On the spatial distribution of hard X-rays from solar flare loops

The aim of this paper is to investigate the spatial structure of the impulsive phase hard X-ray emission from solar flares. This work is motivated by the Pohkoh and the forthcoming High-Energy Solar Spectroscopic Imager (HESSI) observations. Summarizing past results, it is shown that the transport effects can account for the observations in inhomogeneous loop models where there is a strong held convergence and/or density enhancement at the top of the flaring loop. Scattering by plasma turbulence at the acceleration site or a pancake-type pitch-angle distribution of the accelerated electrons can also give rise to enhanced emission at the looptops. These could be a natural consequence of acceleration by plasma waves. This paper considers a general case of stochastic scattering and acceleration that leads to an isotropic pitch-angle distribution and an enhanced emission from the looptops or the acceleration site. Following the formalism developed in earlier papers, the strength and the spectrum of the radiation expected from the acceleration site and the footpoints are evaluated and their dependence on the parameters describing the acceleration process and the hare plasma are determined. The theoretical ratio of these two intensities and relative values of their spectral indices are compared with Yohkoh observations, demonstrating that the above-mentioned parameters can be constrained with such observations. It is shown that future high spatial and spectral resolution observations, for example those expected from HESSI, will be able to begin to distinguish between different models and constrain their parameters.