronal holes and solar wind density. We study
  magnetic and thermal properties, location and motions of EUV and X-ray
  bright points in coronal holes using observations from SOHO EIT (171,
  195 and 304 A) and Yohkoh SXT.

Title: Vorticity patterns of sunspot H-alpha whirls
Authors: Pevtsov, A.; Balasubramaniam, K.; Rogers, J.
Bibcode: 2002cosp...34E1105P
Altcode: 2002cosp.meetE1105P
  We study vorticity patterns of 1003 superpenumbral filaments using H
  (BBSO) data of 145 sunspots observed from July 2000 - April 2001. A
  majority (67%) of filaments show noticeable curvature in clockwise (CW)
  or counter-clockwise (CCW) sense. The 75% of all sunspots contain both
  CW and CCW filaments in their superpenumbrae. Only 25% of sunspots have
  all their superpenumbral filaments twisted in the same direction. The
  average sunspot vorticity exhibits well known hemispheric preference,
  but the dependence is weaker than in previous studies. The 42 (58%)
  out of 73 sunspots in the northern hemisphere exhibit CCW pattern of
  superpenumbral whirls. The 43 (61%) of 70 sunspots in the southern
  hemisphere show CW pattern. The weak hemispheric asymmetry suggests
  that the mechanisms with strong hemispheric dependency (e.g., Coriolis
  force, solar differential rotation) may not play a dominant role in
  the hemispheric chirality (helicity) rule. Mr. J. Rogers was supported
  by the 2001 NSF/RET Program at NSO/SP.

Title: Property and evolution of EUV and X-ray bright points in
    coronal holes
Authors: Sattarov, I.; Pevtsov, A.; Hojaev, A.; Begimqulov, U.;
   Saydalieva, M.; Sherdonov, C.
Bibcode: 2002cosp...34E1155S
Altcode: 2002cosp.meetE1155S
  It has been shown (e.g, Sattarov et al. 2002, ApJ, 564, 1042) that
  solar cycle variation of number of X-ray bright points (XBPs) can be
  caused by the visibility effect, i.e., the increased background due to
  scattering from bright active regions alters identification of XBPs. In
  contrast, the number of photospheric bipoles - the magnetic counterparts
  of bright points - is independent of solar cycle. On the other hand,
  several previous studies reported enhanced density of XBPs in coronal
  holes. It has also been shown that there is positive correlation
  between XBP number in coronal holes and solar wind density. We study
  magnetic and thermal properties, location and motions of EUV and X-ray
  bright points in coronal holes using observations from SOHO EIT (171,
  195 and 304 A) and Yohkoh SXT.

Title: Property and evolution of EUV and X-ray bright points in
    coronal holes
Authors: Sattarov, I.; Pevtsov, A.; Hojaev, A.; Begimqulov, U.;
   Saydalieva, M.; Sherdonov, C.
Bibcode: 2002cosp...34E1154S
Altcode: 2002cosp.meetE1154S
  It has been shown (e.g, Sattarov et al. 2002, ApJ, 564, 1042) that
  solar cycle variation of number of X-ray bright points (XBPs) can be
  caused by the visibility effect, i.e., the increased background due to
  scattering from bright active regions alters identification of XBPs. In
  contrast, the number of photospheric bipoles - the magnetic counterparts
  of bright points - is independent of solar cycle. On the other hand,
  several previous studies reported enhanced density of XBPs in coronal
  holes. It has also been shown that there is positive correlation
  between XBP number in coronal holes and solar wind density. We study
  magnetic and thermal properties, location and motions of EUV and X-ray
  bright points in coronal holes using observations from SOHO EIT (171,
  195 and 304 A) and Yohkoh SXT.

Title: Sinuous Coronal Loops at the Sun [Invited]
Authors: Pevtsov, A. A.
Bibcode: 2002mwoc.conf..125P
Altcode:
  The sinuous coronal loops - sigmoids, first noted in the Skylab X-ray
  observations in association with a CME, - are commonly observed in
  Yohkoh soft X-ray telescope (SXT) data. The S-shape of these loops
  is a manifestation of the helical structure of the coronal magnetic
  fields and, hence, follows the hemispheric helicity (chirality)
  rule established for quiescent filaments and photospheric magnetic
  fields. The forward-S (inverse-S) sigmoids prevail in southern
  (northern) hemisphere, independent of the solar cycle. Sigmoids
  are often associated with the CMEs; they exist prior to eruption and
  disappear after. In addition, active regions that exhibit sinuous loops
  are more likely to be eruptive than non-sigmoidal regions. Once erupted,
  sigmoids tend to produce stronger geomagnetic storms, and often the
  orientation of magnetic field in interplanetary disturbance can be
  directly linked to the coronal field of a sigmoid. In this talk we
  review the observational properties of sigmoids, current theoretical
  models and application of sinuous loops to space weather forecasting.

Title: Helicity transport and creation in the solar convection zone
Authors: Longcope, D.; Pevtsov, A.
Bibcode: 2002cosp...34E3177L
Altcode: 2002cosp.meetE3177L
  Magnetic helicity provides a theoretical tool for characterizing the
  solar dynamo and the evolution of the coronal field. The magnetic
  helicity may be inferred from several types of observation including
  vector magnetograms of the photospehric magnetic fields. The helicty
  of an active region reflects, to some degree, that produced by the
  solar cycle dyanmo which is believed to be operating at the base of
  the convection zone, where the Rossby number is small. The helicty of
  the active region is affected by the turbulence through which it rises,
  and this process must be taken into account when interpreting helicity
  observations. The subsequent dispersal of the active region magnetic
  field will further affect the observed helicty of the photospheric
  field. This transport process suggests an observational method of
  identifying, through helicty measurements, the source of quiet Sun
  field from either a surface (non-helical) dynamo or the fragmentation
  of helical active region fields.

Title: Solar magnetic fields and geomagnetic events
Authors: Pevtsov, Alexei A.; Canfield, Richard C.
Bibcode: 2001JGR...10625191P
Altcode:
  Some interplanetary studies lead one to expect that the toroidal fields
  of individual active regions are directly related to their heliospheric
  structure. Other studies conclude that the large-scale solar dipolar
  field is more important. We have carried out two studies that bear on
  these apparently conflicting views. We first studied geomagnetic events
  temporally associated with the eruption of 18 individual coronal X-ray
  sigmoids, which occurred while the large-scale solar dipolar magnetic
  field pointed southward. We found that if a coronal flux rope model is
  used to interpret magnetic structure, eruptions with a southward leading
  magnetic field are associated with stronger geomagnetic storms, but
  those with a northward leading field are associated with more storms. We
  next studied three full magnetic cycles, solar cycles 17-22. We examined
  the temporal variation of the ratio of the geomagnetic A<SUB>p</SUB>
  index to the sunspot number. We found no statistically compelling
  fluctuations of this quantity on solar cycle time scales that are in
  phase with the reversal of active region polarities. On the other hand,
  we found a weak tendency for fluctuations that are in phase with the
  reversal of the large-scale solar dipole field. From these two studies
  we infer that the magnetic structure of individual active regions
  plays a role in geomagnetic events, but their geoeffectiveness is
  complicated by asymmetries in the leading and following magnetic field
  and density. We conclude that simple cycle-dependent generalizations
  have only statistical significance, and cannot dependably be used to
  predict the geomagnetic effects of a given solar eruption.

Title: Soft X-Ray Luminosity and Photospheric Magnetic Field in
    Quiet Sun
Authors: Pevtsov, Alexei A.; Acton, Loren W.
Bibcode: 2001ApJ...554..416P
Altcode:
  We use full-disk soft X-ray data from Yohkoh and Kitt Peak daily
  magnetograms to study the coronal irradiance and photospheric magnetic
  field remote from active regions between 1991 November and 1998
  December. For every image of our data set we extract three areas
  4°×4° in size centered at N00° W00°, N50° W00°, and S50°
  W00° and compute X-ray irradiance and unsigned magnetic flux for
  each of these areas. Between 1991 (active Sun) and 1996 (quiet Sun)
  the X-ray irradiance at the heliographic center decreased by more than
  a factor of 7, while the magnetic flux decreased by only a factor of
  2. A similar tendency is observed for our high-latitude samples. Apart
  from the cycle-related variations, all three areas of quiet Sun
  exhibit significant nonperiodic changes in X-ray irradiance. These
  variations occur on 9-12 month intervals and clearly correlate with
  changes in sunspot activity. Similar variations are present in the
  total X-ray irradiance averaged over the solar disk. By contrast, the
  magnetic fluxes from the photosphere beneath these same areas show no
  corresponding variations on this time scale. In our opinion, coronal
  heating models based on the reconnection of quiet-Sun magnetic elements
  (variously called chromospheric network, ``magnetic carpet,'' or ``salt
  and pepper'' field) can at best account for a minimal contribution to
  heating the million-degree corona observed by the Yohkoh soft X-ray
  telescope. We conclude that the X-ray irradiance in the quiet Sun
  (at least in the Yohkoh temperature range, &gt;2 MK) is primarily
  associated with the strong magnetic fields of active regions, not with
  weak photospheric fields. The association, however, is not direct. We
  interpret the enhanced X-ray irradiance above the quiet Sun not as a
  result of the coronal ``canopy'' formed by the active-region magnetic
  field above the quiet-Sun areas, but as the large-scale relaxation
  process in the corona triggered by the evolution of active regions. To
  further support this conclusion, we show examples of active and quiet
  hemispheres in 1996 with similar weak-field properties but greatly
  different global X-ray emission and a pronounced change in X-ray
  irradiance over the entire visible hemisphere that was associated with
  the emergence of a single small active region.

Title: Where the Quiet Sun Magnetic Field Comes From?
Authors: Pevtsov, A. A.; Longcope, D. W.
Bibcode: 2001AGUSM..SP41C06P
Altcode:
  It has been recently suggested that there is two separate dynamo
  operating on the Sun. A subphotospheric (e.g. overshoot region) dynamo
  is responsible for strong magnetic fields of active regions, while the
  quiet Sun field is generated by the local (surface) dynamo driven by
  granular flows (Cattaneo 1999). Compelling observational evidence in
  support of the surface dynamo is still lacking. Because of the small
  characteristic size and lifetime of granular flows, the Coriolis force
  has no significant effect on them. Consequently the kinetic helicity of
  granules will not depend of hemisphere or vary with latitude; it will
  almost certainly average to zero. Magnetic field generated by such
  a (non-helical) dynamo should exhibit no hemispheric helicity rule
  either. In contrast, the sub-photospheric dynamo flows have non-zero
  kinetic helicity that changes sign across the solar equator. This
  dynamo will introduce hemispheric asymmetry in magnetic field's twist:
  positive helicity in southern hemisphere and negative in northern
  hemisphere. An observed hemispheric helicity rule for active region
  magnetic fields is well documented (e.g. Pevtsov et al 1995). Thus, the
  helicity approach can be used to distinguish between sub-photospheric
  (helical) and surface (non-helical) solar dynamos. Using vector
  magnetograms from the Advanced Stokes Polarimeter we measure current
  helicity density α <SUB>z</SUB> = J<SUB>z</SUB> / B<SUB>z</SUB>
  of photospheric field in the quiet Sun at few fixed latitudes. Our
  results indicate a weak hemispheric asymmetry in distribution of α
  <SUB>z</SUB> with a tendency for averaged helicity to be negative in
  the northern hemisphere and positive in the southern hemisphere. We
  interpret this asymmetry in a framework of the sub-photospheric origin
  of the photospheric field in the quiet Sun.

Title: Evidence of Separator Reconnection in a Survey of X-Ray
    Bright Points
Authors: Longcope, D. W.; Kankelborg, C. C.; Nelson, J. L.; Pevtsov,
   A. A.
Bibcode: 2001ApJ...553..429L
Altcode:
  X-ray bright points are among the simplest coronal structures
  hypothesized to be powered by magnetic reconnection. Their
  magnetic field appears to consist of a simple loop of field lines
  connecting positive to negative photospheric sources. Quantitative
  three-dimensional models of reconnection in this geometry are therefore
  expected to apply directly to X-ray bright points. We assemble a survey
  from archival Solar and Heliospheric Observatory data of 764 X-ray
  bright points (EUV Imaging Telescope) along with their associated
  photospheric magnetic fields (Solar Oscillation Imager/Michelson
  Doppler Imager). Measurements are made of each quantity relevant to
  the simple three-dimensional reconnection model. These data support
  several predictions of a magnetic reconnection model providing further
  evidence in favor of the hypothesis that magnetic reconnection supplies
  heating power to the quiet solar corona.

Title: Properties of Magnetic Clouds Resulting from Eruption of
    Coronal Sigmoids
Authors: Leamon, R. J.; Canfield, R. C.; Pevtsov, A. A.
Bibcode: 2001AGUSM..SH31C08L
Altcode:
  We study over 40 eruptions which originated with coronal sigmoids seen
  in Yohkoh SXT images, with subsequently observed in situ magnetic
  clouds (MCs) and geomagnetic storms at 1~AU. We correlate solar and
  interplanetary features so as to infer terrestrial event properties
  from their solar sources. A collateral result from studying this
  database is that CMEs and MCs resulting from erupting sigmoids seem
  not to adhere to rules such as leading B<SUB>z</SUB> versus solar
  dipole orientation and, as such, form a distinct class of events. %
  Instead of a large-scale dipole rule, we find there is a weak (3:2)
  solar cycle (Hale polarity)-based rule for leading interplanetary
  field in MCs. We find that the helicity of magnetic clouds is much more
  strongly correlated (&gt;90%) with launch hemisphere than the 60--70%\
  rule of photospheric active region helicity. This rule appears to hold
  for all CMEs, taking the 28 years of events of Bothmer &amp;\ Rust
  [``Coronal Mass Ejections,'' AGU Monograph Series 99, 139, 1997]. %
  At least half of Bothmer &amp;\ Rust's events are associated with
  disparitions brusques, outside active regions. We therefore suggest
  that active region sigmoids and disappearing filaments are the origins
  of two different classes of CMEs.

Title: Hemispheric Helicity Trend for Solar Cycle 23
Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Latushko, Sergei M.
Bibcode: 2001ApJ...549L.261P
Altcode:
  Applying the same methods we used in solar cycle 22, we study active
  region vector magnetograms, full-disk X-ray images, and full-disk
  line-of-sight magnetograms to derive the helicity of solar magnetic
  fields in the first 4 years of solar cycle 23. We find that these
  three data sets all exhibit the same two key tendencies-significant
  scatter and weak hemispheric asymmetry-as were observed in solar cycle
  22. This supports the interpretation of these tendencies as signatures
  of the writhing of magnetic flux by turbulence in the convection zone.

Title: Origin of Helicity in the Quiet Sun
Authors: Pevtsov, A. A.; Longcope, D. W.
Bibcode: 2001ASPC..236..423P
Altcode: 2001aspt.conf..423P
  No abstract at ADS

Title: Using X-ray Bright Points to Infer the Large-Scale Magnetic
    Field of the Quiet Sun
Authors: Nelson, J. L.; Longcope, D. W.; Pevtsov, A. A.
Bibcode: 2000SPD....31.0143N
Altcode: 2000BAAS...32.1289N
  X-ray bright points (XBPs) form above magnetic bipoles in the quiet
  Sun, often at the site of convergence. According to models, the power
  radiated by the XBP is supplied by magnetic reconnection as flux
  is transfered from some overlying field into the bipole itself. It
  follows that the morphology of an XBP depends on both the bipole and
  on the large-scale overlying field. We demonstrate a novel technique
  which exploits this fact to map the horizontal component of the Sun's
  large-scale field using the morphology of observed XBPs. We test this
  technique using data from SOHO's Michaelson Doppler Interferometer (MDI)
  and EUV Imaging Telescope (EIT). The resulting measurements are compared
  to masurements made using standard polarimetric methods, and to models
  of the Sun's diffuse field. This material is based upon work supported
  by the National Science Foundation under Grant No. ATM-9733424.

Title: Helicity Computation Using Observations From two Different
    Polarimetric Instruments
Authors: Bao, S. D.; Pevtsov, A. A.; Wang, T. J.; Zhang, H. Q.
Bibcode: 2000SoPh..195...75B
Altcode:
  We compare vector magnetograms of active region NOAA 5747 observed
  by two very different polarimetric instruments: the imaging vector
  magnetograph of Huairou Solar Observing Station (HSOS) and the Haleakala
  Stokes Polarimeter of Mees Solar Observatory (MSO). Unlike previous
  comparative studies, we concentrate our attention on differences in
  observations and data reduction techniques that can affect the helicity
  computation. Overall, we find a qualitative agreement between the HSOS
  and MSO vector magnetograms. The HSOS data show slightly higher field
  strength, but the distribution of inclination angles is similar in
  measurements from the two instruments. There is a systematic difference
  (up to ∼20°) in the azimuths of transverse fields, which is roughly
  proportional to the longitudinal field strength. We estimate that
  Faraday rotation in the HSOS magnetograms contributes ∼12° in
  the azimuth difference if possible sources of error are taken into
  account. Next, we apply two independent methods to both data sets to
  resolve 180° azimuth ambiguity and to compute two helicity measures
  - the force-free field parameter α<SUB>best</SUB> and the current
  helicity fractional imbalance ρ<SUB>h</SUB>. The methods agree
  reasonably well in sign and value of the helicity measures, but the
  HSOS magnetograms show systematically smaller values of ρ<SUB>h</SUB>
  and α<SUB>best</SUB> in agreement with an expected contribution of
  Faraday rotation. Finally, we discuss the role of Faraday rotation
  in computation of α<SUB>best</SUB> and ρ<SUB>h</SUB> and conclude
  that it does not affect the strength of the hemispheric helicity
  rule. The strength of the rule appears to be related to a helicity
  parameter: α<SUB>best</SUB> shows weaker hemispheric asymmetry than
  ρ<SUB>h</SUB>.

Title: Vector Magnetic Fields, Sub-surface Stresses and Evolution
    of Magnetic Helicity
Authors: Canfield, R. C.; Pevtsov, A. A.
Bibcode: 2000JApA...21..213C
Altcode:
  No abstract at ADS

Title: Coronal Structures as Tracers of Sub-surface Processes
Authors: Pevtsov, A. A.; Canfield, R. C.
Bibcode: 2000JApA...21..185P
Altcode:
  No abstract at ADS

Title: Soft X-ray Luminosity and Photospheric Magnetic Field in
    Quiet Sun.
Authors: Pevtsov, A. A.; Acton, L. W.
Bibcode: 2000SPD....31.1306P
Altcode: 2000BAAS...32..845P
  We are using full disk soft X-ray data from Yohkoh and Kitt Peak
  daily magnetograms to study the coronal luminosity and photospheric
  magnetic field in the quiet Sun between 1991 November and 1998
  December. For every image of our data set we extract three areas
  4 by 4 solar degrees in size centered at 00N00W, 50N00W and 50S00W
  and compute X-ray luminosity and unsigned magnetic flux for each of
  these areas. Between 1991 (active Sun) and 1996 (quiet Sun) the X-ray
  luminosity at the heliographic center decreases by more than a factor
  of 7...while the magnetic flux decreases by only a factor of 2. A
  similar tendency is observed for our high latitude samples. Apart
  from the cycle-related variations, all three areas of quiet Sun
  exhibit significant non-periodic changes in X-ray luminosity. These
  variations occur on 9-12 month intervals and clearly correlate
  with increase/decrease in sunspot activity. Similar variations are
  present in the total X-ray irradiance averaged over the solar disk. On
  the contrary, the magnetic fluxes from the same areas of quiet Sun
  show no corresponding variations on this time scale. In our opinion,
  coronal heating models based on the reconnection of quiet sun magnetic
  elements (variously called chromospheric network, "magnetic carpet"
  or "salt and pepper" field) can not explain the million degree corona
  observed by the Yohkoh soft X-ray telescope. We conclude that the X-ray
  luminosity in the quiet Sun (at least in the Yohkoh temperature range,
  &gt;2 MK) is primarily associated with the strong magnetic fields of
  active regions, not with weak photospheric fields. To further support
  this conclusion, we show one example of a dramatic change in X-ray
  luminosity over the entire visible corona that was associated with
  the emergence of a single small active region.

Title: Transequatorial Loops in the Solar Corona
Authors: Pevtsov, Alexei A.
Bibcode: 2000ApJ...531..553P
Altcode:
  Using X-ray coronal images, full-disk longitudinal magnetograms,
  and vector magnetograms of active regions, we study active regions
  connected across the solar equator. We survey the Yohkoh data set
  between 1991 October and 1998 December and find 87 transequatorial loop
  systems (TLSs). We classify these loops in four different categories
  and study a separation between regions, their rotation rates, and the
  sign of the current helicity (chirality) of the magnetic fields. We
  find that approximately one-third of all active regions on the Sun
  exhibit transequatorial loops. The fraction of TLSs is solar-cycle
  independent. Transequatorial loops may develop between existing active
  regions or between mature regions and new magnetic flux shortly after
  flux emergence. Observations suggest, however, that formation of TLSs
  is not a random process-a connection between two areas may exist well
  before the reconnection takes place. We find that the reconnected
  regions have approximately the same rotation rate and tend to appear
  on certain longitudes, similar to the complexes of activity. In most
  cases transequatorial interconnected regions have the same handedness
  of their magnetic field.

Title: The Solar Dynamo and Emerging Flux - (Invited Review)
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. G.;
   Pevtsov, A. A.
Bibcode: 2000SoPh..192..119F
Altcode:
  The largest concentrations of magnetic flux on the Sun occur in
  active regions. In this paper, the properties of active regions are
  investigated in terms of the dynamics of magnetic flux tubes which
  emerge from the base of the solar convection zone, where the solar cycle
  dynamo is believed to operate, to the photosphere. Flux tube dynamics
  are computed using the `thin flux tube' approximation, and by using
  MHD simulation. Simulations of active region emergence and evolution,
  when compared with the known observed properties of active regions,
  have yielded the following results: (1) The magnetic field at the
  base of the convection zone is confined to an approximately toroidal
  geometry with a field strength in the range (3-10)×10<SUP>4</SUP>
  G. The latitude distribution of the toroidal field at the base of
  the convection zone is more or less mirrored by the observed active
  latitudes; there is not a large poleward drift of active regions as
  they emerge. The time scale for emergence of an active region from the
  base of the convection zone to the surface is typically 2-4 months. The
  equatorial gap in the distribution of active regions has two possible
  origins; if the toroidal field strength is close to 10<SUP>5</SUP> G,
  it is due to the lack of equilibrium solutions at low latitude; if it
  is closer to 3×10<SUP>4</SUP> G, it may be due to modest poleward drift
  during emergence. (2) The tilt of active regions is due primarily to the
  Coriolis force acting to twist the diverging flows of the rising flux
  loops. The dispersion in tilts is caused primarily by the buffeting of
  flux tubes by convective motions as they rise through the interior. (3)
  The Coriolis force also bends the active region flux tube shape toward
  the following (i.e., anti-rotational) direction, resulting in a steeper
  leg on the following side as compared to the leading side of an active
  region. When the active region emerges through the photosphere, this
  results in a more rapid separation of the leading spots away from the
  magnetic neutral line as compared to the following spots. This bending
  motion also results in the neutral line being closer to the following
  magnetic polarity. (4) Active regions behave kinematically after they
  emerge because of `dynamic disconnection', which occurs because of the
  lack of a solution to the hydrostatic equilibrium equation once the flux
  loop has emerged. This could explain why active regions decay once they
  have emerged, and why the advection-diffusion description of active
  regions works well after emergence. Smaller flux tubes may undergo
  `flux tube explosion', a similar process, and provide a source for the
  constant emergence of small-scale magnetic fields. (5) The slight trend
  of most active regions to have a negative magnetic twist in the northern
  hemisphere and positive twist in the south can be accounted for by the
  action of Coriolis forces on convective eddies, which ultimately writhes
  active region flux tubes to produce a magnetic twist of the correct
  sign and amplitude to explain the observations. (6) The properties of
  the strongly sheared, flare productive δ-spot active regions can be
  accounted for by the dynamics of highly twisted Ω loops that succumb to
  the helical kink instability as they emerge through the solar interior.

Title: Current Helicity of the Large-Scale Photospheric Magnetic Field
Authors: Pevtsov, Alexei A.; Latushko, Sergei M.
Bibcode: 2000ApJ...528..999P
Altcode:
  This paper presents the first results of a study of current helicity
  of the large-scale magnetic field (LSMF) using full-disk longitudinal
  B<SUB>long</SUB> magnetograms from the Michelson Doppler Imager on board
  the SOHO spacecraft. We employ a known technique to reconstruct the
  vector magnetic field under the assumption of relatively slow evolution
  of the LSMF and treat the variations of B<SUB>long</SUB>, occurring
  during a defined period of time as the result of the changing projection
  angle only. We compute the synoptic maps of radial (B<SUB>r</SUB>)
  and toroidal (B<SUB>λ</SUB>) magnetic fields and the latitudinal
  profile of current helicity density h<SUB>c</SUB>=B<SUB>r</SUB>(\b.nabla
  XB)<SUB>r</SUB>. For eight solar rotations (1910-1917) that have been
  analyzed, h<SUB>c</SUB> shows significant variations within +/-40° of
  solar latitude with no strong hemispheric sign asymmetry. Asymmetry
  is present, however, in high latitudes, where h<SUB>c</SUB> is
  negative/positive in the northern/southern hemisphere.

Title: Studies of Solar Helicity Using Vector Magnetograms
Authors: Hagyard, M. J.; Pevtsov, A. A.
Bibcode: 1999SoPh..189...25H
Altcode:
  Observations of photospheric magnetic fields made with vector
  magnetographs have been used recently to study helicity on the Sun. In
  this paper we indicate what can and cannot be derived from vector
  magnetograms, and point out some potential problems in these data that
  could affect the calculations of `helicity'. Among these problems
  are magnetic saturation, Faraday rotation, low spectral resolution,
  and the method of resolving the ambiguity in the azimuth.

Title: Interconnecting Active Regions - Where, When, Why?
Authors: Pevtsov, A.
Bibcode: 1999AAS...194.5504P
Altcode: 1999BAAS...31..910P
  It has been shown albeit in a limited dataset that the transequatorial
  loops are observed preferentially between active regions of same
  chirality. One can argue that such selectivity is a natural result
  of the reconnection, which simply "averages" helicities of involved
  regions and redistribute new chirality over the system. According to
  this scenario, the reconnected regions may have either same or opposite
  chirality before the reconnection. On the other hand, the reconnection
  rate depends on the magnetic helicity of involved fields. In presence
  of electric currents, the magnetic fields of same chirality are more
  likely to reconnect (electric currents closure argument), and hence the
  magnetic field should have same chirality before the reconnection. Which
  of these scenarios takes place on the Sun? We are using Yohkoh soft
  X-ray observations and HSP vector magnetograms to study a history of few
  selected areas on the Sun, where the transequatorial interconnecting
  regions were observed. In several cases, we find a clear indication
  of pre-existing connection between the areas before the active
  regions were formed. The distribution of the magnetic field implies
  a close relation between the interconnecting regions and complexes
  of activity. We speculate that the fact that the magnetic field has
  the same chirality within a complex of activity implies a long-term
  organization of the dynamo flows in the convection zone.

Title: A Survey of X-ray Bright Points: Implications for a
    Reconnection Model
Authors: Kankelborg, C. C.; Nelson, J.; Longcope, D. W.; Pevtsov, A. A.
Bibcode: 1999AAS...194.1601K
Altcode: 1999BAAS...31..849K
  We present a survey of over 350 bright points from archival SOHO
  data. Extreme ultraviolet images were measured to determine orientation,
  length, and brightness in the EIT 171 angstrom (Fe X, 1 MK) and 195
  angstrom (Fe XII, 1.5 MK) passbands. MDI data were analyzed to obtain
  the size, orientation, and magnetic flux of the corresponding magnetic
  bipoles. The three-dimensional reconnection theory of Longcope (1998)
  makes several predictions that may be tested with these data. For this,
  the first phase of the study, we concentrate on the scaling of EUV
  brightness with magnetic flux and the distribution of displacement
  angles between EUV bright points and their magnetic counterparts. We
  also verify the assumption of Longcope &amp; Kankelborg (1999) that
  the distribution of magnetic orientations is random and independent
  of latitude.

Title: The Origin and Role of Twist in Active Regions
Authors: Fisher, G. H.; Longcope, D. W.; Linton, M. G.; Fan, Y.;
   Pevtsov, A. A.
Bibcode: 1999soho....9E..56F
Altcode:
  The implications of twist in active region magnetic fields is considered
  in this paper. The latitudinal distribution of twist that has been
  derived from recent vector magnetogram observations may be explained by
  the effects of convective turbulence with a non-zero kinetic helicity
  acting on active region scale magnetic flux tubes as they rise through
  the convection zone. Highly twisted, kink unstable flux tubes are then
  discussed as a possible explanation for many of the observed properties
  of flare productive, "d-spot'' active regions.

Title: Magnetic helicity attracts interdisciplinary participation
    at chapman conference
Authors: Canfield, R. C.; Pevtsov, A. A.
Bibcode: 1999EOSTr..80...13C
Altcode:
  During the last 5 years interest in magnetic helicity has grown
  dramatically in solar physics as a result of improved capabilities to
  measure and image magnetic fields. Magnetic helicity was introduced by
  K. Moffatt in the late 1950s as a topological invariant that describes
  the complexity of a magnetic field. The topological aspect of helicity
  is readily visualized in a Moebius strip, in which the system of
  interest is closed and helicity takes two forms, the writhing of the
  central axis of the strip and the twisting of the edges of the strip
  about that axis.In many plasmas (but not in atmospheres like that
  of Earth, for example), helicity is conserved, just as the sum of
  twist and writhe is conserved in a Moebius strip. Mathematically, it
  is related to linking integrals, which K. F. Gauss employed to study
  asteroid paths on the sky. In the late 1970s the concept of magnetic
  helicity was introduced in laboratory plasma physics, turbulence theory,
  space physics, and statistical theory.

Title: Twisted Flux Tubes and How They Get That Way
Authors: Longcope, Dana; Linton, Mark; Pevtsov, Alexei; Fisher,
   George; Klapper, Isaac
Bibcode: 1999GMS...111...93L
Altcode:
  According to present theories, the Sun's magnetic field rises through
  the convection zone in the form of slender strands known as flux
  tubes, traditionally studied using "thin flux tube" models. While
  these models have been remarkably successful they have only recently
  begun to account for tubes with twisted magnetic flux, in spite of
  observational evidence for such twist. In this work we review the
  recent developments pertaining to twisted magnetic flux tubes and
  compare quantitative predictions to observations. Hydrodynamic theory
  predicts a role for twist in preventing fragmentation. Excessive twist
  can also lead to magnetohydrodynamic instability affecting the dynamics
  of the tube's axis. A thin tube model for a twisted tube suggests
  several possibilities for the origin of twist. The most successful
  of these is the Sigma-effect whereby twist arises from deformation of
  the tube's axis by turbulence. Simulations show that the Sigma-effect
  agrees with observations in magnitude as well as latitudinal dependence.

Title: The Origin and Role of Twist in Active Regions
Authors: Fisher, G. H.; Longcope, D. W.; Linton, M. G.; Fan, Y.;
   Pevtsov, A. A.
Bibcode: 1999ASPC..178...35F
Altcode: 1999sdnc.conf...35F
  No abstract at ADS

Title: The Solar Dynamo and Emerging Flux
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. G.;
   Pevtsov, A. A.
Bibcode: 1999soho....9E..18F
Altcode:
  Much has been learned about the dynamics of magnetic flux tubes in the
  solar interior over the past decade. By using theoretical models for
  the dynamics of active region flux ropes, it is possible to estimate
  observable properties of active regions, such as their orientation,
  position on the disk, and morphology, and then compare these properties
  with active region observations. By varying conditions of the magnetic
  flux ropes as the base of the convection zone until observed properties
  are matched, one can deduce properties of the magnetic field in the
  dynamo layer, such as the magnetic field strength. Observed properties
  such as the active region tilt angle, the dispersion of the tilt angle,
  and magnetic helicity in active regions will be discussed in terms
  of the dynamics of flux tubes rising through the convection zone and
  their interaction with convective motions. Properties of Delta spot
  active regions will be discussed in terms of the kink instability of
  magnetic flux ropes.

Title: Magnetic Helicity in Space and Laboratory Plasmas
Authors: Brown, Michael R.; Canfield, Richard C.; Pevtsov, Alexei A.
Bibcode: 1999GMS...111.....B
Altcode: 1999mhsl.conf.....B
  Using the concept of magnetic helicity, physicists and mathematicians
  describe the topology of magnetic fields: twisting, writhing, and
  linkage. Mathematically, helicity is related to linking integrals,
  which Gauss introduced in the 19th century to describe the paths
  of asteroids in the sky. In the late 1970s the concept proved to
  be critical to understand laboratory plasma experiments on magnetic
  reconnection, dynamos, and magnetic field relaxation. In the late 1980s
  it proved equally important in understanding turbulence in the solar
  wind and the interplanetary magnetic field. During the last five years
  interest in magnetic helicity has grown dramatically in solar physics,
  and it will continue to grow as observations of vector magnetic fields
  become increasingly sophisticated.

Title: Coronal Heating in Active Regions as a Function of Global
    Magnetic Variables
Authors: Fisher, George H.; Longcope, Dana W.; Metcalf, Thomas R.;
   Pevtsov, Alexei A.
Bibcode: 1998ApJ...508..885F
Altcode:
  A comparison of X-ray images of the Sun and full disk magnetograms
  shows a correlation between the locations of the brightest X-ray
  emission and the locations of bipolar magnetic active regions. This
  correspondence has led to the generally accepted idea that magnetic
  fields play an essential role in heating the solar corona. <P />To
  quantify the relationship between magnetic fields and coronal
  heating, the X-ray luminosity of many different active regions
  is compared with several global (integrated over entire active
  region) magnetic quantities. The X-ray measurements were made with
  the SXT Telescope on the Yohkoh spacecraft; magnetic measurements
  were made with the Haleakala Stokes Polarimeter at the University
  of Hawaii's Mees Solar Observatory. <P />The combined data set
  consists of 333 vector magnetograms of active regions taken between
  1991 and 1995; X-ray luminosities are derived from time averages
  of SXT full-frame desaturated (SFD) images of the given active
  region taken within +/-4 hours of each magnetogram. Global magnetic
  quantities include the total unsigned magnetic flux Φ<SUB>tot</SUB>
  ≡ \smallint dA|B<SUB>z</SUB>|, B<SUP>2</SUP><SUB>z,tot</SUB>≡
  dAB<SUP>2</SUP><SUB>z</SUB>, J<SUB>tot</SUB> ≡ \smallint
  dA|J<SUB>z</SUB>|, and B<SUP>2</SUP><SUB>⊥,tot</SUB>≡
  dAB<SUP>2</SUP><SUB>⊥</SUB>, where J<SUB>z</SUB> is the vertical
  current density and B<SUB>z</SUB> and B<SUB>⊥</SUB> are the vertical
  and horizontal magnetic field amplitudes, respectively. <P />The
  X-ray luminosity L<SUB>X</SUB> is highly correlated with all of the
  global magnetic variables, but it is best correlated with the total
  unsigned magnetic flux Φ<SUB>tot</SUB>. The correlation observed
  between L<SUB>X</SUB> and the other global magnetic variables
  can be explained entirely by the observed relationship between
  those variables and Φ<SUB>tot</SUB>. In particular, no evidence
  is found that coronal heating is affected by the current variable
  J<SUB>tot</SUB> once the observed relationship between L<SUB>X</SUB>
  and Φ<SUB>tot</SUB> is accounted for. A fit between L<SUB>X</SUB>
  and Φ<SUB>tot</SUB> yields the relationship L<SUB>X</SUB> ~= 1.2 ×
  10<SUP>26</SUP> ergs s<SUP>-1</SUP>(Φ<SUB>tot</SUB>/10<SUP>22</SUP>
  Mx)<SUP>1.19</SUP>. <P />The observed X-ray luminosities are compared
  with the behavior predicted by several different coronal heating
  theories. The Alfvén wave heating model predicts a best relationship
  between L<SUB>X</SUB> and Φ<SUB>tot</SUB>, similar to what is found,
  but the observed relationship implies a heating rate greater than the
  model can accommodate. The ``Nanoflare Model'' of Parker predicts a best
  relationship between L<SUB>X</SUB> and B<SUP>2</SUP><SUB>z,tot</SUB>
  rather than Φ<SUB>tot</SUB>, but the level of heating predicted by the
  model can still be compared to the observed data. The result is that
  for a widely used choice of the model parameters, the nanoflare model
  predicts 1.5 orders of magnitude more heating than is observed. The
  ``Minimum Current Corona'' model of Longcope predicts a qualitative
  variation of L<SUB>X</SUB> with Φ<SUB>tot</SUB> that agrees with
  what is observed, but the model makes no quantitative prediction
  that can be tested with the data. A comparison between L<SUB>X</SUB>
  and the magnetic energy E<SUB>mag</SUB> in each active region leads
  to a timescale that is typically 1 month, or about the lifetime of
  an active region, placing an important observational constraint on
  coronal heating models. <P />Comparing the behavior of solar active
  regions with nearby active stars suggests that the relationship observed
  between L<SUB>X</SUB> and Φ<SUB>tot</SUB> may be a fundamental one that
  applies over a much wider range of conditions than is seen on the Sun.

Title: NOAA 7926: A Kinked Ω-Loop?
Authors: Pevtsov, Alexei A.; Longcope, Dana W.
Bibcode: 1998ApJ...508..908P
Altcode:
  Using vector magnetograms and X-ray images, we study the evolution
  of the decaying active region NOAA AR 7926. The active region had
  bipolar structure with a leading sunspot of positive (northern [N])
  polarity--non-Hale polarity of cycle 22. Observations suggest that the
  following (southern [S]) polarity of this active region was in fact the
  leading (S) polarity of Active Region 7918 (AR 7918) of the previous
  solar rotation. Analyzing the rotation rate of both active regions
  and their magnetic field topology, we conclude that they form a single
  magnetic system resembling a kinked Ω-loop. During the first rotation,
  the upper part of the loop was exposed, forming a bipolar active region
  of normal (Hale) polarity. The rest of the Ω-loop had emerged by the
  time of the second rotation, giving the appearance of non-Hale polarity.

Title: Flux-Tube Twist Resulting from Helical Turbulence: The
    Σ-Effect
Authors: Longcope, D. W.; Fisher, G. H.; Pevtsov, A. A.
Bibcode: 1998ApJ...507..417L
Altcode:
  Recent observational studies suggest that active region magnetic
  flux emerges in a twisted state and that the sense of twist depends
  weakly on solar hemisphere. We propose that this twist is imparted
  to the flux through its interaction with turbulent velocities in the
  convection zone. This process, designated the Σ-effect, operates on
  isolated magnetic flux tubes subjected to buffeting by turbulence with
  a nonvanishing kinetic helicity &lt;u \b.dot \b.nabla × u&gt;. The
  Σ-effect leads to twist of the same sense inferred from observation
  and opposite to that predicted by the α-effect. A series of numerical
  calculations are performed to estimate the magnitude of the Σ-effect
  in the solar convective zone. The results compare favorably with
  observations in both mean value and statistical dispersion. We find
  a further relationship with total magnetic flux that can be tested in
  future observations. The model also predicts that twist is uncorrelated
  with the tilt angle of the active region.

Title: Helicity of Solar Active-Region Magnetic Fields
Authors: Canfield, Richard C.; Pevtsov, Alexei A.
Bibcode: 1998ASPC..140..131C
Altcode: 1998ssp..conf..131C
  No abstract at ADS

Title: On the Origin of Helicity in Active Region Magnetic Fields
Authors: Pevtsov, A. A.; Canfield, R. C.
Bibcode: 1998ASSL..229...85P
Altcode: 1998opaf.conf...85P
  No abstract at ADS

Title: On the Origin of Helicity in Active Region Magnetic Fields
Authors: Canfield, Richard C.; Pevtsov, Alexei A.
Bibcode: 1997SPD....28.1705C
Altcode: 1997BAAS...29..921C
  The magnetic helicity of flux tubes consists of twist and writhe. If
  flux bundles at the base of the convection zone are simple axisymmetric
  toroids, and rise in omega-shaped loops through the convection zone,
  they acquire writhe through the effect of the Coriolis force on flows
  within them. The tilt of active regions with respect to the equator is
  an observable manifestation of this writhe, at photospheric levels. As
  a consequence of helicity conservation, we expect active regions to
  acquire twist to compensate for this writhe. The non-zero curl of
  the large-scale magnetic fields in active-region vector magnetograms
  is the observable manifestation of twist. Recent observations have
  revealed many interesting properties of the helicity of solar
  magnetic fields. Hemispheric preferences exist: active region
  fields predominantly have left-handed topology in the Northern
  hemisphere. Prominence structures also predominantly have left-handed
  writhe in the Northern hemisphere. Large-scale structures have been
  observed to exist in the distribution of both twist and writhe with
  longitude and latitude, and to persist for many solar rotations. To
  study the physical origin of these properties, we have used a dataset
  of about 100 active regions for which vector magnetograms were obtained
  at Mees Solar Observatory. We have measured both the overall tilt and
  the overall twist of these active regions. The dataset clearly shows
  Joy's law, the well-known dependence of tilt on latitude, as well as
  the hemispheric dependence of twist. However, our analysis shows that
  twist and tilt are not related as they should be if both twist and
  tilt are a consequence of the Coriolis force. Hence, we conclude that
  the twist seen in active regions is the consequence of a deep-seated
  phenomenon, presumably that of the solar dynamo itself.

Title: NOAA 7926: A Kinked, Submerging Omega -loop ?
Authors: Pevtsov, Alexei A.; Canfield, Richard C.
Bibcode: 1997SPD....28.0238P
Altcode: 1997BAAS...29..900P
  Using vector magnetograms and X-ray images, we study the evolution
  of decaying active region NOAA AR 7926. This active region had
  bipolar structure with non-Hale polarity. Observations suggest
  that the following polarity of this active region was in fact the
  leading polarity of active region NOAA AR 7918 on the previous solar
  rotation. Analyzing the rotation rate of both active regions and the
  topology of their magnetic field, we conclude that they belong to a
  single magnetic system resembling a kinked Omega loop. During the first
  rotation the upper part of the loop was exposed, forming the bipolar
  active region of normal (Hale) polarity. The rest of the Omega loop
  had emerged on the time by the second rotation, giving the appearance
  of non-Hale polarity. The major soft X-ray coronal loops of NOAA AR
  7926 closely relate to the weak magnetic field in the middle part of
  the region. In that area, the photospheric field stayed concave-down
  in the course of dissipation of the active region, and the coronal
  loops decreased their length. We also find that coronal loops decrease
  their height derived using E and W limb observations of the two active
  regions NOAA AR 7926 and NOAA AR 7918. Hence, we concluded that the
  magnetic field of the active region submerged under the photosphere.

Title: On the Subphotospheric Origin of Coronal Electric Currents
Authors: Pevtsov, Alexei A.; Canfield, Richard C.; McClymont,
   Alexander N.
Bibcode: 1997ApJ...481..973P
Altcode:
  Using photospheric vector magnetograms from the Haleakala Stokes
  Polarimeter and coronal X-ray images from the Yohkoh Soft X-Ray
  Telescope (SXT), we infer values of the force-free field parameter α
  at both photospheric and coronal levels within 140 active regions. We
  determine the value of α for a linear force-free field that best
  fits each magnetogram in a least-squares sense. We average values
  from all available magnetograms to obtain a single mean photospheric
  α-value &lt;α<SUB>p</SUB>&gt; for each active region. From the SXT
  images we estimate α in the corona by determining (π/L) sin γ for
  individual loops, where γ is the observed shear angle of X-ray loops
  of length L. We then average these values of α to obtain a single
  coronal α value, &lt;α<SUB>c</SUB>&gt;, for each active region. <P
  />In active regions for which the photospheric α-map is predominantly
  of one sign, we find that the values of &lt;α<SUB>p</SUB>&gt; and
  &lt;α<SUB>c</SUB>&gt; are well correlated. Only for active regions in
  which both signs of α are well represented, and in which our method of
  analysis therefore breaks down, are the values of &lt;α<SUB>p</SUB>&gt;
  and &lt;α<SUB>c</SUB>&gt; poorly correlated. The former correlation
  implies that coronal electric currents typically extend down to at
  least the photosphere. However, other studies imply subphotospheric
  origin of the currents, and even current systems, that are observed in
  the photosphere. We therefore conclude that the currents responsible
  for sinuous coronal structures are of subphotospheric origin.

Title: Coronal Structure as a Diagnostic of the Solar Dynamo
Authors: Sandborgh, S.; Canfield, R. C.; Pevtsov, A. A.
Bibcode: 1997SPD....28.0151S
Altcode: 1997BAAS...29..888S
  This paper is motivated by the discovery of large-scale structures
  in the distribution of twist of magnetic fields with solar longitude
  and latitude. These structures are observed to persist for many
  solar rotations (Pevtsov and Latushko, BAAS 27, 978, 1995; Pevtsov,
  Canfield, and Metcalf, 4th SOHO Workshop: Helioseismology, Asilomar,
  2-6 April 1995; Pevstov, Canfield, and Glatzmaier, Geophysical and
  Astrophysical Convection, NCAR, 10-13 October 1995). The large scale of
  these patterns, in space and time, implies that they are a deep-seated
  phenomenon, presumably that of the solar dynamo itself. Recent
  observations of twist and tilt of active regions (Pevstov and Canfield,
  Yohkoh Fifth Anniversary Symposium, Yoyogi, 6-8 November 1996, and
  Canfield and Pevtsov, this meeting) and the relationship between
  photospheric and coronal manifestations of twist (Pevstov, Canfield,
  and McClymont, ApJ 481, in press) confirm this conclusion. The Yohkoh
  Soft X-Ray Telescope images from the first five years of operations
  often clearly show the presence of topologically distinct regions. We
  have developed and used IDL software that allows us to identify and
  transfer to Carrington coordinates the boundaries of these systems,
  which we identify with magnetic flux systems. In this paper we present
  the results of our studies of these systems over many solar rotations,
  and relate them to the previously discovered persistent large-scale
  structures in the distribution of twist of active region magnetic
  fields with longitude and latitude.

Title: Reconnection and Helicity in a Solar Flare
Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Zirin, Harold
Bibcode: 1996ApJ...473..533P
Altcode:
  Using X-ray images, Hα images, and vector magnetograms, we have
  studied the evolution of the coronal structure and magnetic field of
  NOAA Active Region 7154 during 1992 May 5-12. A two-ribbon 4B/M7.4
  flare associated with an Hα filament eruption was observed on May 8,
  15:13-19:16 UT. An interesting feature of the region was a long,
  twisted X-ray structure, which formed shortly before the flare
  and disappeared after it, being replaced by a system of unsheared
  postflare loops. Neither the X-ray nor Hα morphology nor the photo
  spheric magnetic field shows any indication of gradual buildup of
  nonpotential energy prior to the flare. Rather, the long structure
  appears to result from the reconnection of two shorter ones just tens
  of minutes before the filament eruption and flare. <P />Using vector
  magnetograms and X-ray morphology, we determine the helicity density
  of the magnetic field using the force-free field parameter α. The
  observations show that the long structure retained the same helicity
  density as the two shorter structures, but its greater length implies
  a higher coronal twist. The measured length and α value combine to
  imply a twist that exceeds the threshold for the MHD kink instability
  in a force-free cylindrical flux tube. We conclude that theoretical
  studies of such simple models, which have found that the MHD kink
  instability does not lead to global dissipation, do not adequately
  address the physical processes that govern coronal magnetic fields.

Title: Coronal Heating in Active Regions as a Function of Global
    Magnetic Variables
Authors: Fisher, G. H.; Longcope, D. W.; Metcalf, T. R.; Pevtsov, A. A.
Bibcode: 1996AAS...188.3304F
Altcode: 1996BAAS...28..868F
  A comparison of X-ray images of the Sun and full disk magnetograms shows
  a correlation between the locations of the brightest X-ray emission
  and the locations of bipolar magnetic regions. This correspondence
  has led to the generally accepted idea that magnetic fields play an
  essential role in heating the Solar corona. To quantify the relationship
  between magnetic fields and coronal heating, we have compared the
  X-ray luminosity of many different Active Regions with several global
  (integrated over entire active region) magnetic quantities. The X-ray
  measurements were made with the SXT Telescope on the Yohkoh spacecraft;
  magnetic measurements were made with the Haleakala Stokes Polarimeter
  at the University of Hawaii's Mees Solar Observatory. Our combined
  dataset consists of 333 vector magnetograms of active regions taken
  between 1991 and 1995; SXT luminosities consist of time averages of
  SFD images of the given active region taken within +/- 4 hours of each
  magnetogram. Global magnetic quantities include the total unsigned
  magnetic flux, area integrals of B(2) , J_z(2) (J_z is the vertical
  component of the electric current density), and the best-fit alpha
  of the linear force-free field for the entire active region (nabla x
  B = alphaB ). Our results show clear and unmistakable relationships
  between the X-ray luminosity and most of these magnetic variables. The
  relationship between total unsigned magnetic flux and X-ray luminosity
  is especially compelling, holding over 2 orders magnitude in both
  quantities. These measurements provide important contraints on coronal
  heating mechanisms. This work was supported in part by NASA grant
  NAGW-3429, NSF grant AST-9218085, and Cal Space grant CS-17-95.

Title: Magnetic Chirality and Coronal Reconnection
Authors: Pevtsov, A. A.; Canfield, R. C.; McClymont, A. N.
Bibcode: 1996AAS...188.3503P
Altcode: 1996BAAS...28..871P
  We have used Mees Solar observatory vector magnetograms and
  Yohkoh Soft X-Ray Telescope images to study the role of magnetic
  chirality (handedness) in the trans-equatorial reconnection of
  active regions. Transequatorial reconnections are identified using
  SXT images. The chirality of the active regions is inferred from
  vector magnetograms and SXT images. Our results indicate that active
  regions reconnect preferentially with others of the same chirality. We
  have identified the 9 closest pairs of active regions separated by
  up to +/- 20° in latitude and longitude. All six pairs of active
  regions having the same sign of chirality showed transequatorial
  connection. All three pairs of active regions of opposite chirality
  indicated no inter-connection. Less close pairs of active regions,
  with separation up to +/- 40° in latitude and +/- 20° in longitude,
  showed the same tendency: 15 of 17 regions with the same chirality were
  inter-connected in the corona, and 8 of 10 regions having the opposite
  chirality did not show inter-connection. We explain this result with
  a simple model of current-system closure.

Title: Magnetic Chirality and Coronal Reconnection
Authors: Canfield, Richard C.; Pevtsov, Alexei A.; McClymont,
   Alexander N.
Bibcode: 1996ASPC..111..341C
Altcode: 1997ASPC..111..341C
  The authors have used Mees Solar Observatory vector magnetograms
  and Yohkoh Soft X-ray Telescope images to study the role of magnetic
  chirality in the trans-equatorial reconnection of active regions. They
  conclude that active regions reconnect preferentially with others of
  the same chirality. They explain this result with a simple model of
  the closure of their current systems.

Title: Helicity of Large Scale Photospheric Magnetic Fields
Authors: Pevtsov, A. A.; Latushko, S. M.
Bibcode: 1995SPD....26.1008P
Altcode: 1995BAAS...27..978P
  No abstract at ADS

Title: Latitudinal Variation of Helicity of Photospheric Magnetic
    Fields
Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Metcalf, Thomas R.
Bibcode: 1995ApJ...440L.109P
Altcode:
  Using a 1988-1994 data set of original photospheric vector magnetograms
  as well as published data, we have studied the average magnetic
  helicity of 69 diverse active regions, adopting the linear force-free
  field parameter alpha as a measure. This average value was determined
  by minimizing the differences between the computed constant-alpha
  force-free and observed horizontal magnetic fields. The average
  magnetic helicity shows a sign difference at the 2 sigma level in
  opposite hemispheres. In our data set, 76% of the active regions in
  the northern hemisphere have negative helicity, and 69% in the southern
  hemisphere, positive. Although the data show considerable variation from
  one active region to the next, the data set as a whole suggest that
  the magnitude of the average helicity increases with solar latitude,
  starting at zero near the equator, reaches a maximum near 15 deg - 25
  deg in both hemispheres, and drops back toward smaller values avove 35
  deg - 40 deg. Qualitative comparison with published models shows that
  such latitudinal variation of the average magnetic helicity may result
  from either turbulent convective motions or differential rotation,
  although our studies of rotating sunspots lead us to favor the former.

Title: Patterns of Helicity in Solar Active Regions
Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Metcalf, Thomas R.
Bibcode: 1994ApJ...425L.117P
Altcode:
  Using 46 vector magnetograms from the Stokes Polarimeter of Mees
  Solar Observatory (MSO), we studied patterns of local helicity
  in three diverse solar active regions. From these magnetograms
  we computed maps of the local helicity parameter alpha =
  J<SUB>z</SUB>/B<SUB>z</SUB>. Although such maps are noisy, we
  found patterns at the level approximately 2 to 3 sigma<SUB>J(sub
  z</SUB>), which repeat in successive magnetograms for up to
  several days. Typically, the alpha maps of any given active region
  contain identifiable patches with both positive and negative values
  of alpha. Even within a single sunspot complex, several such alpha
  patches can often be seen. We followed 68 alpha patches that could be
  identified on at least two successive alpha maps. We found that the
  persistence fraction of such patches decrease exponentially, with a
  characteristic time approximately 27 hr.

Title: Line-of-sight velocity measurements using a
    dissector-tube. II. Time variations of the tangential velocity
    component in the Evershed effect
Authors: Druzhinin, S. A.; Pevtsov, A. A.; Levkovsky, V. L.; Nikonova,
   M. V.
Bibcode: 1993A&A...277..242D
Altcode:
  We present the results of measurements of sunspot torsional
  oscillations. For six sunspots, a study was made of the spectral
  composition of Doppler velocity signals from two areas of penumbra,
  symmetric about the sunspot umbra located near the limb. The
  spectrograph slit was directed parallel to the nearest solar limb. The
  observations were made in the lines Fe i 543.45 nm and Hβ 486.13 nm
  using a dissector tube. Its electronic scanning was controlled in such
  a way that two channels measure spectral line shifts in two different
  parts of the photo-cathode. Attention mainly was paid to periods from
  1 min to several hours. The results of the work indicate that the
  sunspot penumbra exhibits several kinds of oscillations: quasi-five-min
  vertical oscillations of small areas of penumbra (4 arcsec), vertical
  oscillations of large areas of penumbra (periods from 20 min to 1
  h), and the sunspot torsional oscillations. Periodic variations in
  sunspot position are observed to have an amplitude of about 1 arcsec
  and periods close to those of the sunspot torsional oscillations.

Title: Line-of-sight velocity measurements using a
    dissector-tube. 1. an instrument description
Authors: Druzhinin, S. A.; Pevtsov, A. A.
Bibcode: 1993A&A...272..378D
Altcode:
  A description of an electronic device for solar Doppler velocity
  measurements based on a TV dissector- tube is given. As to the
  principle of measurements, this instrument is the electronic analog of
  the magnetograph Doppler compensator. The advantages of this device
  over existing Doppler compensators using a tilting glass plate are
  the absence of moving parts, the flexibility of the scan parameters,
  and the high speed. The instrument provides quasi-simultaneous
  measurements of (a) the shifts of two neighbouring spectral lines, (b)
  the shifts in a single spectral line at two intensity levels (nearer
  to the core and the wings), and (c) the spectral line shifts at two
  points spaced along spectrum height. <P />The non-uniformity of the
  photocathode sensitivity must be regarded as the disadvantage of the
  device. The functional principles of this instrument are described,
  and some specific errors of measurements are considered. We discuss
  the influence of: (a) the non-uniformity of photocathode sensitivity,
  (b) spectral-line inclination in the spectrum, (c) the illumination
  non-uniformity of the spectrograph entrance slit, and (d) interference
  and stray light.

Title: Line-of-sight velocity measurements using a dissector-tube. III
    - Prominence oscillations
Authors: Mashnich, G. P.; Druzhinin, S. A.; Pevtsov, A. A.; Levkovskij,
   V. I.
Bibcode: 1993A&A...269..503M
Altcode:
  We discuss the results of line-of-sight velocity observations in an
  active prominence and in three quiescent prominences. The observations
  were made by using a TV dissector-tube. At 2 x 3 arcsec spatial
  resolution the prominences showed oscillations in a wide range, from
  short-period (2-10 min) to long-period ones, covering a time span of
  40-200 min. Short-period oscillations seem to refer to small-scale
  ones and are in their character close to oscillations occurring with
  the same periods in the chromosphere. Some interesting features of
  the line-of-sight velocity oscillations in the active prominence
  have been revealed. At the time of activation the amplitude of the
  short-period oscillations increases to exceed the level of quasi-hour
  oscillations. The range 2-20 min exhibits a transformation of the
  oscillation spectrum with height, from short to longer periods.

Title: Relation Between the Longitudinal Field and Radial Velocity
    in Sunspots
Authors: Pevtsov, A. A.
Bibcode: 1993ASPC...46...78P
Altcode: 1993IAUCo.141...78P; 1993mvfs.conf...78P
  No abstract at ADS

Title: The relationship between the longitudinal magnetic field and
    the line-of-sight velocity at different angular positions of sunspots
Authors: Pevtsov, A. A.
Bibcode: 1992SoPh..141...65P
Altcode:
  Using observational data on 14 sunspots from the Sayan Observatory
  vector magnetograph, a study was made of the relationship between the
  sunspot magnetic field and the Evershed motions. It is shown that the
  central area of the solar disk is dominated by an anti-correlation of
  the longitudinal magnetic field B<SUB>∥</SUB> and the line-of-sight
  velocity V<SUB>∥</SUB> when a maximum of V<SUB>∥</SUB> corresponds
  to the neutral line of the longitudinal field. Near the limb there
  usually is a coincidence of the field and velocity neutral lines. There
  is evidence for the possible asymmetric character of the effect with
  respect to the central meridian.

Title: Spatial distribution of line-of-sight velocities in a quiescent
    prominence.
Authors: Druzhinin, S. A.; Pevtsov, A. A.; Mashnich, G. P.
Bibcode: 1992ESASP.344..161D
Altcode: 1992spai.rept..161D
  In 26 - 29 June 1990 Doppler velocity observations in a quiescent
  prominence were carried out. Using a device based on a image dissector,
  the Doppler velocity profile in prominence height were measured
  quasi-simultaneously at two intensity levels in the line Hβ. It was
  found that through the entire height of the prominence, there are 3 -
  4 elements with quasi-hourly oscillations. Relative to each other,
  the oscillations in these elements show a random phase. The distance
  between such elements is about 20″ Quasi-five-minute oscillations
  have a train-like character and a random phase at neighbouring points
  of the prominence (separation ≍4″.

Title: Time variations of the tangential component of velocity in
    the Evershed effect.
Authors: Druzhinin, S. A.; Pevtsov, A. A.; Levkovskii, V. I.; Nikonova,
   M. V.
Bibcode: 1991KFNT....7...51D
Altcode: 1991KNFT....7...51D
  Results of observations of the tangential component of velocity in the
  Evershed effect (torsional oscillations of sunspots) are presented. For
  six sunspots, the spectral composition of line-of-sight velocity
  signals from two areas of the penumbra located symmetrically relative
  to the sunspot umbra has been studied. The areas chosen are located
  perpendicular to the direction toward the solar image centre, i.e.,
  gas rotation velocities in a sunspot were measured. The observations
  were made in lines of Fe I λ543.45 nm and H<SUB>β</SUB> λ486.13
  nm using a dissector-tube, whose electronic scanning control permits
  measuring (quasi-simultaneously in two channels) spectral line shifts
  on two sections of its photocathode. The results obtained confirm the
  presence of torsional oscillations of sunspots at the photospheric level
  with a period of about one hour. For the first time such oscillations
  have been measured at the chromospheric level (with a period of about
  30 minutes). Based on observations of two sunspots during 3 and 4 days,
  respectively, it is concluded that these sunspots involve torsional
  oscillations with a period of several days.

Title: Time variations of the tangential velocity component in the
    Evershed effect.
Authors: Druzhinin, S. A.; Pevtsov, A. A.; Levkovskij, V. I.; Nikonova,
   M. V.
Bibcode: 1991KPCB....7e..46D
Altcode: 1991KPCB....7...46D
  Observations of the tangential velocity component in the Evershed
  effect (torsional oscillations of sunspots) are reported. The spectral
  composition of the radial-velocity signals from two penumbral areas
  at positions symmetric about the umbra was investigated for six
  sunspots. The selected fields were on a line perpendicular to the
  direction to the center of the Sun's image, i.e., the rotational
  velocities of the gas in the sunspot were measured. The observations
  were made in the lines Fe I λ543.45 nm and H<SUB>β</SUB> 486.13
  nm. The results of the study confirm the existence of torsional
  oscillations of sunspots at photosphere level with a period of
  about an hour. These oscillations were measured for the first time
  at chromosphere level (period about 30 min). It is concluded from
  observations of two sunspots over three and four days that torsional
  oscillations with a period of several days occurred in these sunspots.

Title: Direct measurements of short-period torsional oscillations
    of sunspots. I. First results.
Authors: Druzhinin, S. A.; Pevtsov, A. A.; Levkovskii, V. I.; Nikonova,
   M. V.
Bibcode: 1990KFNT....6...29D
Altcode: 1990KNFT....6...29D
  The first results of direct measurements of one of varieties of
  proper motions of sunspots, i. e., their torsional oscillations are
  presented. For three sunspots a study has been made of the spectral
  composition of line-of-sight velocity signals from two penumbra areas
  symmetric relative to the sunspot umbra near the limb. The penumbra
  areas chosen were located on a line perpendicular to the direction of
  the solar image. Main attention is paid to periods from one minute
  to several hours. The results reported here do not give convincing
  evidence for the presence of torsional oscillations of sunspots with
  a period of about an hour as detected by Gopasyuk, 1985 and Berton
  and Rayrode, 1985. Finally the authors discuss some typical features
  of quasi-five-minute oscillations in the sunspot penumbra and of
  non-periodic single velocity fluctuations in different penumbra areas.

Title: Electric currents in a unipolar sunspot
Authors: Pevtsov, A. A.; Peregud, N. L.
Bibcode: 1990GMS....58..161P
Altcode:
  A study is made of longitudinal electric currents of a unipolar
  sunspot (NOAA No. 4744). The pattern of longitudinal currents in
  the sunspot umbra indicates the presence of a common current system
  with predominance of the tangential component. The direction of
  this component, together with the sunspot's polarity, suggests the
  conclusion that currents flowing in the umbra sustain, at least,
  the sunspot magnetic field. The Lorentz force that is calculated in
  cylindrical symmetry is directed largely in radius out of the sunspot
  and exceeds considerably the horizontal gas pressure gradient. The
  nature of electric currents is discussed.

Title: The system of electric currents of a unipolar sunspot.
Authors: Pevtsov, A. A.; Peregud, N. L.
Bibcode: 1989KFNT....5...12P
Altcode: 1989KNFT....5...12P
  The structure of longitudinal electric currents of a unipolar sunspot
  (group SD 42/86) is studied. The pattern of longitudinal currents in
  the sunspot umbra shows evidence for the presence of a common system of
  currents with the tangential component predominance. The direction of
  this component and the sunspot polarity suggest that currents flowing
  in the umbra, at least, sustain the sunspot magnetic field. Under the
  assumption of cylindrical symmetry three components of the electric
  current density vector are calculated.

Title: Ring Structures of the Transverse Magnetic Field in the
    Solar Photosphere
Authors: Pevtsov, A. A.
Bibcode: 1989ATsir1535...21P
Altcode:
  No abstract at ADS

Title: A two-component model of the magnetic field and velocity
    field in a sunspot
Authors: Pevtsov, A. A.
Bibcode: 1988IGAFS..83...85P
Altcode:
  It is suggested that a sunspot penumbra consists of two types of
  elements: (1) a dark component with a nearly horizontal magnetic field
  of greater strength and (2) a light component with a nonhorizontal
  field of lesser strength. In the dark component material flows out
  of the spot (at a maximum velocity of 4 km/s), whereas in the light
  component material falls into the spot (at a velocity of 1 km/s).

Title: A photoelectric guide for sunspot images
Authors: Druzhinin, S. A.; Maslov, I. L.; Pevtsov, A. A.
Bibcode: 1988IGAFS..83..149D
Altcode:
  A photoelectric guide designed to directly handle sunspot images is
  described. A quadrant photodiode (serving as the photodetector) is
  placed in the beam reflected from the Dove prism face. The actuators
  are two plane-parallel plates attached to low-inertia electromagnetic
  drives, ensuring a high frequency (up to 3 Hz) of control action
  in response to image motion. The guide is placed in front of the
  spectrograph entrance slit and can be used with any standard AZU-5
  telescope.

Title: An evaluation of the stray light influence upon line-of sight
    velocity measurements in sunspots.
Authors: Pevtsov, A. A.
Bibcode: 1988BSolD1987...89P
Altcode:
  Using Fe I 525.022 nm line profiles as inferred by J. Staude, the
  relative errors of line-of-sight velocity measurements were estimated
  for different areas of a sunspot and various fractions of stray
  light. The presence of 10% stray light reduces penumbral velocities by
  20 to 30% and umbral velocities by 50%. A correction was made for the
  stray light of a sunspot velocity field map obtained with the Sayan
  Observatory vector magnetograph on 25 August 1986.

Title: Unusual Behavior of Averaged K CAII Line Profile in Solar
    Disk Center
Authors: Druzhinin, S. A.; Pevtsov, A. A.; Teplitskaja, R. B.
Bibcode: 1987ATsir1512....5D
Altcode:
  Averaged profiles of the solar Ca II K line were observed at the Solar
  Telescope on the Sayansk Observatory on July 6 and 9, 1986. At the limb
  the profile has the usual structure with the K<SUB>3</SUB> minimum and
  two K<SUB>2</SUB> emission peaks. In the centre of the disc, the profile
  has an unusual asymmetry with the red emission peak K<SUB>2R</SUB>
  completely lacking. The disappearance of the K<SUB>2R</SUB> peak
  was earlier observed in some fine-structure features on the Sun but
  it was never seen in averaged profiles. This effect seems to be not
  instrumental but of solar origin.

Title: On the relationship between magnetic field and velocity field
    in a sunspot.
Authors: Grigor'ev, V. M.; Pevtsov, A. A.
Bibcode: 1987KFNT....3Q...3G
Altcode: 1987KNFT....3Q...3G
  The paper deals with studying the relationship between magnetic
  fields and velocity fields in sunspots at the early stage of their
  development using observations with the vector magnetograph of
  the Sayan Observatory. It is pointed out that when the maps of the
  magnetic field's longitudinal component and the radial velocity are
  superimposed, the maximum velocity in the sunspot region corresponds
  mainly to the magnetic field zero line. A dependence of the radial
  velocity on the slope of the magnetic field vector to the line of sight
  is investigated. The approximation of cylindrical symmetry is used to
  reconstruct the velocity and magnetic field vectors for the leading
  sunspot of the group SD 135/1984. The results obtained testify the
  existence of two systems of motions. The matter within the sunspot is
  moving mainly across the magnetic field lines, but the sunspot umbra
  involves the region in which the directions of the magnetic field and
  of mass motion are the same.

Title: A Study of Helical Oscillations in Sunspots
Authors: Pevtsov, A. A.; Sattarov, I. S.
Bibcode: 1985BSolD...3...65P
Altcode:
  On the basis of observations and published data of 17 sunspots helical
  oscillations of different type have been detected: close to harmonic,
  and increasing and decreasing with time (Figure 1a). The mean period of
  oscillations is T=7.1 ± 3.0 days, mean amplitude A=40+/- 23 degrees,
  mean rotation rate C=17+/- 15 degrees per day. The depth of the
  penetration of the sunspot under the photosphere has been evaluated
  as 7500 km and the mass of oscillating portion of sunspot as M=2 x
  10<SUP>23</SUP> g using a simple mathematics model. The kinetic energy
  of rotation of sunspot is 1x10<SUP>31</SUP> erg.

Title: A study of helical oscillations in sunspots.
Authors: Pevtsov, A. A.; Sattarov, I. S.
Bibcode: 1985BSolD1985...65P
Altcode:
  On the basis of observations and published data on 17 sunspots helical
  oscillations of different types have been detected: close to harmonic,
  and increasing and decreasing with time.

