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Author name code: kleint
ADS astronomy entries on 2022-09-14
author:"Kleint, Lucia" 

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Title: IRIS burst properties in active regions
Authors: Nelson, C. J.; Kleint, L.
2022arXiv220811013N    Altcode:
  Interface Region Imaging Spectrograph (IRIS) bursts are localised
  features thought to be driven by magnetic reconnection. Although
  these events are well-studied, it remains unknown whether their
  properties vary as their host active regions (ARs) evolve. Here, we
  aim to understand whether the measurable properties of IRIS bursts
  are consistent during the evolution of their host ARs. We study 42
  dense 400-step rasters sampled by IRIS. These rasters each covered
  one of seven ARs, with each AR being sampled at least four times over
  a minimum of 48 hours. An automated detection algorithm is used to
  identify IRIS burst profiles. Data from the Solar Dynamics Observatory's
  Helioseismic and Magnetic Imager are also used to provide context about
  the co-spatial line-of-sight magnetic field. Of the rasters studied,
  36 were found to contain IRIS burst profiles. Five ARs (11850, 11909,
  11916, 12104, and 12139) contained IRIS burst profiles in each raster
  that sampled them whilst one AR (11871) was found to contain no such
  spectra at any time. A total of 4019 IRIS burst profiles belonging
  to 752 connected objects, which we define as parent IRIS bursts,
  were identified. IRIS burst profiles were only detected within compact
  regions in each raster, with these regions appearing to increase in size
  as the host ARs aged. No systematic changes in the frequency of IRIS
  burst profiles or the spectral characteristic of IRIS burst profiles
  through time were found for these ARs. Finally, 93 % of parent IRIS
  bursts with areas between 1 arcsec^2 and 4 arcsec^2 occurred co-spatial
  to bi-poles in the photosphere. Overall, IRIS bursts have remarkably
  consistent spectral and spatial properties throughout the evolution of
  ARs. These events predominantly form within the cores of larger and
  more complex ARs, with the regions containing these events appearing
  to increase in size as the host region itself evolves.

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Title: Occurrence and statistics of IRIS bursts
Authors: Kleint, Lucia; Panos, Brandon
2022A&A...657A.132K    Altcode: 2021arXiv211012957K
  Small reconnection events in the lower solar atmosphere can lead
  to its heating, but whether such heating can propagate into higher
  atmospheric layers and potentially contribute to coronal heating is
  an open question. We carry out a large statistical analysis of all
  IRIS observations from 2013 and 2014. We identified "IRIS burst"
  (IB) spectra using a k-means analysis that entails classifying and
  selecting Si IV spectra with superimposed blend lines on top of
  bursts, which indicate low atmospheric heating. We find that ∼8%
  of all observations show IBs with about 0.01% of all recorded IRIS
  spectra being IB spectra. We find varying blend absorption levels,
  which may indicate different depths of the reconnection event and
  heating. IRIS bursts are statistically visible with similar properties
  and timings in the spectral lines Mg II, C II, and Si IV, but invisible
  in Fe XXI. By statistically analyzing co-spatial AIA light curves,
  we found systematic enhancements in AIA 1600 and AIA 1700, but no
  clear response to bursts in all other AIA wavelengths (94, 131, 171,
  193, 211, 304, 335) in a time-frame of ±6 min around the burst. This
  may indicate that heating due to IBs is confined within the lower
  atmosphere and dissipates before reaching temperatures or formation
  heights covered by the hotter AIA lines. Our developed methods are
  applicable for statistical analyses of any co-observed data sets and
  allow us to efficiently analyze millions of spectra and light curves
  simultaneously. <P />Movie associated to Fig. 11 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202142235/olm">https://www.aanda.org</A>

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Title: Chromospheric Heating Mechanisms in a Plage Region Constrained
    by Comparison of Magnetic Field and Mg II h &amp; k Flux Measurements
    with Theoretical Studies
Authors: Anan, Tetsu; Schad, Thomas; Kitai, Reizaburo; Dima, Gabriel;
   Jaeggli, Sarah; Tarr, Lucas; Collados, Manuel; Dominguez-Tagle,
   Carlos; Kleint, Lucia
2021AGUFMSH44A..05A    Altcode:
  The strongest quasi-steady heating in the solar atmosphere from the
  photosphere through the corona occurs in plage regions. As many
  chromospheric heating mechanisms have been proposed, important
  discriminators of the possible mechanisms are the location of the
  heating and the correlation between the magnetic field properties in
  the chromosphere and the local heating rate. We observed a plage region
  with the He I 1083.0 nm and Si I 1082.7 nm lines on 2018 October 3
  using the integral field unit mode of the GREGOR Infrared Spectrograph
  (GRIS) installed at the GREGOR telescope. During the GRIS observation,
  the Interface Region Imaging Spectrograph (IRIS) obtained spectra of the
  ultraviolet Mg II h &amp; k doublet emitted from the same region. In
  the periphery of the plage region, within the limited field of view
  seen by GRIS, we find that the Mg II radiative flux increases with the
  magnetic field in the chromosphere. The positive correlation implies
  that magnetic flux tubes can be heated by Alfvén wave turbulence
  or by collisions between ions and neutral atoms relating to Alfvén
  waves. Within the plage region itself, the radiative flux was large
  between patches of strong magnetic field strength in the photosphere, or
  at the edges of magnetic patches. On the other hand, we do not find any
  significant spatial correlation between the enhanced radiative flux and
  the chromospheric magnetic field strength or the electric current. In
  addition to the Alfvén wave turbulence or collisions between ions
  and neutral atoms relating to Alfvén waves, other heating mechanisms
  related to magnetic field perturbations produced by interactions of
  magnetic flux tubes could be at work in the plage chromosphere.

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Title: STIX X-ray microflare observations during the Solar Orbiter
    commissioning phase
Authors: Battaglia, Andrea Francesco; Saqri, Jonas; Massa, Paolo;
   Perracchione, Emma; Dickson, Ewan C. M.; Xiao, Hualin; Veronig,
   Astrid M.; Warmuth, Alexander; Battaglia, Marina; Hurford, Gordon J.;
   Meuris, Aline; Limousin, Olivier; Etesi, László; Maloney, Shane A.;
   Schwartz, Richard A.; Kuhar, Matej; Schuller, Frederic; Senthamizh
   Pavai, Valliappan; Musset, Sophie; Ryan, Daniel F.; Kleint, Lucia;
   Piana, Michele; Massone, Anna Maria; Benvenuto, Federico; Sylwester,
   Janusz; Litwicka, Michalina; Stȩślicki, Marek; Mrozek, Tomasz;
   Vilmer, Nicole; Fárník, František; Kašparová, Jana; Mann,
   Gottfried; Gallagher, Peter T.; Dennis, Brian R.; Csillaghy, André;
   Benz, Arnold O.; Krucker, Säm
2021A&A...656A...4B    Altcode: 2021arXiv210610058B
  Context. The Spectrometer/Telescope for Imaging X-rays (STIX) is the
  hard X-ray instrument onboard Solar Orbiter designed to observe solar
  flares over a broad range of flare sizes. <BR /> Aims: We report
  the first STIX observations of solar microflares recorded during
  the instrument commissioning phase in order to investigate the STIX
  performance at its detection limit. <BR /> Methods: STIX uses hard
  X-ray imaging spectroscopy in the range between 4-150 keV to diagnose
  the hottest flare plasma and related nonthermal electrons. This first
  result paper focuses on the temporal and spectral evolution of STIX
  microflares occuring in the Active Region (AR) AR12765 in June 2020,
  and compares the STIX measurements with Earth-orbiting observatories
  such as the X-ray Sensor of the Geostationary Operational Environmental
  Satellite (GOES/XRS), the Atmospheric Imaging Assembly of the Solar
  Dynamics Observatory, and the X-ray Telescope of the Hinode mission. <BR
  /> Results: For the observed microflares of the GOES A and B class, the
  STIX peak time at lowest energies is located in the impulsive phase of
  the flares, well before the GOES peak time. Such a behavior can either
  be explained by the higher sensitivity of STIX to higher temperatures
  compared to GOES, or due to the existence of a nonthermal component
  reaching down to low energies. The interpretation is inconclusive
  due to limited counting statistics for all but the largest flare
  in our sample. For this largest flare, the low-energy peak time is
  clearly due to thermal emission, and the nonthermal component seen at
  higher energies occurs even earlier. This suggests that the classic
  thermal explanation might also be favored for the majority of the
  smaller flares. In combination with EUV and soft X-ray observations,
  STIX corroborates earlier findings that an isothermal assumption
  is of limited validity. Future diagnostic efforts should focus on
  multi-wavelength studies to derive differential emission measure
  distributions over a wide range of temperatures to accurately describe
  the energetics of solar flares. <BR /> Conclusions: Commissioning
  observations confirm that STIX is working as designed. As a rule of
  thumb, STIX detects flares as small as the GOES A class. For flares
  above the GOES B class, detailed spectral and imaging analyses can
  be performed.

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Title: Measurements of Photospheric and Chromospheric Magnetic
    Field Structures Associated with Chromospheric Heating over a Solar
    Plage Region
Authors: Anan, Tetsu; Schad, Thomas A.; Kitai, Reizaburo; Dima,
   Gabriel I.; Jaeggli, Sarah A.; Tarr, Lucas A.; Collados, Manuel;
   Dominguez-Tagle, Carlos; Kleint, Lucia
2021ApJ...921...39A    Altcode: 2021arXiv210807907A
  In order to investigate the relation between magnetic structures and
  the signatures of heating in plage regions, we observed a plage region
  with the He I 1083.0 nm and Si I 1082.7 nm lines on 2018 October 3
  using the integral field unit mode of the GREGOR Infrared Spectrograph
  (GRIS) installed at the GREGOR telescope. During the GRIS observation,
  the Interface Region Imaging Spectrograph obtained spectra of the
  ultraviolet Mg II doublet emitted from the same region. In the periphery
  of the plage region, within the limited field of view seen by GRIS,
  we find that the Mg II radiative flux increases with the magnetic
  field in the chromosphere with a factor of proportionality of 2.38 ×
  10<SUP>4</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> G<SUP>-1</SUP>. The
  positive correlation implies that magnetic flux tubes can be heated
  by Alfvén wave turbulence or by collisions between ions and neutral
  atoms relating to Alfvén waves. Within the plage region itself,
  the radiative flux was large between patches of strong magnetic field
  strength in the photosphere or at the edges of magnetic patches. On
  the other hand, we do not find any significant spatial correlation
  between the enhanced radiative flux and the chromospheric magnetic
  field strength or the electric current. In addition to the Alfvén
  wave turbulence or collisions between ions and neutral atoms relating
  to Alfvén waves, other heating mechanisms related to magnetic field
  perturbations produced by interactions of magnetic flux tubes could
  be at work in the plage chromosphere.

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Title: Measuring the Magnetic Origins of Solar Flares, Coronal Mass
    Ejections, and Space Weather
Authors: Judge, Philip; Rempel, Matthias; Ezzeddine, Rana; Kleint,
   Lucia; Egeland, Ricky; Berdyugina, Svetlana V.; Berger, Thomas; Bryans,
   Paul; Burkepile, Joan; Centeno, Rebecca; de Toma, Giuliana; Dikpati,
   Mausumi; Fan, Yuhong; Gilbert, Holly; Lacatus, Daniela A.
2021ApJ...917...27J    Altcode: 2021arXiv210607786J
  We take a broad look at the problem of identifying the magnetic
  solar causes of space weather. With the lackluster performance
  of extrapolations based upon magnetic field measurements in the
  photosphere, we identify a region in the near-UV (NUV) part of the
  spectrum as optimal for studying the development of magnetic free energy
  over active regions. Using data from SORCE, the Hubble Space Telescope,
  and SKYLAB, along with 1D computations of the NUV spectrum and numerical
  experiments based on the MURaM radiation-magnetohydrodynamic and
  HanleRT radiative transfer codes, we address multiple challenges. These
  challenges are best met through a combination of NUV lines of bright Mg
  II, and lines of Fe II and Fe I (mostly within the 4s-4p transition
  array) which form in the chromosphere up to 2 × 10<SUP>4</SUP>
  K. Both Hanle and Zeeman effects can in principle be used to derive
  vector magnetic fields. However, for any given spectral line the τ
  = 1 surfaces are generally geometrically corrugated owing to fine
  structure such as fibrils and spicules. By using multiple spectral
  lines spanning different optical depths, magnetic fields across nearly
  horizontal surfaces can be inferred in regions of low plasma β, from
  which free energies, magnetic topology, and other quantities can be
  derived. Based upon the recently reported successful sub-orbital space
  measurements of magnetic fields with the CLASP2 instrument, we argue
  that a modest space-borne telescope will be able to make significant
  advances in the attempts to predict solar eruptions. Difficulties
  associated with blended lines are shown to be minor in an Appendix.

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Title: Comparison of active region upflow and core properties using
    simultaneous spectroscopic observations from IRIS and Hinode
Authors: Barczynski, Krzysztof; Harra, Louise; Kleint, Lucia; Panos,
   Brandon; Brooks, David H.
2021A&A...651A.112B    Altcode: 2021arXiv210410234B
  Context. The origin of the slow solar wind is still an open issue. It
  has been suggested that upflows at the edge of active regions are
  a possible source of the plasma outflow and therefore contribute
  to the slow solar wind. <BR /> Aims: We investigate the origin and
  morphology of the upflow regions and compare the upflow region and
  the active region core properties. <BR /> Methods: We studied how the
  plasma properties of flux, Doppler velocity, and non-thermal velocity
  change throughout the solar atmosphere, from the chromosphere via the
  transition region to the corona in the upflow region and the core
  of an active region. We studied limb-to-limb observations of the
  active region (NOAA 12687) obtained from 14 to 25 November 2017. We
  analysed spectroscopic data simultaneously obtained from IRIS and
  Hinode/EIS in the six emission lines Mg II 2796.4Å, C II 1335.71Å,
  Si IV 1393.76Å, Fe XII 195.12Å, Fe XIII 202.04Å, and Fe XIV
  270.52Å and 274.20Å. We studied the mutual relationships between the
  plasma properties for each emission line, and we compared the plasma
  properties between the neighbouring formation temperature lines. To
  find the most characteristic spectra, we classified the spectra in
  each wavelength using the machine learning technique k-means. <BR />
  Results: We find that in the upflow region the Doppler velocities of
  the coronal lines are strongly correlated, but the transition region
  and coronal lines show no correlation. However, their fluxes are
  strongly correlated. The upflow region has a lower density and lower
  temperature than the active region core. In the upflow region, the
  Doppler velocity and non-thermal velocity show a strong correlation in
  the coronal lines, but the correlation is not seen in the active region
  core. At the boundary between the upflow region and the active region
  core, the upflow region shows an increase in the coronal non-thermal
  velocity, the emission obtained from the DEM, and the domination
  of the redshifted regions in the chromosphere. <BR /> Conclusions:
  The obtained results suggest that at least three parallel mechanisms
  generate the plasma upflow: (1) The reconnection between closed loops
  and open magnetic field lines in the lower corona or upper chromosphere;
  (2) the reconnection between the chromospheric small-scale loops and
  open magnetic field; and (3) the expansion of the magnetic field lines
  that allows the chromospheric plasma to escape to the solar corona.

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Title: Exploring Mutual Information between IRIS Spectral
    Lines. II. Calculating the Most Probable Response in all Spectral
    Windows
Authors: Panos, Brandon; Kleint, Lucia
2021ApJ...915...77P    Altcode: 2021arXiv210603463P
  A three-dimensional picture of the solar atmosphere's thermodynamics
  can be obtained by jointly analyzing multiple spectral lines that
  span many formation heights. In Paper I, we found strong correlations
  between spectral shapes from a variety of different ions during solar
  flares in comparison to the quiet Sun. We extend these techniques to
  address the following questions: which regions of the solar atmosphere
  are most connected during a solar flare, and what are the most likely
  responses across several spectral windows based on the observation of
  a single Mg II spectrum? Our models are derived from several million
  IRIS spectra collected from 21 M- and X-class flares. We applied this
  framework to archetypal Mg II flare spectra and analyzed the results
  from a multiline perspective. We find that (1) the line correlations
  from the photosphere to the transition region are highest in flare
  ribbons. (2) Blueshifted reversals appear simultaneously in Mg II, C II,
  and Si IV during the impulsive phase, with Si IV displaying possible
  optical depth effects. Fe II shows signs of strong emission, indicating
  deep early heating. (3) The Mg II line appears to typically evolve
  a blueshifted reversal that later returns to line center and becomes
  single peaked within 1-3 minutes. The widths of these single-peaked
  profiles slowly erode with time. During the later flare stages, strong
  red-wing enhancements indicating coronal rain are evident in Mg II, C
  II, and Si IV. Our framework is easily adaptable to any multiline data
  set and enables comprehensive statistical analyses of the atmospheric
  behavior in different spectral windows.

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Title: A Journey from Quiet Sun Magnetic Fields to Flares
Authors: Kleint, L.
2021AAS...23822301K    Altcode:
  Even though solar physics is a well-established field, the number
  of unsolved fundamental questions is surprisingly large. What is the
  strength and direction of magnetic fields on different scales and at
  different heights? Why is the corona hot? When will the next solar
  flare occur? To probe these open questions, we rely on a combination
  of challenging observations, complex modeling, and analyzing a wealth
  of data. In this talk, we will discuss some of the recent advances in
  observations, modeling, and data analysis. Using GREGOR as an example,
  we will explore how our observing capabilities can be improved, discuss
  approaches to measuring turbulent quiet Sun fields via high-precision
  spectropolarimetry, and how techniques from computer science can help us
  cope with vast amounts of data that can no longer be analyzed manually.

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Title: Energy Budget of Plasma Motions, Heating, and Electron
    Acceleration in a Three-loop Solar Flare
Authors: Fleishman, Gregory D.; Kleint, Lucia; Motorina, Galina G.;
   Nita, Gelu M.; Kontar, Eduard P.
2021ApJ...913...97F    Altcode: 2021arXiv210400811F
  Nonpotential magnetic energy promptly released in solar flares is
  converted to other forms of energy. This may include nonthermal energy
  of flare-accelerated particles, thermal energy of heated flaring
  plasma, and kinetic energy of eruptions, jets, upflows/downflows,
  and stochastic (turbulent) plasma motions. The processes or parameters
  governing partitioning of the released energy between these components
  are an open question. How these components are distributed between
  distinct flaring loops and what controls these spatial distributions
  are also unclear. Here, based on multiwavelength data and 3D modeling,
  we quantify the energy partitioning and spatial distribution
  in the well-observed SOL2014-02-16T064620 solar flare of class
  C1.5. Nonthermal emission of this flare displayed a simple impulsive
  single-spike light curve lasting about 20 s. In contrast, the thermal
  emission demonstrated at least three distinct heating episodes, only
  one of which was associated with the nonthermal component. The flare
  was accompanied by upflows and downflows and substantial turbulent
  velocities. The results of our analysis suggest that (i) the flare
  occurs in a multiloop system that included at least three distinct flux
  tubes; (ii) the released magnetic energy is divided unevenly between
  the thermal and nonthermal components in these loops; (iii) only
  one of these three flaring loops contains an energetically important
  amount of nonthermal electrons, while two other loops remain thermal;
  (iv) the amounts of direct plasma heating and that due to nonthermal
  electron loss are comparable; and (v) the kinetic energy in the flare
  footpoints constitutes only a minor fraction compared with the thermal
  and nonthermal energies.

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Title: Magnetic field structures associated with chromospheric
    heating in a plage region
Authors: Anan, T.; Schad, T.; Kitai, R.; Dima, G.; Jaeggli, S.;
   Collados, M.; Dominguez-Tagle, C.; Kleint, L.
2021AAS...23821222A    Altcode:
  The strongest quasi-steady heating in the solar atmosphere occurs in
  the active chromosphere and in particular within plage regions. Our
  aim is to investigate the relation between magnetic structures and
  the signatures of heating in the plage regions so as to clarify what
  mechanisms are at work. We observed a plage region in NOAA active
  region 12723 in the near infrared He I triplet and Si I 1082.7 nm
  on 2018 October 3 using the Integral Field Unit mode of the GREGOR
  Infrared Spectrograph (GRIS) installed at the GREGOR telescope. At the
  same time, the Interface Region Imaging Spectrograph (IRIS) obtained
  spectra in the ultra-violet Mg II h &amp; k doublet emitted from the
  same region. We applied the HAnle and ZEeman Light v2.0 inversion
  code (HAZEL v2.0) to the GRIS data to infer the photospheric and
  chromospheric magnetic field. We find that the radiative flux of the Mg
  II was large between patches of strong magnetic field strength in the
  photosphere, or at edges of the magnetic patches. On the other hand,
  the spatial correspondences between the Mg II flux and the magnetic
  field strength in the chromosphere and between the Mg II flux and the
  electric current are not so clear. In conclusion, chromospheric heatings
  in the plage region can be related to magnetic field perturbations
  produced by interactions of magnetic flux tubes.

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Title: Exploring Mutual Information between IRIS Spectral
    Lines. I. Correlations between Spectral Lines during Solar Flares
    and within the Quiet Sun.
Authors: Panos, Brandon; Kleint, Lucia; Voloshynovskiy, Sviatoslav
2021ApJ...912..121P    Altcode: 2021arXiv210412161P
  Spectral lines allow us to probe the thermodynamics of the solar
  atmosphere, but the shape of a single spectral line may be similar for
  different thermodynamic solutions. Multiline analyses are therefore
  crucial, but computationally cumbersome. We investigate correlations
  between several chromospheric and transition region lines to restrain
  the thermodynamic solutions of the solar atmosphere during flares. We
  used machine-learning methods to capture the statistical dependencies
  between six spectral lines sourced from 21 large solar flares observed
  by NASA's Interface Region Imaging Spectrograph. The techniques are
  based on an information-theoretic quantity called mutual information
  (MI), which captures both linear and nonlinear correlations between
  spectral lines. The MI is estimated using both a categorical and
  numeric method, and performed separately for a collection of quiet Sun
  and flaring observations. Both approaches return consistent results,
  indicating weak correlations between spectral lines under quiet Sun
  conditions, and substantially enhanced correlations under flaring
  conditions, with some line-pairs such as Mg II and C II having a
  normalized MI score as high as 0.5. We find that certain spectral
  lines couple more readily than others, indicating a coherence in
  the solar atmosphere over many scale heights during flares, and that
  all line-pairs are correlated to the GOES derivative, indicating a
  positive relationship between correlation strength and energy input. Our
  methods provide a highly stable and flexible framework for quantifying
  dependencies between the physical quantities of the solar atmosphere,
  allowing us to obtain a three-dimensional picture of its state.

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Title: High Resolution Solar Flare Observations
Authors: Kleint, Lucia
2021cosp...43E1771K    Altcode:
  Solar flare observations are complex because of the unpredictability of
  the events and their fast evolution. I will present recent spectroscopic
  and polarimetric observations of solar flares. Such observations allow
  us to investigate the flare-related changes of the magnetic field,
  their relation to continuum emission, and to understand the energetics
  of flares. By additionally employing machine learning methods, we can
  utilize flare spectra to attempt to predict flares and first results
  indicate that there are spectral features that appear a few minutes
  before a flare starts.

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Title: Photospheric and Chromospheric Polarimetry
Authors: Kleint, Lucia
2021cosp...43E1787K    Altcode:
  Polarimetry is the key to understanding the magnetic nature of the Sun
  and stars. Directed magnetic fields are detected via the Zeeman effect,
  which causes spectral lines to split in the presence of magnetic
  fields. The polarization of the different components allows us to
  infer the direction and strength of the magnetic field. But even the
  very weak turbulent magnetic field is measurable by investigating the
  scattering polarization and its changes due to the Hanle effect. I
  will review photospheric and chromospheric polarimetry and how such
  observations help us understand the scales, strength, and variability
  of magnetic fields on the Sun.

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Title: A Comparison of the Active Region Upflow and Core Morphologies
    Using Simultaneous Spectroscopic Observations from IRIS and Hinode.
Authors: Barczynski, K.; Harra, L. K.; Kleint, L.; Panos, B.
2020AGUFMSH004..05B    Altcode:
  The origin of the slow solar wind is still an open issue. It has
  been suggested that upflows at the edge of the active region are the
  source of the plasma outflow, and therefore contribute to the slow
  solar wind . However, the origin and morphology of the upflow region
  remain open questions. We investigated how the plasma properties
  (flux, Doppler velocity, and non-thermal velocity) change throughout
  the solar atmosphere, from the chromosphere via the transition
  region to the corona. We compared the upflow region and the core of
  an active region. We studied limb-to-limb observation of the active
  region (NOAA 12687) obtained between 14th and 25th November 2017. We
  analyzed spectroscopic data simultaneously obtained from Hinode/EIS
  and IRIS in six wavelengths (MgII, CII, SiIV, FeXII, FeXIII, and
  FeXIV). After the high-precision alignment (accuracy of the Hinode
  pixel size) of the raster maps, we studied the mutual relation between
  the plasma properties for each line, as well as compared the plasma
  properties in the close formation temperature lines. To find the most
  characteristic spectra, we classified the spectra in each wavelength
  using the machine learning technique k-means . We found that the
  fluxes of the lines formed in the close temperatures are highly
  correlated in the chromosphere via transition region to the corona. In
  the corona, the Doppler velocities are well correlated too. Despite
  high-correlation between the transition region and coronal fluxes,
  the Doppler velocities are independent in our active region. In
  coronal lines, the average non-thermal velocity is higher in the
  upflow region than the active region core. In the transition region,
  the velocities are similar; thus the non-thermal motions are essential
  in the coronal upflow. We found several mutual relations between the
  plasma parameters in different spectral lines. These relations and
  the spectra classification results suggest that the plasma upflow
  begins in the solar corona, but the nature of the upflow region can
  be determined from the underlying layers.

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Title: The Statistical Relationship between White-light Emission
    and Photospheric Magnetic Field Changes in Flares
Authors: Castellanos Durán, J. Sebastián; Kleint, Lucia
2020ApJ...904...96C    Altcode: 2020arXiv200702954C
  Continuum emission, also called white-light emission (WLE), and
  permanent changes of the magnetic field (ΔB<SUB>LOS</SUB>) are often
  observed during solar flares. However, their relation and precise
  mechanisms are still unknown. We study statistically the relationship
  between ΔB<SUB>LOS</SUB> and WLE during 75 solar flares of different
  strengths and locations on the solar disk. We analyze SDO/HMI data
  and determine for each pixel in each flare if it exhibited WLE and/or
  ΔB<SUB>LOS</SUB>. We then investigate the occurrence, strength,
  and spatial size of the WLE, its dependence on flare energy, and
  its correlation to the occurrence of ΔB<SUB>LOS</SUB>. We detected
  WLE in 44/75 flares and ΔB<SUB>LOS</SUB> in 59/75 flares. We find
  that WLE and ΔB<SUB>LOS</SUB> are related, and their locations
  often overlap between 0% and 60%. Not all locations coincide, thus
  potentially indicating differences in their origin. We find that the
  WL area is related to the flare class by a power law, and extend the
  findings of previous studies, that the WLE is related to the flare
  class by a power law, to also be valid for C-class flares. To compare
  unresolved (Sun-as-a-star) WL measurements with our data, we derive
  a method to calculate temperatures and areas of such data under the
  blackbody assumption. The calculated unresolved WLE areas improve,
  but still differ to the resolved flaring area by about a factor of
  5-10 (previously 10-20), which could be explained by various physical
  or instrumental causes. This method could also be applied to stellar
  flares to determine their temperatures and areas independently.

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Title: Coordination within the remote sensing payload on the Solar
    Orbiter mission
Authors: Auchère, F.; Andretta, V.; Antonucci, E.; Bach, N.;
   Battaglia, M.; Bemporad, A.; Berghmans, D.; Buchlin, E.; Caminade,
   S.; Carlsson, M.; Carlyle, J.; Cerullo, J. J.; Chamberlin, P. C.;
   Colaninno, R. C.; Davila, J. M.; De Groof, A.; Etesi, L.; Fahmy,
   S.; Fineschi, S.; Fludra, A.; Gilbert, H. R.; Giunta, A.; Grundy,
   T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.;
   Howard, R. A.; Hurford, G.; Kleint, L.; Kolleck, M.; Krucker, S.;
   Lagg, A.; Landini, F.; Long, D. M.; Lefort, J.; Lodiot, S.; Mampaey,
   B.; Maloney, S.; Marliani, F.; Martinez-Pillet, V.; McMullin, D. R.;
   Müller, D.; Nicolini, G.; Orozco Suarez, D.; Pacros, A.; Pancrazzi,
   M.; Parenti, S.; Peter, H.; Philippon, A.; Plunkett, S.; Rich, N.;
   Rochus, P.; Rouillard, A.; Romoli, M.; Sanchez, L.; Schühle, U.;
   Sidher, S.; Solanki, S. K.; Spadaro, D.; St Cyr, O. C.; Straus, T.;
   Tanco, I.; Teriaca, L.; Thompson, W. T.; del Toro Iniesta, J. C.;
   Verbeeck, C.; Vourlidas, A.; Watson, C.; Wiegelmann, T.; Williams,
   D.; Woch, J.; Zhukov, A. N.; Zouganelis, I.
2020A&A...642A...6A    Altcode:
  Context. To meet the scientific objectives of the mission, the Solar
  Orbiter spacecraft carries a suite of in-situ (IS) and remote sensing
  (RS) instruments designed for joint operations with inter-instrument
  communication capabilities. Indeed, previous missions have shown that
  the Sun (imaged by the RS instruments) and the heliosphere (mainly
  sampled by the IS instruments) should be considered as an integrated
  system rather than separate entities. Many of the advances expected
  from Solar Orbiter rely on this synergistic approach between IS and
  RS measurements. <BR /> Aims: Many aspects of hardware development,
  integration, testing, and operations are common to two or more
  RS instruments. In this paper, we describe the coordination effort
  initiated from the early mission phases by the Remote Sensing Working
  Group. We review the scientific goals and challenges, and give an
  overview of the technical solutions devised to successfully operate
  these instruments together. <BR /> Methods: A major constraint for the
  RS instruments is the limited telemetry (TM) bandwidth of the Solar
  Orbiter deep-space mission compared to missions in Earth orbit. Hence,
  many of the strategies developed to maximise the scientific return from
  these instruments revolve around the optimisation of TM usage, relying
  for example on onboard autonomy for data processing, compression,
  and selection for downlink. The planning process itself has been
  optimised to alleviate the dynamic nature of the targets, and an
  inter-instrument communication scheme has been implemented which can
  be used to autonomously alter the observing modes. We also outline the
  plans for in-flight cross-calibration, which will be essential to the
  joint data reduction and analysis. <BR /> Results: The RS instrument
  package on Solar Orbiter will carry out comprehensive measurements
  from the solar interior to the inner heliosphere. Thanks to the close
  coordination between the instrument teams and the European Space
  Agency, several challenges specific to the RS suite were identified
  and addressed in a timely manner.

---------------------------------------------------------
Title: The Spectrometer/Telescope for Imaging X-rays (STIX)
Authors: Krucker, Säm; Hurford, G. J.; Grimm, O.; Kögl, S.;
   Gröbelbauer, H. -P.; Etesi, L.; Casadei, D.; Csillaghy, A.; Benz,
   A. O.; Arnold, N. G.; Molendini, F.; Orleanski, P.; Schori, D.; Xiao,
   H.; Kuhar, M.; Hochmuth, N.; Felix, S.; Schramka, F.; Marcin, S.;
   Kobler, S.; Iseli, L.; Dreier, M.; Wiehl, H. J.; Kleint, L.; Battaglia,
   M.; Lastufka, E.; Sathiapal, H.; Lapadula, K.; Bednarzik, M.; Birrer,
   G.; Stutz, St.; Wild, Ch.; Marone, F.; Skup, K. R.; Cichocki, A.; Ber,
   K.; Rutkowski, K.; Bujwan, W.; Juchnikowski, G.; Winkler, M.; Darmetko,
   M.; Michalska, M.; Seweryn, K.; Białek, A.; Osica, P.; Sylwester, J.;
   Kowalinski, M.; Ścisłowski, D.; Siarkowski, M.; Stęślicki, M.;
   Mrozek, T.; Podgórski, P.; Meuris, A.; Limousin, O.; Gevin, O.; Le
   Mer, I.; Brun, S.; Strugarek, A.; Vilmer, N.; Musset, S.; Maksimović,
   M.; Fárník, F.; Kozáček, Z.; Kašparová, J.; Mann, G.; Önel,
   H.; Warmuth, A.; Rendtel, J.; Anderson, J.; Bauer, S.; Dionies, F.;
   Paschke, J.; Plüschke, D.; Woche, M.; Schuller, F.; Veronig, A. M.;
   Dickson, E. C. M.; Gallagher, P. T.; Maloney, S. A.; Bloomfield, D. S.;
   Piana, M.; Massone, A. M.; Benvenuto, F.; Massa, P.; Schwartz, R. A.;
   Dennis, B. R.; van Beek, H. F.; Rodríguez-Pacheco, J.; Lin, R. P.
2020A&A...642A..15K    Altcode:
  <BR /> Aims: The Spectrometer Telescope for Imaging X-rays (STIX)
  on Solar Orbiter is a hard X-ray imaging spectrometer, which
  covers the energy range from 4 to 150 keV. STIX observes hard X-ray
  bremsstrahlung emissions from solar flares and therefore provides
  diagnostics of the hottest (⪆10 MK) flare plasma while quantifying
  the location, spectrum, and energy content of flare-accelerated
  nonthermal electrons. <BR /> Methods: To accomplish this, STIX applies
  an indirect bigrid Fourier imaging technique using a set of tungsten
  grids (at pitches from 0.038 to 1 mm) in front of 32 coarsely pixelated
  CdTe detectors to provide information on angular scales from 7 to 180
  arcsec with 1 keV energy resolution (at 6 keV). The imaging concept of
  STIX has intrinsically low telemetry and it is therefore well-suited
  to the limited resources available to the Solar Orbiter payload. To
  further reduce the downlinked data volume, STIX data are binned on
  board into 32 selectable energy bins and dynamically-adjusted time
  bins with a typical duration of 1 s during flares. <BR /> Results:
  Through hard X-ray diagnostics, STIX provides critical information
  for understanding the acceleration of electrons at the Sun and their
  transport into interplanetary space and for determining the magnetic
  connection of Solar Orbiter back to the Sun. In this way, STIX serves
  to link Solar Orbiter's remote and in-situ measurements.

---------------------------------------------------------
Title: GREGOR: Optics redesign and updates from 2018-2020
Authors: Kleint, Lucia; Berkefeld, Thomas; Esteves, Miguel; Sonner,
   Thomas; Volkmer, Reiner; Gerber, Karin; Krämer, Felix; Grassin,
   Olivier; Berdyugina, Svetlana
2020A&A...641A..27K    Altcode: 2020arXiv200611875K
  The GREGOR telescope was inaugurated in 2012. In 2018, we began
  a complete upgrade, involving optics, alignment, instrumentation,
  mechanical upgrades for vibration reduction, updated control systems,
  and building enhancements, and in addition, adapted management
  and policies. This paper describes all major updates performed
  during this time. Since 2012, all powered mirrors except for M1 were
  exchanged. Since March 2020, GREGOR observes with diffraction-limited
  performance and a new optics and instrument layout.

---------------------------------------------------------
Title: New Light on an Old Problem of the Cores of Solar Resonance
    Lines
Authors: Judge, Philip G.; Kleint, Lucia; Leenaarts, Jorrit;
   Sukhorukov, Andrii V.; Vial, Jean-Claude
2020ApJ...901...32J    Altcode: 2020arXiv200801250J
  We reexamine a 50+ yr old problem of deep central reversals predicted
  for strong solar spectral lines, in contrast to the smaller reversals
  seen in observations. We examine data and calculations for the resonance
  lines of H I, Mg II, and Ca II, the self-reversed cores of which form
  in the upper chromosphere. Based on 3D simulations, as well as data for
  the Mg II lines from the Interface Region Imaging Spectrograph (IRIS),
  we argue that the resolution lies not in velocity fields on scales in
  either of the micro- or macroturbulent limits. Macroturbulence is ruled
  out using observations of optically thin lines formed in the upper
  chromosphere, and by showing that it would need to have unreasonably
  special properties to account for critical observations of the Mg
  II resonance lines from the IRIS mission. The power in "turbulence"
  in the upper chromosphere may therefore be substantially lower than
  earlier analyses have inferred. Instead, in 3D calculations horizontal
  radiative transfer produces smoother source functions, smoothing out
  intensity gradients in wavelength and in space. These effects increase
  in stronger lines. Our work will have consequences for understanding
  the onset of the transition region, for understanding the energy in
  motions available for heating the corona, and for the interpretation
  of polarization data in terms of the Hanle effect applied to resonance
  line profiles.

---------------------------------------------------------
Title: High-resolution observations of the solar photosphere,
    chromosphere, and transition region. A database of coordinated IRIS
    and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
   M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
   Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
   V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
   Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
   Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
   Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
   M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
2020A&A...641A.146R    Altcode: 2020arXiv200514175R
  NASA's Interface Region Imaging Spectrograph (IRIS) provides
  high-resolution observations of the solar atmosphere through ultraviolet
  spectroscopy and imaging. Since the launch of IRIS in June 2013, we
  have conducted systematic observation campaigns in coordination with
  the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
  complementary high-resolution observations of the photosphere and
  chromosphere. The SST observations include spectropolarimetric imaging
  in photospheric Fe I lines and spectrally resolved imaging in the
  chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
  a database of co-aligned IRIS and SST datasets that is open for
  analysis to the scientific community. The database covers a variety
  of targets including active regions, sunspots, plages, the quiet Sun,
  and coronal holes.

---------------------------------------------------------
Title: Determining the dynamics and magnetic fields in He I 10830
    Å during a solar filament eruption
Authors: Kuckein, C.; González Manrique, S. J.; Kleint, L.; Asensio
   Ramos, A.
2020A&A...640A..71K    Altcode: 2020arXiv200610473K
  <BR /> Aims: We investigate the dynamics and magnetic properties of
  the plasma, including the line-of-sight velocity (LOS) and optical
  depth, as well as the vertical and horizontal magnetic fields,
  belonging to an erupted solar filament. <BR /> Methods: The filament
  eruption was observed with the GREGOR Infrared Spectrograph at
  the 1.5-meter GREGOR telescope on July 3, 2016. We acquired three
  consecutive full-Stokes slit-spectropolarimetric scans in the He
  I 10830 Å spectral range. The Stokes I profiles were classified
  using the machine learning k-means algorithm and then inverted with
  different initial conditions using the HAZEL code. <BR /> Results: The
  erupting-filament material presents the following physical conditions:
  (1) ubiquitous upward motions with peak LOS velocities of ∼73 km
  s<SUP>-1</SUP>; (2) predominant large horizontal components of the
  magnetic field, on average, in the range of 173-254 G, whereas the
  vertical components of the fields are much lower, on average between
  39 and 58 G; (3) optical depths in the range of 0.7-1.1. The average
  azimuth orientation of the field lines between two consecutive
  raster scans (&lt;2.5 min) remained constant. <BR /> Conclusions:
  The analyzed filament eruption belongs to the fast rising phase, with
  total velocities of about 124 km s<SUP>-1</SUP>. The orientation of the
  magnetic field lines does not change from one raster scan to the other,
  indicating that the untwisting phase has not yet started. The untwisting
  appears to start about 15 min after the beginning of the filament
  eruption. <P />Movies attached to Figs. 1 and 3 are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202038408/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Determining the dynamics and magnetic fields in the
    chromospheric He I 10830 Å triplet during a solar filament eruption
Authors: Kuckein, C.; González Manrique, S. J.; Kleint, L.; Asensio
   Ramos, A.
2020sea..confE.202K    Altcode:
  We investigate the dynamics and magnetic properties of the plasma, such
  as line-of-sight velocity (LOS), optical depth, vertical and horizontal
  magnetic fields, belonging to an erupted solar filament. The filament
  eruption was observed with the GREGOR Infrared Spectrograph (GRIS)
  at the 1.5-meter GREGOR telescope on 2016 July 3. Three consecutive
  full-Stokes slit-spectropolarimetric scans in the He I 10830 Å
  spectral range were acquired. The Stokes I profiles were classified
  using the machine learning k-means algorithm and then inverted with
  different initial conditions using the inversion code HAZEL. The
  erupting-filament material presents the following physical conditions:
  (i) ubiquitous upward motions with peak LOS velocities of ∼73 km/s;
  (ii) predominant large horizontal components of the magnetic field, on
  average, in the range of 173-254 G, whereas the vertical components of
  the fields are much lower, on average between 39-58 G; (iii) optical
  depths in the range of 0.7-1.1. The average azimuth orientation of
  the field lines between two consecutive raster scans (&lt;2.5 minutes)
  remained constant. The analyzed filament eruption belonged to the fast
  rising phase, with total velocities of about 124 km/s.

---------------------------------------------------------
Title: Real-time Flare Prediction Based on Distinctions between
    Flaring and Non-flaring Active Region Spectra
Authors: Panos, Brandon; Kleint, Lucia
2020ApJ...891...17P    Altcode: 2019arXiv191112621P
  With machine learning entering into the awareness of the heliophysics
  community, solar flare prediction has become a topic of increased
  interest. Although machine-learning models have advanced with each
  successive publication, the input data has remained largely fixed on
  magnetic features. Despite this increased model complexity, results
  seem to indicate that photospheric magnetic field data alone may not
  be a wholly sufficient source of data for flare prediction. For the
  first time, we have extended the study of flare prediction to spectral
  data. In this work, we use Deep Neural Networks to monitor the changes
  of several features derived from the strong resonant Mg II h and
  k lines observed by the Interface Region Imaging Spectrograph. The
  features in descending order of predictive capability are: the triplet
  emission at 2798.77 Å, line core intensity, total continuum emission
  between the h and k line cores, the k/h ratio, line width, followed by
  several other line features such as asymmetry and line center. Regions
  that are about to flare generate spectra that are distinguishable
  from non-flaring active region spectra. Our algorithm can correctly
  identify pre-flare spectra approximately 35 minutes before the start of
  the flare, with an AUC of 86% and an accuracy, precision, and recall
  of 80%. The accuracy and AUC monotonically increase to 90% and 97%,
  respectively, as we move closer in time to the start of the flare. Our
  study indicates that spectral data alone can lead to good predictive
  models and should be considered an additional source of information
  alongside photospheric magnetograms.

---------------------------------------------------------
Title: Joint X-Ray, EUV, and UV Observations of a Small Microflare
Authors: Hannah, Iain G.; Kleint, Lucia; Krucker, Säm; Grefenstette,
   Brian W.; Glesener, Lindsay; Hudson, Hugh S.; White, Stephen M.;
   Smith, David M.
2019ApJ...881..109H    Altcode: 2018arXiv181209214H
  We present the first joint observation of a small microflare in X-rays
  with the Nuclear Spectroscopic Telescope ARray (NuSTAR), in UV with the
  Interface Region Imaging Spectrograph (IRIS), and in EUV with the Solar
  Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA). These
  combined observations allow us to study the hot coronal and cooler
  chromospheric/transition region emission from the microflare. This small
  microflare peaks from 2016 July 26 23:35 to 23:36 UT, in both NuSTAR,
  SDO/AIA, and IRIS. Spatially, this corresponds to a small loop visible
  in the SDO/AIA Fe XVIII emission, which matches a similar structure
  lower in the solar atmosphere seen by IRIS in SJI1330 and 1400 Å. The
  NuSTAR emission in both 2.5-4 and 4-6 keV is located in a source at
  this loop location. The IRIS slit was over the microflaring loop,
  and fits show little change in Mg II but do show intensity increases,
  slight width enhancements, and redshifts in Si IV and O IV, indicating
  that this microflare had most significance in and above the upper
  chromosphere. The NuSTAR microflare spectrum is well fitted by a
  thermal component of 5.1 MK and 6.2 × 10<SUP>44</SUP> cm<SUP>-3</SUP>,
  which corresponds to a thermal energy of 1.5 × 10<SUP>26</SUP> erg,
  making it considerably smaller than previously studied active region
  microflares. No non-thermal emission was detected but this could be
  due to the limited effective exposure time of the observation. This
  observation shows that even ordinary features seen in UV can remarkably
  have a higher-energy component that is clear in X-rays.

---------------------------------------------------------
Title: Measurement of the Evolution of the Magnetic Field of the
    Quiet Photosphere over a Solar Cycle
Authors: Ramelli, R.; Bianda, M.; Berdyugina, S.; Belluzzi, L.;
   Kleint, L.
2019ASPC..526..283R    Altcode:
  The solar photosphere is filled by magnetic fields tangled at scales
  much smaller than the resolution capability of solar telescopes. These
  hidden magnetic fields can be investigated via the Hanle effect. In
  2007, we started a synoptic program to explore whether the magnetic flux
  of the quiet photosphere varies with the solar cycle. For this purpose
  we applied a differential Hanle effect technique based on observations
  of scattering polarization in C<SUB>2</SUB> molecular lines around
  514.0 nm, taken with a cadence of approximately one month. Our results
  now span almost one complete solar cycle.

---------------------------------------------------------
Title: Mapping the Magnetic Field of Flare Coronal Loops
Authors: Kuridze, D.; Mathioudakis, M.; Morgan, H.; Oliver, R.; Kleint,
   L.; Zaqarashvili, T. V.; Reid, A.; Koza, J.; Löfdahl, M. G.; Hillberg,
   T.; Kukhianidze, V.; Hanslmeier, A.
2019ApJ...874..126K    Altcode: 2019arXiv190207514K
  Here, we report on the unique observation of flaring coronal loops at
  the solar limb using high-resolution imaging spectropolarimetry from
  the Swedish 1 m Solar Telescope. The vantage position, orientation,
  and nature of the chromospheric material that filled the flare loops
  allowed us to determine their magnetic field with unprecedented accuracy
  using the weak-field approximation method. Our analysis reveals coronal
  magnetic field strengths as high as 350 G at heights up to 25 Mm above
  the solar limb. These measurements are substantially higher than a
  number of previous estimates and may have considerable implications
  for our current understanding of the extended solar atmosphere.

---------------------------------------------------------
Title: Heating of the solar photosphere during a white-light flare
Authors: Jurčák, Jan; Kašparová, Jana; Švanda, Michal; Kleint,
   Lucia
2018A&A...620A.183J    Altcode: 2018arXiv181107794J
  Context. The Fe I lines observed by the Hinode/SOT spectropolarimeter
  were always seen in absorption, apart from the extreme solar limb. Here
  we analyse a unique dataset capturing these lines in emission during
  a solar white-light flare. <BR /> Aims: We analyse the temperature
  stratification in the solar photosphere during a white-light flare and
  compare it with the post-white-light flare state. <BR /> Methods: We
  used two scans of the Hinode/SOT spectropolarimeter to infer, by means
  of the LTE inversion code Stokes Inversion based on Response function
  (SIR), the physical properties in the solar photosphere during and
  after a white-light flare. The resulting model atmospheres are compared
  and the changes are related to the white-light flare. <BR /> Results:
  We show that the analysed white-light flare continuum brightening is
  probably not caused by the temperature increase at the formation height
  of the photospheric continuum. However, the photosphere is heated
  by the flare approximately down to log τ = -0.5 and this results
  in emission profiles of the observed Fe I lines. From the comparison
  with the post-white-light flare state of the atmosphere, we estimate
  that the major contribution to the increase in the continuum intensity
  originates in the heated chromosphere.

---------------------------------------------------------
Title: High-density Off-limb Flare Loops Observed by SDO
Authors: Jejčič, S.; Kleint, L.; Heinzel, P.
2018ApJ...867..134J    Altcode: 2018arXiv181002431J
  The density distribution of flare loops and the mechanisms of their
  emission in the continuum are still open questions. On 2017 September
  10, a prominent loop system appeared during the gradual phase of an X8.2
  flare (SOL2017-09-10), visible in all passbands of SDO/AIA and in the
  white-light continuum of SDO/HMI. We investigate its electron density
  by taking into account all radiation processes in the flare loops,
  i.e., the Thomson continuum, hydrogen Paschen and Brackett recombination
  continua, as well as free-free continuum emission. We derive a quadratic
  function of the electron density for a given temperature and effective
  loop thickness. By absolutely calibrating SDO/HMI intensities,
  we convert the measured intensities into electron density at each
  pixel in the loops. For a grid of plausible temperatures between cool
  (6000 K) and hot (10<SUP>6</SUP> K) structures, the electron density
  is computed for representative effective thicknesses between 200 and
  20,000 km. We obtain a relatively high maximum electron density, about
  10<SUP>13</SUP> cm<SUP>-3</SUP>. At such high electron densities, the
  Thomson continuum is negligible and therefore one would not expect a
  significant polarization degree in dense loops. We conclude that the
  Paschen and Brackett recombination continua are dominant in cool flare
  loops, while the free-free continuum emission is dominant for warmer
  and hot loops.

---------------------------------------------------------
Title: Nonlinear Force-free Modeling of Flare-related Magnetic Field
    Changes at the Photosphere and Chromosphere
Authors: Kleint, Lucia; Wheatland, Michael S.; Mastrano, Alpha;
   McCauley, Patrick I.
2018ApJ...865..146K    Altcode: 2018arXiv180807079K
  Rapid and stepwise changes of the magnetic field are often observed
  during flares but cannot be explained by models yet. Using a 45 minute
  sequence of Solar Dynamics Observatory/Helioseismic and Magnetic
  Imager 135 s fast-cadence vector magnetograms of the X1 flare on 2014
  March 29 we construct, at each timestep, nonlinear force-free models
  for the coronal magnetic field. Observed flare-related changes in the
  line-of-sight magnetic field B <SUB>LOS</SUB> at the photosphere and
  chromosphere are compared with changes in the magnetic fields in the
  models. We find a moderate agreement at the photospheric layer (the
  basis for the models), but no agreement at chromospheric layers. The
  observed changes at the photosphere and chromosphere are surprisingly
  different, and are unlikely to be reproduced by a force-free model. The
  observed changes are likely to require a change in the magnitude of
  the field, not just in its direction.

---------------------------------------------------------
Title: Solar Polarimetry - from Turbulent Magnetic Fields to Sunspots
Authors: Kleint, Lucia
2018cosp...42E1772K    Altcode:
  Polarimetric measurements are essential to investigate the solar
  magnetic field. Scattering polarization and the Hanle effect allow us to
  probe the turbulent magnetic field and the still open questions of its
  strength and variability. Directed magnetic fields can be detected via
  the Zeeman effect. To derive their orientation and strength, so-called
  inversion codes are used, which iteratively modify a model atmosphere
  and calculate the resulting polarization profiles that are then compared
  to the observations. While photospheric polarimetry is well-established,
  chromospheric polarimetry is still in its infancy, especially because
  it requires a treatment in non-LTE, making it a complex non-linear
  problem. But some of the most important open questions concern the
  strength and geometry of the chromospheric magnetic field. In this
  talk, I will review different polarimetric analysis techniques and
  recent advances in magnetic field measurements going from the small
  scales of turbulent magnetic fields to changes of magnetic fields in
  an active region during flares.

---------------------------------------------------------
Title: Identifying Typical Mg II Flare Spectra Using Machine Learning
Authors: Panos, Brandon; Kleint, Lucia; Huwyler, Cedric; Krucker,
   Säm; Melchior, Martin; Ullmann, Denis; Voloshynovskiy, Sviatoslav
2018ApJ...861...62P    Altcode: 2018arXiv180510494P
  The Interface Region Imaging Spectrograph (IRIS) performs
  solar observations over a large range of atmospheric heights,
  including the chromosphere where the majority of flare energy is
  dissipated. The strong Mg II h&amp;k spectral lines are capable of
  providing excellent atmospheric diagnostics, but have not been fully
  utilized for flaring atmospheres. We aim to investigate whether the
  physics of the chromosphere is identical for all flare observations by
  analyzing if there are certain spectra that occur in all flares. To
  achieve this, we automatically analyze hundreds of thousands of Mg
  II h&amp;k-line profiles from a set of 33 flares and use a machine
  learning technique, which we call supervised hierarchical k-means, to
  cluster all profile shapes. We identify a single peaked Mg II profile,
  in contrast to the double-peaked quiet Sun profiles, appearing in
  every flare. Additionally, we find extremely broad profiles with
  characteristic blueshifted central reversals appearing at the front of
  fast-moving flare ribbons. These profiles occur during the impulsive
  phase of the flare, and we present results of their temporal and spatial
  correlation with non-thermal hard X-ray signatures, suggesting that
  flare-accelerated electrons play an important role in the formation
  of these profiles. The ratio of the integrated Mg II h&amp;k lines
  can also serve as an opacity diagnostic, and we find higher opacities
  during each flare maximum. Our study shows that machine learning is
  a powerful tool for large scale statistical solar analyses.

---------------------------------------------------------
Title: Understanding the HMI Pseudocontinuum in White-light Solar
    Flares
Authors: Švanda, Michal; Jurčák, Jan; Kašparová, Jana; Kleint,
   Lucia
2018ApJ...860..144S    Altcode: 2018arXiv180503369S
  We analyze observations of the X9.3 solar flare (SOL2017-09-06T11:53)
  observed by SDO/HMI and Hinode/Solar Optical Telescope. Our aim is to
  learn about the nature of the HMI pseudocontinuum I <SUB>c</SUB> used as
  a proxy for the white-light continuum. From model atmospheres retrieved
  by an inversion code applied to the Stokes profiles observed by the
  Hinode satellite, we synthesize profiles of the Fe I 617.3 nm line and
  compare them to HMI observations. Based on a pixel-by-pixel comparison,
  we show that the value of I <SUB>c</SUB> represents the continuum level
  well in quiet-Sun regions only. In magnetized regions, it suffers from
  a simplistic algorithm that is applied to a complex line shape. During
  this flare, both instruments also registered emission profiles in the
  flare ribbons. Such emission profiles are poorly represented by the
  six spectral points of HMI and the MDI-like algorithm does not account
  for emission profiles in general; thus, the derived pseudocontinuum
  intensity does not approximate the continuum value properly.

---------------------------------------------------------
Title: NuSTAR X-ray observations of tiny solar flares
Authors: Hannah, Iain G.; Krucker, Sam; Grefenstette, Brian; Glesener,
   Lindsay; Kuhar, Matej; Miles Smith, David; Kleint, Lucia
2018tess.conf40801H    Altcode:
  NuSTAR is an astrophysics X-ray telescope, with direct imaging
  spectroscopy providing a unique sensitivity for observing the Sun
  above 2.5keV. This is ideal for capturing the response of the solar
  atmosphere to the energy released in the smallest flares. NuSTAR has
  observed the Sun several times since Sep 2014 and we present some of
  the initial observations of tiny microflares within active regions,
  events down to the GOES A0.1 equivalent level. These microflares
  show thermal emission up to 10MK, and the possibility of non-thermal
  emission powering this heating. In conjunction with observations at
  lower energy wavelengths (Hinode/XRT, SDO/AIA and IRIS) we are able
  to present a more complete picture of the solar atmosphere's thermal
  response to these small flares. We also show tiny impulsive events
  outside of active regions, that are orders of magnitude smaller,
  and yet still emit a clear X-ray signature.

---------------------------------------------------------
Title: Observation and Modeling of Mg II lines during an M6.5 Flare
Authors: Huang, Nengyi; Xu, Yan; Jing, Ju; Sadykov, Viacheslav M.;
   Kleint, Lucia; Wang, Haimin
2018tess.conf11403H    Altcode:
  Recent studies have shown special properties of flare emission at the
  precipitating site of electrons, such as the enhanced absorption in He
  I lines and strong Doppler shift in H-alpha. Using the high resolution
  imaging spectroscopic data obtained by IRIS, we investigate the Mg
  II emission lines during an M6.5 flare (SOL2015-06-22T18:23), which
  was well covered by the joint observation of IRIS and BBSO/GST. On
  the leading edge of the propagating ribbon, Mg II h and k lines are
  characterized by strong broadening (~100 km/s) and weak blue shift
  (~5km/s), cospatial with red shifted H-alpha spectra. Additionally,
  we carry out numerical simulation of the special Mg II lines using RH
  code, taking inputs of flare atmosphere calculated by the radiative
  hydrodynamic code RADYN. By iterating the parameters such as velocity
  fields, plasma temperature and electron density, we investigate the
  dominant factors in generating the Mg II broadening and blue shift. Our
  preliminary results indicate that nonuniform velocity field and strong
  microturbulence can enhance the line broadening, but the cause of blue
  shift is not conclusive.

---------------------------------------------------------
Title: A Statistical Study of Photospheric Magnetic Field Changes
    During 75 Solar Flares
Authors: Castellanos Durán, J. S.; Kleint, L.; Calvo-Mozo, B.
2018ApJ...852...25C    Altcode: 2017arXiv171108631S; 2017arXiv171108631C
  Abrupt and permanent changes of photospheric magnetic fields have
  been observed during solar flares. The changes seem to be linked
  to the reconfiguration of magnetic fields, but their origin is
  still unclear. We carried out a statistical analysis of permanent
  line-of-sight magnetic field ({B}<SUB>{LOS</SUB>}) changes during
  18 X-, 37 M-, 19 C-, and 1 B-class flares using data from the
  Solar Dynamics Observatory/Helioseismic and Magnetic Imager. We
  investigated the properties of permanent changes, such as frequency,
  areas, and locations. We detected changes of {B}<SUB>{LOS</SUB>}
  in 59/75 flares. We find that strong flares are more likely to show
  changes, with all flares ≥M1.6 exhibiting them. For weaker flares,
  permanent changes are observed in 6/17 C-flares. 34.3% of the permanent
  changes occurred in the penumbra and 18.9% in the umbra. Parts of
  the penumbra appeared or disappeared in 23/75 flares. The area where
  permanent changes occur is larger for stronger flares. Strong flares
  also show a larger change of flux, but there is no dependence of the
  magnetic flux change on the heliocentric angle. The mean rate of change
  of flare-related magnetic field changes is 20.7 Mx cm<SUP>-2</SUP>
  min<SUP>-1</SUP>. The number of permanent changes decays exponentially
  with distance from the polarity inversion line. The frequency of the
  strength of permanent changes decreases exponentially, and permanent
  changes up to 750 Mx cm<SUP>-2</SUP> were observed. We conclude that
  permanent magnetic field changes are a common phenomenon during flares,
  and future studies will clarify their relation to accelerated electrons,
  white-light emission, and sunquakes to further investigate their origin.

---------------------------------------------------------
Title: Prospects of Solar Magnetometry—From Ground and in Space
Authors: Kleint, Lucia; Gandorfer, Achim
2018smf..book..397K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The connection between X-ray and coronal emission measure in
    solar limb flares as a diagnostic of non-thermal particle acceleration
    and heating processes
Authors: Rubio da Costa, F.; Effenberger, F.; Kleint, L.
2017AGUFMSH41A2747R    Altcode:
  Using RHESSI X-ray observations and EUV differential emission measures
  (DEM) inferred from SDO/AIA observations, we investigate thermal and
  non-thermal heating processes associated with coronal emission. We
  focus on partially occulted flares located near the solar limb, without
  contamination of the strong non-thermal footpoint emission, which allows
  us to investigate non-thermal sources at/near the loop top.This study
  allows us to temporally and spatially correlate the non-thermal hard
  X-ray signatures with temperature dependent heating processes, with
  the goal of constraining the physical processes of energy release in
  the upper corona. This leads to a better understanding of the thermal
  response of the upper atmosphere to non-thermal processes during solar
  flares. Our preliminary results show that low coronal loops are denser
  and cooler than higher coronal emissions. Higher coronal emissions are
  associated to low energy (6-12 keV) thermal emission and lower loops,
  to non-thermal (24-26 keV) emission.

---------------------------------------------------------
Title: Joint NuSTAR and IRIS observation of a microflaring active
    region
Authors: Hannah, I. G.; Kleint, L.; Krucker, S.; Glesener, L.;
   Grefenstette, B.
2017AGUFMSH41A2743H    Altcode:
  We present observations of a weakly microflaring active region observed
  in X-rays with NuSTAR, UV with IRIS and EUV with SDO/AIA. NuSTAR
  was pointed at this unnamed active region near the East limb between
  23:27UT and 23:37UT 26-July-2016, finding mostly quiescent emission
  except for a small microflare about 23:35UT. The NuSTAR spectrum
  for the pre-microflare time (23:27UT to 23:34UT) is well fitted by a
  single thermal component of about 3MK and combined with SDO/AIA we
  can determine the differential emission measure (DEM), finding it,
  as expected, drops very sharply to higher temperatures. During the
  subsequent microflare, the increase in NuSTAR counts matches a little
  brightening loop observed with IRIS SJI 1400Å and SDO/AIA. Fortuitously
  the IRIS slit crosses this microflaring loop and we find an increased
  emission in Si IV 1394Å, Si IV 1403Å and O IV 1402Å but only
  average line widths and velocities. The NuSTAR microflare spectrum
  shows heating to higher temperatures and also allows us to investigate
  the energetics of this event.

---------------------------------------------------------
Title: On the Nature of Off-limb Flare Continuum Sources Detected
    by SDO/HMI
Authors: Heinzel, P.; Kleint, L.; Kašparová, J.; Krucker, S.
2017ApJ...847...48H    Altcode: 2017arXiv170906377H
  The Helioseismic and Magnetic Imager on board the Solar Dynamics
  Observatory has provided unique observations of off-limb flare
  emission. White-light continuum enhancements were detected in the
  “continuum” channel of the Fe 6173 Å line during the impulsive
  phase of the observed flares. In this paper we aim to determine which
  radiation mechanism is responsible for such enhancement being seen above
  the limb, at chromospheric heights around or below 1000 km. Using a
  simple analytical approach, we compare two candidate mechanisms, the
  hydrogen recombination continuum (Paschen) and the Thomson continuum
  due to scattering of disk radiation on flare electrons. Both mechanisms
  depend on the electron density, which is typically enhanced during the
  impulsive phase of a flare as the result of collisional ionization (both
  thermal and also non-thermal due to electron beams). We conclude that
  for electron densities higher than 10<SUP>12</SUP> cm<SUP>-3</SUP>,
  the Paschen recombination continuum significantly dominates the
  Thomson scattering continuum and there is some contribution from the
  hydrogen free-free emission. This is further supported by detailed
  radiation-hydrodynamical (RHD) simulations of the flare chromosphere
  heated by the electron beams. We use the RHD code FLARIX to compute the
  temporal evolution of the flare-heating in a semi-circular loop. The
  synthesized continuum structure above the limb resembles the off-limb
  flare structures detected by HMI, namely their height above the limb,
  as well as the radiation intensity. These results are consistent with
  recent findings related to hydrogen Balmer continuum enhancements,
  which were clearly detected in disk flares by the IRIS near-ultraviolet
  spectrometer.

---------------------------------------------------------
Title: Prospects of Solar Magnetometry—From Ground and in Space
Authors: Kleint, Lucia; Gandorfer, Achim
2017SSRv..210..397K    Altcode: 2015SSRv..tmp..114K; 2015arXiv151003763K
  In this review we present an overview of observing facilities for solar
  research, which are planned or will come to operation in near future. We
  concentrate on facilities, which harbor specific potential for solar
  magnetometry. We describe the challenges and science goals of future
  magnetic measurements, the status of magnetic field measurements at
  different major solar observatories, and provide an outlook on possible
  upgrades of future instrumentation.

---------------------------------------------------------
Title: Measurement of the evolution of the magnetic field of the
    quiet photosphere during a solar cycle
Authors: Ramelli, Renzo; Bianda, Michele; Berdyugina, Svetlana;
   Belluzzi, Luca; Kleint, Lucia
2017arXiv170803287R    Altcode:
  The solar photosphere is filled by a magnetic field which is tangled
  on scales much smaller than the resolution capability of solar
  telescopes. This hidden magnetic field can be investigated via the
  Hanle effect. In 2007 we started a synoptic program to explore if the
  magnetic flux of the quiet photosphere varies with the solar cycle. For
  this purpose we applied a differential Hanle effect technique based
  on observations of scattering polarization in C$_2$ molecular lines
  around 514.0 nm, taken generally every month. Our results now span
  almost one complete solar cycle.

---------------------------------------------------------
Title: NuSTAR's X-ray observations of a microflaring active region
Authors: Hannah, Iain; Kleint, Lucia; Krucker, Sam; Wright, Paul James;
   Glesener, Lindsay; Grefenstette, Brian
2017SPD....4820101H    Altcode:
  We present observations of a weakly microflaring active region observed
  in X-rays with NuSTAR, UV with IRIS and EUV with SDO/AIA. NuSTAR
  was pointed at this unnamed active region near the East limb between
  23:27UT and 23:37UT 26-July-2016, finding mostly quiescent emission
  except for a small microflare about 23:35UT. The NuSTAR spectrum for
  the pre-microflare time (23:27UT to 23:34UT) is well fitted by a single
  thermal of about 3MK and combined with SDO/AIA we can determine the
  differential emission measure (DEM), finding it, as expected, drops
  very sharply to higher temperatures. During the subsequent microflare,
  the increase in NuSTAR counts matches a little brightening loop observed
  with IRIS SJI 1400Å and SDO/AIA 94Å/Fe XVIII. Fortuitously the IRIS
  slit was on this microflaring loop and we find that the IRIS spectrum
  shows increased emission in Si IV 1394Å, O IV 1402Å and Si IV 1403Å
  but only average line widths and velocities. The NuSTAR microflare
  spectrum shows heating to higher temperatures and also allows us to
  investigate the energetics of this event.

---------------------------------------------------------
Title: A Parameter Study for Modeling Mg II h and k Emission during
    Solar Flares
Authors: Rubio da Costa, Fatima; Kleint, Lucia
2017ApJ...842...82R    Altcode: 2017arXiv170405874R
  Solar flares show highly unusual spectra in which the thermodynamic
  conditions of the solar atmosphere are encoded. Current models are
  unable to fully reproduce the spectroscopic flare observations,
  especially the single-peaked spectral profiles of the Mg II h and k
  lines. We aim to understand the formation of the chromospheric and
  optically thick Mg II h and k lines in flares through radiative
  transfer calculations. We take a flare atmosphere obtained from
  a simulation with the radiative hydrodynamic code RADYN as input
  for a radiative transfer modeling with the RH code. By iteratively
  changing this model atmosphere and varying thermodynamic parameters
  such as temperature, electron density, and velocity, we study their
  effects on the emergent intensity spectra. We reproduce the typical
  single-peaked Mg II h and k flare spectral shape and approximate the
  intensity ratios to the subordinate Mg II lines by increasing either
  densities, temperatures, or velocities at the line core formation height
  range. Additionally, by combining unresolved upflows and downflows up to
  ∼250 km s<SUP>-1</SUP> within one resolution element, we reproduce the
  widely broadened line wings. While we cannot unambiguously determine
  which mechanism dominates in flares, future modeling efforts should
  investigate unresolved components, additional heat dissipation, larger
  velocities, and higher densities and combine the analysis of multiple
  spectral lines.

---------------------------------------------------------
Title: On the Origin of the Flare Emission in IRIS’ SJI 2832
Filter:Balmer Continuum or Spectral Lines?
Authors: Kleint, Lucia; Heinzel, Petr; Krucker, Säm
2017ApJ...837..160K    Altcode: 2017arXiv170207167K
  Continuum (“white-light,” WL) emission dominates the energetics of
  flares. Filter-based observations, such as the IRIS SJI 2832 filter,
  show WL-like brightenings during flares, but it is unclear whether
  the emission arises from real continuum emission or enhanced spectral
  lines, possibly turning into emission. The difficulty in filter-based
  observations, contrary to spectral observations, is to determine which
  processes contribute to the observed brightening during flares. Here
  we determine the contribution of the Balmer continuum and the spectral
  line emission to IRIS’ SJI 2832 emission by analyzing the appropriate
  passband in simultaneous IRIS NUV spectra. We find that spectral line
  emission can contribute up to 100% to the observed slitjaw images (SJI)
  emission, that the relative contributions usually temporally vary,
  and that the highest SJI enhancements that are observed are most likely
  because of the Balmer continuum. We conclude that care should be taken
  when calling SJI 2832 a continuum filter during flares, because the
  influence of the lines on the emission can be significant.

---------------------------------------------------------
Title: First Detection of Chromospheric Magnetic Field Changes during
    an X1-Flare
Authors: Kleint, Lucia
2017ApJ...834...26K    Altcode: 2016arXiv160802552K
  Stepwise changes of the photospheric magnetic field, which
  often becomes more horizontal, have been observed during many
  flares. Previous interpretations include coronal loops that contract,
  and it has been speculated that such jerks could be responsible for
  sunquakes. Here we report the detection of stepwise chromospheric
  line-of-sight magnetic field (B{}<SUB>{LOS</SUB>}) changes obtained
  through spectropolarimetry of Ca II 8542 Å with DST/IBIS during the
  X1-flare SOL20140329T17:48. These changes are stronger (&lt;640 Mx
  cm<SUP>-2</SUP>) and appear in larger areas than their photospheric
  counterparts (&lt;320 Mx cm<SUP>-2</SUP>). The absolute value of
  {B}<SUB>{LOS</SUB>} more often decreases than increases. Photospheric
  changes are predominantly located near a polarity inversion line, and
  chromospheric changes near the footpoints of loops. The locations
  of changes are near, but not exactly co-spatial to hard X-ray
  emission and neither to enhanced continuum emission nor to a small
  sunquake. Enhanced chromospheric and coronal emission is observed in
  nearly all locations that exhibit changes of {B}<SUB>{LOS</SUB>},
  but the emission also occurs in many locations without any
  {B}<SUB>{LOS</SUB>} changes. Photospheric and chromospheric changes
  of {B}<SUB>{LOS</SUB>} show differences in timing, sign, and size and
  seem independent of each other. A simple model of contracting loops
  yields changes of the opposite sign to those observed. An explanation
  for this discrepancy could be increasing loop sizes or loops that
  untwist in a certain direction during the flare. It is yet unclear
  which processes are responsible for the observed changes and their
  timing, size, and location, especially considering the incoherence
  between the photosphere and the chromosphere.

---------------------------------------------------------
Title: Multi-wavelength Study of Transition Region Penumbral
    Subarcsecond Bright Dots Using IRIS and NST
Authors: Deng, Na; Yurchyshyn, Vasyl; Tian, Hui; Kleint, Lucia; Liu,
   Chang; Xu, Yan; Wang, Haimin
2016ApJ...829..103D    Altcode: 2016arXiv160700306D
  Using high-resolution transition region (TR) observations taken by
  the Interface Region Imaging Spectrograph (IRIS) mission, Tian et
  al. revealed numerous short-lived subarcsecond bright dots (BDs)
  above sunspots (mostly located in the penumbrae), which indicate yet
  unexplained small-scale energy releases. Moreover, whether or not
  these subarcsecond TR brightenings have any signature in the lower
  atmosphere and how they are formed are still not fully resolved. This
  paper presents a multi-wavelength study of the TR penumbral BDs using a
  coordinated observation of a near disk center sunspot with IRIS and the
  1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory. NST
  provides high-resolution chromospheric and photospheric observations
  with narrowband Hα imaging spectroscopy and broadband TiO images,
  respectively, complementary to IRIS TR observations. A total of 2692
  TR penumbral BDs are identified from a 37 minute time series of IRIS
  1400 Å slit-jaw images. Their locations tend to be associated more
  with downflowing and darker fibrils in the chromosphere, and weakly
  associated with bright penumbral features in the photosphere. However,
  temporal evolution analyses of the BDs show that there is no consistent
  and convincing brightening response in the chromosphere. These results
  are compatible with a formation mechanism of the TR penumbral BDs
  by falling plasma from coronal heights along more vertical and dense
  magnetic loops. The BDs may also be produced by small-scale impulsive
  magnetic reconnection taking place sufficiently high in the atmosphere
  that has no energy release in the chromosphere.

---------------------------------------------------------
Title: Data-driven Radiative Hydrodynamic Modeling of the 2014 March
    29 X1.0 Solar Flare
Authors: Rubio da Costa, Fatima; Kleint, Lucia; Petrosian, Vahé;
   Liu, Wei; Allred, Joel C.
2016ApJ...827...38R    Altcode: 2016arXiv160304951R; 2016ApJ...827...38D
  Spectroscopic observations of solar flares provide critical diagnostics
  of the physical conditions in the flaring atmosphere. Some key
  features in observed spectra have not yet been accounted for in
  existing flare models. Here we report a data-driven simulation of
  the well-observed X1.0 flare on 2014 March 29 that can reconcile some
  well-known spectral discrepancies. We analyzed spectra of the flaring
  region from the Interface Region Imaging Spectrograph (IRIS) in Mg
  II h&amp;k, the Interferometric BIdimensional Spectropolarimeter at
  the Dunn Solar Telescope (DST/IBIS) in Hα 6563 Å and Ca II 8542 Å,
  and the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI)
  in hard X-rays. We constructed a multithreaded flare loop model and
  used the electron flux inferred from RHESSI data as the input to
  the radiative hydrodynamic code RADYN to simulate the atmospheric
  response. We then synthesized various chromospheric emission lines
  and compared them with the IRIS and IBIS observations. In general, the
  synthetic intensities agree with the observed ones, especially near the
  northern footpoint of the flare. The simulated Mg II line profile has
  narrower wings than the observed one. This discrepancy can be reduced
  by using a higher microturbulent velocity (27 km s<SUP>-1</SUP>) in a
  narrow chromospheric layer. In addition, we found that an increase of
  electron density in the upper chromosphere within a narrow height range
  of ≈800 km below the transition region can turn the simulated Mg II
  line core into emission and thus reproduce the single peaked profile,
  which is a common feature in all IRIS flares.

---------------------------------------------------------
Title: Solar Polarimetry - from Turbulent Magnetic Fields to Sunspots
Authors: Kleint, Lucia
2016cosp...41E1013K    Altcode:
  Polarimetric measurements are essential to investigate the solar
  magnetic field. Scattering polarization and the Hanle effect allow us to
  probe the turbulent magnetic field and the still open questions of its
  strength and variability. Directed magnetic fields can be detected via
  the Zeeman effect. To derive their orientation and strength, so-called
  inversion codes are used, which iteratively modify a model atmosphere
  and calculate the resulting polarization profiles that are then compared
  to the observations. While photospheric polarimetry is well-established,
  chromospheric polarimetry is still in its infancy, especially because
  it requires a treatment in non-LTE, making it a complex non-linear
  problem. But some of the most important open questions concern the
  strength and geometry of the chromospheric magnetic field. In this
  talk, I will review different polarimetric analysis techniques and
  recent advances in magnetic field measurements going from the small
  scales of turbulent magnetic fields to changes of magnetic fields in
  an active region during flares.

---------------------------------------------------------
Title: Multi-wavelength Solar Flare Observations with Ground- and
    Space-based Observatories
Authors: Kleint, Lucia
2016cosp...41E1012K    Altcode:
  Solar flares affect a wide range of atmospheric heights from the
  corona to the photosphere. Solar instruments are generally designed
  for high-resolution observations in limited spectral windows and
  therefore only capture part of the flare. To obtain a more complete
  flare picture from coronal reconnection to the atmospheric response of
  the chromosphere and photosphere, it is necessary to combine data from
  multiple instruments. I will review multi-wavelength flare observations
  with ground- and space-based observatories. By taking the X1 flare on
  March 29, 2014 as an example, which was observed with an unprecedented
  number of telescopes, I will demonstrate how to investigate the origin
  of the flare by looking at a filament eruption, the chromospheric
  evaporation by means of spectroscopy, the flare heating by analyzing
  continuum emission, and the changes of chromospheric magnetic fields
  using polarimetric data.

---------------------------------------------------------
Title: Ultra-Narrow Negative Flare Front Observed in Helium-10830
    Å Using the1.6m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
   Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
   Kyuhyoun; Cho, Kyung-Suk; Gary, Dale E.; Wang, Haimin
2016SPD....47.0633X    Altcode:
  Solar flares are sudden flashes of brightness on the Sun and are often
  associated with coronal mass ejections and solar energetic particles
  that have adverse effects on the near-Earth environment. By definition,
  flares are usually referred to as bright features resulting from excess
  emission. Using the newly commissioned 1.6-m New Solar Telescope at
  Big Bear Solar Observatory, we show a striking “negative” flare
  with a narrow but unambiguous “dark” moving front observed in He I
  10830 Å, which is as narrow as 340 km and is associated with distinct
  spectral characteristics in Hα and Mg II lines. Theoretically, such
  negative contrast in He I 10830 Å can be produced under special
  circumstances by nonthermal electron collisions or photoionization
  followed by recombination. Our discovery, made possible due to
  unprecedented spatial resolution, confirms the presence of the required
  plasma conditions and provides unique information in understanding
  the energy release and radiative transfer in solar flares.

---------------------------------------------------------
Title: Understanding the formation of the Mg II h&amp;k lines during
    solar flares
Authors: Rubio Da Costa, Fatima; Kleint, Lucia; Petrosian, Vahe';
   Liu, Wei; Allred, Joel C.
2016SPD....4740304R    Altcode:
  The Mg II h&amp;k lines are useful diagnostics for physical processes
  in the solar chromosphere. Understanding the line formation is
  crucial for the correct interpretation of spectral observations
  and characteristics such as line asymmetries or how their central
  reversals in the line cores disappear and turn into emission during
  flares are manifestations of various physical processes.Focusing on
  the well-observed X1.0 flare on 2014 March 29, we carried out a joint
  observational and modeling study to analyze the Mg II h&amp;k spectra
  observed by IRIS. We constructed a multi-threaded flare loop model and
  used the time-dependent electron flux inferred from the RHESSI hard
  X-ray data as the input to the radiative hydrodynamic code RADYN to
  simulate the atmospheric response. Using the RH code we conducted a
  detailed modeling on line shape and evolution to derive how different
  atmospheric parameters may affect the MgII line emission.We successfully
  simulated the single-peaked Mg II h&amp;k line profiles by increasing
  electron density in the upper chromosphere within a narrow height range
  of ≈ 800 km below the transition region. To our knowledge, this is the
  first successful attempt in reproducing such line-profile shapes under
  flaring conditions. We will discuss the implications of this result for
  diagnosing atmospheric dynamics and energy transport in solar flares.

---------------------------------------------------------
Title: Multi-wavelength Study of Transition Region Penumbral
    Bright Dots Using Interface Region Imaging Spectrograph and New
    Solar Telescope
Authors: Deng, Na; Yurchyshyn, Vasyl B.; Tian, Hui; Kleint, Lucia;
   Liu, Chang; Xu, Yan; Wang, Haimin
2016SPD....47.0101D    Altcode:
  Using high-resolution transition region (TR) observations taken by
  the Interface Region Imaging Spectrograph (IRIS) mission, Tian et
  al. (2014b) revealed numerous short-lived sub-arcsecond bright dots
  above sunspots (mostly located in the penumbrae), which indicate yet
  unexplained small-scale energy releases. Moreover, whether these TR
  brightenings have any signature in the lower atmosphere and how they
  are formed are still not fully resolved. This paper presents a study of
  these bright dots using a coordinated observation of a near disk-center
  sunspot with IRIS and the 1.6 m New Solar Telescope (NST) at the Big
  Bear Solar Observatory. NST provides high-resolution chromospheric
  and photospheric observations with narrow-band H-alpha imaging
  spectroscopy and broad-band TiO images, respectively, complementary
  to IRIS TR observations. A total of 2692 TR penumbral bright dots
  are identified from a 37-minute time series of IRIS 1400 A slitjaw
  images. Their locations tend to be associated more with downflowing and
  darker fibrils in the chromosphere, and weakly associated with bright
  penumbral features in the photosphere. However, temporal evolution
  analyses of the dots show that there is no consistent and convincing
  brightening response in the chromosphere. These results are compatible
  with a formation mechanism of the TR penumbral bright dots by falling
  plasma from coronal heights along more vertical and dense magnetic
  loops. The dots may also be produced by small-scale impulsive magnetic
  reconnection taking place sufficiently high in the atmosphere that
  has no energy release in the chromosphere.Acknowledgement: This work
  is mainly supported by NASA grants NNX14AC12G, NNX13AF76G and by NSF
  grant AGS 1408703.

---------------------------------------------------------
Title: Amplitudes of MHD Waves in Sunspots
Authors: Norton, Aimee Ann; Cally, Paul; Baldner, Charles; Kleint,
   Lucia; Tarbell, Theodore D.; De Pontieu, Bart; Scherrer, Philip H.;
   Rajaguru, Paul
2016SPD....47.1009N    Altcode:
  The conversion of p-modes into MHD waves by strong magnetic fields
  occurs mainly in the sub-photospheric layers. The photospheric
  signatures of MHD waves are weak due to low amplitudes at the beta=1
  equipartion level where mode-conversion occurs. We report on small
  amplitude oscillations observed in the photosphere with Hinode SOT/SP
  in which we analyze time series for sunspots ARs 12186 (11.10.2014)
  and 12434 (17.10.2015). No significant magnetic field oscillations
  are recovered in the umbra or penumbra in the ME inversion. However,
  periodicities in the inclination angle are found at the umbral/penumbral
  boundary with 5 minute periods. Upward propagating waves are indicated
  in the intensity signals correlated between HMI and AIA at different
  heights. We compare SP results with the oscillations observed in HMI
  data. Simultaneous IRIS data shows transition region brightening above
  the umbral core.

---------------------------------------------------------
Title: The dynamics and magnetism of the X1 flare on 2014-03-29
Authors: Kleint, Lucia; Heinzel, Petr; Philip, Judge; Krucker, Sam
2016SPD....47.0613K    Altcode:
  The X1 flare on 2014-03-29 was observed with an unprecedented number
  of instruments including chromospheric polarimetry and spectroscopy
  from the UV to the IR. By combining data from these instruments, we can
  answer several open questions: Where is the observed continuum emission
  during flares formed and through which physical processes? How does
  the magnetic field structure in the photosphere and in the chromosphere
  change during a flare? We discuss the implications of our findings on
  standard flare models.

---------------------------------------------------------
Title: Ultra-narrow Negative Flare Front Observed in Helium-10830
    Å Using the 1.6 m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
   Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
   Kyuhyoun; Cho, Kyungsuk; Gary, Dale; Wang, Haimin
2016ApJ...819...89X    Altcode: 2016arXiv160104729X
  Solar flares are sudden flashes of brightness on the Sun and are often
  associated with coronal mass ejections and solar energetic particles
  that have adverse effects on the near-Earth environment. By definition,
  flares are usually referred to as bright features resulting from excess
  emission. Using the newly commissioned 1.6 m New Solar Telescope at
  Big Bear Solar Observatory, we show a striking “negative” flare
  with a narrow but unambiguous “dark” moving front observed in He I
  10830 Å, which is as narrow as 340 km and is associated with distinct
  spectral characteristics in Hα and Mg II lines. Theoretically, such
  negative contrast in He I 10830 Å can be produced under special
  circumstances by nonthermal electron collisions or photoionization
  followed by recombination. Our discovery, made possible due to
  unprecedented spatial resolution, confirms the presence of the required
  plasma conditions and provides unique information in understanding
  the energy release and radiative transfer in astronomical objects.

---------------------------------------------------------
Title: Correlation of Hard X-Ray and White Light Emission in Solar
    Flares
Authors: Kuhar, Matej; Krucker, Säm; Martínez Oliveros, Juan Carlos;
   Battaglia, Marina; Kleint, Lucia; Casadei, Diego; Hudson, Hugh S.
2016ApJ...816....6K    Altcode: 2015arXiv151107757K
  A statistical study of the correlation between hard X-ray and white
  light emission in solar flares is performed in order to search for a
  link between flare-accelerated electrons and white light formation. We
  analyze 43 flares spanning GOES classes M and X using observations
  from the Reuven Ramaty High Energy Solar Spectroscopic Imager and
  Helioseismic and Magnetic Imager. We calculate X-ray fluxes at 30
  keV and white light fluxes at 6173 Å summed over the hard X-ray
  flare ribbons with an integration time of 45 s around the peak hard-X
  ray time. We find a good correlation between hard X-ray fluxes and
  excess white light fluxes, with a highest correlation coefficient
  of 0.68 for photons with energy of 30 keV. Assuming the thick target
  model, a similar correlation is found between the deposited power by
  flare-accelerated electrons and the white light fluxes. The correlation
  coefficient is found to be largest for energy deposition by electrons
  above ∼50 keV. At higher electron energies the correlation decreases
  gradually while a rapid decrease is seen if the energy provided by
  low-energy electrons is added. This suggests that flare-accelerated
  electrons of energy ∼50 keV are the main source for white light
  production.

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Title: Continuum Enhancements in the Ultraviolet, the Visible and
    the Infrared during the X1 Flare on 2014 March 29
Authors: Kleint, Lucia; Heinzel, Petr; Judge, Phil; Krucker, Säm
2016ApJ...816...88K    Altcode: 2015arXiv151104161K
  Enhanced continuum brightness is observed in many flares (“white
  light flares”), yet it is still unclear which processes contribute to
  the emission. To understand the transport of energy needed to account
  for this emission, we must first identify both the emission processes
  and the emission source regions. Possibilities include heating in the
  chromosphere causing optically thin or thick emission from free-bound
  transitions of Hydrogen, and heating of the photosphere causing enhanced
  H<SUP>-</SUP> continuum brightness. To investigate these possibilities,
  we combine observations from Interface Region Imaging Spectrograph
  (IRIS), SDO/Helioseismic and Magnetic Imager, and the ground-based
  Facility Infrared Spectrometer instrument, covering wavelengths in
  the far-UV, near-UV (NUV), visible, and infrared during the X1 flare
  SOL20140329T17:48. Fits of blackbody spectra to infrared and visible
  wavelengths are reasonable, yielding radiation temperatures ∼6000-6300
  K. The NUV emission, formed in the upper photosphere under undisturbed
  conditions, exceeds these simple fits during the flare, requiring
  extra emission from the Balmer continuum in the chromosphere. Thus,
  the continuum originates from enhanced radiation from photosphere
  (visible-IR) and chromosphere (NUV). From the standard thick-target
  flare model, we calculate the energy of the nonthermal electrons
  observed by Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI)
  and compare it to the energy radiated by the continuum emission. We
  find that the energy contained in most electrons &gt;40 keV, or
  alternatively, of ∼10%-20% of electrons &gt;20 keV is sufficient to
  explain the extra continuum emission of ∼(4-8) × 10<SUP>10</SUP>
  erg s<SUP>-1</SUP> cm<SUP>-2</SUP>. Also, from the timing of the RHESSI
  HXR and the IRIS observations, we conclude that the NUV continuum is
  emitted nearly instantaneously when HXR emission is observed with a
  time difference of no more than 15 s.

---------------------------------------------------------
Title: Observed IRIS Profiles of the h and k Doublet of Mg II and
    Comparison with Profiles from Quiescent Prominence NLTE Models
Authors: Vial, Jean-Claude; Pelouze, Gabriel; Heinzel, Petr; Kleint,
   Lucia; Anzer, Ulrich
2016SoPh..291...67V    Altcode: 2015SoPh..tmp..173V
  With the launch of the Interface Region Imaging Spectrograph (IRIS)
  mission, it is now possible to obtain high-resolution solar prominence
  spectra and to begin to distinguish the contributions of the many
  (apparent or not) threads that structure prominences. We aim at
  comparing unique observations obtained in the Mg II h and k lines of a
  polar crown prominence with the radiative outputs from one-dimensional
  models built with non-local-thermodynamic equilibrium codes (Heinzel et
  al.Astron. Astrophys.564, A132, 2014). We characterize the profiles
  obtained through thorough calibration procedures, with attention
  paid to the absolute values, full-width at half-maximum, and the
  ratio of k to h intensities. We also show that at the top of some
  structures, line-of-sight velocities of about 9 kms−<SUP>1</SUP> can
  be detected. We find a range of static, low-pressure, low-thickness,
  low-temperature models that could fit k or h observed values, but that
  cannot satisfy the low observed k/h ratio. We investigate whether these
  low values might be explained by the inclusion of horizontal flows in
  small-scale threads. These flows are also necessary in another class
  of models, where the pressure is kept low but thickness and temperature
  are increased up to the observed thickness and up to 15 000 K.

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Title: Mg II Lines Observed During the X-class Flare on 29 March
    2014 by the Interface Region Imaging Spectrograph
Authors: Liu, W.; Heinzel, P.; Kleint, L.; Kašparová, J.
2015SoPh..290.3525L    Altcode: 2015SoPh..tmp..166L; 2015arXiv151100480L
  Mg II lines represent one of the strongest emissions from the
  chromospheric plasma during solar flares. In this article, we
  studied the Mg II lines observed during the X1 flare on 29 March 2014
  (SOL2014-03-29T17:48) by the Interface Region Imaging Spectrograph
  (IRIS). IRIS detected large intensity enhancements of the Mg II h and
  k lines, subordinate triplet lines, and several other metallic lines
  at the flare footpoints during this flare. We have used the advantage
  of the slit-scanning mode (rastering) of IRIS and performed, for the
  first time, a detailed analysis of spatial and temporal variations
  of the spectra. Moreover, we were also able to identify positions
  of strongest hard X-ray (HXR) emissions using the Reuven Ramaty
  High Energy Solar Spectroscopic Imager (RHESSI) observations and to
  correlate them with the spatial and temporal evolution of IRIS Mg
  II spectra. The light curves of the Mg II lines increase and peak
  contemporarily with the HXR emissions but decay more gradually. There
  are large red asymmetries in the Mg IIh and k lines after the flare
  peak. We see two spatially well-separated groups of Mg II line profiles,
  non-reversed and reversed. In some cases, the Mg II footpoints with
  reversed profiles are correlated with HXR sources. We show the spatial
  and temporal behavior of several other line parameters (line metrics)
  and briefly discuss them. Finally, we have synthesized the Mg IIk line
  using our non-LTE code with the Multilevel Accelerated Lambda Iteration
  (MALI) technique. Two kinds of models are considered, the flare model
  F2 of Machado et al. (Astrophys. J.242, 336, 1980) and the models of
  Ricchiazzi and Canfield (Astrophys. J.272, 739, 1983, RC models). Model
  F2 reproduces the peak intensity of the non-reversed Mg IIk profile
  at flare maximum, but does not account for high wing intensities. On
  the other hand, the RC models show the sensitivity of Mg II line
  intensities to various electron-beam parameters. Our simulations also
  show that the microturbulence produces a broader line core, while the
  intense line wings are caused by an enhanced line source function.

---------------------------------------------------------
Title: A self-consistent combined radiative transfer hydrodynamic and
    particle acceleration model for the X1.0 class flare on March 29, 2014
Authors: Rubio da Costa, F.; Kleint, L.; Sainz Dalda, A.; Petrosian,
   V.; Liu, W.
2015AGUFMSH31B2419R    Altcode:
  The X1.0 flare on March 29, 2014 was well observed, covering its
  emission at several wavelengths from the photosphere to the corona. The
  RHESSI spectra images allow us to estimate the temporal variation of
  the electron spectra using regularized inversion techniques. Using
  this as input for a combined particle acceleration and transport
  (Stanford-Flare) and radiative transfer hydrodynamic (Radyn) code, we
  calculate the response of the atmosphere to the electron heating. We
  will present the evolution of the thermal continuum and several line
  emissions. Comparing them with GOES soft X-ray and high resolution
  observations from IRIS, SDO and DST/IBIS allows us to test the basic
  mechanism(s) of acceleration and to constrain its characteristics. We
  will also present perspectives on how to apply this methodology and
  related diagnostics to other flares.

---------------------------------------------------------
Title: On Helium 1083 nm Line Polarization during the Impulsive
    Phase of an X1 Flare
Authors: Judge, Philip G.; Kleint, Lucia; Sainz Dalda, Alberto
2015ApJ...814..100J    Altcode: 2015arXiv151009218J
  We analyze spectropolarimetric data of the He i 1083 nm multiplet
  (1s2s{}<SUP>3</SUP>{S}<SUB>1</SUB>-1s2p{}<SUP>3</SUP>{P}<SUB>2,1,0</SUB><SUP>o</SUP>)
  during the X1 flare SOL2014-03-29T17:48, obtained with the Facility
  Infrared Spectrometer (FIRS) at the Dunn Solar Telescope. While scanning
  active region NOAA 12017, the FIRS slit crossed a flare ribbon during
  the impulsive phase, when the helium line intensities turned into
  emission at ≲twice the continuum intensity. Their linear polarization
  profiles are of the same sign across the multiplet including 1082.9
  nm, intensity-like, at ≲5% of the continuum intensity. Weaker
  Zeeman-induced linear polarization is also observed. Only the strongest
  linear polarization coincides with hard X-ray (HXR) emission at 30-70
  keV observed by RHESSI. The polarization is generally more extended and
  lasts longer than the HXR emission. The upper J = 0 level of the 1082.9
  nm component is unpolarizable thus, lower-level polarization is the
  culprit. We make non-LTE radiative transfer calculations in thermal
  slabs optimized to fit only intensities. The linear polarizations
  are naturally reproduced, through a systematic change of sign with
  wavelength of the radiation anisotropy when slab optical depths of
  the 1082.9 component are ≲1. Neither are collisions with beams of
  particles needed, nor can they produce the same sign of polarization
  of the 1082.9 and 1083.0 nm components. The He i line polarization
  merely requires heating sufficient to produce slabs of the required
  thickness. Widely different polarizations of Hα, reported previously,
  are explained by different radiative anisotropies arising from slabs
  of different optical depths.

---------------------------------------------------------
Title: How Important Are Electron Beams in Driving Chromospheric
    Evaporation in the 2014 March 29 Flare?
Authors: Battaglia, Marina; Kleint, Lucia; Krucker, Säm; Graham, David
2015ApJ...813..113B    Altcode: 2015arXiv150909186B
  We present high spatial resolution observations of chromospheric
  evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging
  Spectrograph observations of the Fe xxi λ 1354.1 line indicate
  evaporating plasma at a temperature of 10 MK along the flare ribbon
  during the flare peak and several minutes into the decay phase with
  upflow velocities between 30 and 200 km s<SUP>-1</SUP>. Hard X-ray (HXR)
  footpoints were observed by the Ramaty High Energy Solar Spectroscopic
  Imager for two minutes during the peak of the flare. Their locations
  coincided with the locations of the upflows in parts of the southern
  flare ribbon but the HXR footpoint source preceded the observation
  of upflows in Fe xxi by 30-75 s. However, in other parts of the
  southern ribbon and in the northern ribbon, the observed upflows were
  not coincident with an HXR source in time or space, most prominently
  during the decay phase. In this case evaporation is likely caused by
  energy input via a conductive flux that is established between the hot
  (25 MK) coronal source, which is present during the whole observed
  time-interval, and the chromosphere. The presented observations suggest
  that conduction may drive evaporation not only during the decay phase
  but also during the flare peak. Electron beam heating may only play
  a role in driving evaporation during the initial phases of the flare.

---------------------------------------------------------
Title: A Circular-ribbon Solar Flare Following an Asymmetric Filament
    Eruption
Authors: Liu, Chang; Deng, Na; Liu, Rui; Lee, Jeongwoo; Pariat,
   Étienne; Wiegelmann, Thomas; Liu, Yang; Kleint, Lucia; Wang, Haimin
2015ApJ...812L..19L    Altcode: 2015arXiv150908414L
  The dynamic properties of flare ribbons and the often associated
  filament eruptions can provide crucial information on the flaring
  coronal magnetic field. This Letter analyzes the GOES-class X1.0 flare
  on 2014 March 29 (SOL2014-03-29T17:48), in which we found an asymmetric
  eruption of a sigmoidal filament and an ensuing circular flare
  ribbon. Initially both EUV images and a preflare nonlinear force-free
  field model show that the filament is embedded in magnetic fields with
  a fan-spine-like structure. In the first phase, which is defined by a
  weak but still increasing X-ray emission, the western portion of the
  sigmoidal filament arches upward and then remains quasi-static for
  about five minutes. The western fan-like and the outer spine-like
  fields display an ascending motion, and several associated ribbons
  begin to brighten. Also found is a bright EUV flow that streams down
  along the eastern fan-like field. In the second phase that includes the
  main peak of hard X-ray (HXR) emission, the filament erupts, leaving
  behind two major HXR sources formed around its central dip portion
  and a circular ribbon brightened sequentially. The expanding western
  fan-like field interacts intensively with the outer spine-like field,
  as clearly seen in running difference EUV images. We discuss these
  observations in favor of a scenario where the asymmetric eruption of
  the sigmoidal filament is initiated due to an MHD instability and
  further facilitated by reconnection at a quasi-null in corona; the
  latter is in turn enhanced by the filament eruption and subsequently
  produces the circular flare ribbon.

---------------------------------------------------------
Title: Advances in high-resolution observations of solar flares
Authors: Kleint, Lucia
2015IAUGA..2252932K    Altcode:
  I will review recent advances in high-resolution flare observations on
  the Sun. High resolution is important both in space (sub-arcsecond)
  and in time (seconds), in order to tackle open questions of flare
  physics. For example, the small-scale structure of flare ribbons,
  which may still be unresolved, has large implications on the energy
  deposit calculations. I will focus on the X1 flare SOL20140329T17:48,
  which was observed with an unprecedented number of different satellites
  and ground-based instruments. Combining the data from these instruments
  allows us to probe many different atmospheric layers: the photosphere,
  chromosphere, transition region and corona and their evolution during
  the flare.

---------------------------------------------------------
Title: IRIS observations of MgII lines in solar flares
Authors: none Heinzel, Petr; Liu, Wenjuan; Kleint, Lucia; Kasparova,
   Jana
2015IAUGA..2258503N    Altcode:
  We present the results of first analysis of IRIS NUV spectra in an
  X-class flare of29 March 2014.

---------------------------------------------------------
Title: The Fast Filament Eruption Leading to the X-flare on 2014
    March 29
Authors: Kleint, Lucia; Battaglia, Marina; Reardon, Kevin; Sainz Dalda,
   Alberto; Young, Peter R.; Krucker, Säm
2015ApJ...806....9K    Altcode: 2015arXiv150400515K
  We investigate the sequence of events leading to the solar X1 flare
  SOL2014-03-29T17:48. Because of the unprecedented joint observations of
  an X-flare with the ground-based Dunn Solar Telescope and the spacecraft
  IRIS, Hinode, RHESSI, STEREO, and the Solar Dynamics Observatory, we can
  sample many solar layers from the photosphere to the corona. A filament
  eruption was observed above a region of previous flux emergence, which
  possibly led to a change in magnetic field configuration, causing
  the X-flare. This was concluded from the timing and location of the
  hard X-ray emission, which started to increase slightly less than a
  minute after the filament accelerated. The filament showed Doppler
  velocities of ∼2-5 km s<SUP>-1</SUP> at chromospheric temperatures
  for at least one hour before the flare occurred, mostly blueshifts,
  but also redshifts near its footpoints. Fifteen minutes before the
  flare, its chromospheric Doppler shifts increased to ∼6-10 km
  s<SUP>-1</SUP> and plasma heating could be observed before it lifted
  off with at least 600 km s<SUP>-1</SUP> as seen in IRIS data. Compared
  to previous studies, this acceleration (∼3-5 km s<SUP>-2</SUP>) is
  very fast, while the velocities are in the common range for coronal
  mass ejections. An interesting feature was a low-lying twisted second
  filament near the erupting filament, which did not seem to participate
  in the eruption. After the flare ribbons started on each of the second
  filament’s sides, it seems to have untangled and vanished during the
  flare. These observations are some of the highest resolution data of
  an X-class flare to date and reveal some small-scale features yet to
  be explained.

---------------------------------------------------------
Title: Solar Flare Chromospheric Line Emission: Comparison Between
    IBIS High-resolution Observations and Radiative Hydrodynamic
    Simulations
Authors: Rubio da Costa, Fatima; Kleint, Lucia; Petrosian, Vahé;
   Sainz Dalda, Alberto; Liu, Wei
2015ApJ...804...56R    Altcode: 2015ApJ...804...56D; 2014arXiv1412.1815R
  Solar flares involve impulsive energy release, which results in enhanced
  radiation over a broad spectral range and a wide range of heights. In
  particular, line emission from the chromosphere can provide critical
  diagnostics of plasma heating processes. Thus, a direct comparison
  between high-resolution spectroscopic observations and advanced
  numerical modeling results could be extremely valuable, but has not
  yet been attempted. In this paper, we present such a self-consistent
  investigation of an M3.0 flare observed by the Dunn Solar Telescope’s
  Interferometric Bi-dimensional Spectrometer (IBIS) on 2011 September 24
  which we have modeled using the radiative hydrodynamic code RADYN. We
  obtained images and spectra of the flaring region with IBIS in Hα
  6563 Å and Ca ii 8542 Å, and with RHESSI in X-rays. The latter
  observations were used to infer the non-thermal electron population,
  which was passed to RADYN to simulate the atmospheric response to
  electron collisional heating. We then synthesized spectral lines and
  compared their shapes and intensities to those observed by IBIS and
  found a general agreement. In particular, the synthetic Ca ii 8542
  Å profile fits well to the observed profile, while the synthetic Hα
  profile is fainter in the core than for the observation. This indicates
  that Hα emission is more responsive to the non-thermal electron flux
  than the Ca ii 8542 Å emission. We suggest that it is necessary to
  refine the energy input and other processes to resolve this discrepancy.

---------------------------------------------------------
Title: Internetwork Chromospheric Bright Grains Observed With IRIS
    and SST
Authors: Martínez-Sykora, Juan; Rouppe van der Voort, Luc; Carlsson,
   Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, Paul; Hurlburt,
   Neal; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan;
   Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean;
   Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli,
   Sarah; Kankelborg, Charles
2015ApJ...803...44M    Altcode: 2015arXiv150203490M
  The Interface Region Imaging Spectrograph (IRIS) reveals small-scale
  rapid brightenings in the form of bright grains all over coronal holes
  and the quiet Sun. These bright grains are seen with the IRIS 1330,
  1400, and 2796 Å slit-jaw filters. We combine coordinated observations
  with IRIS and from the ground with the Swedish 1 m Solar Telescope
  (SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H
  3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å,
  Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps
  in Fe i 6302 Å at high spatial (0\buildrel{\prime\prime}\over{.}
  33), temporal, and spectral resolution. We conclude that the IRIS
  slit-jaw grains are the counterpart of so-called acoustic grains,
  i.e., resulting from chromospheric acoustic waves in a non-magnetic
  environment. We compare slit-jaw images (SJIs) with spectra from the
  IRIS spectrograph. We conclude that the grain intensity in the 2796
  Å slit-jaw filter comes from both the Mg ii k core and wings. The
  signal in the C ii and Si iv lines is too weak to explain the presence
  of grains in the 1300 and 1400 Å SJIs and we conclude that the grain
  signal in these passbands comes mostly from the continuum. Although
  weak, the characteristic shock signatures of acoustic grains can often
  be detected in IRIS C ii spectra. For some grains, a spectral signature
  can be found in IRIS Si iv. This suggests that upward propagating
  acoustic waves sometimes reach all the way up to the transition region.

---------------------------------------------------------
Title: Electron Acceleration and Radiative Hydrodynamic Simulations
    of the 29 March 2014 X1.0 flare
Authors: Rubio da Costa, Fatima; Kleint, Lucia; Petrosian, Vahe
2015TESS....130205R    Altcode:
  The X1.0 flare on 29 March 2014 presents a unique opportunity to use
  its observations to better understand the origin of the white light
  emission and the evolution of the spectral line profiles. RHESSI
  observed the whole flare including the impulsive phase, allowing us to
  estimate the variation of the spectral parameters of the accelerated
  electrons using the Stanford acceleration code. Using this as input
  to the radiative RADYN code, we determine the hydrodynamic response
  of the solar atmosphere and the spectrum of the continuum and line
  emission. Using this self consistent results and observations we
  constrain the characteristics of the acceleration mechanism.

---------------------------------------------------------
Title: Co-Spatial White Light and Hard X-Ray Flare Footpoints Seen
    Above the Solar Limb
Authors: Krucker, Säm; Saint-Hilaire, Pascal; Hudson, Hugh S.;
   Haberreiter, Margit; Martinez-Oliveros, Juan Carlos; Fivian, Martin
   D.; Hurford, Gordon; Kleint, Lucia; Battaglia, Marina; Kuhar, Matej;
   Arnold, Nicolas G.
2015ApJ...802...19K    Altcode:
  We report analysis of three solar flares that occur within 1° of limb
  passage, with the goal to investigate the source height of chromospheric
  footpoints in white light (WL) and hard X-rays (HXR). We find the
  WL and HXR (≥30 keV) centroids to be largely co-spatial and from
  similar heights for all events, with altitudes around 800 km above
  the photosphere or 300-450 km above the limb height. Because of the
  extreme limb location of the events we study, emissions from such
  low altitudes are influenced by the opacity of the atmosphere and
  projection effects. STEREO images reveal that for SOL2012-11-20T12:36
  the projection effects are smallest, giving upper limits of the absolute
  source height above the nominal photosphere for both wavelengths of
  ∼1000 km. To be compatible with the standard thick target model,
  these rather low altitudes require very low ambient densities within
  the flare footpoints, in particular if the HXR-producing electrons
  are only weakly beamed. That the WL and HXR emissions are co-spatial
  suggests that the observed WL emission mechanism is directly linked
  to the energy deposition by flare accelerated electrons. If the WL
  emission is from low-temperature (≤slant {{10}<SUP>4</SUP>} K) plasma
  as currently thought, the energy deposition by HXR-producing electrons
  above ∼30 keV seems only to heat chromospheric plasma to such low
  temperatures. This implies that the energy in flare-accelerated
  electrons above ∼30 keV is not responsible for chromospheric
  evaporation of hot (\gt {{10}<SUP>6</SUP>} K) plasma, but that their
  energy is lost through radiation in the optical range.

---------------------------------------------------------
Title: Photon Mean Free Paths, Scattering, and Ever-Increasing
    Telescope Resolution
Authors: Judge, P. G.; Kleint, L.; Uitenbroek, H.; Rempel, M.;
   Suematsu, Y.; Tsuneta, S.
2015SoPh..290..979J    Altcode: 2014arXiv1409.7866J; 2015SoPh..tmp....3J
  We revisit an old question: what are the effects of observing stratified
  atmospheres on scales below a photon mean free path λ? The mean free
  path of photons emerging from the solar photosphere and chromosphere
  is ≈ 10<SUP>2</SUP> km. Using current 1 m-class telescopes, λ is
  on the order of the angular resolution. But the Daniel K. Inoue Solar
  Telescope will have a diffraction limit of 0.020″ near the atmospheric
  cutoff at 310 nm, corresponding to 14 km at the solar surface. Even
  a small amount of scattering in the source function leads to physical
  smearing due to this solar "fog", with effects similar to a degradation
  of the telescope point spread function. We discuss a unified picture
  that depends simply on the nature and amount of scattering in the
  source function. Scalings are derived from which the scattering in the
  solar atmosphere can be transcribed into an effective Strehl ratio,
  a quantity useful to observers. Observations in both permitted (e.g.,
  Fe I 630.2 nm) and forbidden (Fe I 525.0 nm) lines will shed light on
  both instrumental performance as well as on small-scale structures in
  the solar atmosphere.

---------------------------------------------------------
Title: Homologous Helical Jets: Observations By IRIS, SDO, and Hinode
    and Magnetic Modeling With Data-Driven Simulations
Authors: Cheung, Mark C. M.; De Pontieu, B.; Tarbell, T. D.; Fu, Y.;
   Tian, H.; Testa, P.; Reeves, K. K.; Martínez-Sykora, J.; Boerner,
   P.; Wülser, J. P.; Lemen, J.; Title, A. M.; Hurlburt, N.; Kleint,
   L.; Kankelborg, C.; Jaeggli, S.; Golub, L.; McKillop, S.; Saar, S.;
   Carlsson, M.; Hansteen, V.
2015ApJ...801...83C    Altcode: 2015arXiv150101593C
  We report on observations of recurrent jets by instruments on board
  the Interface Region Imaging Spectrograph, Solar Dynamics Observatory
  (SDO), and Hinode spacecraft. Over a 4 hr period on 2013 July 21,
  recurrent coronal jets were observed to emanate from NOAA Active Region
  11793. Far-ultraviolet spectra probing plasma at transition region
  temperatures show evidence of oppositely directed flows with components
  reaching Doppler velocities of ±100 km s<SUP>-1</SUP>. Raster Doppler
  maps using a Si iv transition region line show all four jets to have
  helical motion of the same sense. Simultaneous observations of the
  region by SDO and Hinode show that the jets emanate from a source
  region comprising a pore embedded in the interior of a supergranule. The
  parasitic pore has opposite polarity flux compared to the surrounding
  network field. This leads to a spine-fan magnetic topology in the
  coronal field that is amenable to jet formation. Time-dependent
  data-driven simulations are used to investigate the underlying drivers
  for the jets. These numerical experiments show that the emergence of
  current-carrying magnetic field in the vicinity of the pore supplies
  the magnetic twist needed for recurrent helical jet formation.

---------------------------------------------------------
Title: Hmi and Rhessi Measurements of the Radial Location of Solar
    Flare Footpoints to Subarcsecond Accuracy
Authors: Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Haberreiter,
   M.; Kleint, L.; Hurford, G. J.; Fivian, M. D.; Battaglia, M.; Martinez
   Oliveros, J. C.
2014AGUFMSH31C..05K    Altcode:
  We report analysis of three solar flares that occur within one degree
  of limb passage, with the goal to investigate the source height of
  chromospheric footpoints in white light (WL) and hard X-rays (HXR). The
  optical observations are from the Helioseismic and Magnetic Imager
  (HMI) around 617.3 nm, providing high precision observations with an
  absolute positional accuracy in the radial direction below 0.1 arcsec
  (~70 km), as referred to the adjacent limb. The Reuven Ramaty Higher
  Energy Solar Spectroscopic Imager (RHESSI) gives HXR source centroids to
  a similar accuracy depending on counting statistics. The observed height
  of the emissions at either wavelength is influenced by the opacity of
  the atmosphere at that wavelength and the height must correspond to a
  radial distance from Sun center that is greater than the solar limb at
  that wavelength (~350 km for WL and ~450 km for HXR). We find the WL
  and HXR (~30 keV) centroids to be largely co-spatial and from similar
  heights for all events, with altitudes around 800 km above the height of
  the photosphere. The observed altitudes are limited by the uncertainty
  of the precise heliographic locations near the limb and the resulting
  projection effects. STEREO images reveal that for SOL2012-11-20T12:36
  the projection effects are smallest, giving upper limits of the absolute
  source height of 979+-70 km for the WL emission and 926+-51 km for HXR
  source. Hence, the peak of the WL and HXR must be below 1000 km. To
  be compatible with the standard thick target model, these rather low
  altitudes require low ambient densities within the flare footpoints, in
  particular if the HXR-producing electrons are only weakly beamed. That
  the WL and HXR emissions are co-spatial suggests that the observed WL
  emission mechanism is directly linked to the energy deposition by flare
  accelerated electrons with energies above ~30 keV. If the WL emission
  is from low-temperature (~10 000 K) plasma as currently thought, the
  energy deposition by HXR-producing electrons above ~30 keV seems only
  to heat chromospheric plasma to such low temperatures. This implies
  that the energy in flare-accelerated electrons above ~30 keV is lost
  through radiation in the optical range rather than heating chromospheric
  plasma to coronal (&gt; MK) temperatures.

---------------------------------------------------------
Title: Comparison between IBIS Observations and Radiative Transfer
    Hydrodynamic Simulations of a Solar Flare
Authors: Rubio da Costa, F.; Kleint, L.; Liu, W.; Sainz Dalda, A.;
   Petrosian, V.
2014AGUFMSH13B4104R    Altcode:
  High-resolution spectroscopic observations of solar flares are
  rare but can provide valuable diagnostics. On September 24, 2011 an
  M3.0 class flare was observed by the Interferometric BIdimensional
  Spectropolarimeter (IBIS) in chromospheric Hα and CaII 8542 Å
  lines and by the Reuven Ramaty High Energy Solar Spectroscopic Imager
  (RHESSI) in X-rays. We fitted the RHESSI spectra at different times
  with a power-law plus isothermal component. We then used the fitted
  real-time spectral parameters of nonthermal electrons as the input to
  the RADYN radiative hydrodynamic code (Carlsson et al, 1992, 1996;
  Allred et al, 2005) to simulate the low-chromospheric response to
  collisional heating by energetic electrons. We synthesized both the
  Hα and CaII 8542 Å lines from the simulation results and compare
  them with the IBIS observations. We discuss the constraints from this
  comparison on particle acceleration mechanisms in solar flares.

---------------------------------------------------------
Title: High-resolution Observations of the X-flare on 2014-03-29
Authors: Kleint, L.; Battaglia, M.; Krucker, S.; Reardon, K.; Sainz
   Dalda, A.
2014AGUFMSH31C..06K    Altcode:
  We investigate the sequence of events leading to the X1 flare
  SOL2014-03-29T17:48. Because of the unprecedented joint observations of
  an X-flare with the ground-based Dunn Solar Telescope and the spacecraft
  IRIS, Hinode, RHESSI, STEREO, and SDO, we can sample many solar layers
  from the photosphere to the corona. We find that a filament eruption,
  which was possibly caused by a thermal instability, was the cause of
  this X-flare. The filament was rising in the chromosphere for at least
  one hour before the flare occurred with a velocity of ∼2--5 km/s. 15
  minutes before the flare, its chromospheric rise velocity increased to
  ∼6--10 km/s, before it lifted off with at least 600 km/s, as seen by
  IRIS in the transition region. Doppler velocities from H-alpha images
  reveal intriguing small scale flows along the filament and enable us to
  derive its probable shape. An unusual feature was a low-lying twisted
  flux rope near the filament, which did not participate in the filament
  eruption. After the flare ribbons started on each of its sides, the
  flux rope seems to have untangled and vanished during the flare. We
  present a comprehensive overview of the flare, including polarimetric
  and spectroscopic data at subarcsecond resolution.

---------------------------------------------------------
Title: On the Origin of a Sunquake during the 2014 March 29 X1 Flare
Authors: Judge, Philip G.; Kleint, Lucia; Donea, Alina; Sainz Dalda,
   Alberto; Fletcher, Lyndsay
2014ApJ...796...85J    Altcode: 2014arXiv1409.6268J
  Helioseismic data from the Helioseismic Magnetic Imager instrument have
  revealed a sunquake associated with the X1 flare SOL2014-03-29T17:48
  in active region NOAA 12017. We try to discover if acoustic-like
  impulses or actions of the Lorentz force caused the sunquake. We
  analyze spectropolarimetric data obtained with the Facility Infrared
  Spectrometer (FIRS) at the Dunn Solar Telescope (DST). Fortunately,
  the FIRS slit crossed the flare kernel close to the acoustic source
  during the impulsive phase. The infrared FIRS data remain unsaturated
  throughout the flare. Stokes profiles of lines of Si I 1082.7 nm and He
  I 1083.0 nm are analyzed. At the flare footpoint, the Si I 1082.7 nm
  core intensity increases by a factor of several, and the IR continuum
  increases by 4% ± 1%. Remarkably, the Si I core resembles the classical
  Ca II K line's self-reversed profile. With nLTE radiative models of
  H, C, Si, and Fe, these properties set the penetration depth of flare
  heating to 100 ± 100 km (i.e., photospheric layers). Estimates of the
  non-magnetic energy flux are at least a factor of two less than the
  sunquake energy flux. Milne-Eddington inversions of the Si I line show
  that the local magnetic energy changes are also too small to drive the
  acoustic pulse. Our work raises several questions. Have we missed the
  signature of downward energy propagation? Is it intermittent in time
  and/or non-local? Does the 1-2 s photospheric radiative damping time
  discount compressive modes? <P />The National Center for Atmospheric
  Research is sponsored by the National Science Foundation.

---------------------------------------------------------
Title: Hot explosions in the cool atmosphere of the Sun
Authors: Peter, H.; Tian, H.; Curdt, W.; Schmit, D.; Innes, D.;
   De Pontieu, B.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
   Tarbell, T. D.; Wuelser, J. P.; Martínez-Sykora, Juan; Kleint,
   L.; Golub, L.; McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.;
   Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V.
2014Sci...346C.315P    Altcode: 2014arXiv1410.5842P
  The solar atmosphere was traditionally represented with a simple
  one-dimensional model. Over the past few decades, this paradigm shifted
  for the chromosphere and corona that constitute the outer atmosphere,
  which is now considered a dynamic structured envelope. Recent
  observations by the Interface Region Imaging Spectrograph (IRIS) reveal
  that it is difficult to determine what is up and down, even in the cool
  6000-kelvin photosphere just above the solar surface: This region hosts
  pockets of hot plasma transiently heated to almost 100,000 kelvin. The
  energy to heat and accelerate the plasma requires a considerable
  fraction of the energy from flares, the largest solar disruptions. These
  IRIS observations not only confirm that the photosphere is more complex
  than conventionally thought, but also provide insight into the energy
  conversion in the process of magnetic reconnection.

---------------------------------------------------------
Title: Hydrogen Balmer Continuum in Solar Flares Detected by the
    Interface Region Imaging Spectrograph (IRIS)
Authors: Heinzel, P.; Kleint, L.
2014ApJ...794L..23H    Altcode: 2014arXiv1409.5680H
  We present a novel observation of the white light flare (WLF) continuum,
  which was significantly enhanced during the X1 flare on 2014 March
  29 (SOL2014-03-29T17:48). Data from the Interface Region Imaging
  Spectrograph (IRIS) in its near-UV channel show that at the peak of
  the continuum enhancement, the contrast at the quasi-continuum window
  above 2813 Å reached 100%-200% and can be even larger closer to Mg
  II lines. This is fully consistent with the hydrogen recombination
  Balmer-continuum emission, which follows an impulsive thermal and
  non-thermal ionization caused by the precipitation of electron beams
  through the chromosphere. However, a less probable photospheric
  continuum enhancement cannot be excluded. The light curves of the
  Balmer continuum have an impulsive character with a gradual fading,
  similar to those detected recently in the optical region on the
  Solar Optical Telescope on board Hinode. This observation represents
  a first Balmer-continuum detection from space far beyond the Balmer
  limit (3646 Å), eliminating seeing effects known to complicate the
  WLF detection. Moreover, we use a spectral window so far unexplored
  for flare studies, which provides the potential to study the Balmer
  continuum, as well as many metallic lines appearing in emission
  during flares. Combined with future ground-based observations of the
  continuum near the Balmer limit, we will be able to disentangle various
  scenarios of the WLF origin. IRIS observations also provide a critical
  quantitative measure of the energy radiated in the Balmer continuum,
  which constrains various models of the energy transport and deposit
  during flares.

---------------------------------------------------------
Title: The unresolved fine structure resolved: IRIS observations of
    the solar transition region
Authors: Hansteen, V.; De Pontieu, B.; Carlsson, M.; Lemen, J.; Title,
   A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Pereira,
   T. M. D.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
   S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
   Martínez-Sykora, J.
2014Sci...346E.315H    Altcode: 2014arXiv1412.3611H
  The heating of the outer solar atmospheric layers, i.e., the transition
  region and corona, to high temperatures is a long-standing problem
  in solar (and stellar) physics. Solutions have been hampered by an
  incomplete understanding of the magnetically controlled structure of
  these regions. The high spatial and temporal resolution observations
  with the Interface Region Imaging Spectrograph (IRIS) at the solar
  limb reveal a plethora of short, low-lying loops or loop segments
  at transition-region temperatures that vary rapidly, on the time
  scales of minutes. We argue that the existence of these loops solves
  a long-standing observational mystery. At the same time, based on
  comparison with numerical models, this detection sheds light on a
  critical piece of the coronal heating puzzle.

---------------------------------------------------------
Title: Evidence of nonthermal particles in coronal loops heated
    impulsively by nanoflares
Authors: Testa, P.; De Pontieu, B.; Allred, J.; Carlsson, M.; Reale,
   F.; Daw, A.; Hansteen, V.; Martinez-Sykora, J.; Liu, W.; DeLuca, E. E.;
   Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Tian, H.; Lemen, J.;
   Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
   Kleint, L.; Kankelborg, C.; Jaeggli, S.
2014Sci...346B.315T    Altcode: 2014arXiv1410.6130T
  The physical processes causing energy exchange between the Sun’s
  hot corona and its cool lower atmosphere remain poorly understood. The
  chromosphere and transition region (TR) form an interface region between
  the surface and the corona that is highly sensitive to the coronal
  heating mechanism. High-resolution observations with the Interface
  Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to
  60 seconds) of intensity and velocity on small spatial scales (≲500
  kilometers) at the footpoints of hot and dynamic coronal loops. The
  observations are consistent with numerical simulations of heating by
  beams of nonthermal electrons, which are generated in small impulsive
  (≲30 seconds) heating events called “coronal nanoflares.” The
  accelerated electrons deposit a sizable fraction of their energy
  (≲10<SUP>25 </SUP>erg) in the chromosphere and TR. Our analysis
  provides tight constraints on the properties of such electron beams
  and new diagnostics for their presence in the nonflaring corona.

---------------------------------------------------------
Title: Prevalence of small-scale jets from the networks of the solar
    transition region and chromosphere
Authors: Tian, H.; DeLuca, E. E.; Cranmer, S. R.; De Pontieu, B.;
   Peter, H.; Martínez-Sykora, J.; Golub, L.; McKillop, S.; Reeves,
   K. K.; Miralles, M. P.; McCauley, P.; Saar, S.; Testa, P.; Weber,
   M.; Murphy, N.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
   Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli,
   S.; Carlsson, M.; Hansteen, V.; McIntosh, S. W.
2014Sci...346A.315T    Altcode: 2014arXiv1410.6143T
  As the interface between the Sun’s photosphere and corona, the
  chromosphere and transition region play a key role in the formation and
  acceleration of the solar wind. Observations from the Interface Region
  Imaging Spectrograph reveal the prevalence of intermittent small-scale
  jets with speeds of 80 to 250 kilometers per second from the narrow
  bright network lanes of this interface region. These jets have lifetimes
  of 20 to 80 seconds and widths of ≤300 kilometers. They originate from
  small-scale bright regions, often preceded by footpoint brightenings
  and accompanied by transverse waves with amplitudes of ~20 kilometers
  per second. Many jets reach temperatures of at least ~10<SUP>5</SUP>
  kelvin and constitute an important element of the transition region
  structures. They are likely an intermittent but persistent source of
  mass and energy for the solar wind.

---------------------------------------------------------
Title: On the prevalence of small-scale twist in the solar
    chromosphere and transition region
Authors: De Pontieu, B.; Rouppe van der Voort, L.; McIntosh, S. W.;
   Pereira, T. M. D.; Carlsson, M.; Hansteen, V.; Skogsrud, H.; Lemen,
   J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser,
   J. P.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
   S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
   Martinez-Sykora, J.
2014Sci...346D.315D    Altcode: 2014arXiv1410.6862D
  The solar chromosphere and transition region (TR) form an interface
  between the Sun’s surface and its hot outer atmosphere. There,
  most of the nonthermal energy that powers the solar atmosphere
  is transformed into heat, although the detailed mechanism remains
  elusive. High-resolution (0.33-arc second) observations with NASA’s
  Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere
  and TR that are replete with twist or torsional motions on sub-arc
  second scales, occurring in active regions, quiet Sun regions, and
  coronal holes alike. We coordinated observations with the Swedish
  1-meter Solar Telescope (SST) to quantify these twisting motions and
  their association with rapid heating to at least TR temperatures. This
  view of the interface region provides insight into what heats the low
  solar atmosphere.

---------------------------------------------------------
Title: An Interface Region Imaging Spectrograph First View on Solar
    Spicules
Authors: Pereira, T. M. D.; De Pontieu, B.; Carlsson, M.; Hansteen,
   V.; Tarbell, T. D.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt,
   N.; Wülser, J. P.; Martínez-Sykora, J.; Kleint, L.; Golub, L.;
   McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.; Tian, H.; Jaeggli,
   S.; Kankelborg, C.
2014ApJ...792L..15P    Altcode: 2014arXiv1407.6360P
  Solar spicules have eluded modelers and observers for decades. Since
  the discovery of the more energetic type II, spicules have become
  a heated topic but their contribution to the energy balance of the
  low solar atmosphere remains unknown. Here we give a first glimpse of
  what quiet-Sun spicules look like when observed with NASA's recently
  launched Interface Region Imaging Spectrograph (IRIS). Using IRIS
  spectra and filtergrams that sample the chromosphere and transition
  region, we compare the properties and evolution of spicules as
  observed in a coordinated campaign with Hinode and the Atmospheric
  Imaging Assembly. Our IRIS observations allow us to follow the thermal
  evolution of type II spicules and finally confirm that the fading
  of Ca II H spicules appears to be caused by rapid heating to higher
  temperatures. The IRIS spicules do not fade but continue evolving,
  reaching higher and falling back down after 500-800 s. Ca II H type
  II spicules are thus the initial stages of violent and hotter events
  that mostly remain invisible in Ca II H filtergrams. These events
  have very different properties from type I spicules, which show lower
  velocities and no fading from chromospheric passbands. The IRIS spectra
  of spicules show the same signature as their proposed disk counterparts,
  reinforcing earlier work. Spectroheliograms from spectral rasters also
  confirm that quiet-Sun spicules originate in bushes from the magnetic
  network. Our results suggest that type II spicules are indeed the
  site of vigorous heating (to at least transition region temperatures)
  along extensive parts of the upward moving spicular plasma.

---------------------------------------------------------
Title: Observations of Subarcsecond Bright Dots in the Transition
    Region above Sunspots with the Interface Region Imaging Spectrograph
Authors: Tian, H.; Kleint, L.; Peter, H.; Weber, M.; Testa, P.;
   DeLuca, E.; Golub, L.; Schanche, N.
2014ApJ...790L..29T    Altcode: 2014arXiv1407.1060T
  Observations with the Interface Region Imaging Spectrograph (IRIS)
  have revealed numerous sub-arcsecond bright dots in the transition
  region above sunspots. These bright dots are seen in the 1400 Å and
  1330 Å slit-jaw images. They are clearly present in all sunspots we
  investigated, mostly in the penumbrae, but also occasionally in some
  umbrae and light bridges. The bright dots in the penumbrae typically
  appear slightly elongated, with the two dimensions being 300-600 km and
  250-450 km, respectively. The long sides of these dots are often nearly
  parallel to the bright filamentary structures in the penumbrae but
  sometimes clearly deviate from the radial direction. Their lifetimes
  are mostly less than one minute, although some dots last for a few
  minutes or even longer. Their intensities are often a few times stronger
  than the intensities of the surrounding environment in the slit-jaw
  images. About half of the bright dots show apparent movement with
  speeds of ~10-40 km s<SUP>-1</SUP> in the radial direction. Spectra of
  a few bright dots were obtained and the Si IV 1402.77 Å line profiles
  in these dots are significantly broadened. The line intensity can be
  enhanced by one to two orders of magnitude. Some relatively bright
  and long-lasting dots are also observed in several passbands of the
  Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory,
  and they appear to be located at the bases of loop-like structures. Many
  of these bright dots are likely associated with small-scale energy
  release events at the transition region footpoints of magnetic loops.

---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.;
   Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou,
   C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman,
   C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish,
   D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.;
   Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons,
   R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.;
   Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.;
   Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.;
   Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski,
   W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.;
   Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.;
   Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.;
   Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson,
   M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu,
   K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora,
   J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.;
   Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N.
2014SoPh..289.2733D    Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D
  The Interface Region Imaging Spectrograph (IRIS) small explorer
  spacecraft provides simultaneous spectra and images of the photosphere,
  chromosphere, transition region, and corona with 0.33 - 0.4 arcsec
  spatial resolution, two-second temporal resolution, and 1 km
  s<SUP>−1</SUP> velocity resolution over a field-of-view of up to
  175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous
  orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a
  19-cm UV telescope that feeds a slit-based dual-bandpass imaging
  spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å,
  1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines
  formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and
  transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw
  images in four different passbands (C II 1330, Si IV 1400, Mg II k
  2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral
  rasters that sample regions up to 130 arcsec × 175 arcsec at a variety
  of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
  emission from plasma at temperatures between 5000 K and 10 MK and will
  advance our understanding of the flow of mass and energy through an
  interface region, formed by the chromosphere and transition region,
  between the photosphere and corona. This highly structured and dynamic
  region not only acts as the conduit of all mass and energy feeding
  into the corona and solar wind, it also requires an order of magnitude
  more energy to heat than the corona and solar wind combined. The
  IRIS investigation includes a strong numerical modeling component
  based on advanced radiative-MHD codes to facilitate interpretation of
  observations of this complex region. Approximately eight Gbytes of data
  (after compression) are acquired by IRIS each day and made available
  for unrestricted use within a few days of the observation.

---------------------------------------------------------
Title: Detection of Supersonic Downflows and Associated Heating
    Events in the Transition Region above Sunspots
Authors: Kleint, L.; Antolin, P.; Tian, H.; Judge, P.; Testa, P.;
   De Pontieu, B.; Martínez-Sykora, J.; Reeves, K. K.; Wuelser, J. P.;
   McKillop, S.; Saar, S.; Carlsson, M.; Boerner, P.; Hurlburt, N.; Lemen,
   J.; Tarbell, T. D.; Title, A.; Golub, L.; Hansteen, V.; Jaeggli, S.;
   Kankelborg, C.
2014ApJ...789L..42K    Altcode: 2014arXiv1406.6816K
  Interface Region Imaging Spectrograph data allow us to study the solar
  transition region (TR) with an unprecedented spatial resolution of
  0.”33. On 2013 August 30, we observed bursts of high Doppler shifts
  suggesting strong supersonic downflows of up to 200 km s<SUP>-1</SUP>
  and weaker, slightly slower upflows in the spectral lines Mg II h
  and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated
  with brightenings in the slitjaw images (SJIs). The bursty behavior
  lasts throughout the 2 hr observation, with average burst durations
  of about 20 s. The locations of these short-lived events appear to
  be the umbral and penumbral footpoints of EUV loops. Fast apparent
  downflows are observed along these loops in the SJIs and in the
  Atmospheric Imaging Assembly, suggesting that the loops are thermally
  unstable. We interpret the observations as cool material falling
  from coronal heights, and especially coronal rain produced along the
  thermally unstable loops, which leads to an increase of intensity
  at the loop footpoints, probably indicating an increase of density
  and temperature in the TR. The rain speeds are on the higher end of
  previously reported speeds for this phenomenon, and possibly higher
  than the free-fall velocity along the loops. On other observing days,
  similar bright dots are sometimes aligned into ribbons, resembling
  small flare ribbons. These observations provide a first insight into
  small-scale heating events in sunspots in the TR.

---------------------------------------------------------
Title: A particular seismic event generated during the solar flare
    2014 March 29
Authors: Donea, Alina C.; Judge, P.; Kleint, L.; Sainz-Dalda, Alberto
2014shin.confE..49D    Altcode:
  The X1.3 solar flare of 2014 March 29 from AR 2017 was extremely well
  observed, from both space and the ground. Helioseismic observations
  from the Helioseismic Magnetic Imager (HMI) aboard the Solar Dynamics
  Observatory (SDO) indicate that this flare generated a weak seismic
  transient. All previous strong seismic transients to date have
  emanated from sunspot penumbrae, but the source of this transient
  lay outside the active-region penumbra close to a magnetic pore. <P
  />Uniquely, Kleint and Sainz Dalda captured ground based imaging
  and slit spectropolarimetry of this flare using the IBIS and FIRS
  instruments respectively, at the Dunn Solar Telescope in Sunspot,
  New Mexico. Here we report only on FIRS data along with space-based
  data, IBIS data will be reported elsewhere. The FIRS infrared data
  are not saturated even during the flare, which was observed through
  the rise and decay phases. We are still investigating the origins of
  peculiar, Zeeman-induced polarization in the He I 1083 nm multiplet. <P
  />Using spectropolarimetric data of Si I and He I lines from FIRS, we
  investigate the evolution of both photosphere and chromosphere above
  and around the seismic source. Together with data from AIA and from
  RHESSI, these data offer unique new insight into how the flare energy
  is channeled into and through the photosphere into the Sun's interior
  as a seismic transient. We present acoustic properties of the seismic
  event and their relationship to photospheric and chromospheric plasma
  and magnetic fields from FIRS, and to the evolving plasmas seen from
  space from UV to X-ray wavelengths.

---------------------------------------------------------
Title: Relationship between unusual features in umbrae and flares
Authors: Sainz Dalda, Alberto; Kleint, Lucia
2014AAS...22412314S    Altcode:
  The influence of photospheric and chromospheric dynamics and
  morphologies on flare activity are still unclear. We present a study
  of two flaring active regions (ARs) with complementary instruments
  (DST/IBIS, Hinode/SOT-SP, SDO/HMI and SDO/AIA) to investigate the
  temporal evolution of the sunspots and their magnetic and thermodynamic
  properties. In spite of vast differences in flare occurrence and flare
  magnitudes, both ARs show similar features in the lower solar atmosphere
  during flares. We investigate common magnetic topologies and dynamics,
  which may favor flare activity.

---------------------------------------------------------
Title: IRIS observations of the transition region above sunspots:
    oscillations and moving penumbral dots
Authors: Tian, Hui; DeLuca, Ed; Weber, Mark A.; McKillop, Sean;
   Reeves, Kathy; Kleint, Lucia; Martinez-Sykora, Juan; De Pontieu,
   Bart; Carlsson, Mats
2014AAS...22431306T    Altcode:
  NASA's IRIS mission is providing high-cadence and high-resolution
  observations of the solar transition region and chromosphere. We
  present results from IRIS observation of the transition region above
  sunspots. The major findings can be summarized as following: (1) The C
  II and Mg II line profiles are almost Gaussian in the sunspot umbra and
  clearly exhibit a deep reversal at the line center in the plage region,
  suggesting a greatly reduced opacity in the sunspot atmosphere. (2)
  Strongly nonlinear sunspot oscillations can be clearly identified
  in not only the slit jaw images of 2796Å, 1400Å and 1330Å, but
  also in spectra of the bright Mg II, C II and Si IV lines. The Si
  iv oscillation lags those of C ii and Mg ii by 3 and 12 seconds,
  respectively. The temporal evolution of the line core is dominated by
  the following behavior: a rapid excursion to the blue side, accompanied
  by an intensity increase, followed by a linear decrease of the velocity
  to the red side. The maximum intensity slightly lags the maximum blue
  shift in Si iv , whereas the intensity enhancement slightly precedes the
  maximum blue shift in Mg ii . We find a positive correlation between
  the maximum velocity and deceleration. These results are consistent
  with numerical simulations of upward propagating magneto-acoustic
  shock waves. We also demonstrate that the strongly nonlinear line
  width oscillation, reported both previously and here, is spurious. (3)
  Many small-scale bright dots are present in the penumbral filaments and
  light bridges in SJI 1330Å and 1400Å images obtained in high-cadence
  observations. They are usually smaller than 1" and often just a couple
  of pixels wide. Some bright dots show apparent movement with a speed of
  20-60 km/s(either outward or inward). The lifetime of these penumbral
  dots is mostly less than 1 min. The most obvious feature of the Si IV
  profiles in the bright dots is the enhanced line width. Besides that,
  the profile looks normal and no obvious fast flows are detected. The
  bright dots in the light bridges even show oscillation patterns. It's
  not clear whether these oscillations are triggered by the umbral
  oscillations or not.

---------------------------------------------------------
Title: Hybrid Kinetic and Radiative Hydrodynamic Simulations of
    Solar Flares and Comparison With Multiwavelength Observations
Authors: Rubio Da Costa, Fatima; Petrosian, Vahe; Liu, Wei; Carlsson,
   Mats; Kleint, Lucia
2014AAS...22440906R    Altcode:
  We present a unified simulation which combines two physical processes:
  how the particles are accelerated and the energy is transported along
  a coronal loop, and how the atmosphere responds. The “flare”
  code from Stanford University (Petrosian et al, 2001) models the
  stochastic acceleration and transport of particles and radiation of
  solar flares. It includes pitch angle diffusion and energy loss, and
  computes collisional heating to the background plasma and bremsstrahlung
  emission along the loop. The radiative hydrodynamic RADYN Code
  (Carlsson et al, 1992, 1996; Allred et al, 2005) computes the energy
  transport by the injected non-thermal electrons at the top of a 1D
  coronal loop. Recently, we have combined the two codes by updating the
  non-thermal heating in the RADYN code from the "flare" code, allowing us
  to develop a self-consistent simulation. In addition, we can now model
  more realistically the multi-wavelength emission of solar flares and
  compare it with observations, e.g., at optical wavelengths from IBIS at
  the Dunn Solar Telescope and in X-rays from RHESSI. The high resolution
  UV observations from the recently launched IRIS imaging spectrograph
  will be particularly useful in this regard. These will allow us to
  compare numerically modeled and observed emissions of solar flares in
  several lines using more robust simulations than possible before.

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Title: High-resolution Observations of the Shock Wave Behavior for
    Sunspot Oscillations with the Interface Region Imaging Spectrograph
Authors: Tian, H.; DeLuca, E.; Reeves, K. K.; McKillop, S.; De Pontieu,
   B.; Martínez-Sykora, J.; Carlsson, M.; Hansteen, V.; Kleint, L.;
   Cheung, M.; Golub, L.; Saar, S.; Testa, P.; Weber, M.; Lemen, J.;
   Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
   Kankelborg, C.; Jaeggli, S.; McIntosh, S. W.
2014ApJ...786..137T    Altcode: 2014arXiv1404.6291T
  We present the first results of sunspot oscillations from observations
  by the Interface Region Imaging Spectrograph. The strongly nonlinear
  oscillation is identified in both the slit-jaw images and the
  spectra of several emission lines formed in the transition region and
  chromosphere. We first apply a single Gaussian fit to the profiles of
  the Mg II 2796.35 Å, C II 1335.71 Å, and Si IV 1393.76 Å lines in the
  sunspot. The intensity change is ~30%. The Doppler shift oscillation
  reveals a sawtooth pattern with an amplitude of ~10 km s<SUP>-1</SUP>
  in Si IV. The Si IV oscillation lags those of C II and Mg II by ~3 and
  ~12 s, respectively. The line width suddenly increases as the Doppler
  shift changes from redshift to blueshift. However, we demonstrate
  that this increase is caused by the superposition of two emission
  components. We then perform detailed analysis of the line profiles at
  a few selected locations on the slit. The temporal evolution of the
  line core is dominated by the following behavior: a rapid excursion
  to the blue side, accompanied by an intensity increase, followed by a
  linear decrease of the velocity to the red side. The maximum intensity
  slightly lags the maximum blueshift in Si IV, whereas the intensity
  enhancement slightly precedes the maximum blueshift in Mg II. We find
  a positive correlation between the maximum velocity and deceleration,
  a result that is consistent with numerical simulations of upward
  propagating magnetoacoustic shock waves.

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Title: A comparison between observed IRIS profiles of the h &amp;
    k doublet of Mg II and profiles from quiescent prominence NLTE models
Authors: Vial, Jean-Claude; Anzer, Ulrich; Heinzel, Petr; Kleint, Lucia
2014cosp...40E3515V    Altcode:
  With the advent of IRIS, it is now possible to investigate the cool core
  of prominences through the detailed profiles of the Mg II resonance
  lines with an unprecedented spatial resolution of 0.33 arcsecond. The
  slit-jaw camera also allows to record the temporal evolution of the
  prominence fine structure. We present IRIS observations of quiescent
  prominence profiles that we analyse in terms of reversal (if any),
  width, k/h line ratio, prominence/ quiet Sun line ratio. Comparing these
  parameters with the results of NLTE modelling (see Heinzel et al. 2014),
  we can derive thermodynamic parameters of the cool prominence plasma,
  along with the (line-of-sight) velocities and mass flows.

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Title: Coronal rain observed with IRIS
Authors: Antolin, Patrick; Katsukawa, Yukio; De Pontieu, Bart; Kleint,
   Lucia; Pereira, Tiago
2014cosp...40E.105A    Altcode:
  New IRIS observations in upper chromospheric and TR lines show abundance
  of coronal rain in active regions. The wide range of spectral lines in
  which it is observed together with co-observations in cool chromospheric
  lines with SOT and SST show clearly that coronal rain has a broad
  multi-thermal character. This picture agrees well with the thermal
  instability scenario in which the plasma cools down catastrophically
  from coronal temperatures. A statistical analysis of the line widths
  in the rain provides estimates of the non-thermal line broadening and
  temperature. Mainly, we find Gaussian-like distributions of non-thermal
  line broadening between 0 and 17 km/s with a peak at 7 km/s and a small
  upper tail spanning up to 25 km/s. We also report on short-lived heating
  events in umbrae and penumbrae at the end of thermally unstable coronal
  loops. Bursts of high redshifts up to 200 km/s in TR lines are found,
  accompanied by milder blue shifts. The bright dots sometimes display
  coherent structure into a "string of pearls" with striking similarity
  to flare ribbons, suggesting a strong heating correlation between the
  loops. We discuss these results within the coronal rain scenario.

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Title: MgII lines in solar flares: IRIS observations and NLTE modeling
Authors: Heinzel, Petr; Kasparova, Jana; Kleint, Lucia; Dzifcakova,
   Elena
2014cosp...40E1182H    Altcode:
  Chromospheric flares have been recently observed in MgII resonance
  lines by the IRIS instrument. Apart from the resonance lines h and k,
  also subordinate line emissions due to transitions between the MgII
  levels 3P and 3D have been now detected by IRIS during flares. We apply
  the NLTE radiative-transfer code to synthesize all these MgII lines
  under typical flare conditions. In particular, we focus on the role
  of the non-thermal excitations and ionizations, which are due to the
  presence of the electron beams and corresponding return currents. The
  results of this modeling are compared with new IRIS data.

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Title: Unusual Filaments inside the Umbra
Authors: Kleint, L.; Sainz Dalda, A.
2013ApJ...770...74K    Altcode: 2013arXiv1305.7263K
  We analyze several unusual filamentary structures which appeared in
  the umbra of one of the sunspots in AR 11302. They do not resemble
  typical light bridges in morphology or in evolution. We analyze data
  from SDO/HMI to investigate their temporal evolution, Hinode/SP
  for photospheric inversions, IBIS for chromospheric imaging, and
  SDO/AIA for the overlying corona. Photospheric inversions reveal a
  horizontal, inverse Evershed flow along these structures, which we
  call umbral filaments. Chromospheric images show brightenings and
  energy dissipation, while coronal images indicate that bright coronal
  loops seem to end in these umbral filaments. These rapidly evolving
  features do not seem to be common, and are possibly related to the
  high flare-productivity of the active region. Their analysis could
  help to understand the complex evolution of active regions.

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Title: Spectropolarimetry of a Limb Active Region and its Cool
    Coronal Structures
Authors: Judge, Philip G.; Kleint, L.; Casini, R.; Schad, T.
2012AAS...22052119J    Altcode:
  During the SDO mission we have regularly used the IBIS and FIRS
  spectropolarimeters at the Dunn Solar Telescope to measure magnetic
  fields and plasma parameters from photosphere up to the coronal
  base. Here we analyze data of a region at and above the east limb (later
  named NOAA 11302) obtained on September 22nd 2011. The measurements
  show an erupting prominence, remarkably uniform cool plumes and some
  material seemingly draining into the active region along post-flare
  loops. The imaging Fabry-Perot instrument IBIS obtained 30 scans of
  intensity spectra (30s cadence) and 40 scans of Stokes parameters
  (90s cadence) in lines of Fe I 630 nm, Na I 596 nm, Ca II 852 nm and
  H-alpha 656 nm, with an angular resolution near 0.2", over a 40"x80"
  field of view. The FIRS slit was scanned across the solar image to
  obtain Stokes profiles including lines of Si I 1028.7 nm and He I 1083
  nm. We obtained 3 FIRS scans covering a 90"x75" area with cadences of
  between half an hour and an hour simultaneously with IBIS, at a lower
  angular resolution. Simultaneous broad band Ca II K and G-band data
  were obtained with a cadence of 5s. We discuss the vector magnetic
  fields and plasma properties of NOAA 11302, with emphasis on cool <P
  />plasma structures extending many Mm into the corona.

---------------------------------------------------------
Title: Spectropolarimetry of C-class Flare Footpoints
Authors: Kleint, L.
2012ApJ...748..138K    Altcode: 2012arXiv1201.6312K
  We investigate the decay phase of a C-class flare in full-Stokes
  imaging spectropolarimetry with quasi-simultaneous measurements in the
  photosphere (6302.5 Å line) and in the chromosphere (8542 Å line)
  with the IBIS instrument. We analyze data from two fields of view,
  each spanning about 40” × 80” and targeting the two footpoints of
  the flare. A region of interest is identified from V/I images: a patch
  of opposite polarity in the smaller sunspot's penumbra. We find unusual
  flows in this patch at photospheric levels: a Doppler shift of -4 km
  s<SUP>-1</SUP>, but also a possible radial inflow into the sunspot of
  4 km s<SUP>-1</SUP>. Such patches seem to be common during flares, but
  only high-resolution observations allowed us to see the inflow, which
  may be related to future flares observed in this region. Chromospheric
  images show variable overlying emission and flows and unusual Stokes
  profiles. We also investigate the irregular penumbra, whose formation
  may be blocked by the opposite polarity patch and flux emergence. The
  40 minute temporal evolution depicts the larger of the flare ribbons
  becoming fainter and changing its shape. Measurable photospheric
  magnetic fields remain constant and we do not detect flare energy
  transport down from the chromosphere. We find no clear indications
  of impact polarization in the 8542 Å line. We cannot exclude the
  possibility of impact polarization, because weaker signals may be
  buried in the prominent Zeeman signatures or it may have been present
  earlier during the flare.

---------------------------------------------------------
Title: Spectropolarimetry of the photosphere and the chromosphere
    with IBIS
Authors: Kleint, L.; Sainz Dalda, A.
2012decs.confE...4K    Altcode:
  We have obtained quasi-simultaneous spectropolarimetric imaging
  observations of various chromospheric and photospheric features in the
  lines Fe I 6302 A, Ca II 8542 A, H-alpha 6563 A and Na I 5896 A with
  the IBIS instrument at Sac Peak. Our targets include the quiet Sun,
  pores, sunspots, and flaring regions and our goal is to analyze the
  3D magnetic field structure of the solar atmosphere. We carry out
  NTLE inversions with the NICOLE code to investigate interpretation
  techniques for chromospheric spectropolarimetric observations. The very
  faint polarization signatures make chromospheric inversions of the
  quiet Sun challenging. On the other hand, they are quite pronounced
  during flares and show us that the chromospheric magnetic structure
  is seemingly unrelated to the photosphere during these events.

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Title: Solar turbulent magnetic fields: Non-LTE modeling of the
    Hanle effect in the C<SUB>2</SUB> molecule
Authors: Kleint, L.; Shapiro, A. I.; Berdyugina, S. V.; Bianda, M.
2011A&A...536A..47K    Altcode:
  Context. Scattering polarization measurements contain a wealth of
  information that needs a thorough interpretation. This often requires
  accounting for the non-local origin of photons with different
  frequencies and at different limb positions. Currently, modeling
  scattering polarization in several molecular C<SUB>2</SUB> lines
  simultaneously is only successful for lines with similar quantum
  numbers. More sophisticated models are needed to understand the
  dependence on quantum numbers and to reliably derive the strength of
  the turbulent magnetic fields using the differential Hanle effect. <BR
  /> Aims: We have developed a non-LTE analyzing technique for the
  C<SUB>2</SUB> lines to determine the strength of turbulent magnetic
  fields and have applied it to observations obtained during our synoptic
  program at the Istituto Ricerche SOlari Locarno (IRSOL). <BR /> Methods:
  The influence of magnetic fields on scattering polarization can be
  interpreted differentially, i.e., by comparing several spectral lines
  within one spectral region. Through the application of the differential
  Hanle effect and non-LTE 1D radiative transfer, we are able to infer
  a magnetic field strength from the photospheric C<SUB>2</SUB> lines
  around 5141 Å. Compared to previous models we include the effect
  of collisions and investigate their dependence on the total angular
  momentum number J. <BR /> Results: We carry out a detailed parameter
  study to investigate the influence of model parameters on the resulting
  scattering polarization. A good fit can now be obtained for spectral
  lines from different C<SUB>2</SUB> triplets. For the 78 measurements
  obtained during the solar minimum in 2007-2009 we infer a mean magnetic
  field strength of 7.41 G with a standard deviation of 0.76 G.

---------------------------------------------------------
Title: Spectropolarimetry Of The Footpoints Of A C-class Flare In
    The Chromosphere
Authors: Kleint, Lucia; Judge, P.
2011SPD....42.0308K    Altcode: 2011BAAS..43S.0308K
  Flares are well-known solar phenomena but have rarely been
  imaged in high resolution polarimetry and even less often in the
  chromosphere. We observed the declining phase of a C-class flare in
  NOAA 10940 on January 29, 2007 with the IBIS instrument (0.17"/px),
  taking quasi-simultaneous spectropolarimetric images in the chromosphere
  (8542 [[Unable to Display Character: &amp;#506]]) and in the photosphere
  (6302 [[Unable to Display Character: &amp;#506]]). <P />Only the inner
  wings and core of the chromospheric line are seen to brighten in IBIS,
  the underlying photosphere remaining undisturbed. TRACE images reveal
  the connectivity of the chromospheric flaring plasma to the overlying
  corona: IBIS fortuitously captured the chromospheric flares associated
  with both footpoints of a loop systems seen in TRACE. <P />Our hour-long
  image sequence shows the evolution and weakening of the chromospheric
  flare, and reveals unresolved opposite magnetic field components with
  large velocities with respect to the average Sun. In the chromosphere,
  we find redshifted components but in the photosphere we see observe
  blueshifts. We will present high resolution movies of the flaring
  plasma seen in both footpoints of the loop system. We will discuss the
  implications of these measurements for models of the storage and release
  of energy for this class of small flare, and possible connections to
  the formation of the penumbra that appears later at this location.

---------------------------------------------------------
Title: Imaging spectropolarimetry with two LiNbO<SUB>3</SUB> Fabry
    Pérot interferometers and a spectrograph
Authors: Kleint, L.; Feller, A.; Gisler, D.
2011A&A...529A..78K    Altcode:
  Context. Narrow-band spectropolarimetry is used to obtain
  information about the velocity and magnetic field structure of the
  solar atmosphere. Several types of instruments are suited to these
  observations, each with different advantages and drawbacks. <BR /> Aims:
  We set up a novel instrument combination using two LiNbO<SUB>3</SUB>
  Fabry Pérot interferometers (FPI), a high-resolution grating
  spectrograph, and the ZIMPOL system for polarimetry at IRSOL. With this
  system, we can carry out imaging spectropolarimetry of any spectral
  line from 390 to 660 nm, with a spectral resolution of 30 mÅ at
  630 nm. <BR /> Methods: We describe the setup, its properties, and
  calculate the limitations induced by the FPI and the spectrograph. We
  carry out spectropolarimetric observations of the sunspot AR 11087 in
  different spectral lines with suitable Landé factors that could be
  used to derive the magnetic field strength in different height ranges
  of the solar atmosphere. <BR /> Results: The main advantage of our
  instrument compared to similar systems is that no special prefilters
  are required for each spectral line. A slight disadvantage is the
  spatial smearing induced by the dispersion of the finite transmission
  profiles of the FPI, which however is of the same magnitude as the
  seeing-limited resolution of 1-2″ at IRSOL. <BR /> Conclusions:
  We demonstrate that this particular instrument combination is well
  suited to spectropolarimetry at IRSOL.

---------------------------------------------------------
Title: Solar turbulent magnetic fields: surprisingly homogeneous
    distribution during the solar minimum
Authors: Kleint, L.; Berdyugina, S. V.; Shapiro, A. I.; Bianda, M.
2010A&A...524A..37K    Altcode:
  Context. Small-scale, weak magnetic fields are ubiquitous in the
  quiet solar atmosphere. Yet their properties and temporal and spatial
  variations are not well known. <BR /> Aims: We have initiated a
  synoptic program, carried out at the Istituto Ricerche Solari Locarno
  (IRSOL), to investigate both turbulent, mixed-polarity magnetic
  fields and nearly horizontal, directed fields and their variation
  with the solar cycle. <BR /> Methods: Through spectropolarimetric
  observations we monitor linear and circular polarization at the solar
  limb (5” on the disk) at five positional angles (N, NW, S, SW, W)
  with the sensitivity of ~10<SUP>-5</SUP>. In addition, we analyzed
  measurements taken at different limb distances. We measure signatures
  in the 5141 Å region including two C<SUB>2</SUB> triplets and three
  Fe i lines. Linear polarization in these lines arises from scattering
  and can be modified via the Hanle effect in the presence of turbulent
  magnetic fields. Through the application of the differential Hanle
  effect to the C<SUB>2</SUB> R-triplet line ratios and the use of a
  simplified line formation model, we are able to infer a strength of
  turbulent magnetic fields while using the P-triplet to further restrict
  it. A Zeeman analysis of Fe i Stokes V/I is used to evaluate flux
  densities of horizontally directed fields. <BR /> Results: We conclude
  that weak fields were evenly distributed over the Sun during this solar
  minimum. The turbulent field strength was at least 4.7 ± 0.2 G, and it
  did not vary during the last two years. This result was complemented
  with earlier, mainly unpublished measurements in the same region,
  which extend our set to nearly one decade. A statistical analysis of
  these all data suggests that there could be a very small variation of
  the turbulent field strength (3σ-limit) since the solar maximum in
  2000. The Zeeman analysis of Fe i Stokes V/I reveals weak horizontal
  flux densities of 3-8 G. <BR /> Conclusions: Our results demonstrate
  the potential of long-term observations of small-scale magnetic fields,
  which may vary with the solar cycle in both mean strength and spatial
  distribution. This provides important constraints on the energy budget
  of the solar cycle. Extending this synoptic program to many spectral
  lines would provide a sample of heights in the solar atmosphere.

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Title: A synoptic program for large solar telescopes: Cyclic variation
    of turbulent magnetic fields
Authors: Kleint, L.; Berdyugina, S. V.; Gisler, D.; Shapiro, A. I.;
   Bianda, M.
2010AN....331..644K    Altcode:
  Upcoming large solar telescopes will offer the possibility of
  unprecedented high resolution observations. However, during periods
  of non-ideal seeing such measurements are impossible and alternative
  programs should be considered to best use the available observing
  time. We present a synoptic program, currently carried out at
  the Istituto Ricerche Solari Locarno (IRSOL), to monitor turbulent
  magnetic fields employing the differential Hanle effect in atomic and
  molecular lines. This program can be easily adapted for the use at large
  telescopes exploring new science goals, nowadays impossible to achieve
  with smaller telescopes. The current, interesting scientific results
  prove that such programs are worthwhile to be continued and expanded in
  the future. We calculate the approximately achievable spatial resolution
  at a large telescope like ATST for polarimetric measurements with a
  noise level below 5 × 10<SUP>-5</SUP> and a temporal resolution which
  is sufficient to explore variations on the granular scale. We show
  that it would be important to optimize the system for maximal photon
  throughput and to install a high-speed camera system to be able to
  study turbulent magnetic fields with unprecedented accuracy.

---------------------------------------------------------
Title: Turbulent Magnetic Fields in the Quiet Sun: A Search for
    Cyclic Variations
Authors: Kleint, L.; Berdyugina, S. V.; Shapiro, A. I.; Bianda, M.
2010ASPC..428..103K    Altcode: 2010arXiv1003.4103K
  Turbulent magnetic fields fill most of the volume of the solar
  atmosphere. However, their spatial and temporal variations are
  still unknown. Since 2007, during the current solar minimum, we
  are periodically monitoring several wavelength regions in the solar
  spectrum to search for variations of the turbulent magnetic field in
  the quiet Sun. These fields, which are below the resolution limit,
  can be detected via the Hanle effect which influences the scattering
  polarization signatures (Q/I) in the presence of magnetic fields. We
  present a description of our program and first results showing that
  such a synoptic program is complementary to the daily SOHO magnetograms
  for monitoring small-scale magnetic fields.

---------------------------------------------------------
Title: Exploring solar turbulent magnetic fields and advancing
    instrumentation for spectropolarimetry
Authors: Kleint, Lucia
2010PhDT.......562K    Altcode:
  No abstract at ADS

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Title: Spectropolarimetry of Ca II 8542: Probing the Chromospheric
    Magnetic Field
Authors: Kleint, L.; Reardon, K.; Stenflo, J. O.; Uitenbroek, H.;
   Tritschler, A.
2009ASPC..405..247K    Altcode:
  We present spectropolarimetric observations of the chromospheric Ca II
  8542 and photospheric Fe I 6302 lines obtained with the Interferometric
  Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope. The
  high spatial resolution over a large field of view (FOV) allows us to
  connect the observed profiles to the overall topology of the target
  region. After suitable calibrations we can extract Stokes profiles
  for each point in the FOV. The Stokes V profiles observed in the Ca II
  line show a large variety of shapes, indicating widely varying vertical
  behavior of the field strength, velocity, and temperature. We examine
  the center-of-gravity method for determining a representative field
  strength from the observed profiles and use it to directly compare
  photospheric and chromospheric magnetic fields.

---------------------------------------------------------
Title: Synoptic program - Variations of the Turbulent magnetic field
Authors: Kleint, L.; Berdyugina, S.; Bianda, M.
2008ESPM...12.2.71K    Altcode:
  We have initiated a synoptic program at the Istituto Ricerche Solari
  Locarno (IRSOL) to observe variations of the turbulent magnetic field
  with the solar cycle. Our main target are C2 molecular lines at 5141
  A which are excellent for employing the differential Hanle effect to
  determine the strength of the turbulent magnetic field (see Berdyugina
  &amp; Fluri 2004). These lines are monitored about twice per month
  at five position angles around the solar limb (N, NW, W, SW, S) at
  mu=0.1. Several other lines, for example the Cr I triplet at 5206 A,
  have also been chosen for our observations. This is the first systematic
  study of temporal variations of the second solar spectrum, and we have
  already noticed significant differences between single measurements
  taken at the solar maximum and minimum. We present a description of
  the synoptic program and first observations.

---------------------------------------------------------
Title: Combination of two Fabry-Perot etalons and a grating
    spectrograph for imaging polarimetry of the Sun
Authors: Kleint, Lucia; Feller, Alex; Bianda, Michele
2008SPIE.7014E..14K    Altcode: 2008SPIE.7014E..37K
  Imaging spectroscopy of the Sun is a challenging task usually performed
  with Fabry-Perot etalons. The common setup is a combination of two or
  three etalons in series and a narrow-band prefilter. The requirement of
  one, usually expensive prefilter for every desired wavelength limits
  the number of spectral regions that can be observed. We present a
  novel instrument combination consisting of two Fabry-Perot etalons
  and a grating spectrograph, which allows for observations in any
  wavelength between 390 nm and 660 nm without the need for narrow-band
  prefilters. Furthermore, two or more adjacent monochromatic images are
  projected on the detector, each image corresponding to a different
  spectral transmission peak of the Fabry-Perot filtergraph. Together
  with our Zurich Imaging Polarimeter (ZIMPOL) the system is installed
  at the telescope of the Istituto Ricerche Solari Locarno (IRSOL) where
  it will be used for two-dimensional spectropolarimetry. We present a
  description of the instrument and test observations.

---------------------------------------------------------
Title: Two-dimensional Spectropolarimetry At The Dunn Solar Tower
Authors: Uitenbroek, Han; Tritshler, A.; Reardon, K.; Kleint, L.
2007AAS...210.2605U    Altcode: 2007BAAS...39..324U
  Measurement of the solar magnetic field within individual atmospheric
  structures is a desirable, but persistently challenging goal, in
  particular in chromospheric layers. Successful measurements over
  different heights would provide an important contribution to our
  understanding of the solar atmosphere and would provide valuable
  input for theoretical modeling. We provide a short description of the
  capabilities of the Interferometric BIdimensional Spectrometer (IBIS),
  which has recently been upgraded to full Stokes capabilities. IBIS
  is installed at the Dunn Solar Tower (DST) at the Sacramento Peak
  observatory operated by NSO. Using IBIS we achieve high spatial
  resolution over a large field of view in both the photosphere and
  the chromosphere, which allows us to connect the observed profiles
  to the overall topology of the target region. After performing
  suitable calibrations for the telescope and instrument polarization
  properties, we can extract Stokes profiles for each point in the
  field of view. Stokes V profiles observed in the Ca II 854.2 nm line
  show a large variety of forms, indicating widely varying vertical
  behavior of the field strength, velocity, and temperature. We examine
  the center-of-gravity method for determining a representative field
  strength from the observed profiles looking at observations and
  comparing with simulated profiles.