AbstractIn the past several years, X-ray observations of the Sun made from rockets and satellites have demonstrated the existence of high temperature (~20 × 106 – ~100 × 106 K), low density plasmas associated with solar flare phenomena. In the hard X-ray range (λ≲ 1 Å), spectra of the flaring plasma have been obtained using proportional and scintillation counter detectors. It is possible from these data to determine the evolution of the hard X-ray flare spectrum as the burst progresses; and by assuming either a non-thermal or thermal (Maxwellian) electron distribution function, characteristic plasma parameters such as emission measure and temperature (for a thermal interpretation) can be determined. Thermal interpretations of hard X-ray data require temperatures of ~100 × 106 K.Incontrasi, the soft X-ray flare spectrum (1 Å<λ<30Å) exhibits line emission from hydrogenlike and helium-like ions, e.g. Ne, Mg, Al, Si, … Fe, that indicates electron energies more characteristic of temperatures of ~20 × 106 K. Furthermore, line intensity ratios obtained during the course of an event show that the flare plasma can only be described satisfactorily by assuming a source composed of several different temperature regions; and that the emission measures and temperatures of these regions appear to change as the flare evolves. Temperatures are determined from line ratios of hydrogen-like to helium-like ions for a number of different elements, e.g., S, Si, and Mg, and from the slope of the X-ray continuum which is assumed to be due to free-free and free-bound emission. There is no obvious indication in soft X-ray flare spectra of non-thermal processes, although accurate continuum measurements are difficult with the data obtained to date because of higher order diffraction effects due to the use of crystal spectrometers.Soft X-ray flare spectra also show satellite lines of the hydrogen-like and helium-like ions, notably the 1s22s2S-1s2s2p2P transition of the lithium-like ion, and support the contention that in low density plasmas these lines are formed by dielectronic recombination to the helium-like ion. Also, series of allowed transitions of hydrogen-like and helium-like ions are strong, e.g., the Lyman series of S up to Lyman-ε, and ratios of the higher member lines to the Lyman-α line can be compared with theoretical calculations of the relative line strengths obtained by assuming various processes of line formation.This review will discuss the X-ray spectrum of solar flares from ~250 keV to ~0.4 keV, but will be primarily concerned with the soft X-ray spectrum and the interpretation of emission lines and continuum features that lie in this spectral range.
Thermal characteristics of a B8.3 flare observed on July 04, 2009