Hard X-Ray Imaging Observations by YOHKOH of the 1991 November 15 Solar Flare

We report on hard X-ray imaging observations of the 1991 November 15 flare with the HXT instrument aboard {Yohkoh}. Distributions of the hard X-ray sources at various stages of the flare, together with an overlay of the white-light flare, are presented. Attention is concentrated on the behavior of hard X-ray sources during the impulsive phase. The hard X-ray source appeared initially as a single source near the magnetic neutral line, then evolved into a double-source shape with the separation increasing with time. We believe that this is evidence for a multiple loop system flaring successively with a rising energy-release site. At the minima between the individual spikes of the time profile, the hard X-rays at 20--30 keV were concentrated near the apex of the flaring loop, whereas the hard X-rays above 30 keV originated from the footpoints. These observations are compared with the existing models.