On the hard X-ray spatial structure during the impulsive phase of solar flares

A simplified form of the bremsstrahlung cross-section is used to obtain an analytic expression for the intensity of electron-beam-produced hard X-ray emission with depth in solar flares which can be used in a first-order analysis of imaging data. The results indicate that the conditions for the appearance of bright footpoint emission, in terms of loop parameters such as density and length, are much less restrictive than previously suggested. The analysis shows that the observed footpoint structure of many flares in hard X-rays is consistent with the thick-target bombardment model, and that the intensity of the footpoint emission relative to the spatially integrated flux can be used as a diagnostic tool of coronal column density.