Microwave and Hard X-Ray Observations of Footpoint Emission from Solar Flares

Abstract

We investigate radio and X-ray imaging data for two solar flares in order to test the idea that asymmetric precipitation of nonthermal electrons at the two ends of a magnetic loop is consistent with the magnetic mirroring explanation. The events we present were observed in 1993 May by the HXT and SXT X-ray telescopes on the Yohkoh spacecraft and by the Nobeyama 17 GHz radioheliograph. The hard X-ray images in one case show two well-separated sources; the radio images indicate circularly polarized, nonthermal radio emission with opposite polarities from these two sources, indicating oppositely directed fields and consistent with a single-loop model. In the second event there are several sources in the HXT images which appear to be connected by soft X-ray loops. The strongest hard X-ray source has unpolarized radio emission, whereas the strongest radio emission lies over strong magnetic fields and is polarized. In both events the strongest radio emission is highly polarized and not coincident with the strongest hard X-ray emission. This is consistent with asymmetric loops in which the bulk of the precipitation (and hence the X-ray emission) occurs at the weaker field footpoint.