R. Galloway, A.L. MacKinnon, J.C. Brown
Department of Physics and Astronomy, University of Glasgow,
Glasgow G12 8QQ, U.K.
Most theoretical descriptions of the production of flare hard X-ray emission assume it to be due to dilute fast particles colliding with a cold, dense background plasma. These approaches neglect collisions between the accelerated particles themselves - this may be inadequate for situations where these fast particles are not sufficiently few in number in comparison to the background plasma, for example in low-density coronal sources. We present a model of the evolution of a population of fast electrons as a result of self-interactions, and in the absence of a dense background plasma, using a Fokker-Planck approach. We generate synthetic bremsstrahlung spectra from this population, and study their transition from initial purely power-law spectral forms to final Maxwellian spectra via self-consistent intermediate forms. We compare these model spectra to solar X-ray observations, and show that our model qualitatively reproduces the behaviour seen in many flares, particularly those from coronal sources. We also discuss the impact of such particle evolution on the solar flare energy budget, since this thermalisation process affords a more efficient way to generate the observed X-ray emission than in the classical cold, thick target model.