YOHKOH remnants: partially occulted flares in hard X-rays

Context. Modern solar X-ray imagers did not breakthrough the problem of detailed diagnostics of faint hard X-ray sources in the presence of stronger ones. This is the case of the impulsive phase of solar flares in which footpoint sources are usually stronger than loop-top ones. Aims. For this aim, flares being partially occulted by the solar limb, are the best reservoir of our knowledge about hard X-ray loop-top sources. Recently, the survey of partially occulted flares observed by the RHESSI has been published (Krucker & Lin 2008). The extensive Yohkoh database still awaits such activities. This work is an attempt to fill this gap. Methods. Among from 1286 flares in the Yohkoh Hard X-ray Telescope Flare Catalogue (Sato et al. 2006), for which the hard X-ray images had been enclosed, we identified 98 events that occurred behind the solar limb. We investigated their hard X-ray spectra and spatial structure. Results. We found that in most cases the hard X-ray spectrum of partially occulted flares consists of two components, non-thermal and thermal, which are co-spatial. The photon energy spectra of the partially occulted flares are systematically steeper than spectra of the non-occulted flares. Such a difference we explain as a consequence of intrinsically dissimilar conditions ruling in coronal parts of flares, in comparison with the footpoints which usually dominate the hard X-ray emission of disk flares. At least two reasons of the difference should be taken into consideration: (1) stronger contamination of hard X-rays with emission of thermal plasma, (2) different mechanism in which non-thermal electrons radiate their energy. For events unbiased with the thermal component the difference, ∆γ = γ̄LT − γ̄FP, is equal to 1.5. Conclusions. A schematic picture, in which thin-target mechanism is responsible for hard X-ray emission of loop-top sources and thick-target mechanism – for emission of footpoint sources, is modified by the presence of some coronal thick-target sources. At least a part of them suggests a magnetic trapping. Investigated flares do not respond the overall (global) magnetic configuration of the solar corona. For their characteristics conclusive is rather the local magnetic configuration in which they were developed.