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X-ray observations are a powerful diagnostic tool for transport,acceleration, and heating of electrons in solar flares. Height and sizemeasurements of X-ray footpoints sources can be used to determine thechromospheric density and constrain the parameters of magnetic fieldconvergence and electron pitch-angle evolution. We investigate the influence ofthe chromospheric density, magnetic mirroring and collisional pitch-anglescattering on the size of X-ray sources. The time-independent Fokker-Planckequation for electron transport is solved numerically and analytically to findthe electron distribution as a function of height above the photosphere. Fromthis distribution, the expected X-ray flux as a function of height, its peakheight and full width at half maximum are calculated and compared with RHESSIobservations. A purely instrumental explanation for the observed source sizewas ruled out by using simulated RHESSI images. We find that magnetic mirroringand collisional pitch-angle scattering tend to change the electron flux suchthat electrons are stopped higher in the atmosphere compared with the simplecase with collisional energy loss only. However, the resulting X-ray flux isdominated by the density structure in the chromosphere and only marginalincreases in source width are found. Very high loop densities (>10^{11}cm^{-3}) could explain the observed sizes at higher energies, but areunrealistic and would result in no footpoint emission below about 40 keV,contrary to observations. We conclude that within a monolithic density modelthe vertical sizes are given mostly by the density scale-height and arepredicted smaller than the RHESSI results show.

Numerical simulations of chromospheric hard X-ray source sizes in solar flares