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Positions and sizes of X-ray solar flare sources  

Eduard P. Kontar   Submitted: 2010-04-16 07:20

Aims. The positions and source sizes of X-ray sources taking into account Compton backscattering (albedo) are investigated. Methods. Using a Monte Carlo simulation of X-ray photon transport including photo-electric absorption and Compton scattering, we calculate the apparent source sizes and positions of X-ray sources at the solar disk for various source sizes, spectral indices and directivities of the primary source. Results. We show that the albedo effect can alter the true source positions and substantially increase the measured source sizes. The source positions are shifted by up to ~0.5? radially towards the disk centre and 5 arcsec source sizes can be two times larger even for an isotropic source (minimum albedo effect) at 1 Mm above the photosphere. The X-ray sources therefore should have minimum observed sizes, and thus their FWHM source size (2.35 times second-moment) will be as large as ~7? in the 20-50 keV range for a disk-centered point source at a height of 1 Mm (~1.4?) above the photosphere. The source size and position change is greater for flatter primary X-ray spectra, a stronger downward anisotropy, for sources closer to the solar disk centre, and between the energies of 30 and 50 keV. Conclusions. Albedo should be taken into account when X-ray footpoint positions, footpoint motions or source sizes from e.g. RHESSI or Yohkoh data are interpreted, and we suggest that footpoint sources should be larger in X-rays than in either optical or EUV ranges.

Authors: E. P. Kontar and N. L. S. Jeffrey
Projects: None,RHESSI,Yohkoh-HXT

Publication Status: A&A Letters in press
Last Modified: 2010-04-16 14:33
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Electron-Electron Bremsstrahlung Emission and the Inference of Electron Flux Spectra in Solar Flares  

Eduard P. Kontar   Submitted: 2007-08-02 07:13

Although both electron-ion and electron-electron bremsstrahlung contribute to the hard X-ray emission from solar flares, the latter is normally ignored. Such an omission is not justified at electron (and photon) energies above sim 300 keV, and inclusion of the additional electron-electron bremsstrahlung in general makes the electron spectrum required to produce a given hard X-ray spectrum steeper at high energies. Unlike electron-ion bremsstrahlung, electron-electron bremsstrahlung cannot produce photons of all energies up to the maximum electron energy involved. The maximum possible photon energy depends on the angle between the direction of the emitting electron and the emitted photon, and this suggests a diagnostic for an upper cutoff energy and/or for the degree of beaming of the accelerated electrons. We analyze the large event of January 17, 2005 observed by RHESSI and show that the upward break around 400 keV in the observed hard X-ray spectrum is naturally accounted for by the inclusion of electron-electron bremsstrahlung. Indeed, the mean source electron spectrum recovered through a regularized inversion of the hard X-ray spectrum, using a cross-section that includes both electron-ion and electron-electron terms, has a relatively constant spectral index delta over the range from electron kinetic energy E = 200 keV to E = 1 MeV. However, the level of detail discernible in the recovered electron spectrum is not sufficient to determine whether or not any upper cutoff energy exists.

Authors: Kontar, Eduard P., Emslie, A. Gordon, Massone, Anna Maria, Piana, Michele, Brown, John C., and Prato, Marco
Projects: RHESSI

Publication Status: Submitted to Astrophysical Journal
Last Modified: 2007-08-02 11:11
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Positions and sizes of X-ray solar flare sources
Electron-Electron Bremsstrahlung Emission and the Inference of Electron Flux Spectra in Solar Flares

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