Author name code: kleint ADS astronomy entries on 2022-09-14 author:"Kleint, Lucia" ------------------------------------------------------------------------ Title: IRIS burst properties in active regions Authors: Nelson, C. J.; Kleint, L. Bibcode: 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. Title: Occurrence and statistics of IRIS bursts Authors: Kleint, Lucia; Panos, Brandon Bibcode: 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.

Movie associated to Fig. 11 is available at https://www.aanda.org Title: Chromospheric Heating Mechanisms in a Plage Region Constrained by Comparison of Magnetic Field and Mg II h & 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 Bibcode: 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 & 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. 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 Bibcode: 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.
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.
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.
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.
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. 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 Bibcode: 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 × 104 erg cm-2 s-1 G-1. 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. 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. Bibcode: 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 × 104 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. 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. Bibcode: 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.
Aims: We investigate the origin and morphology of the upflow regions and compare the upflow region and the active region core properties.
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.
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.
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. Title: Exploring Mutual Information between IRIS Spectral Lines. II. Calculating the Most Probable Response in all Spectral Windows Authors: Panos, Brandon; Kleint, Lucia Bibcode: 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. Title: A Journey from Quiet Sun Magnetic Fields to Flares Authors: Kleint, L. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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 & 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. 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 Bibcode: 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. Title: High Resolution Solar Flare Observations Authors: Kleint, Lucia Bibcode: 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. Title: Photospheric and Chromospheric Polarimetry Authors: Kleint, Lucia Bibcode: 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. 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. Bibcode: 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. Title: The Statistical Relationship between White-light Emission and Photospheric Magnetic Field Changes in Flares Authors: Castellanos Durán, J. Sebastián; Kleint, Lucia Bibcode: 2020ApJ...904...96C Altcode: 2020arXiv200702954C Continuum emission, also called white-light emission (WLE), and permanent changes of the magnetic field (ΔBLOS) are often observed during solar flares. However, their relation and precise mechanisms are still unknown. We study statistically the relationship between ΔBLOS 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 ΔBLOS. We then investigate the occurrence, strength, and spatial size of the WLE, its dependence on flare energy, and its correlation to the occurrence of ΔBLOS. We detected WLE in 44/75 flares and ΔBLOS in 59/75 flares. We find that WLE and ΔBLOS 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. 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. Bibcode: 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.
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.
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.
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. Bibcode: 2020A&A...642A..15K Altcode:
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.
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.
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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 2020A&A...640A..71K Altcode: 2020arXiv200610473K
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.
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.
Results: The erupting-filament material presents the following physical conditions: (1) ubiquitous upward motions with peak LOS velocities of ∼73 km s-1; (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 (<2.5 min) remained constant.
Conclusions: The analyzed filament eruption belongs to the fast rising phase, with total velocities of about 124 km s-1. 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.

Movies attached to Figs. 1 and 3 are available at https://www.aanda.org 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. Bibcode: 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 (<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 Bibcode: 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. Bibcode: 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 × 1044 cm-3, which corresponds to a thermal energy of 1.5 × 1026 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. Bibcode: 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 C2 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. Bibcode: 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 Bibcode: 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.
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.
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.
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. Bibcode: 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 (106 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 1013 cm-3. 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. Bibcode: 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 LOS 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 Bibcode: 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 Bibcode: 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&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&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&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 Bibcode: 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 c 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 c 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 Bibcode: 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 Bibcode: 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. Bibcode: 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}{LOS}) 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}{LOS} 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-2 min-1. 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-2 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 1012 cm-3, 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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-1 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 Bibcode: 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 Bibcode: 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{}{LOS}) changes obtained through spectropolarimetry of Ca II 8542 Å with DST/IBIS during the X1-flare SOL20140329T17:48. These changes are stronger (<640 Mx cm-2) and appear in larger areas than their photospheric counterparts (<320 Mx cm-2). The absolute value of {B}{LOS} 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}{LOS}, but the emission also occurs in many locations without any {B}{LOS} changes. Photospheric and chromospheric changes of {B}{LOS} 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 Bibcode: 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. Bibcode: 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&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-1) 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 Bibcode: 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 Bibcode: 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 Bibcode: 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&k lines during solar flares Authors: Rubio Da Costa, Fatima; Kleint, Lucia; Petrosian, Vahe'; Liu, Wei; Allred, Joel C. Bibcode: 2016SPD....4740304R Altcode: The Mg II h&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&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&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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. 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 Bibcode: 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- 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 >40 keV, or alternatively, of ∼10%-20% of electrons >20 keV is sufficient to explain the extra continuum emission of ∼(4-8) × 1010 erg s-1 cm-2. 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 Bibcode: 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−1 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. 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. Bibcode: 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. Bibcode: 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 Bibcode: 2015ApJ...814..100J Altcode: 2015arXiv151009218J We analyze spectropolarimetric data of the He i 1083 nm multiplet (1s2s{}3{S}1-1s2p{}3{P}2,1,0o) 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 Bibcode: 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-1. 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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-1 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-1 and plasma heating could be observed before it lifted off with at least 600 km s-1 as seen in IRIS data. Compared to previous studies, this acceleration (∼3-5 km s-2) 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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}4} 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}6} 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. Bibcode: 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 ≈ 102 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. Bibcode: 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-1. 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. Bibcode: 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 (> 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. Bibcode: 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. Bibcode: 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 Bibcode: 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?

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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 (≲1025 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. Bibcode: 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 ~105 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. Bibcode: 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. Bibcode: 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. Bibcode: 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-1 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. Bibcode: 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−1 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. Bibcode: 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-1 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 Bibcode: 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.

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.

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 Bibcode: 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 Bibcode: 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 Bibcode: 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. 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. Bibcode: 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-1 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. Title: A comparison between observed IRIS profiles of the h & k doublet of Mg II and profiles from quiescent prominence NLTE models Authors: Vial, Jean-Claude; Anzer, Ulrich; Heinzel, Petr; Kleint, Lucia Bibcode: 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. Title: Coronal rain observed with IRIS Authors: Antolin, Patrick; Katsukawa, Yukio; De Pontieu, Bart; Kleint, Lucia; Pereira, Tiago Bibcode: 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. Title: MgII lines in solar flares: IRIS observations and NLTE modeling Authors: Heinzel, Petr; Kasparova, Jana; Kleint, Lucia; Dzifcakova, Elena Bibcode: 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. Title: Unusual Filaments inside the Umbra Authors: Kleint, L.; Sainz Dalda, A. Bibcode: 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. Title: Spectropolarimetry of a Limb Active Region and its Cool Coronal Structures Authors: Judge, Philip G.; Kleint, L.; Casini, R.; Schad, T. Bibcode: 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

plasma structures extending many Mm into the corona. Title: Spectropolarimetry of C-class Flare Footpoints Authors: Kleint, L. Bibcode: 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-1, but also a possible radial inflow into the sunspot of 4 km s-1. 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. Bibcode: 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. Title: Solar turbulent magnetic fields: Non-LTE modeling of the Hanle effect in the C2 molecule Authors: Kleint, L.; Shapiro, A. I.; Berdyugina, S. V.; Bianda, M. Bibcode: 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 C2 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.
Aims: We have developed a non-LTE analyzing technique for the C2 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).
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 C2 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.
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 C2 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. Bibcode: 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: &#506]]) and in the photosphere (6302 [[Unable to Display Character: &#506]]).

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.

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 LiNbO3 Fabry Pérot interferometers and a spectrograph Authors: Kleint, L.; Feller, A.; Gisler, D. Bibcode: 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.
Aims: We set up a novel instrument combination using two LiNbO3 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.
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.
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.
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. Bibcode: 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.
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.
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 ~10-5. In addition, we analyzed measurements taken at different limb distances. We measure signatures in the 5141 Å region including two C2 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 C2 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.
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.
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. 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. Bibcode: 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 × 10-5 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. Bibcode: 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 Bibcode: 2010PhDT.......562K Altcode: No abstract at ADS Title: Spectropolarimetry of Ca II 8542: Probing the Chromospheric Magnetic Field Authors: Kleint, L.; Reardon, K.; Stenflo, J. O.; Uitenbroek, H.; Tritschler, A. Bibcode: 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. Bibcode: 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 & 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 Bibcode: 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. Bibcode: 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.