The YOHKOH survey of partially occulted flares in hard X-rays

Abstract

Context. Modern solar X-ray imagers do not completely resolve the problem of deriving detailed diagnostics of faint hard X-ray sources in the presence of stronger ones. This is the case for the impulsive phase of solar flares in which footpoint sources are usually stronger man loop-top ones. Alms. Hares mat are partially occulted by the solar limb provide the most hopeful source of knowledge about hard X-ray loop-top sources. This work attempts to fill the gap between the published survey of partially occulted flares observed by RHESSI (Krucker & Lin 2008, ApJ, 673, 1181) and the extensive Yohkoh database. Methods. Among the 1286 flares in the Yohkoh Hard X-ray Telescope Flare Catalogue (Sato et al. 2006, Sol. Phys., 236, 351), for which the hard X-ray images were presented, we identified 98 events that occurred behind the solar limb. We investigated their hard X-ray spectra and spatial structure. Results. In most cases, we found that the hard X-ray spectrum of partially occulted flares consists of two components, non-thermal and thermal, which are cospatial to within 4 arcsec. For rest events, the components are separated, the non-thermal component clearly appearing to be situated higher. The photon energy spectra of the partially occulted flares are systematically steeper than spectra of the non-occulted flares. We can explain mis difference as a consequence of intrinsically dissimilar conditions in coronal parts of flares, in comparison with the footpoints that usually dominate the hard X-ray emission of disk flares. At least two reasons for the difference should be taken into consideration: (1) stronger contamination of hard X-rays by emission from thermal plasma; and (2) different mechanisms in which non-thermal electrons radiate their energy. For events unbiased by the thermal component, the difference, Δγ = -γ̄ - γ̄FP, equals 1.5. We found a lack of correlation between the altitude of flares and the hard X-ray power-law index γ. Conclusions. A schematic picture, in which a thin-target mechanism is responsible for the hard X-ray emission of loop-top sources and a thick-target mechanism for emission by footpoint sources, is modified by the presence of some coronal thick-target sources. Some of these sources exhibit evidence of magnetic trapping. For the characteristics of flares is conclusive the local magnetic configuration in which they occur. ©ESO 2009.