Numerical simulations of chromospheric hard X-ray source sizesin solar flares

Battaglia, M., Kontar, E.P. , Fletcher, L. and MacKinnon, A.L. (2012) Numerical simulations of chromospheric hard X-ray source sizesin solar flares. Astrophysical Journal, 752(1), (doi: 10.1088/0004-637X/752/1/4)

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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 footpoint 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.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Fletcher, Professor Lyndsay and Battaglia, Dr Marina and Kontar, Professor Eduard and MacKinnon, Dr Alexander
Authors: Battaglia, M., Kontar, E.P., Fletcher, L., and MacKinnon, A.L.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Astrophysical Journal
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