Author name code: kasparova ADS astronomy entries on 2022-09-14 author:Kasparova, Jana ------------------------------------------------------------------------ Title: Filament Leg--Leg Reconnection as a Source of Prominent Supra-Arcade Downflows Authors: Dudik, Jaroslav; Aulanier, Guillaume; Kasparova, Jana; Karlicky, Marian; Zemanova, Alena; Lorincik, Juraj; Druckmuller, Miloslav Bibcode: 2022arXiv220900306D Altcode: We report on interaction of the legs of the erupting filament of 2012 August 31 and associated prominent supra-arcade downflows (P-SADs) as observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. We employ a number of image processing techniques to enhance weak interacting features. As the filament erupts, both legs stretch outwards. The positive-polarity leg also untwists and splits into two parts. The first part runs into the conjugate (negative-polarity) leg, tearing it apart. The second part then converges into the remnant of the conjugate leg, after which both weaken and finally disappear. All these episodes of interaction of oppositely-oriented filament legs are followed by appearance of P-SADs, seen in the on-disk projection to be shaped as loop-tops, along with many weaker SADs. All SADs are preceded by hot supra-arcade downflowing loops. This observed evolution is consistent with the three-dimensional rr-rf (leg-leg) reconnection, where the erupting flux rope reconnects with itself. In our observations, as well as in some models, the reconnection in this geometry is found to be long-lasting. It plays a substantial role in the evolution of the flux rope of the erupting filament and leads to prominent supra-arcade downflows. Title: A first comparison of FLARIX flare simulations with the MSDP H-alpha spectral observations on very short time scales Authors: Heinzel, Petr; Kasparova, Jana; Berlicki, Arkadiusz; Radziszewski, Krzysztof; Rudawy, Pawel Bibcode: 2022cosp...44.2560H Altcode: We present first results of a comparative analysis of high temporal resolution MSDP H$\alpha$ observations of a compact solar flare with results of radiation-hydrodynamical simulations performed using the FLARIX code. X-ray spectral observations of a compact C1 GOES-class flare obtained from RHESSI satellite were used to estimate physical parameters of the electron beams transporting the energy from the reconnection site down to the chromosphere. These parameters are then used for data-driven FLARIX simulations. A small size of the flare and its simple structure allowed us to assume a single-loop geometry which significantly simplified the modelling and analysis of the whole event. Using the MSDP imaging spectrograph at Bia{\l}k\'{o}w observatory, it was possible to compare the recorded H$\alpha$ line emission with that obtained from FLARIX simulations, on short time-scales down to 50 ms. Title: Narrowband Spikes Observed During the 13 June 2012 Flare in the 800 - 2000 MHz Range Authors: Karlický, Marian; Rybák, Ján; Benáček, Jan; Kašparová, Jana Bibcode: 2022SoPh..297...54K Altcode: 2022arXiv220409327K Narrowband (∼5 MHz) and short-lived (∼0.01 s) spikes with three different distributions in the 800 - 2000 MHz radio spectrum of the 13 June 2012 flare are detected and analyzed. We designate them as SB (spikes distributed in a broad band or bands), SZ (spikes distributed in zebra-like bands) and SBN (spikes distributed in broad and narrow bands). On analyzing AIA/SDO images of the active region NOAA 11504, a rough correspondence between groups of the spikes observed at 1000 MHz and peaks in the time profiles of AIA channels taken from the flare subarea close to the leading sunspot is found. Among the types of spikes the SZ type is the most interesting because it resembles zebras. Therefore, using autocorrelation and crosscorrelation methods we compare SZ and SBN spikes with the typical zebra observed in the same frequency range. While the ratio of SZ band frequencies with their frequency separation (220 MHz) is about 4, 5, and 6, in the zebra the frequency stripe separation is about 24 MHz and the ratio is around 50. Moreover, the bandwidth of SZ bands, which consists of clouds of narrowband spikes, is much broader than that of zebra stripes. This comparison indicates that SZ spikes are generated in a different way from the zebra, but in a similar way to SBN spikes. We successfully fit the SZ band frequencies by the Bernstein modes. Based on this fitting we interpret SZ and SBN spikes as those generated in the model of Bernstein modes. Thus, the magnetic field and plasma density in the SZ spike source are estimated to be about 79 G and 8.4 × 109 cm−3, respectively. 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: On the importance of Ca II photoionization by the hydrogen lyman transitions in solar flare models Authors: Osborne, C. M. J.; Heinzel, P.; Kašparová, J.; Fletcher, L. Bibcode: 2021MNRAS.507.1972O Altcode: 2021MNRAS.tmp.1947O; 2021arXiv210711145O The forward fitting of solar flare observations with radiation-hydrodynamic simulations is a common technique for learning about energy deposition and atmospheric evolution during these explosive events. A frequent spectral line choice for this process is Ca II 854.2 nm due to its formation in the chromosphere and substantial variability. It is important to ensure that this line is accurately modelled to obtain the correct interpretation of observations. Here, we investigate the importance of photoionization of Ca II to Ca III by the hydrogen Lyman transitions, whilst the Lyman continuum is typically considered in this context in simulations, the associated bound-bound transitions are not. This investigation uses two RADYN flare simulations and reprocesses the radiative transfer using the Lightweaver framework which accounts for the overlapping of all active transitions. The Ca II 854.2 nm line profiles are found to vary significantly due to photoionization by the Lyman lines, showing notably different shapes and even reversed asymmetries. Finally, we investigate to what extent these effects modify the energy balance of the simulation and the implications on future radiation-hydrodynamic simulations. There is found to be a 10-15 per cent change in detailed optically thick radiative losses from considering these photoionization effects on the calcium lines in the two simulations presented, demonstrating the importance of considering these effects in a self-consistent way. Title: Flare Expansion to a Magnetic Rope Accompanied by Rare Radio Bursts Authors: Zemanová, Alena; Karlický, Marian; Kašparová, Jana; Dudík, Jaroslav Bibcode: 2020ApJ...905..111Z Altcode: 2021arXiv210108633Z We present multispectral analysis (radio, Hα, ultraviolet (UV)/extreme ultraviolet (EUV), and hard X-ray) of a confined flare from 2015 March 12. This flare started within the active region NOAA 12 297 and then it expanded into a large preexisting magnetic rope embedded with a cold filament. The expansion started with several brightenings located along the rope. This process was accompanied by a group of slowly positively drifting bursts in the 0.8-2 GHz range. The frequency drift of these bursts was 45-100 MHz s-1. One of the bursts had an S-like form. During the brightening of the rope we observed a unique bright EUV structure transverse to the rope axis. The structure was observed in a broad range of temperatures and it moved along the rope with the velocity of about 240 km s-1. When the structure dissipated, we saw a plasma further following twisted threads in the rope. The observed slowly positively drifting bursts were interpreted considering particle beams and we show that one with the S-like form could be explained by the beam propagating through the helical structure of the magnetic rope. The bright structure transverse to the rope axis was interpreted considering line-of-sight effects and the dissipation-spreading process, which we found to be more likely. 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: Exploiting Solar Visible-Range Observations by Inversion Techniques: From Flows in the Solar Subsurface to a Flaring Atmosphere Authors: Švanda, Michal; Jurčák, Jan; Korda, David; Kašparová, Jana Bibcode: 2020rfma.book..349S Altcode: Observations of the Sun in the visible spectral range belong to standard measurements obtained by instruments both on the ground and in the space. Nowadays, both nearly continuous full-disc observations with medium resolution and dedicated campaigns of high spatial, spectral and/or temporal resolution constitute a holy grail for studies that can capture (both) the long- and short-term changes in the dynamics and energetics of the solar atmosphere. Observations of photospheric spectral lines allow us to estimate not only the intensity at small regions, but also various derived data products, such as the Doppler velocity and/or the components of the magnetic field vector. We show that these measurements contain not only direct information about the dynamics of solar plasmas at the surface of the Sun but also imprints of regions below and above it. Here, we discuss two examples: First, the local time-distance helioseismology as a tool for plasma dynamic diagnostics in the near subsurface and second, the determination of the solar atmosphere structure during flares. The methodology in both cases involves the technique of inverse modelling. Title: Drifting Pulsation Structure at the Very Beginning of the 2017 September 10 Limb Flare Authors: Karlický, Marian; Chen, Bin; Gary, Dale E.; Kašparová, Jana; Rybák, Jan Bibcode: 2020ApJ...889...72K Altcode: 2019arXiv191212518K Drifting pulsation structures (DPSs) are important radio fine structures usually observed at the beginning of eruptive solar flares. It has been suggested that DPSs carry important information on the energy release processes in solar flares. We study DPS observed in an X8.2-class flare on 2017 September 10 in the context of spatial and spectral diagnostics provided by microwave, EUV, and X-ray observations. We describe DPS and its substructures that were observed for the first time. We use a new wavelet technique to reveal characteristic periods in DPS and their frequency bands. Comparing the periods of pulsations found in this DPS with those in previous DPSs, we found new very short periods in the 0.09-0.15 s range. We present Expanded Owens Valley Solar Array images and spectra of microwave sources observed during the DPS. This DPS at its very beginning has pulsations in two frequency bands (1000-1300 MHz and 1600-1800 MHz), which are interconnected by fast drifting bursts. We show that these double-band pulsations started just at the moment when the ejected filament splits apart in a tearing motion at the location where a signature of the flare current sheet later appeared. Using the standard flare model and previous observations of DPSs, we interpret these double-band pulsations as a radio signature of superthermal electrons trapped in the rising magnetic rope and flare arcade at the moment when the flare magnetic reconnection starts. The results are discussed in a scenario with the plasmoid in the rising magnetic rope. Title: Exploiting solar visible-range observations by inversion techniques: from flows in the solar subsurface to a flaring atmosphere Authors: Švanda, Michal; Jurčák, Jan; Korda, David; Kašparová, Jana Bibcode: 2020arXiv200103874S Altcode: Observations of the Sun in the visible spectral range belong to standard measurements obtained by instruments both on the ground and in the space. Nowadays, both nearly continuous full-disc observations with medium resolution and dedicated campaigns of high spatial, spectral and/or temporal resolution constitute a holy grail for studies that can capture (both) the long- and short-term changes in the dynamics and energetics of the solar atmosphere. Observations of photospheric spectral lines allow us to estimate not only the intensity at small regions, but also various derived data products, such as the Doppler velocity and/or the components of the magnetic field vector. We show that these measurements contain not only direct information about the dynamics of solar plasmas at the surface of the Sun but also imprints of regions below and above it. Here, we discuss two examples: First, the local time-distance helioseismology as a tool for plasma dynamic diagnostics in the near subsurface and second, the determination of the solar atmosphere structure during flares. The methodology in both cases involves the technique of inverse modelling. Title: Radio, EUV, and X-Ray Observations during a Filament Rise in the 2011 June 7 Solar Flare Authors: Karlický, Marian; Kašparová, Jana; Sych, Robert Bibcode: 2020ApJ...888...18K Altcode: 2020arXiv200400122K The most energetic flares start with a filament rise followed by magnetic reconnection below this filament. The start of the reconnection corresponds to the beginning of the flare impulsive phase. In this paper we study processes before this phase. During the filament rise we recognize an unusual radio continuum with a starting boundary drifting toward lower frequencies. The estimated velocity of the agent generating this continuum boundary is about 400 km s-1, similar to that of the rising filament. In association with this filament rise, transient X-ray sources and extreme ultraviolet (EUV) brightenings are found near the filament footpoint and outside the locations where later two parallel flare ribbons appear. Moreover, oscillations with a ∼30 s period are found simultaneously in radio, EUV, and X-ray observations. Around the end of these oscillations the flare impulsive phase starts as seen in observations of the drifting pulsation structure and X-ray source located at the upper part of the rising filament. We interpret the unusual radio continuum and transient X-ray sources, which are located outside the two parallel flare ribbons, as those generated during an interaction of the rising filament with the above-lying magnetic loops. The EUV brightening at the filament footpoint could be a signature of the magnetic reconnection inside the magnetic rope carrying the filament. Possible scenarios of the ∼30 s period oscillations in radio, X-ray, and EUV are discussed. Title: Modelling of Flare Processes: A Comparison of the Two RHD Codes FLARIX and RADYN Authors: Kašparová, Jana; Carlsson, Mats; Heinzel, Petr; Varady, Michal Bibcode: 2019ASPC..519..141K Altcode: We present a comparison of two autonomous, methodologically different radiation hydrodynamic codes, FLARIX and RADYN, and their use to model the solar flare processes. Both codes can model the time evolution of a 1D atmosphere heated by a specified process, e.g. by electron beams propagating from the injection site in the corona down to the lower atmosphere. In such a scenario time scales can be rather short and lead to fast heating on even sub-second time scales. Our aim is to compare the FLARIX and RADYN codes using exactly the same setup and model conditions. Although such a comparison has never been done successfully before for this type of codes, we will present a close agreement between the time evolution of the modelled atmospheric structure for a test case of electron beam heating. 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: Fast velocities of flare ribbon kernels and ribbon elongation in a quescent filament eruption of 2012 August 31 observed by SDO/AIA Authors: Lörinčík, Juraj; Dudík, Jaroslav; Kašparová, Jana; Aulanier, Guillaume; Zemanová, Alena; Dzifčáková, Elena Bibcode: 2018csc..confE..63L Altcode: We report on SDO observations of an eruption of a quiescent filament from 2012 August 31. In the 1600 Å filter channel of AIA, flare ribbons were observed to elongate at velocities up to 480 km s^{-1} and flare kernels move along a ribbon at velocity of ≈ 260 km s^{-1}. In order to investigate the emission observed in the 1600 Å channel, we used synthetic spectra modeled using CHIANTI and RADYN models of flare atmospheres with beam parameters constrained using fits of RHESSI spectra. We found out that depending on parameters of heating of a flare model, thickness of a region where the emission of the 1600 Å filter channel originates ranges between 10^{-2} and 10^{2} km. Information on dimensions of the formation region were then utilized to estimate densities in flare ribbons using inversions of the emission measure. These were found to range between 10^{10} - 4.10^{12} cm^{-3} for flare atmospheres heated by beams of different parameters. Together with B_{LOS} data from SDO/HMI, diagnosed densities were used to calculate Alfvén velocities in observed ribbons. These can be as small as 17 km s^{-1} for flare ribbons observed in region of weak magnetic field at latter stages of heating. This finding suggests that elongation of ribbons and motion of kernels might not be related to waves. Motions along the PIL are well-described in the 3D model of solar eruptions of Aulanier et al. 2013 (A&A, 543, 110). However, EUV observations of flare loops revealed that velocity of their apparent slipping motion is much lower than velocity of elongation of a ribbon, which is observed in a close vicinity. Therefore, observed phenomena can not be directly related to super-Alvénic regime of magnetic slipping reconnection introduced in the 3D model. Title: Broad Non-Gaussian Fe XXIV Line Profiles in the Impulsive Phase of the 2017 September 10 X8.3-class Flare Observed by Hinode/EIS Authors: Polito, Vanessa; Dudík, Jaroslav; Kašparová, Jana; Dzifčáková, Elena; Reeves, Katharine K.; Testa, Paola; Chen, Bin Bibcode: 2018ApJ...864...63P Altcode: 2018arXiv180709361P We analyze the spectra of high-temperature Fe XXIV lines observed by the Hinode/Extreme-Ultraviolet Imaging Spectrometer (EIS) during the impulsive phase of the X8.3-class flare on 2017 September 10. The line profiles are broad, show pronounced wings, and clearly depart from a single-Gaussian shape. The lines can be well fitted with κ distributions, with values of κ varying between ≈1.7 and 3. The regions where we observe the non-Gaussian profiles coincide with the location of high-energy (≈100-300 keV) hard X-ray (HXR) sources observed by RHESSI, suggesting the presence of particle acceleration or turbulence, also confirmed by the observations of nonthermal microwave sources with the Expanded Owens Valley Solar Array at and above the HXR loop-top source. We also investigate the effect of taking into account κ distributions in the temperature diagnostics based on the ratio of the Fe XXIII λ263.76 and Fe XXIV λ255.1 EIS lines. We found that these lines can be formed at much higher temperatures than expected (up to log(T[K]) ≈ 7.8) if departures from Maxwellian distributions are taken into account. Although larger line widths are expected because of these higher formation temperatures, the observed line widths still imply nonthermal broadening in excess of 200 km s-1. The nonthermal broadening related to HXR emission is better interpreted by turbulence than by chromospheric evaporation. Title: Broad Non-Gaussian fe XXIV Line Profiles in the Impulsive Phase of the 2017 September 10 X8.3-CLASS Flare Observed by Hinode/eis Authors: Polito, Vanessa; Dudik, Jaroslav; Kasparova, Jana; Dzifcakova, Elena; Reeves, Katharine K.; Testa, Paola; Chen, Bin Bibcode: 2018shin.confE.212P Altcode: We analyze the spectra of high temperature Fe XXIV lines observed by Hinode/EIS during the impulsive phase the X8.3-class flare on September 10, 2017. The line profiles are broad, show pronounced wings, and clearly depart from a single Gaussian shape. The lines can be well fitted with the ? distributions, with values of ? varying between ?1.7 to 3. The region where we observe the non-Gaussian profiles coincides with the location of high-energy (?100-300 keV) HXR sources observed by RHESSI, suggesting the presence of particle acceleration or turbulence, also confirmed by the observations of a non-thermal microwave sources with EOVSA at and above the HXR looptop source. We also investigate the effect of taking into account ? distributions in the temperature diagnostics based on the ratio of the Fe XXIII 263.76 ?A and Fe XXIV 255.1 ?A EIS lines. We found that these lines can be formed at much higher temperatures than expected (up to log(T [K]) ? 7.8), if departures from Maxwellian distributions are taken into account. Although larger line widths are expected because of these higher formation temperatures, the observed line widths still imply non-thermal broadening in excess of 200kms?1. 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: 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: Hybrid simulations of chromospheric HXR flare sources Authors: Moravec, Z.; Varady, M.; Kašparová, J.; Kramoliš, D. Bibcode: 2016AN....337.1020M Altcode: 2016arXiv160107026M Recent measurements of vertical extents and positions of the chromospheric hard X-ray (HXR) flare sources based on Ramaty High-Energy Spectroscopic Imager (RHESSI) observations show a significant inconsistency with the theoretical predictions based on the standard collisional thick target model (CTTM). Using the hybrid flare code Flarix, we model simultaneously and self-consistently the propagation, scattering and energy losses of electron beams with power-law energy spectra and various initial pitch-angle distributions in a purely collisional approximation and concurrently the dynamic response of the heated chromosphere on timescales typical for RHESSI image reconstruction. The results of the simulations are used to model the time evolution of the vertical distribution of chromospheric HXR source within a singular (compact) loop. Adopting the typical RHESSI imaging times scales, energy dependent vertical sizes and positions as could be observed by RHESSI are presented. Title: Numerical RHD simulations of flaring chromosphere with Flarix Authors: Heinzel, Petr; Kašparová, Jana; Varady, Michal; Karlický, Marian; Moravec, Zdeněk Bibcode: 2016IAUS..320..233H Altcode: 2016arXiv160200016H Flarix is a radiation-hydrodynamical (RHD) code for modeling of the response of the chromosphere to a beam bombardment during solar flares. It solves the set of hydrodynamic conservation equations coupled with NLTE equations of radiative transfer. The simulations are driven by high energy electron beams. We present results of the Flarix simulations of a flaring loop relevant to the problem of continuum radiation during flares. In particular we focus on properties of the hydrogen Balmer continuum which was recently detected by IRIS. 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: Numerical simulations of flaring loops with Flarix Authors: none Heinzel, Petr; Karlicky, Marian; Varady, Michal; Kasparova, Jana; Moravec, Zdenek Bibcode: 2015IAUGA..2258466N Altcode: Flarix is the radiation-hydrodynamical code for simulation of the flare evolution. It solves the set of hydrodynamicval equations coupled to NLTE equations of radiative transfer. The simulation is driven by the accelerated electron beams. We present new results of Flarix simulations for various types of flare loops, incorporating new features like effects of the return current and particle re-acceleration in the chromosphere. 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: Modifications of thick-target model: re-acceleration of electron beams by static and stochastic electric fields Authors: Varady, M.; Karlický, M.; Moravec, Z.; Kašparová, J. Bibcode: 2014A&A...563A..51V Altcode: 2014arXiv1401.3329V Context. The collisional thick-target model (CTTM) of the impulsive phase of solar flares, together with the famous Carmichael, Sturrock, Hirayama, and Kopp-Pneuman (CSHKP) model, presented for many years a "standard" model, which straightforwardly explained many observational aspects of flares. On the other hand, many critical issues appear when the concept is scrutinised theoretically or with the new generation of hard X-ray (HXR) observations. The famous "electron number problem" or problems related to transport of enormous particle fluxes though the corona represent only two of them. To resolve the discrepancies, several modifications of the CTTM appeared.
Aims: We study two of them based on the global and local re-acceleration of non-thermal electrons by static and stochastic electric fields during their transport from the coronal acceleration site to the thick-target region in the chromosphere. We concentrate on a comparison of the non-thermal electron distribution functions, chromospheric energy deposits, and HXR spectra obtained for both considered modifications with the CTTM itself.
Methods: The results were obtained using a relativistic test-particle approach. We simulated the transport of non-thermal electrons with a power-law spectrum including the influence of scattering, energy losses, magnetic mirroring, and also the effects of the electric fields corresponding to both modifications of the CTTM.
Results: We show that both modifications of the CTTM change the outcome of the chromospheric bombardment in several aspects. The modifications lead to an increase in chromospheric energy deposit, change of its spatial distribution, and a substantial increase in the corresponding HXR spectrum intensity.
Conclusions: The re-acceleration in both models reduces the demands on the efficiency of the primary coronal accelerator, on the electron fluxes transported from the corona downwards, and on the total number of accelerated coronal electrons during flares. 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: A new approach to model particle acceleration and energy transfer in solar flares Authors: Rubio Da Costa, Fatima; Zuccarello, F.; Fletcher, L.; Labrosse, N.; Kasparova, J.; Prosecký, T.; Carlsson, M.; Petrosian, V.; Liu, W. Bibcode: 2013SPD....4440401R Altcode: Motivated by available observations of two different flares in Lyα and Hα, we model the conditions of the solar atmosphere using a radiation hydrodynamics code (RADYN, Carlsson & Stein, 1992) and analyze the energy transport carried by a beam of non-thermal electrons injected at the top of a 1D coronal loop. The numerical Lyα and Hα intensities match with the observations. The electron energy distribution is assumed to follow a power law of the form (E/Ec )-δ for energies greater than a cutoff value of Ec. Abbett & Hawley (1999) and Allred et al. (2005) assumed that the non-thermal electrons flux injected at the top of a flaring loop, the cut-off energy and the power law index are constant over time. An improvement was achieved by Allred & Hawley (2006), who modified the RADYN code in such a way that the input parameters were time dependent. Their inputs were based on observations of a flare obtained with RHESSI. By combining RADYN with the “flare” code from Stanford University which models the acceleration and transport of particles and radiation of solar flares in non-LTE regime, we can calculate the non-thermal electrons flux, the cut-off energy and the power law index at every simulated time step. The atmospheric parameters calculated by RADYN could in turn be used as updated inputs for "flare", providing several advantages over the results from Liu et al. (2009), who combined the particle acceleration code with a 1-D hydrodynamic code, improving the atmospheric conditions. Title: Observational consequences of the local re-acceleration thick-target model Authors: Varady, M.; Moravec, Z.; Karlický, M.; Kašparová, J. Bibcode: 2013JPhCS.440a2013V Altcode: In our contribution we compare the efficiency of the hard X-ray production and the vertical sizes and positions of the hard X-ray sources for the classical collisional thick-target model and for its recently proposed modification, the local re-acceleration thick-target model. The latter model has been proposed in order to ease some of the severe theoretical problems of the collisional thick-target model related to interpretation of the observational properties of the foot-point HXR sources in solar flares. The results are obtained using a relativistic test-particle approach for a fully ionised atmosphere with a converging magnetic field and a single (compact) flare loop. Title: Solar flares at submillimeter wavelengths Authors: Krucker, Säm; Giménez de Castro, C. G.; Hudson, H. S.; Trottet, G.; Bastian, T. S.; Hales, A. S.; Kašparová, J.; Klein, K. -L.; Kretzschmar, M.; Lüthi, T.; Mackinnon, A.; Pohjolainen, S.; White, S. M. Bibcode: 2013A&ARv..21...58K Altcode: We discuss the implications of the first systematic observations of solar flares at submillimeter wavelengths, defined here as observing wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The events observed thus far show that this wave band requires a new understanding of high-energy processes in solar flares. Several events, including observations from two different observatories, show during the impulsive phase of the flare a spectral component with a positive (increasing) slope at the highest observable frequencies (up to 405 GHz). To emphasize the increasing spectra and the possibility that these events could be even more prominent in the THz range, we term this spectral feature a "THz component". Here we review the data and methods, and critically assess the observational evidence for such distinct component(s). This evidence is convincing. We also review the several proposed explanations for these feature(s), which have been reported in three distinct flare phases. These data contain important clues to flare development and particle acceleration as a whole, but many of the theoretical issues remain open. We generally have lacked systematic observations in the millimeter-wave to far-infrared range that are needed to complete our picture of these events, and encourage observations with new facilities. Title: Simulations of HXR Foot-point Source Sizes for Modified Thick-target Models Authors: Moravec, Z.; Varady, M.; Karlický, M.; Kašparová, J. Bibcode: 2013CEAB...37..535M Altcode: We study vertical sizes of foot-point hard X-ray (HXR) sources using a relativistic test particle approach in a flare loop with a converging magnetic field. We compare results for the Collisional Thick Target Model (CTTM) with recently proposed modifications of the CTTM comprising a secondary acceleration of beam electrons. Our preliminary results indicate that none of the proposed modifications of the CTTM can explain the observed sizes of the HXR sources in a single loop flare scenario. Title: The spectrometer telescope for imaging x-rays on board the Solar Orbiter mission Authors: Benz, A. O.; Krucker, S.; Hurford, G. J.; Arnold, N. G.; Orleanski, P.; Gröbelbauer, H. -P.; Klober, S.; Iseli, L.; Wiehl, H. J.; Csillaghy, A.; Etesi, L.; Hochmuth, N.; Battaglia, M.; Bednarzik, M.; Resanovic, R.; Grimm, O.; Viertel, G.; Commichau, V.; Meuris, A.; Limousin, O.; Brun, S.; Vilmer, N.; Skup, K. R.; Graczyk, R.; Stolarski, M.; Michalska, M.; Nowosielski, W.; Cichocki, A.; Mosdorf, M.; Seweryn, K.; Przepiórka, A.; Sylwester, J.; Kowalinski, M.; Mrozek, T.; Podgorski, P.; Mann, G.; Aurass, H.; Popow, E.; Onel, H.; Dionies, F.; Bauer, S.; Rendtel, J.; Warmuth, A.; Woche, M.; Plüschke, D.; Bittner, W.; Paschke, J.; Wolker, D.; Van Beek, H. F.; Farnik, F.; Kasparova, J.; Veronig, A. M.; Kienreich, I. W.; Gallagher, P. T.; Bloomfield, D. S.; Piana, M.; Massone, A. M.; Dennis, B. R.; Schwarz, R. A.; Lin, R. P. Bibcode: 2012SPIE.8443E..3LB Altcode: The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, a confirmed Mclass mission of the European Space Agency (ESA) within the Cosmic Vision program scheduled to be launched in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The status of the instrument reviewed in this paper is based on the design that passed the Preliminary Design Review (PDR) in early 2012. Particular emphasis is given to the first light of the detector system called Caliste-SO. Title: Formation of Balmer Lines in Impulsively Heated Flare Atmosphere by Neutral Beams Authors: Varady, M.; Kašparová, J.; Moravec, Z.; Karlický, M.; Heinzel, P. Bibcode: 2012ASPC..454..341V Altcode: In the context of interpreting non-thermal hard X-ray emission and γ lines emanating from the footpoints of flare loops, most contemporary flare models assign a fundamental role during the flare energy release, transport and deposition to the high energy non-thermal particle beams. In this contribution we concentrate on modelling of the spectroscopic properties of chromospheric flare emission in optical hydrogen lines generated due to the bombardment of the chromosphere and photosphere by neutral beams with power-law spectra. In order to obtain an estimate of the neutral beam flare heating in the solar atmosphere we produced a simple model describing the propagation and thermalisation of neutral beams. We compare the neutral beam flare heating with the flare heating produced by corresponding pure electron and proton beams. Further we compare the contribution functions for Hα line obtained for neutral and electron beam heating. Title: Diagnostics of Non-Thermal Distribution from RESIK and RHESSI Flare Spectra Authors: Kulinová, A.; Kašparová, J.; Dzifčáková, E.; Sylwester, J.; Sylwester, B. Bibcode: 2012ASPC..454..329K Altcode: Solar flare spectra observed by the X-ray spectrometers RESIK and RHESSI with high energy resolution enabled us to analyse possible non-thermality of plasma electron distribution in the keV range. For RESIK diagnostics (in the 2-4 keV range) we assumed that the bulk of the plasma is represented by the so-called n-distribution, which describes the deviations from the Maxwellian distribution by two parameters: n and T. Using thick-target approximation for RHESSI spectral analysis, we obtained characteristics of injected electron power-law distribution in the deka-keV range. The event presented here shows a very good time correlation of non-thermality obtained from the RESIK spectra with appearance of non-thermal component in RHESSI and/or radio spectra. However, a thermal component was still present in RHESSI. Both spectral and imaging information in RHESSI soft and hard X-ray ranges were used for the estimation of the ratio of thermal to non-thermal electron densities of the X-ray emitting plasma. Title: Influence of Static and Stochastic Electric Fields on Electron Beams Bombarding the Chromosphere Authors: Varady, M.; Karlický, M.; Moravec, Z.; Kašparová, J. Bibcode: 2012ASPC..456..203V Altcode: Using a relativistic test-particle code we study and compare the influence of static and stochastic electric fields on propagation of electron beams along the magnetic fieldlines through the solar atmosphere given by the VAL C model from the primary acceleration site in the corona downwards to the chromosphere. The results are compared with the most common, classical model of electron beam propagation, scattering and thermalisation given by Emslie (1978). The effects of the fields on the chromospheric heating and hard X-ray emission are discussed. Title: The non-Maxwellian continuum in the X-ray, UV, and radio range Authors: Dudík, J.; Kašparová, J.; Dzifčáková, E.; Karlický, M.; Mackovjak, Š. Bibcode: 2012A&A...539A.107D Altcode:
Aims: We investigate the X-ray, UV, and also the radio continuum arising from plasmas with a non-Maxwellian distribution of electron energies. The two investigated types of distributions are the κ- and n-distributions.
Methods: We derived analytical expressions for the non-Maxwellian bremsstrahlung and free-bound continuum spectra. The spectra were calculated using available cross-sections. Then we compared the bremsstrahlung spectra arising from the different bremsstrahlung cross-sections that are routinely used in solar physics.
Results: The behavior of the bremsstrahlung spectra for the non-Maxwellian distributions is highly dependent on the assumed type of the distribution. At flare temperatures and hard X-ray energies, the bremsstrahlung is greatly increased for κ-distributions and exhibits a strong high-energy tail. With decreasing κ, the maximum of the bremsstrahlung spectrum decreases and moves to higher wavelengths. In contrast, the maximum of the spectra for n-distributions increases with increasing n, and the spectrum then falls off very steeply with decreasing wavelength. In the millimeter radio range, the non-Maxwellian bremsstrahlung spectra are almost parallel to the thermal bremsstrahlung. Therefore, the non-Maxwellian distributions cannot be detected by off-limb observations made by the ALMA instrument. The free-bound continua are also highly dependent on the assumed type of the distribution. For n-distributions, the ionization edges disappear and a smooth continuum spectrum is formed for n ≧ 5. Opposite behavior occurs for κ-distributions where the ionization edges are in general significantly enhanced, with details depending on κ and T through the ionization equilibrium. We investigated how the non-Maxwellian κ-distributions can be determined from the observations of the continuum and conclude that one can sample the low-energy part of the distribution from the continuum.

Appendix A is available in electronic form at http://www.aanda.org Title: Solar flares in Halpha and Ly-alpha : observations vs simulations. Authors: Rubio da Costa, F.; Zuccarello, F.; Fletcher, L.; Labrosse, N.; Prosecký, T.; Kašparová, J. Bibcode: 2012MSAIS..19..117R Altcode: In order to study the properties of faint, moderate and bright flares, we simulate the conditions of the solar atmosphere using a radiative hydrodynamic model \citep{2005ApJ...630..573A}. A constant beam of non-thermal electrons is injected at the apex of a 1D coronal loop and heating from thermal soft X-ray and UV emission is included. We study the contribution of different processes to the total intensity of different lines at different atmospheric layers. We obtain the total integrated intensity of different lines and we compare those of the Ly-alpha and Halpha lines with the observational values for Ly-alpha (using TRACE 1216 and 1600 Å data and estimating the ``pure'' Ly-alpha emission) and Halpha (using data from the Ondřejov Observatory). We inferred from the analysis of the values obtained by simulation that the X-ray energy of the different kind of flares does not strongly affect the Ly-alpha results; the Halpha results are comparable to the observed ones, concluding that the simulated solar atmosphere fits better at lower layers of the chromosphere than at upper layers. Title: The Diagnostics of the Shape of the Electron Distribution Function during the Solar Flares Authors: Dzifčáková, E.; Kulinová, A.; Kašparová, J. Bibcode: 2011ASPC..448.1095D Altcode: 2011csss...16.1095D The non-thermal electrons accelerated during the flares interact with surrounding plasma and the electron distribution of the flaring plasma becomes non-Maxwellian. X-ray spectrometers RESIK and RHESSI with high energy resolution give an opportunity to diagnose the presence of the non-thermal electron distribution. RESIK X-line spectra with high fluxes of satellite lines can be explained by presence of the non-thermal n-distribution in a plasma bulk in the 2-2.5 keV range. The RHESSI spectrometer enables us to diagnose the non-thermal high-energy tail of the electron distribution in deka-keV energy range. This high-energy tail can be described by a power-law distribution. We have analyzed three solar flares to get non-thermal characteristics of both non-thermal parts of the electron distribution. The ratios of the intensities of allowed to satellite lines have been used to estimate the parameters of the n-distribution. RHESSI data has been used to obtain the temporal changes of the parameters of Maxwellian and power-law distributions and also for determination of the parameters of n-distribution in two specific cases. The parameters of n-distribution obtained from RHESSI analysis agree within the errors with those derived from RESIK observations. Finally, the synthetic soft X-ray line spectra has been computed for diagnosed parameters of distributions and have been compared with RESIK X-ray observations. Title: Diagnostics of non-thermal distributions in solar flare spectra observed by RESIK and RHESSI Authors: Kulinová, A.; Kašparová, J.; Dzifčáková, E.; Sylwester, J.; Sylwester, B.; Karlický, M. Bibcode: 2011A&A...533A..81K Altcode: 2011arXiv1109.1011K Context. During solar flares an enormous amount of energy is released, and the charged particles, like electrons, are accelerated. These non-thermal electrons interact with the plasma in various parts of solar flares, where the distribution function of electrons can therefore be non-Maxwellian.
Aims: We focus on the non-thermal components of the electron distribution in the keV range and analyse high-energy resolution X-ray spectra detected by RESIK and RHESSI for three solar flares.
Methods: In the 2-4 keV range we assume that the electron distribution can be modelled by an n-distribution. Using a method of line-intensity ratios, we analyse allowed and satellite lines of Si observed by RESIK and estimate the parameters of this n-distribution. At higher energies we explore RHESSI bremsstrahlung spectra. Adopting a forward-fitting approach and thick-target approximation, we determine the characteristics of injected electron beams.
Results: RHESSI non-thermal component associated with the electron beam is correlated well with presence of the non-thermal n-distribution obtained from the RESIK spectra. In addition, such an n-distribution occurs during radio bursts observed in the 0.61-15.4 GHz range. Furthermore, we show that the n-distribution could also explain RHESSI emission below ~5 keV. Therefore, two independent diagnostics methods indicate the flare plasma being affected by the electron beam can have a non-thermal component in the ~2-5 keV range, which is described by the n-distribution well. Finally, spectral line analysis reveals that the n-distribution does not occupy the same location as the thermal component detected by RHESSI at ~10 keV.

Appendix A is available in electronic form at http://www.aanda.org Title: Microflares and the Statistics of X-ray Flares Authors: Hannah, I. G.; Hudson, H. S.; Battaglia, M.; Christe, S.; Kašparová, J.; Krucker, S.; Kundu, M. R.; Veronig, A. Bibcode: 2011SSRv..159..263H Altcode: 2011SSRv..tmp..262H; 2011SSRv..tmp...87H; 2011arXiv1108.6203H; 2011SSRv..tmp..243H; 2011SSRv..tmp..163H This review surveys the statistics of solar X-ray flares, emphasising the new views that RHESSI has given us of the weaker events (the microflares). The new data reveal that these microflares strongly resemble more energetic events in most respects; they occur solely within active regions and exhibit high-temperature/nonthermal emissions in approximately the same proportion as major events. We discuss the distributions of flare parameters (e.g., peak flux) and how these parameters correlate, for instance via the Neupert effect. We also highlight the systematic biases involved in intercomparing data representing many decades of event magnitude. The intermittency of the flare/microflare occurrence, both in space and in time, argues that these discrete events do not explain general coronal heating, either in active regions or in the quiet Sun. Title: Deducing Electron Properties from Hard X-ray Observations Authors: Kontar, E. P.; Brown, J. C.; Emslie, A. G.; Hajdas, W.; Holman, G. D.; Hurford, G. J.; Kašparová, J.; Mallik, P. C. V.; Massone, A. M.; McConnell, M. L.; Piana, M.; Prato, M.; Schmahl, E. J.; Suarez-Garcia, E. Bibcode: 2011SSRv..159..301K Altcode: 2011arXiv1110.1755K; 2011SSRv..tmp..279K X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager ( RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future. Title: Solar flares: observations vs simulations Authors: Rubio da Costa, Fatima; Zuccarello, Francesca; Labrosse, Nicolas; Fletcher, Lyndsay; Prosecký, Tomáš; Kašparová, Jana Bibcode: 2011IAUS..274..182R Altcode: In order to study the properties of faint, moderate and bright flares, we simulate the conditions of the solar atmosphere using a radiative hydrodynamic model (Abbett & Hawley, 1999). A constant beam of non-thermal electrons is injected at the apex of a 1D coronal loop and heating from thermal soft X-ray emission is included. We compare the results with some observational data in Ly-α (using TRACE 1216 and 1600 Å data and estimating the ``pure'' Ly-α emission) and in Hα (data taken with a Multichannel Flare Spectrograph, at the Ondrejov Observatory). Title: Diagnostics of non-thermal distribution from solar flares spectra - RESIK Authors: Kulinová, A.; Kašparová, J.; Dzifčáková, E. Bibcode: 2010nspm.conf...77K Altcode: Solar flares spectra observed by X-ray spectrometers RESIK and RHESSI made possible detecting and diagnosing non-thermal effects in the flare plasma in energy range from 2 keV to several tens of keV. Based on previous studies we have assumed that the core of a non-thermal distribution behaves like a so-called n-distribution mixed with a thermal component, and its high-energy tail can be described by a power-law relation. We used two different diagnostics: spectral lines ratia (RESIK) and a raw target model with a thermal component (RHESSI) to obtain parameters of these parts of the free electron 'model' distribution. Title: Modeling of Solar Flare Plasma and Its Radiation Authors: Varady, Michal; Kasparova, Jana; Moravec, Zdeněk; Heinzel, Petr; Karlicky, Marian Bibcode: 2010ITPS...38.2249V Altcode: No abstract at ADS Title: Electron beam - plasma system with the return current and directivity of its X-ray emission Authors: Karlický, M.; Kašparová, J. Bibcode: 2009A&A...506.1437K Altcode: 2009arXiv0909.0146K Aims: An evolution of the electron distribution function in the beam-plasma system with the return current is computed numerically for different parameters. The X-ray bremsstrahlung corresponding to such an electron distribution is calculated and the directivity of the X-ray emission is studied.
Methods: For computations of the electron distribution functions we used a 3-D particle-in-cell electromagnetic code. The directivity of the X-ray emission was calculated using the angle-dependent electron-ion bremsstrahlung cross-section.
Results: It was found that the resulting electron distribution function depends on the magnetic field assumed along the electron beam propagation direction. For small magnetic fields the electron distribution function becomes broad in the direction perpendicular to the beam propagation due to the Weibel instability and the return current is formed by the electrons in a broad and shifted bulk of the distribution. On the other hand, for stronger magnetic fields the distribution is more extended in the beam-propagation direction and the return current is formed by the electrons in the extended distribution tail. In all cases, the anisotropy of the electron distribution decreases rapidly due to fast collisionless processes. However, the magnetic field reduces this anisotropy decrease. The X-ray directivity shows the same trend and it is always closer to the isotropic case than that in a simple beaming model. Title: Response of optical hydrogen lines to beam heating. I. Electron beams Authors: Kašparová, J.; Varady, M.; Heinzel, P.; Karlický, M.; Moravec, Z. Bibcode: 2009A&A...499..923K Altcode: 2009arXiv0904.2084K Context: Observations of hydrogen Balmer lines in solar flares remain an important source of information on flare processes in the chromosphere during the impulsive phase of flares. The intensity profiles of optically thick hydrogen lines are determined by the temperature, density, and ionisation structure of the flaring atmosphere, by the plasma velocities and by the velocity distribution of particles in the line formation regions.
Aims: We investigate the role of non-thermal electrons in the formation regions of Hα, Hβ, and Hγ lines in order to unfold their influence on the formation of these lines. We concentrate on pulse-beam heating varying on a subsecond timescale. Furthermore, we theoretically explore possibility that a new diagnostic tool exists indicating the presence of non-thermal electrons in the flaring chromosphere based on observations of optical hydrogen lines.
Methods: To model the evolution of the flaring atmosphere and the time-dependent hydrogen excitation and ionisation, we used a 1-D radiative hydrodynamic code combined with a test-particle code that simulates the propagation, scattering, and thermalisation of a power-law electron beam in order to obtain the flare heating and the non-thermal collisional rates due to the interaction of the beam with the hydrogen atoms. To not bias the results by other effects, we calculate only short time evolutions of the flaring atmosphere and neglect the plasma velocities in the radiative transfer.
Results: All calculated models have shown a time-correlated response of the modelled Balmer line intensities on a subsecond timescale, with a subsecond timelag behind the beam flux. Depending on the beam parameters, both line centres and wings can show pronounced intensity variations. The non-thermal collisional rates generally result in an increased emission from a secondary region formed in the chromosphere.
Conclusions: Despite the clear influence of the non-thermal electron beams on the Balmer line intensity profiles, we were not able on the basis of our simulations to produce any unambiguous diagnostic of non-thermal electrons in the line-emitting region, which would be based on comparison of individual Balmer line intensity profiles. However, fast line intensity variations, well-correlated with the beam flux variations, represent an indirect indication of pulsating beams. Title: Examples of Science Cases and Requirements for EST Authors: Sobotka, M.; Heinzel, P.; Kašparová, J. Bibcode: 2009ASPC..405..455S Altcode: We present various suggestions for the study of flares (fast emission variations, heating mechanisms), prominences (fine structure, magnetic field, energy balance), and sunspots (umbral structure, heating mechanisms, depth and structure of the penumbra, Evershed flow). From these cases we derive the requirements for the future European Solar Telescope: spectropolarimetric capabilities, high signal-to-noise ratio, multi-line spectroscopy, high spatial and temporal resolution. Title: Kappa distribution and hard X-ray emission of solar flares Authors: Kašparová, J.; Karlický, M. Bibcode: 2009A&A...497L..13K Altcode: 2009arXiv0902.3574K Aims: We investigate whether the so-called kappa distribution, often used to fit electron distributions detected in situ in the solar wind, can describe electrons producing the hard X-ray emission in solar flares.
Methods: Using Ramaty High Energy Solar Spectroscopic imager (RHESSI) flare data we fit spatially- and feature-integrated spectra, assuming a kappa distribution for the mean electron flux spectrum.
Results: We show that a single kappa distribution generally cannot describe spatially integrated X-ray emission composed of both footpoint and coronal sources. In contrast, the kappa distribution is consistent with mean electron spectra producing hard X-ray emission in some coronal sources. Title: Far-IR and Radio Thermal Continua in Solar Flares Authors: Kašparová, J.; Heinzel, P.; Karlický, M.; Moravec, Z.; Varady, M. Bibcode: 2009CEAB...33..309K Altcode: 2009arXiv0901.3465K With the invention of new far-infrared (FIR) and radio mm and sub-mm instruments (DESIR on SMESE satellite, ESO-ALMA), there is a growing interest in observations and analysis of solar flares in this so far unexplored wavelength region. Two principal radiation mechanisms play a role: the synchrotron emission due to accelerated particle beams moving in the magnetic field and the thermal emission due to the energy deposit in the lower atmospheric layers. In this contribution we explore the time-dependent effects of beams on thermal FIR and radio continua. We show how and where these continua are formed in the presence of time dependent beam heating and non-thermal excitation/ionisation of the chromospheric hydrogen plasma. Title: Low-Energy Cutoffs in Electron Spectra of Solar Flares: Statistical Survey Authors: Kontar, E. P.; Dickson, E.; Kašparová, J. Bibcode: 2008SoPh..252..139K Altcode: 2008arXiv0805.1470K; 2008SoPh..tmp..149K The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) X-ray data base (February 2002 - May 2006) has been searched to find solar flares with weak thermal components and flat photon spectra. Using a regularized inversion technique, we determine the mean electron flux distribution from count spectra for a selection of events with flat photon spectra in the 15 - 20 keV energy range. Such spectral behavior is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in a given energy range (e.g., a low-energy cutoff in the mean electron spectra of nonthemal particles). We have found 18 cases that exhibit a statistically significant local minimum (a dip) in the range of 13 - 19 keV. The positions and spectral indices of events with low-energy cutoff indicate that such features are likely to be the result of photospheric albedo. It is shown that if the isotropic albedo correction is applied, all low-energy cutoffs in the mean electron spectrum are removed, and hence the low-energy cutoffs in the mean electron spectrum of solar flares above ∼ 12 keV cannot be viewed as real features. If low-energy cutoffs exist in the mean electron spectra, their energies should be less than ∼ 12 keV. Title: Hydrogen Balmer Lines in the Solar Atmosphere Heated by Electron Beams Authors: Kasparova, J.; Varady, M.; Moravec, Z.; Heinzel, P.; Karlicky, M. Bibcode: 2008ESPM...12.3.16K Altcode: Accelerated particle beam are one of the mechanisms of the flare energy transport from the corona to the transition region and the chromosphere. Such beams heat ambient plasma and may affect atomic population via collisions. Using 1D NLTE radiative hydrodynamics we model influence of electron beams on hydrogen Balmer lines. In the contribution we focus on the effect of so-called non-thermal collisional rates on Halpha and Hbeta lines. Based on our results we discuss diagnostic methods for determination of beam presence in the formation regions of Halpha and Hbeta lines. Title: Far-IR and Radio Continua in Solar Flares Authors: Heinzel, P.; Kasparova, J.; Varady, M.; Karlicky, M.; Moravec, Z. Bibcode: 2008ESPM...12.3.20H Altcode: With the invention of new far-infrared (FIR) and radio mm and sub-mm instruments (DESIR on SMESE satellite, ESO ALMA), there is a growing interest in observations and analysis of solar flares in this so far unexplored wavelength region. Two principal radition mechanisms play a role: the synchrotron emission due to accelerated particle beams moving in the magnetic field and the thermal emission due to energy deposit in the lower atmospheric layers. The latter one was recently explored for the case of semiempirical flare models, without considering the temporal evolution. However, as the radiation-hydrodynamical simulations do show, the lower atmosphere heated by beams exhibits fast temporal changes which are typically reflected in variations of spectral-line intensities. In this contribution we explore the time-dependent effects of beams on FIR and radio continua. We show how and where these continua are formed in the presence of time dependent beam heating and non-thermal excitation/ionization of the chromospheric hydrogen plasma. Our results should contribute to planning of new observations in FIR and radio domain. Title: Fast spectral fitting of hard X-ray bremsstrahlung from truncated power-law electron spectra Authors: Brown, J. C.; Kašparová, J.; Massone, A. M.; Piana, M. Bibcode: 2008A&A...486.1023B Altcode: 2008arXiv0802.0621B Context: Hard X-ray bremsstrahlung continuum spectra, such as from solar flares, are commonly described in terms of power-law fits, either to the photon spectra themselves or to the electron spectra responsible for them. In applications various approximate relations between electron and photon spectral indices are often used for energies both above and below electron low-energy cutoffs.
Aims: We examine the form of the exact relationships in various situations, and for various cross-sections, showing that empirical relations sometimes used can be highly misleading especially at energies below the low-energy cutoff, and consider how to improve fitting procedures.
Methods: We obtain expressions for photon spectra from single, double and truncated power-law electron spectra for a variety of cross-sections and for the thin and thick target models and simple analytic expressions for the non-relativistic Bethe-Heitler case.
Results: We show that below the low-energy cutoff Kramers and other constant spectral index forms commonly used are very poor approximations to accurate results, but that our analytical forms are a good match; and that above a low-energy cutoff, the Kramers and non-relativistic Bethe-Heitler results match reasonably well with results for up to energies around 100 keV.
Conclusions: Analytical forms of the non-relativistic Bethe-Heitler photon spectra from general power-law electron spectra are good match to exact results for both thin and thick targets and they enable much faster spectral fitting than evaluation of the full spectral integrations. Title: Modelling of Optical Emission in Solar Flares Authors: Varady, M.; Kašparová, J.; Heinzel, P.; Karlický, M.; Moravec, Z. Bibcode: 2008CEAB...32..101V Altcode: We present recent progress achieved by our group in the field of flare optical emission modelling. We concentrate on two problems. Firstly, on the possibility of modelling of the time evolution of several Balmer line profiles calculated for real electron fluxes obtained from the hard X--ray observations of a particular flare with the prospect to compare the theoretical results with the observed data for the corresponding flare. Secondly, we discuss the influence of the non--thermal electrons in the line emitting region on the formation and time evolution of Balmer hydrogen line profiles. Title: Low-Energy Cut-Offs In Electron Spectra Of Solar Flares: Statistical Survey Authors: Kontar, Eduard; Dickson, Ewan; Kasparova, Jana Bibcode: 2008cosp...37.1577K Altcode: 2008cosp.meet.1577K Ramaty High Energy Solar Spectroscopic Imager (RHESSI) X-ray data base (February 2002 - May 2006) has been searched to find solar flares with weak thermal component and flat photon spectra. Using a regularised inversion technique, we determine the mean electron flux distribution from count spectra of the events which had flat photon spectra in the 15-20 keV energy range. Such spectral behaviour is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in some energy range, e.g. low-energy cutoff in electron spectra. We have found a number of cases which exhibit a statistically significant dip in the range of 10-20 keV. The positions and spectral indices of events with low-energy cutoffs indicate that such feature could be a result of photospheric albedo. It is shown that if the isotropic albedo correction was applied, all low-energy cutoffs in mean electron spectrum were removed. Title: Hydrogen Balmer line formation in solar flares affected by return currents Authors: Štepán, J. Å.; Kašparová, J.; Karlický, M.; Heinzel, P. Bibcode: 2007A&A...472L..55S Altcode: 2007arXiv0708.0265S Aims:We investigate the effect of the electric return currents in solar flares on the profiles of hydrogen Balmer lines. We consider the monoenergetic approximation for the primary beam and runaway model of the neutralizing return current.
Methods: Propagation of the 10 keV electron beam from a coronal reconnection site is considered for the semiempirical chromosphere model F1. We estimate the local number density of return current using two approximations for beam energy fluxes between 4 × 1011 and 1 × 1012 erg cm-2 s-1. Inelastic collisions of beam and return-current electrons with hydrogen are included according to their energy distributions, and the hydrogen Balmer line intensities are computed using an NLTE radiative transfer approach.
Results: In comparison to traditional NLTE models of solar flares that neglect the return-current effects, we found a significant increase emission in the Balmer line cores due to nonthermal excitation by return current. Contrary to the model without return current, the line shapes are sensitive to a beam flux. It is the result of variation in the return-current energy that is close to the hydrogen excitation thresholds and the density of return-current electrons. Title: Hα with Heating by Particle Beams Authors: Kašparová, J.; Varady, M.; Karlický, M.; Heinzel, P.; Moravec, Z. Bibcode: 2007ASPC..368..441K Altcode: 2007astro.ph..3800K Using 1D NLTE radiative hydrodynamics we model the influence of the particle beams on the Hα line profile treating the beam propagation and the atmosphere evolution self-consistently. We focus on the influence of the non-thermal collisional rates and the return current. Based on our results, we propose a diagnostic method for determination of the particle beam presence in the formation regions of the Hα line. Title: Hard X-ray spectra and positions of solar flares observed by RHESSI: photospheric albedo, directivity and electron spectra Authors: Kašparová, J.; Kontar, E. P.; Brown, J. C. Bibcode: 2007A&A...466..705K Altcode: 2007astro.ph..1871K Aims:We investigate the signature of the photospheric albedo contribution in solar flare hard X-ray spectra, the effect of low energy cutoffs in electron spectra, and the directivity of hard X-ray emission.
Methods: Using Ramaty High Energy Solar Spectroscopic Imager (RHESSI) flare data we perform a statistical analysis of spatially integrated spectra and positions of solar flares.
Results: We demonstrate clear centre-to-limb variation of photon spectral indices in the 15-20 keV energy range and a weaker dependency in the 20-50 keV range which is consistent with photospheric albedo as the cause. The results also suggest that low-energy cutoffs sometimes inferred in mean electron spectra are an artefact of albedo. We also derive the anisotropy (ratio of downward/observer directed photons) of hard X-ray emission in the 15-20 keV range for various heliocentric angles. Title: Return Current and Energy Deposit in Flares Authors: Varady, M.; Karlický, M.; Kašparová, J. Bibcode: 2007ASPC..368..473V Altcode: The return current (RC) related effects represent in flares one of the possible mechanisms of conversion of the kinetic energy of electron beams into the thermal energy of flare plasma. Using a 1-D current-in-cell model, details of formation and properties of the RC driven by monoenergetic electron beams are calculated. Based on results of the simulations, the influence of the RC on the energy deposit in flares is discussed. Title: Optical and UV Emission of Solar Flares: Multiwavelength Observations and Modelling Authors: Kasparova, J.; Heinzel, P. Bibcode: 2007AGUSMSH22A..07K Altcode: Solar flares reveal themselves in various ranges of electromagnetic emission and exhibit temporal variations down to sub-second scale, namely during the so-called impulsive phase. This contribution will focus on spatial and temporal variations of the optical and UV emissions which are thought to be driven by flare energy transport in the form of accelerated particles. We will review our current understanding of the formation of optical and UV emissions during the impulsive phase. Namely, we will discuss recent radiative-hydrodynamic models and various physical processes playing an important role in radiation transfer and formation of optical and UV spectra. We will also report on recent interpretations of observations in various lines and continua and discuss their importance for the diagnostics of the flare atmosphere structure and the presence of accelerated particles in the emission formation regions. Title: RHESSI survey of photospheric albedo and directivity of solar flare hard X-ray spectra Authors: Kasparova, J.; Kontar, E. P. Bibcode: 2006IAUJD...1E..48K Altcode: Hard X-ray spectra of solar flares are generated as bremsstrahlung of accelerated electrons propagating in solar corona. The photons emitted towards the photosphere have a high probability to undergo Compton backscattering into observers direction. They form so called photospheric albedo component and modify the spatially integrated photon spectra of solar flares. Analysing 409 solar flares observed by RHESSI, we show significant centre-to-limb variation of observed photon spectra in energies ~ 20 keV, which is consistent with the photospheric albedo. Moreover, we also show that the low-energy cutoff in the mean electron spectrum can be viewed as an artifact of the albedo component. Noting that the amount of backscattered photons strongly depends on the downward directed photon flux, we determine for the first time the directivity of the downward photon flux. The results favour near-isotropic photon distribution and represent a problem for the models with downward propagating electron beam. Title: Return Current And The Energy Deposit In Flares Authors: Varady, M.; Karlicky, M.; Kasparova, J. Bibcode: 2006IAUJD...1E..51V Altcode: The return current related effects in flares represent one of the possible mechanisms of conversion of the kinetic energy of electron beams into the thermal energy of flare plasma. Using a 1-D particle in cell model details of formation and properties of the return current driven by monoenergetic electron beams are calculated. Using the results of the simulations the influence of the return current on the energy deposit in flares are discussed. Title: Polarization Diagnostics of Proton Beams in Solar Flares Authors: Stepan, J.; Heinzel, P.; Kasparova, J.; Sahal-Brechot, S. Bibcode: 2006IAUJD...1E..55S Altcode: We review the problem of proton beam bombardment of solar chromosphere considering the self-consistent NLTE polarized radiation transfer in hydrogen lines. Several observations indicate a linear polarization of H-alpha line of the order of 5% or higher and preferentially in radial direction. This polarization is often explained as anisotropic collisional excitation of the n= 3 level by vertical proton beams. Our calculations indicate that deceleration of the proton beam with initial power-law energy distribution together with increased electron and proton densities in H-alpha forming layers lead to negligible line polarization. Thus the proton beams seem not to be a good candidate for explanation of the observed polarization degree. Title: On AN Effect of Particle Beams on Correlation Between Balmer Series Lines Authors: Kashapova, L. K.; Kotrč, P.; Kupryakov, Yu. A.; Kašparová, J. Bibcode: 2005ESASP.600E.126K Altcode: 2005dysu.confE.126K; 2005ESPM...11..126K No abstract at ADS Title: Problem of the Return Current in Energy Deposit in Flares Authors: Varady, M.; Karlický, M.; Kašparová, J. Bibcode: 2005ESASP.600E.146V Altcode: 2005ESPM...11..146V; 2005dysu.confE.146V No abstract at ADS Title: Hα Line in Solar Atmosphere Heated by Particle Beams Authors: Kašparová, J.; Varady, M.; Karlický, M.; Moravec, Z.; Heinzel, P. Bibcode: 2005ESASP.600E.127K Altcode: 2005dysu.confE.127K; 2005ESPM...11..127K No abstract at ADS Title: Multi-Wavelength Analysis of High-Energy Electrons in Solar Flares: A Case Study of the August 20, 2002 Flare Authors: Kašparová, Jana; Karlický, Marian; Kontar, Eduard P.; A. Schwartz, Richard; Dennis, Brian R. Bibcode: 2005SoPh..232...63K Altcode: 2005astro.ph..8636K A multi-wavelength spatial and temporal analysis of solar high-energy electrons is conducted using the August 20, 2002 flare of an unusually flat (γ1 = 1.8) hard X-ray spectrum. The flare is studied using RHESSI, Hα, radio, TRACE, and MDI observations with advanced methods and techniques never previously applied in the solar flare context. A new method to account for X-ray Compton backscattering in the photosphere (photospheric albedo) has been used to deduce the primary X-ray flare spectra. The mean electron flux distribution has been analysed using both forward fitting and model-independent inversion methods of spectral analysis. We show that the contribution of the photospheric albedo to the photon spectrum modifies the calculated mean electron flux distribution, mainly at energies below ∼100 keV. The positions of the Hα emission and hard X-ray sources with respect to the current-free extrapolation of the MDI photospheric magnetic field and the characteristics of the radio emission provide evidence of the closed geometry of the magnetic field structure and the flare process in low altitude magnetic loops. In agreement with the predictions of some solar flare models, the hard X-ray sources are located on the external edges of the Hα emission and show chromospheric plasma heated by the non-thermal electrons. The fast changes of Hα intensities are located not only inside the hard X-ray sources, as expected if they are the signatures of the chromospheric response to the electron bombardment, but also away from them. Title: X-Ray and HαEmission of the 20 Aug 2002 Flare* Authors: Kašparová, J.; Karlický, M.; Schwartz, R. A.; Dennis, B. R. Bibcode: 2005ASSL..320..187K Altcode: 2005smp..conf..187K No abstract at ADS Title: Collisional excitation and ionization of hydrogen by return current in solar flares Authors: Karlický, M.; Kašparová, J.; Heinzel, P. Bibcode: 2004A&A...416L..13K Altcode: First a problem of the transport of electron beams with high energy fluxes into the cold chromosphere during the flare is presented. Then it is shown that the problem might be solved by the return current formed by superthermal (runaway) electrons. In such a case the return current electrons could influence hydrogen excitations and ionizations. Therefore, we computed collisional rates of such a return current and compared them with those of the thermal plasma and of a monoenergetic (10 keV) electron beam with the energy flux FE = 1012 erg cm-2 s-1 penetrating into the flare atmosphere described by the F1 model (Machado et al. \cite{Machado1980}). We show that in this situation the return current collisional rates can be dominant for some transitions. Title: Collisional Excitation and Ionization of Hydrogen by Return Current in Solar Flares Authors: Kašparová, J.; Heinzel, P.; Karlický, M. Bibcode: 2004IAUS..219..760K Altcode: No abstract at ADS Title: Analysis and modelling of hard X-ray and optical signatures of electron beams in solar flares Authors: Kasparova, Jana Bibcode: 2004PhDT.......563K Altcode: No abstract at ADS Title: The Unusual Hard X-ray Spectrum of the Flare of 20 August 2002 Authors: Schwartz, R. A.; Kasparova, J.; Dennis, B.; Karlicky, M. Bibcode: 2003AGUFMSH22A0171S Altcode: An M3 Class flare was observed in x-rays with RHESSI and in H-alpha with the Kanzelhohe Solar Observatory. The event was observed to several hundreds of keV in X-rays and was marked by an unusually flat spectrum observed from 20-70 keV. The measured power-law exponent of this component was about 1.7, very close to the theoretical limit for a thick-target injection of energetic electrons implying a near cutoff below 80 keV. We will bound any systematic effects that may be contributing to this result by analyzing the spectrum using multiple techniques. We will also forward model the spatial/spectral x-ray sources to further validate these observations. Title: Time-dependent Flare Models with MALI Authors: Kašparová, J.; Heinzel, P.; Varady, M.; Karlický, M. Bibcode: 2003ASPC..288..544K Altcode: 2003sam..conf..544K Temporal variations of Hα line profile intensities related to electron beams are presented. We show first results of time dependent simulations of a chromospheric response to a 1 sec monoenergetic electron beam. 1-D hydrodynamic code together with particle representation of the beam have been used to calculate atmospheric evolution. Time dependent radiative transfer problem has been solved for the resulting atmosphere in the MALI approach, using the Crank-Nicholson implicit scheme. Non-thermal collisional rates were included in linearised equations of statistical equilibrium. Title: Time dependent flare model with non-LTE radiative transfer Authors: Varady, M.; Karlický, M.; Kašparová, J.; Heinzel, P. Bibcode: 2002ESASP.506..521V Altcode: 2002svco.conf..521V; 2002ESPM...10..521V The first results of a time dependent simulation of chromospheric response to a high energy electron beam are presented. The hybrid code, i.e. a combination of a 1-D hydrodynamic code and a test particle code, has been used to calculate the energy losses of a high energy electron beam propagating through the solar atmosphere and the consequent response of the ambient solar plasma to the energy deposition. The resulting time evolution of the solar plasma temperature, density, velocity and energy deposit on hydrogen has then been used as an input for a time dependent radiative transfer code in the MALI approach to determine the time variation of the Hα line profile. Non-thermal collisional rates have been included in the linearised ESE. Title: Diagnostics of electron bombardment in solar flares from hydrogen Balmer lines Authors: Kašparová, J.; Heinzel, P. Bibcode: 2002A&A...382..688K Altcode: Influence of non-thermal collisional rates, related to an electron beam, on hydrogen Balmer line profiles is investigated. Semi-empirical temperature structure of the flare model F1 has been used for computing non-LTE profiles of Hα , Hβ , and Hγ . Contribution functions and their change due to different values of beam parameters are shown. Unlike the line core intensity, the intensity of line wings considerably depends on beam parameters and it is significantly enhanced for typical values of the beam energy flux. The ratio of line intensities at a selected wavelength is proposed to be used for diagnostics of electron beams during solar flares, particularly at impulsive phases. Obtained Hα line profiles are compared to those of Fang et al. (\cite{fhg93}). Title: Physical Characteristics of the September 23, 1998 Solar Flare Authors: Kotrc, P.; Karlický, M.; Kupryakov, Yu. A.; Kaltman, T. I.; Kasparova, J.; Rompolt, B. Bibcode: 1999ESASP.448..841K Altcode: 1999ESPM....9..841K; 1999mfsp.conf..841K No abstract at ADS Title: New Evidence of Chromospheric Evaporation Authors: Kasparova, J.; Kotrc, P.; Heinzel, P.; Nikulin, I. F.; Rudawy, P. Bibcode: 1998ASPC..150..397K Altcode: 1998IAUCo.167..397K; 1998npsp.conf..397K No abstract at ADS