Looptop and Footpoint Impulsive Hard X-Rays and Stochastic Electron Acceleration in Solar Flares

The discovery of hard X-rays from tops of flaring loops by the HXT of YOHKOH represents a significant progress in the understanding of solar flares. This report describes the properties of 20 limb flares observed by YOHKOH from October 1991 to August 1998, 15 of which show detectable impulsive looptop emission. Considering the finite dynamic range (about a decade) of the detection it can be concluded that looptop emission is a common feature of all flares. The light curves and images of a representative flare are presented and the statistical properties of the footpoint and looptop fluxes and spectral indexes are summarized. The importance of these observations, and those expected from RHESSI with its superior angular, spectral and temporal resolution, in constraining the acceleration models and parameters is discussed. briefly.