A thermal/nonthermal model for solar hard X-ray bursts

A formalism is developed for analyzing high-resolution hard X-ray spectra, incorporating the coexistence of thermal and nonthermal bremsstrahlung. The two processes are physically linked by the presence of electric currents, which both heat the surrounding plasma via Joule dissipation and accelerate electrons via the runaway process. We use this formalism to analyze the flare of 1980 June 27 and find that both the gradual and spike components of the hard X-ray emission are consistent with runaway acceleration. We also find that significant heating is observed only in the gradual component. The electric field is always sub-Dreicer, the maximum total potential drop in the acceleration region is found to be approximately 100 kV in two of the spikes, and the average accelerated electron flux is approximately 10³⁴ electrons/s. We argue that classical resistivity is a valid assumption for this event and find the density in the current channels (approximately 10¹¹/cc) and a lower limit on the volume of the heated plasma. We find that the ratio of the electric field to the Dreicer field (epsilon = E/ED varies systematically, whereas the value of E alone does not. We also find that the acceleration region fragmented into many current/return current pairs, and that the fragmentation varied systematically. We also discuss further implications of this model.