ACCELERATION OF COSMIC-RAY ELECTRONS BY ION-EXCITED WAVES AT QUASI-PERPENDICULAR SHOCKS

The standard model of cosmic ray electron acceleration requires electr ons to be preaccelerated to mildly relativistic energies. It has been suggested that energy transfer from waves, generated by gyrotropic ion s reflected from quasi-perpendicular shocks, could provide the necessa ry pre-acceleration. The distribution of shock-reflected ions at any u pstream point is more likely to consist of two beams rather than a gyr otropic ring. Wave excitation in the presence of both types of ion dis tribution is Studied. It is shown that gyrotropic or beam ions, reflec ted from shocks associated with supernova remnants, can excite waves c apable of accelerating electrons to beyond the required injection ener gies. The wave group velocity along the shock normal can be approximat ely equal to the shock velocity: such waves are not rapidly convected away from the shock, and can thus grow to a high level. Moreover, wave s satisfying this condition which also have phase velocities parallel to the magnetic field ranging from the electron thermal speed to relat ivistic speeds are excited in high Mach number shocks with a low ratio of electron plasma frequency to cyclotron frequency. Bulk electrons c an then be accelerated to the required energies within the region in w hich shock-reflected ions are present. This is consistent with a sugge stion, based on a comparison between Wolf-Rayet stars and radio supern ovae, that there exists a threshold perpendicular shock speed (between 1 and 3 per cent of the speed of light) above which the efficiency of electron injection increases by several orders of magnitude.