Acceleration of runaway electrons and Joule heating in solar flares

Abstract

D.C electric fields provide the simplest and most direct means of accelerating electrons out of a thermal plasma. Most solar flare models result in the production of D.C. electric fields. On the other hand, microwave and hard X-ray observations of flares provide specific requirements for the number and energy of energetic electrons produced during a flare, and the timescales involved in accelerating them. The microwave emission from flares is understood to be gyrosynchrotron radiation from electrons with energies of 100 keV or greater. The hard X-ray emission ≳ 25 keV) can be interpreted as being either thicktarget bremsstrahlung from non-thermal electrons (thin-target radiation may also contribute to the X-ray emission, but the process is less efficient), or thermal bremsstrahlung from hot, impulsively heated plasma. Hence, it is of interest to study the electric field acceleration of “runaway” electrons and the simultaneous Joule heating of the thermal plasma in light of these results from flare observations, without recourse to a specific flare model. Some of the results of such a study are summarized here.