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A Compressed Sensing-based Image Reconstruction Algorithm for Solar Flare X-Ray Observations  

Marina Battaglia   Submitted: 2017-09-26 05:10

One way of imaging X-ray emission from solar flares is to measure Fourier components of the spatial X-ray source distribution. We present a new Compressed Sensing-based algorithm named VIS_CS, which reconstructs the spatial distribution from such Fourier components. We demonstrate the application of the algorithm on synthetic and observed solar flare X-ray data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite and compare its performance with existing algorithms. VIS_CS produces competitive results with accurate photometry and morphology, without requiring any algorithm- and X-ray source-specific parameter tuning. Its robustness and performance make this algorithm ideally suited for generation of quicklook images or large image cubes without user intervention, such as for imaging spectroscopy analysis.

Authors: Simon Felix, Roman Bolzern, Marina Battaglia
Projects: RHESSI

Publication Status: ApJ, in press
Last Modified: 2017-09-26 13:44
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The Solar X-ray Limb  

Marina Battaglia   Submitted: 2017-06-01 01:03

We describe a new technique to measure the height of the X-ray limb with observations from occulted X-ray flare sources as observed by the RHESSI (the Reuven Ramaty High-Energy Spectroscopic Imager) satellite. This method has model dependencies different from those present in traditional observations at optical wavelengths, which depend upon detailed modeling involving radiative transfer in a medium with complicated geometry and flows. It thus provides an independent and more rigorous measurement of the ''true'' solar radius, meaning that of the mass distribution. RHESSI's measurement makes use of the flare X-ray source's spatial Fourier components (the visibilities), which are sensitive to the presence of the sharp edge at the lower boundary of the occulted source. We have found a suitable flare event for analysis, SOL2011-10-20T03:25 (M1.7), and report a first result from this novel technique here. Using a 4-minute integration over the 3-25 keV photon energy range, we find RX-ray=964.05 ? 0.15?0.29 arcsec, where the uncertainties include statistical uncertainties from the method and a systematic error. The standard VAL-C model predicts a value of 963.48 arcsec, about 2σ below our value.

Authors: M. Battaglia, H. S. Hudson, G. J. Hurford, S. Krucker, R. A. Schwartz
Projects: RHESSI

Publication Status: ApJ, accepted
Last Modified: 2017-06-01 10:21
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Multi-thermal representation of the kappa-distribution of solar flare electrons and application to simultaneous X-ray and EUV observations  

Marina Battaglia   Submitted: 2015-11-05 00:54

Acceleration of particles and plasma heating is one of the fundamental problems in solar flare physics. An accurate determination of the spectrum of flare energized electrons over a broad energy range is crucial for our understanding of aspects such as the acceleration mechanism and the total flare energy. Recent years have seen a growing interest in the kappa-distribution as representation of the total spectrum of flare accelerated electrons. In this work we present the kappa-distribution as a differential emission measure. This allows for inferring the electron distribution from X-ray observations and EUV observations by simultaneously fitting the proposed function to RHESSI and SDO/AIA data. This yields the spatially integrated electron spectra of a coronal source between less than 0.1 keV up to several tens of keV. The method is applied to a single-loop GOES C4.1 flare. The results show that the total energy can only be determined accurately by combining RHESSI and AIA observations. Simultaneously fitting the proposed representation of the kappa-distribution reduces the electron number density in the analyzed flare by a factor of ~30 and the total flare energy by a factor of ~5 compared with the commonly used fitting of RHESSI spectra. The spatially integrated electron spectrum of the investigated flare between 0.043 keV and 24 keV is consistent with the combination of a low-temperature (~2 MK) component and a hot (~11 MK) kappa-like component with spectral index 4, reminiscent of solar wind distributions.

Authors: Marina Battaglia, Galina Motorina, Eduard P. Kontar
Projects: RHESSI,SDO-AIA

Publication Status: ApJ, accepted
Last Modified: 2015-11-05 11:05
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How important are electron beams in driving chromospheric evaporation in the 2014 March 29 flare?  

Marina Battaglia   Submitted: 2015-10-20 02:31

We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph (IRIS) observations of the FeXXI 1354.1 A 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 km s-1 and 200 km s-1. Hard X-ray (HXR) footpoints were observed by RHESSI 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 FeXXI by 30-75 seconds. However, in other parts of the southern ribbon and in the northern ribbon the observed upflows were not coincident with a HXR source in time nor 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.

Authors: Marina Battaglia, Lucia Kleint, Säm Krucker, David Graham
Projects: None

Publication Status: ApJ, accepted
Last Modified: 2015-10-20 12:20
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Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop?  

Marina Battaglia   Submitted: 2014-05-20 01:33

Before the onset of a flare is observed in hard X-rays there is often a prolonged pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare with a prolonged pre-heating phase are analysed to study the time evolution of the thermal emission and to determine the onset of particle acceleration. During the 20 minutes duration of the pre-heating phase we find no hint of accelerated electrons, neither in hard X-rays nor in microwave emission. However, the total energy budget during the pre-heating phase suggests that energy must be supplied to the flaring loop to sustain the observed temperature and emission measure. Under the assumption of this energy being transported toward the chromosphere via thermal conduction, significant energy deposition at the chromosphere is expected. However, no detectable increase of the emission in the AIA wavelength channels sensitive to chromospheric temperatures is observed. The observations suggest energy release and deposition in the flaring loop before the onset of particle acceleration, yet a model in which energy is conducted to the chromosphere and subsequent heating of the chromosphere is not supported by the observations.

Authors: Marina Battaglia, Lyndsay Fletcher, Paulo J. A. Simões
Projects: Nobeyama Radioheliograph,RHESSI,SDO-AIA

Publication Status: ApJ, accepted
Last Modified: 2014-05-21 13:39
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Electron Distribution Functions in Solar Flares from combined X-ray and EUV Observations  

Marina Battaglia   Submitted: 2013-10-15 23:23

Simultaneous solar flare observations with SDO and RHESSI provide spatially resolved information about hot plasma and energetic particles in flares. RHESSI allows the properties of both hot (> 8 MK) thermal plasma and nonthermal electron distributions to be inferred, while SDO/AIA is more sensitive to lower temperatures. We present and implement a new method to reconstruct electron distribution functions from SDO/AIA data. The combined analysis of RHESSI and AIA data allows the electron distribution function to be inferred over the broad energy range from ~0.1 keV up to a few tens of keV. The analysis of two well observed flares suggests that the distributions in general agree to within a factor of three when the RHESSI values are extrapolated into the intermediate range 1-3 keV, with AIA systematically predicting lower electron distributions. Possible instrumental and numerical effects, as well as potential physical origins for this discrepancy are discussed. The inferred electron distribution functions in general show one or two nearly Maxwellian components at energies below ~ 15 keV and a non-thermal tail above.

Authors: Marina Battaglia & Eduard P. Kontar
Projects: RHESSI,SDO-AIA

Publication Status: ApJ, accepted
Last Modified: 2013-10-16 12:44
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RHESSI and SDO/AIA observations of the chromospheric and coronal plasma parameters during a solar flare  

Marina Battaglia   Submitted: 2012-10-15 01:51

X-ray and EUV observations are an important diagnostic of various plasma parameters of the solar atmosphere during solar flares. Soft X-ray and EUV observations often show coronal sources near the top of flaring loops, while hard X-ray emission is mostly observed from chromospheric footpoints. Combining RHESSI with simultaneous SDO/AIA observations, it is possible for the first time to determine the density, temperature, and emission profile of the solar atmosphere over a wide range of heights during a flare, using two independent methods. Here we analyze a near limb event during the first of three hard X-ray peaks. The emission measure, temperature, and density of the coronal source is found using soft X-ray RHESSI images while the chromospheric density is determined using RHESSI visibility analysis of the hard X-ray footpoints. A regularized inversion technique is applied to AIA images of the flare to find the differential emission measure (DEM). Using DEM maps we determine the emission and temperature structure of the loop, as well as the density, and compare it with RHESSI results. The soft X-ray and hard X-ray sources are spatially coincident with the top and bottom of the EUV loop, but the bulk of the EUV emission originates from a region without co-spatial RHESSI emission. The temperature analysis along the loop indicates that the hottest plasma is found near the coronal loop top source. The EUV observations suggest that the density in the loop legs increases with increasing height while the temperature remains constant within uncertainties.

Authors: Marina Battaglia, Eduard P. Kontar
Projects: RHESSI,SDO-AIA

Publication Status: ApJ, accepted
Last Modified: 2012-10-15 12:44
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Numerical simulations of chromospheric hard X-ray source sizes in solar flares  

Marina Battaglia   Submitted: 2012-04-06 04:29

X-ray observations are a powerful diagnostic tool for transport, acceleration, and heating of electrons in solar flares. Height and size measurements of X-ray footpoints sources can be used to determine the chromospheric density and constrain the parameters of magnetic field convergence and electron pitch-angle evolution. We investigate the influence of the chromospheric density, magnetic mirroring and collisional pitch-angle scattering on the size of X-ray sources. The time-independent Fokker-Planck equation for electron transport is solved numerically and analytically to find the electron distribution as a function of height above the photosphere. From this distribution, the expected X-ray flux as a function of height, its peak height and full width at half maximum are calculated and compared with RHESSI observations. A purely instrumental explanation for the observed source size was ruled out by using simulated RHESSI images. We find that magnetic mirroring and collisional pitch-angle scattering tend to change the electron flux such that electrons are stopped higher in the atmosphere compared with the simple case with collisional energy loss only. However, the resulting X-ray flux is dominated by the density structure in the chromosphere and only marginal increases in source width are found. Very high loop densities (>1011 cm-3) could explain the observed sizes at higher energies, but are unrealistic and would result in no footpoint emission below about 40 keV, contrary to observations. We conclude that within a monolithic density model the vertical sizes are given mostly by the density scale-height and are predicted smaller than the RHESSI results show.

Authors: M. Battaglia, E. P. Kontar, L. Fletcher, A. L. MacKinnon
Projects: RHESSI

Publication Status: ApJ, accepted
Last Modified: 2012-04-09 08:27
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Height structure of X-ray, EUV and white-light emission in a solar flare  

Marina Battaglia   Submitted: 2011-07-21 03:56

Context. The bulk of solar flare emission originates from very compact sources located in the lower solar atmosphere and observable at a broad range of wavelengths such as near optical, UV, EUV, soft and hard X-rays, and gamma-rays. Nevertheless, very few spatially resolved imaging observations have been performed to determine the structure of these compact regions.
Aims: We investigate the above-the-photosphere heights of hard X-ray (HXR), EUV, and white-light (6173 Å) continuum sources in the low atmosphere and the corresponding densities at these heights. By considering the collisional transport of solar energetic electrons, we also determine where and how much energy is deposited and compare these values with the emissions observed in HXR, EUV, and the continuum.
Methods: Simultaneous EUV/continuum images from AIA/HMI on-board SDO and HXR RHESSI images are compared to study a well-observed gamma-ray limb flare. Using RHESSI X-ray visibilities, we determine the height of the HXR sources as a function of energy above the photosphere. Co-aligning AIA/SDO and HMI/SDO images with RHESSI, we infer, for the first time, the heights and characteristic densities of HXR, EUV, and continuum (white-light) sources in the flaring footpoint of the magnetic loop.
Results: We find 35-100 keV HXR sources at heights of between 1.7 and 0.8 Mm above the photosphere, below the 6173 Å continuum emission that appears at heights 1.5-3 Mm and the peak of EUV emission originating near 3 Mm.
Conclusions: The EUV emission locations are consistent with energy deposition from low energy electrons of ~12 keV occurring in the top layers of the fully ionized chromosphere/low corona and not by ≳ 20 keV electrons that produce HXR footpoints in the lower neutral chromosphere. The maximum of white-light continuum emission appears between the HXR and EUV emission, presumably in the transition between ionized and neutral atmospheres, implying that it consists of free-bound and free-free continuum emission. We note that the energy deposited by low energy electrons is sufficient to explain the energetics of both the optical and UV emissions.

Two movies are available in electronic form at http://www.aanda.org


Authors: Marina Battaglia, Eduard P. Kontar
Projects: RHESSI,SDO-AIA,SDO-HMI

Publication Status: Astronomy & Astrophysics Letters, accepted
Last Modified: 2011-07-21 08:42
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Hard X-ray footpoint sizes and positions as diagnostics of flare accelerated energetic electrons in the low solar atmosphere  

Marina Battaglia   Submitted: 2011-04-17 23:37

The hard X-ray (HXR) emission in solar flares comes almost exclusively from avery small part of the flaring region, the footpoints of magnetic loops. UsingRHESSI observations of solar flare footpoints, we determine the radialpositions and sizes of footpoints as a function of energy in six near-limbevents to investigate the transport of flare accelerated electrons and theproperties of the chromosphere. HXR visibility forward fitting allows to findthe positions/heights and the sizes of HXR footpoints along and perpendicularto the magnetic field of the flaring loop at different energies in the HXRrange. We show that in half of the analyzed events, a clear trend of decreasingheight of the sources with energy is found. Assuming collisional thick-targettransport, HXR sources are located between 600 and 1200 km above thephotosphere for photon energies between 120 and 25 keV respectively. In theother events, the position as a function of energy is constant within theuncertainties. The vertical sizes (along the path of electron propagation)range from 1.3 to 8 arcseconds which is up to a factor 4 larger than predictedby the thick-target model even in events where the positions/heights of HXRsources are consistent with the collisional thick-target model. Magneticmirroring, collisional pitch angle scattering and X-ray albedo are discussed aspotential explanations of the findings.

Authors: Marina Battaglia, Eduard P. Kontar
Projects: RHESSI

Publication Status: ApJ, accepted
Last Modified: 2011-04-18 00:43
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The influence of albedo on the size of hard X-ray flare sources  

Marina Battaglia   Submitted: 2010-10-27 05:33

Context: Hard X-rays from solar flares are an important diagnostic of particle acceleration and transport in the solar atmosphere. Any observed X-ray flux from on-disc sources is composed of direct emission plus Compton backscattered photons (albedo). This affects both the observed spectra and images as well as the physical quantities derived from them such as the spatial and spectral distributions of accelerated electrons or characteristics of the solar atmosphere. Aims: We propose a new indirect method to measure albedo and to infer the directivity of X-rays in imaging using RHESSI data. Methods: Visibility forward fitting is used to determine the size of a disc event observed by RHESSI as a function of energy. This is compared to the sizes of simulated sources from a Monte Carlo simulation code of photon transport in the chromosphere for different degrees of downward directivity and true source sizes to find limits on the true source size and the directivity. Results: The observed full width half maximum of the source varies in size between 7.4 arcsec and 9.1 arcsec with the maximum between 30 and 40 keV. Such behaviour is expected in the presence of albedo and is found in the simulations. A source size smaller than 6 arcsec is improbable for modest directivities and the true source size is likely to be around 7 arcsec for small directivities. Conclusions: While it is difficult to image the albedo patch directly, the effect of backscattered photons on the observed source size can be estimated. The increase in source size caused by albedo has to be accounted for when computing physical quantities that include the size as a parameter such as flare energetics. At the same time, the study of the albedo signature provides vital information about the directivity of X-rays and related electrons.

Authors: M. Battaglia, E. P. Kontar, I. G. Hannah
Projects: RHESSI

Publication Status: A&A (accepted)
Last Modified: 2010-10-28 05:49
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Do solar decimetric spikes originate in coronal X-ray sources?  

Marina Battaglia   Submitted: 2009-04-27 04:40

Context: In the standard solar flare scenario, a large number of particles are accelerated in the corona. Nonthermal electrons emit both X-rays and radio waves. Thus, correlated signatures of the acceleration process are predicted at both wavelengths, coinciding either close to the footpoints of a magnetic loop or near the coronal X-ray source. Aims: We attempt to study the spatial connection between coronal X-ray emission and decimetric radio spikes to determine the site and geometry of the acceleration process. Methods: The positions of radio-spike sources and coronal X-ray sources are determined and analyzed in a well-observed limb event. Radio spikes are identified in observations from the Phoenix-2 spectrometer. Data from the Nancay radioheliograph are used to determine the position of the radio spikes. RHESSI images in soft and hard X-ray wavelengths are used to determine the X-ray flare geometry. Those observations are complemented by images from GOES/SXI. Results: We find that the radio emission originates at altitudes much higher than the coronal X-ray source, having an offset from the coronal X-ray source amounting to 90 arcsec and to 113 arcsec and 131 arcsec from the two footpoints, averaged over time and frequency. Conclusions: Decimetric spikes do not originate from coronal X-ray flare sources contrary to previous expectations. However, the observations suggest a causal link between the coronal X-ray source, related to the major energy release site, and simultaneous activity in the higher corona.

Authors: Marina Battaglia and Arnold O. Benz
Projects: RHESSI

Publication Status: A&AL, accepted
Last Modified: 2009-04-27 10:14
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Observations of conduction driven evaporation in the early rise phase of solar flares  

Marina Battaglia   Submitted: 2009-03-16 07:19

The classical flare picture features a beam of electrons, which were accelerated in a site in the corona, hitting the chromosphere. The electrons are stopped in the dense chromospheric plasma, emitting bremsstrahlung in hard X-rays. The ambient material is heated by the deposited energy and expands into the magnetic flare loops, a process termed chromospheric evaporation. In this view hard X-ray emission from the chromosphere is succeeded by soft-X-ray emission from the hot plasma in the flare loop, the soft X-ray emission being a direct consequence of the impact of the non-thermal particle beam. However, observations of events exist in which a pronounced increase in soft X-ray emission is observed minutes before the onset of the hard X-ray emission. Such pre-flare emission clearly contradicts the classical flare picture. For the first time, the pre-flare phase of such solar flares is studied in detail. The aim is to understand the early rise phase of these events. We want to explain the time evolution of the observed emission by means of alternative energy transport mechanisms such as heat conduction. RHESSI events displaying pronounced pre-flare emission were analyzed in imaging and spectroscopy. The time evolution of images and full sun spectra was investigated and compared to the theoretical expectations from conduction driven chromospheric evaporation. The pre-flare phase is characterized by purely thermal emission from a coronal source with increasing emission measure and density. After this earliest phase, a small non-thermal tail to higher energies appears in the spectra, becoming more and more pronounced. However, images still only display one X-ray source, implying that this non-thermal emission is coronal. The increase of emission measure and density indicates that material is added to the coronal region. The most plausible origin is evaporated material from the chromosphere. Energy provided by a heat flux is capable of driving chromospheric evaporation. We show that the often used classical Spitzer treatment of the conductive flux is not applicable. The conductive flux is saturated. During the preflare-phase, the temperature of the coronal source remains constant or increases. Continuous heating in the corona is necessary to explain this observation. The observations of the pre-flare phase of four solar flares are consistent with chromospheric evaporation driven by a saturated heat flux. Additionally, continuous heating in the corona is necessary to sustain the observed temperature.

Authors: Battaglia, M., Fletcher, L., Benz, A. O.
Projects: RHESSI

Publication Status: A&A, accepted
Last Modified: 2009-03-16 11:54
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Subject will be restored when possible  

Marina Battaglia   Submitted: 2008-02-05 05:55

Context: The common flare scenario comprises an acceleration site in the corona and particle transport to the chromosphere. With the currently available satellites it has become possible to disentangle the two processes of acceleration and transport and study the particle propagation in flare loops in detail, as well as compare them to theory. Aims: The goal of this work is a quantitative comparison of flare hard X-ray spectra observed with RHESSI to theoretical predictions. This allows to distinguish acceleration from transport and to explore the nature of transport effects. Methods: Data from the RHESSI satellite have been used both in full sun spectroscopy as well as in imaging spectroscopy. Coronal source and footpoint spectra of well observed limb events were analyzed and quantitatively compared to theoretical predictions. New concepts are introduced to existing models in order to solve the discrepancy between the observations and predictions. Results: The standard thin-thick target solar flare model cannot explain the observations in all events. In the here presented events, propagation effects in the form of non-collisional energy loss are of importance to explain the observations. We show that those energy losses can be interpreted by an electric field in the flare loop. One event suggests particle propagation or acceleration in lower than average density in the coronal source. Conclusions: We find observational evidence for an electric field in flare loops caused by return currents.

Authors: Marina Battaglia, Arnold O. Benz
Projects: RHESSI

Publication Status: submitted to A&A
Last Modified: 2008-02-05 09:59
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Subject will be restored when possible  

Marina Battaglia   Submitted: 2008-02-05 05:55

Context: The common flare scenario comprises an acceleration site in the corona and particle transport to the chromosphere. With the currently available satellites it has become possible to disentangle the two processes of acceleration and transport and study the particle propagation in flare loops in detail, as well as compare them to theory. Aims: The goal of this work is a quantitative comparison of flare hard X-ray spectra observed with RHESSI to theoretical predictions. This allows to distinguish acceleration from transport and to explore the nature of transport effects. Methods: Data from the RHESSI satellite have been used both in full sun spectroscopy as well as in imaging spectroscopy. Coronal source and footpoint spectra of well observed limb events were analyzed and quantitatively compared to theoretical predictions. New concepts are introduced to existing models in order to solve the discrepancy between the observations and predictions. Results: The standard thin-thick target solar flare model cannot explain the observations in all events. In the here presented events, propagation effects in the form of non-collisional energy loss are of importance to explain the observations. We show that those energy losses can be interpreted by an electric field in the flare loop. One event suggests particle propagation or acceleration in lower than average density in the coronal source. Conclusions: We find observational evidence for an electric field in flare loops caused by return currents.

Authors: Marina Battaglia, Arnold O. Benz
Projects: RHESSI

Publication Status: A&A, in press
Last Modified: 2008-06-11 00:47
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Observational evidence for return currents in solar flare loops  

Marina Battaglia   Submitted: 2008-02-05 05:55

Context: The common flare scenario comprises an acceleration site in the corona and particle transport to the chromosphere. Using satellites available to date it has become possible to distinguish between the two processes of acceleration and transport, and study the particle propagation in flare loops in detail, as well as complete comparisons with theoretical predictions. Aims: We complete a quantitative comparison between flare hard X-ray spectra observed by RHESSI and theoretical predictions. This enables acceleration to be distinguished from transport and the nature of transport effects to be explored. Methods: Data acquired by the RHESSI satellite were analyzed using full sun spectroscopy as well as imaging spectroscopy methods. Coronal source and footpoint spectra of well observed limb events were analyzed and quantitatively compared to theoretical predictions. New concepts are introduced to existing models to resolve discrepancies between observations and predictions. Results: The standard thin-thick target solar flare model cannot explain the observations of all events. In the events presented here, propagation effects in the form of non-collisional energy loss are of importance to explain the observations. We demonstrate that those energy losses can be interpreted in terms of an electric field in the flare loop. One event seems consistent with particle propagation or acceleration in lower than average density in the coronal source. Conclusions: We find observational evidence for an electric field in flare loops caused by return currents.

Authors: Marina Battaglia, Arnold O. Benz
Projects: RHESSI

Publication Status: A&A, in press
Last Modified: 2008-06-11 00:48
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Exploring the connection between coronal and footpoint sources in a thin-thick target solar flare model  

Marina Battaglia   Submitted: 2007-01-26 02:54

Context: Hard X-ray emission of coronal sources in solar flares has been observed and studied since its discovery in Yohkoh observations. Several models have been proposed to explain the physical mechanisms causing this emission and the relations between those sources and simultaneously observed footpoint sources. Aims: We investigate and test one of the models (intermediate thin-thick target model) developed on the basis of Yohkoh observations. The model makes precise predictions on the shape of coronal and footpoint spectra and the relations between them, that can be tested with new instruments such as RHESSI. Methods: RHESSI observations of well observed events are studied in imaging and spectroscopy and compared to the predictions from the intermediate thin-thick target model. Results: The results indicate that such a simple model cannot account for the observed relations between the non-thermal spectra of coronal and footpoint sources. Including non-collisional energy loss of the electrons in the flare loop due to an electric field can solve most of the inconsistencies.

Authors: Marina Battaglia & Arnold. O. Benz
Projects: RHESSI

Publication Status: A&A, accepted
Last Modified: 2007-02-07 01:42
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Relations between concurrent hard X-ray sources in solar flares  

Marina Battaglia   Submitted: 2006-06-15 03:59

Solar flares release a large fraction of their energy into non-thermal electrons, but it is not clear where and how. Bremsstrahlung X-rays are observed from the corona (coronal or looptop source) and chromosphere (footpoints). The spectral time evolution of the sources and the relations between them reflect the geometry and constrict the configuration of the flare. We studied solar flares of GOES class larger than M1 with three hard X-ray sources observed simultaneously in the course of the flare. The events where selected from observations with the X-ray satellite RHESSI from February 2002 until July 2005. We used imaging spectroscopy methods to determine the spectral time evolution of each source in each event. The images of all of the five events show two sources visible only at high energies (footpoints) and one source only visible at low energies (coronal source). We find soft-hard-soft behavior in both, coronal source and footpoints. This is a strong indication, that soft-hard-soft is a feature of the acceleration mechanism rather than a transport effect. The coronal source is nearly always softer than the footpoints. The footpoint spectra differ significantly only in one event out of five.

Authors: Marina Battaglia & Arnold O. Benz
Projects: RHESSI

Publication Status: A&A (in press)
Last Modified: 2006-06-16 11:07
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Size dependence of solar X-ray flare properties  

Marina Battaglia   Submitted: 2005-05-09 06:35

Non-thermal and thermal parameters of 85 solar flares of GOES class B1 to M6 (background subtracted classes A1 to M6) have been compared to each other. The hard X-ray flux has been measured by RHESSI and a spectral fitting provided flux and spectral index of the non-thermal emission, as well as temperature and emission measure of the thermal emission. The soft X-ray flux was taken from GOES measurements. We find a linear correlation in a double logarithmic plot between the non-thermal flux and the spectral index. The higher the acceleration rate of a flare, the harder the non-thermal electron distribution. The relation is similar to the one found by a comparison of the same parameters from several sub-peaks of a single flare. Thus small flares behave like small subpeaks of large flares. Thermal flare properties such as temperature, emission measure and the soft X-ray flux also correlate with peak non-thermal flux. A large non-thermal peak flux entails an enhancement in both thermal parameters. The relation between spectral index and the non-thermal flux is an intrinsic feature of the particle acceleration process, depending on flare size. This property affects the reported frequency distribution of flare energies.

Authors: Marina Battaglia, Paolo C. Grigis and Arnold O. Benz
Projects: RHESSI

Publication Status: A&A (in press)
Last Modified: 2005-05-09 06:35
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Abstracts by Author
A Compressed Sensing-based Image Reconstruction Algorithm for Solar Flare X-Ray Observations
The Solar X-ray Limb
Multi-thermal representation of the kappa-distribution of solar flare electrons and application to simultaneous X-ray and EUV observations
How important are electron beams in driving chromospheric evaporation in the 2014 March 29 flare?
Where is the chromospheric response to conductive energy input from a hot pre-flare coronal loop?
Electron Distribution Functions in Solar Flares from combined X-ray and EUV Observations
RHESSI and SDO/AIA observations of the chromospheric and coronal plasma parameters during a solar flare
Numerical simulations of chromospheric hard X-ray source sizes in solar flares
Height structure of X-ray, EUV and white-light emission in a solar flare
Hard X-ray footpoint sizes and positions as diagnostics of flare accelerated energetic electrons in the low solar atmosphere
The influence of albedo on the size of hard X-ray flare sources
Do solar decimetric spikes originate in coronal X-ray sources?
Observations of conduction driven evaporation in the early rise phase of solar flares
Subject will be restored when possible
Subject will be restored when possible
Observational evidence for return currents in solar flare loops
Exploring the connection between coronal and footpoint sources in a thin-thick target solar flare model
Relations between concurrent hard X-ray sources in solar flares
Size dependence of solar X-ray flare properties

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

Archive Maintainer
Alisdair Davey



© 2003 Solar Physics Group - Montana State University