A Discussion on the physics of the solar atmosphere - The interpretation of hard and soft x-rays from solar flares

Abstract

The present status of observations of hard X-ray bursts is reviewed in terms of the light they shed on alternative source models and on general characteristics of electron acceleration in flares. Special attention is given to the requirements of total energy release, and the time scale of its release, into energetic electrons on the basis of the normal bremsstrahlung interpretation of bursts. It is particularly emphasized that, since these electrons may dominate the energy balance in many flares, they provide on the one hand an attractive heating mechanism for the thermal flare but, on the other, put severe demands on acceleration mechanisms. A reassessment of the relative merits of synchrotron and inverse Compton source mechanisms is suggested, along with other possibilities, as an escape from this apparent difficulty. Observational characteristics of soft X-ray flares are cursorily reviewed. The importance of a non-isothermal approach to the physics of the soft X-ray plasma is then illustrated in terms of flare energy flow. It is argued however, that high spectral resolution is not the key to this problem since ill conditioning of the problem prevents useful inference of temperature structure. Instead high resolution imaging with moderate spectral resolution is advocated.