ELECTRON ACCELERATION DUE TO BEAM FLUX INCREASE IN A CONVERGING MAGNETIC-FIELD

The effects of the magnetic field mirroring and backscattering of beam electrons in a time varying return-current electric field are studied with a 1-D hybrid test particle model of the electron bombardment in flare loops. These effects are analyzed for monoenergetic electron beams with an energy flux increasing with time, for different widths of their pitch-angle distribution. The consequence of a weak linear increase of the magnetic field from the apex to the feet of the loop is considered. The number and therefore the role of mirroring electrons are increasing with the broadening of the pitch-angle distribution. While in the case of bombardment by an unidirectional beam only few particles are reflected by back-scattering in the dense layers of the loop and then accelerated in the return-current electric field, in the case of a broader pitch-angle distribution of beams much more electrons are mirrored and accelerated. Part of them are accelerated to energies higher than the initial ones. Consequently, a collisionless return-current is formed, which decreases the beam return-current losses. The possible role of these effects in the solar flare scenario is shortly discussed.