HARD X-RAY SOURCES AND THE PRIMARY ENERGY-RELEASE SITE IN SOLAR-FLARES

Accurately coaligned hard and soft X-ray images, taken simultaneously with the Hard X-ray Telescope (HXT) and the Soft X-ray Telescope (SXT) aboard Yohkoh, of impulsive solar flares on 1992 January 13 (17:29 UT ), 1992 October 4 (22:21 UT), and 1993 February 17 (10:35 UT), occurri ng near the limb,clearly reveal that, in addition to double-footpoint sources, a hard X-ray source exists well above the corresponding soft X-ray loop structure around the peak time of the impulsive phase. This hard X-ray source shows an intensity variation similar to double-foot point sources and a spectrum that is relatively hard compared with tha t of loop-top gradual source which appeared later in the flare. We bel ieve that this is the first clear evidence that magnetic reconnection, which is responsible for the primary flare energy release, is under p rogress above the soft X-ray flaring loop. May-be this 'loop-top'' har d X-ray source represents the reconnection site itself or the site whe re the downward plasma stream, ejected from the reconnection point far above the hard X-ray source, collides with the underlying closed magn etic loop. The characteristics of this hard X-ray source are quantitat ively discussed in the schemes of thermal (T greater than or similar t o 10(8) K) and nonthermal interpretations of hard X-ray emission.