SPECTROSCOPY OF HIGH-TEMPERATURE SOLAR-FLARE PLASMAS

The past decade has seen great improvements in the quality of X-ray spectra of solar flares obtained from spacecraft. Such spectra show lines emitted by highly ionized atoms of abundant elements which make up high-temperature plasma contained within coronal magnetic flux tubes. This plasma is probably energized at or a little before the flare impulsive stage, as revealed by bursts of hard X-rays. Temperature and density conditions can be deduced from ratios of line intensities, as well as element abundances under certain conditions. In this paper, several examples of line ratios to deduce these are given. Analysis shows that there is a wide range of electron temperatures - generally from 2 x 10(6) K to 20 x 10(6) K - though sometimes even higher. Electron densities of around 10(17)-10(18) m-3 have been derived, higher values occurring at the flare peak or just before, and then declining. The physical conditions of the hot plasma are now precisely enough known from X-ray spectroscopy that models of flares which have been constructed in the past can be constrained. The most profitable direction for research in this area in the near future would in fact appear to be for a much better linking of the findings from X-ray spectra and modelling of plasma in flux tubes to understand better the flare process in general.