Production of a collisionless conduction front by rapid coronal heating and its role in solar hard X-ray bursts

The theoretical and observational evidence for and against an impulsively heated thermal bremsstrahlung source of solar hard X-ray bursts is briefly reviewed. In particular, it is noted that in a collision-dominated plasma of any reasonable density the collisional relaxation time would be much longer, and the conductive cooling time much shorter, than typical burst durations (cf. Kahler). It is then shown, however, that free expansion of impulsively heated electrons into a cool surrounding plasma will generate an ion-sound turbulent front which efficiently scatters the electrons, thus bottling up their heat flux. The conductive cooling time is thus increased by a factor (mt/me)112 over its free expansion limit, compatible with observations. The front thickness is less than about 1 km. Interaction with the turbulence permits the bulk of the electrons to reach a relaxed distribution, without the need for high densities, hence providing an efficient source of hard X-rays. Electrons of U > 2.6Ue, however, are not scattered by the ion-sound turbulence but do not necessarily escape freely either, because of the action of the thermoelectric field present. Finally, some brief remarks are directed to the mechanism of heating and to the compatibility of these conditions in the hard X-ray source with other flare observations. Subject headings: hydromagnetics - Sun: corona - Sun: X-rays