IMAGING SPECTRA OF HARD X-RAYS FROM THE FOOTPOINTS OF SOLAR IMPULSIVELOOP FLARES

The imaging spectra of hard X-rays emitted from one or both footpoints of solar impulsive loop hares seem to frequently show a better fit to an extremely hot thermal spectrum, on the order of 10(8) K, than to a power-law spectrum, in the main phase of the bursts. In the present s tudy this tendency has been verified by the spectra of nine impulsive bursts having a sufficient flux for reliable imaging in the II-band (5 2.7-92.8 keV) of the hard X-ray telescope (HXT) aboard the Yohkoh spac ecraft. The spectral indexes at the footpoints were derived from the h ard X-ray images in four energy bands of the HXT; their reliability wa s verified by mapping simulations. Since the X-ray spectrum depends on the location in the X-ray loop, the spectrum of the total flux is gen erally different from the spatially resolved spectra. The imaging spec trum is thus crucial for studying the nature of the electrons emitting hard X-rays. The heat conduction of an extremely hot quasi-thermal pl asma seems to play an essential role in impulsive loop flares.