Comparison of thermal and nonthermal hard X-ray emission in electron-heated solar flares

Abstract

Using the results of numerical simulations of the solar atmospheric response to heating by nonthermal electron beams during solar flares, we have calculated the spatial and temporal evolution of both (i) the direct (beam-target) nonthermal bremsstrahlung and (ii) the thermal bremsstrahlung arising from the hot plasma energized by the electron beam. Typically, we find that below a certain ‘cross-over’ energy E *, the emission is dominated by the thermal component, while at higher energies the direct bremsstrahlung component becomes more important. This ‘cross-over’ energy is dependent on the position within the loop, generally increasing with height. We have also investigated the dependence of the ‘cross-over’ energy E * on the parameters of the electron energy input. At the time of peak electron flux injection the ‘cross-over’ energy E * can, for plausible parameters, be as high as 52 keV at the top 1″ pixel, and as low as 16 keV at the bottom 1″ pixel. We conclude that a possible reassessment of SMM HXIS data as an indicator of the ‘thermal’ or ‘nonthermal’ character of the primary energy release (based primarily on the geometric properties of the hard X-ray source) is required. Our results also point to the minimum photon energy that future instruments should observe (where practical, giving due consideration to detector sensitivity) in order to be sure that, in the context of the thick-target interpretation, the nonthermal component is not swamped by the self-consistent thermal counterpart created by the beam heating.