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Abstract: We investigate the positions and source sizes of X-ray solar flare sourcestaking into account Compton backscattering albedo. Using a Monte Carlosimulation of X-ray photon transport including photo-electric absorption andCompton scattering, we calculate the apparent source sizes and positions ofX-ray sources at the solar disk for various source sizes, spectral indices anddirectivities of the primary source. We show that the albedo effect will alterthe true source positions and substantially increase the measured source sizes.The source positions are shifted up to $\sim 0.5-$ radially towards the diskcentre and 5 arcsecond source sizes can be two times larger even for anisotropic source minimum albedo effect at 1 Mm above the photosphere. X-raysources therefore should have minimum observed sizes, thus FWHM source size2.35 times second-moment will be as large as $\sim 7-$ in the 20-50 keV rangefor a disk-centered point source at a height of 1 Mm $\sim 1.4-$ above thephotosphere. The source size and position change is the largest for flatterprimary X-ray spectra, stronger downward anisotropy, for sources closer to thesolar disk centre, and between the energies of 30 and 50 keV. Albedo should betaken into account when X-ray footpoint positions, footpoint motions or sourcesizes from e.g. RHESSI or Yohkoh data are interpreted, and suggest thatfootpoint sources should be larger in X-rays than in optical or EUV ranges.

Author: E. P. Kontar, N. L. S. Jeffrey

Source: https://arxiv.org/


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