Collisionless turbulent transport and anisotropic electron heating in coronal flare loops

Abstract

Context. One of the hypotheses about the generation of the hard X-ray emissions (HXR) of the sun is that a strong electron-beam is first accelerated near the looptop, and then propagated down to the chromosphere. There the HXR emissions are generated by the bombardment of electrons via thick-target bremsstrahlung. Recently, the beam-plasma model has been questioned because streaming instabilities make the beam propagation doubtful. Another open question in solar flare models is the generation of anisotropic electron distributions deduced from microwave emissions. The question is whether one can find a mechanism, in addition to the generally considered mirror motion, that may cause the electron anisotropy in flare loops. Aims. To understand the transport of coronal electron beams and the possible generation of electron anisotropic distribution in the course of the beam propagation, we simulated a beam-plasma return-current system. Our aim is to investigate the evolution of predicted streaming instabilities at the nonlinear stage and to determine the intensity of beam transport in coronal loops. Methods. The linear instabilities and wave coupling in the beam-plasma return-current system are investigated by a multi-fluid dispersion analysis. We performed a two-dimensional electromagnetic particle-in-cell simulation to understand the generation of turbulent transport and anisotropic electron heating at the nonlinear stage of the beam evolution. Results. The beam-plasma return-current system is stablized at a late stage of evolution by a combined effect of 1) a relaxation of electron bulk drifts; and 2) a fast thermalization of the beam and return-current electrons in the drift direction. As a result, the downward electron beam continues to propagate in the coronal loop. Distinguishable parallel and perpendicular electron heating is observed, which is caused by turbulent deflection of electron beams. The electron distribution becomes anisotropic through different heating rates. Conclusions. An electron beam injected from a solar flare looptop can continue to propagate stably despite a partial relaxation of its drift velocity. After drift relaxation and anisotropic electron heating, the slowed-down electron drifts and the ambient thermalized plasma create a stable beam-propagation environment because of the Landau damping effect. This electron beam can propagate stably at a modified drift speed down to the chromosphere, where it generates HXR radiation. We conclude that the beam plasma model is feasible for the HXR generation in a solar flare event. The observed anisotropic electron distribution is a direct consequence of turbulent deflections of the electron beams.