The polarization and directivity of solar-flare hard X-ray bremsstrahlung from a thermal source

Abstract

The polarization and directivity of hard X-ray bremsstrahlung from athermal source consisting of a region in which a thermal flux drives apair of steel collisionless conduction fronts were evaluated. Theconduction fronts are symmetrically driven from a central region, heatedby the flare energy dissipation process. By comparing results withsimilar calculations based on a nonthermal thick-target electron beammodel of the source, it was aimed to determine the degree to which theobserved polarization and directivity of solar flare hard X-rays favoreither model. Results which exhibit significant polarization anddirectivity of the hard X-ray radiation emitted by the source areproduced by using Maxwellian electron-phase-space distribution functionsmodified to take into account a directional heat flux and a steadydirect current in the X-ray source, and a fully relativistic treatmentof the bremsstrahlung emission process. The results are consistent withsolar hard X-ray anisotropy and polarization observations to date,although these observations are too crude to be conclusive.