INTERPRETING YOHKOH HARD AND SOFT-X-RAY FLARE OBSERVATIONS

A simple model is presented to account for the Yohkoh Rare observation s of Feldman et al. (1994), and Masuda (1994). Electrons accelerated b y the flare are assumed to encounter the dense, small regions observed by Feldman et al, at the tops of impulsively flaring coronal magnetic loops. The values of electron density and volume inferred by Feldman et al. imply that these dense regions present an intermediate thick-th in target to the energised electrons. Specifically, they present a thi ck (thin) target to electrons with energy much less (greater) than E(c ) where 15 keV < E(c) < 40 keV. The electrons are either stopped at th e loop top or precipitate down the field lines of the loop to the foot points. Collisional losses of the electrons at the loop top produce th e heating observed by Feldman et al. and also some hard X-rays. It is argued that this is the mechanism for the loop-top hard X-ray sources observed in limb flares by Masuda. Adopting a simple model for the ene rgy losses of electrons traversing the dense region and the ambient lo op plasma, hard X-ray spectra are derived for the loop-top source, the footpoint sources and the region between the loop top and footpoints. These spectra are compared with the observations of Masuda. The model spectra are found to qualitatively agree with the data, and in partic ular account for the observed steepening of the loop-top and footpoint spectra between 14 and 53 keV and the relative brightnesses of the lo op-top and footpoint sources.