Hard X-rays of solar flares — thermal versus non-thermal models: a review

Abstract

The experimental results of X-ray bursts about spectral characteristics, spatial distribution, fast time variations and directivity measurements carried out with PVO/ISEE-3 spacecraft, imaging instrument observations of hard X-ray (HXIS) and hard X-ray burst spectrometer (HXRBS) during solar maximum mission have been reviewed. The observed results about the above characteristics are discussed in detail in terms of thermal and non-thermal models. The results are interpreted in terms of beamed thick target model in which electrons stream down to the loop footpoints and produce hard X-rays through electron-ion bremsstrahlung.