Hard X-ray imaging observations of solar hot thermal flares with the HINOTORI spacecraft

Abstract

Two solar hard X-ray bursts of a new type (hot thermal flare) were observed with hard X-ray imaging telescopes and other instruments on Japanese spacecraft Hinotori. The flares have no clear impulsive phase below 40 keV and emit intense hard X-rays (10--50 keV) with extremely steep spectra from a small region with size (FWHM) of 10''--20''. This source contains a hot thermal plasma of (3--3.5) x 10/sup 7/ K with an emission measure of the order of 10/sup 49/ cm/sup -3/. One of the flares occurred just on the limb, and the centroid of the hard X-ray (14-38 keV) source was located at (6 +- 3) x 10/sup 3/ km above the photosphere. It is concluded that the energy continuously released goes into heating rather than acceleration almost throughout the flare. Typical impulsive flares may usually have a similar nature in the later phase (gradual phase) of the flare evolution. The number density of the hard X-ray emitting region was higher than (6-10) x 10/sup 10/ cm/sup -3/ even in the initial phase of the flare, probably due to its low altitude. We infer that this high density made acceleration difficult, and therefore unusually intense heating occurred from the startmore » of the flare.« less