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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
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

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