Spectral characteristics of above-the-looptop hard X-ray source Original Research Article

It is observed in impulsive flares with the Hard X-ray Telescope (HXT) onboard Yohkoh that a hard X-ray source is located above the apex of a soft X-ray flaring loop in addition to double footpoint sources (Masuda et al. 1994, 1995). This observation suggests that the flare energy-release, probably magnetic reconnection, takes place not in the soft X-ray loop, but above the loop. It is important to derive the hard X-ray spectrum of this source accurately in order to understand how electrons are energized there. Using an improved image-synthesis technique with new modulation patterns (Sato 1997), we obtained hard X-ray images of the 13 January 1992 flare in the four HXT energy bands with a common photon-accumulation time. The spectrum of the above-the-looptop source in the energy range of 14–93 keV seems to be well fit by emission from an isothermal plasma whose temperature is about 100 MK, rather than by a non-thermal, single power-law spectrum. Although this is consistent with the observational result that the density of the ambient plasma is not high enough to stop high energy electrons, we need to study other events, especially intense events from which more accurate spectra can be derived, in order to confirm this result.