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Deducing Electron Properties from Hard X-Ray ObservationsX-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.
Document ID
Document Type
Reprint (Version printed in journal)
Kontar, E. P.
(Glasgow Univ. United Kingdom)
Brown, J. C.
(Glasgow Univ. United Kingdom)
Emslie, A. G.
(Oklahoma State Univ. Stillwater, OK, United States)
Hajdas, W.
(Paul Scherrer Inst. Villigen, Switzerland)
Holman, G. D.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Hurford, G. J.
(California Univ. Berkeley, CA, United States)
Kasparova, J.
(Academy of Sciences of the Czech Republic Czech Republic)
Mallik, P. C. V.
(Glasgow Univ. United Kingdom)
Massone, A. M.
(Consiglio Nazionale delle Ricerche Genoa, Italy)
McConnell, M. L.
(New Hampshire Univ. Durham, NH, United States)
Piana, M.
Prato, M.
(Consiglio Nazionale delle Ricerche Genoa, Italy)
Schmahl, E. J.
Suarez-Garcia, E.
(Paul Scherrer Inst. Villigen, Switzerland)
Date Acquired
August 25, 2013
Publication Date
August 11, 2011
Publication Information
Publication: Space Science Reviews
Volume: 159
Issue: 4-Jan
Subject Category
Solar Physics
Report/Patent Number
Funding Number(s)
CONTRACT_GRANT: Czech Republic 205/06/P135
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