Imaging Spectra Of Hard X-Ray From The Footpoints Of Solar Impulsive Loop Flares

Abstract

The imaging spectra of hard X-rays emitted from one or both footpointsof solar impulsive loop flares seem to frequently show a better fit toan extremely hot thermal spectrum, on the order of 108 K,than to a power-law spectrum, in the main phase of the bursts. In thepresent study of tendency has been verified by the spectra of nineimpulsive bursts having a sufficient flux for reliable imaging in theH-band (52.7-92.8 keV) of the hard X-ray telescope (HXT) aboard theYohkoh spacraft. The spectral indexes at the footpoints were derivedfrom the hard X-ray images in four energy bands of the HXT; theirreliability was verified by mapping simulations. Since the X-rayspectrum depends on the location in the X-ray loop, the spectrum of thetotal flux is generally different from the spatially resolved spectra.The imaging spectrum is thus crucial for studying the nature of theelectrons emitting hard X-rays. The heat conduction of an extremely hotquasi-thermal plasma seems to play an essential role in impulsive loopflares.