The analysis of hard X-ray radiation of flares with occulted footpoints

Abstract

14 behind-the-limb flares with occulted footpoints, well-observed by Yohkoh, have been carefully selected. Such a flare location on the Sun offers a unique possibility to investigate loop-top hard X-ray emission sources separately, without the presence of strong footpoint sources which can disturb the observational characteristics of the loop-top sources during any process of image reconstruction. Hard X-ray radiation in all investigated flares has similar features: (1) smooth light curves with several impulsive spikes, (2) low-energy spectra (mainly below 33 keV) which can expressed in terms of temperatures within the interval of about 20-100 MK. The HXR images show two types of loop-top emission sources: type A, co-spatial with the bright loop-top kernels seen in soft X-rays, and type B, having no distinct counterpart in soft X-rays. Arguments are presented that this classification is more physical than the classification considering impulsive and gradual sources proposed by Masuda ([CITE]), who actually separated two phases of evolution of the same loop-top source. The model of the turbulent flare kernel (Jakimiec [CITE]; Jakimiec et al. [CITE]) has been adapted to explain the obtained characteristics of loop-top HXR emission sources of type A.