EDP Sciences
Free access
Volume 366, Number 1, January IV 2001
Page(s) 294 - 305
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:20000204

A&A 366, 294-305 (2001)
DOI: 10.1051/0004-6361:20000204

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

M. Tomczak

Astronomical Institute, Wroclaw University, Kopernika 11, 51-622 Wroclaw, Poland

(Received 30 May 2000 / Accepted 26 October 2000)

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 (1994), who actually separated two phases of evolution of the same loop-top source. The model of the turbulent flare kernel (Jakimiec 1998; Jakimiec et al. 1998) has been adapted to explain the obtained characteristics of loop-top HXR emission sources of type A.

Key words: Sun: corona -- flares -- X -- rays, gamma-rays

© ESO 2001