EXCITATION OF KINETIC ALFVÉN WAVES BY FAST ELECTRON BEAMS

Abstract

Energetic electron beams, which are ubiquitous in a large variety of active phenomena in space and astrophysical plasmas, are one of the most important sources that drive plasma instabilities. In this paper, taking account of the return-current effect of fast electron beams, kinetic Alfven wave (KAW) instability driven by a fast electron beam is investigated in a finite-beta plasma of Q < beta < 1 (where beta is the kinetic-to-magnetic pressure ratio and Q = m(e)/m(i) is the mass ratio of electrons to ions). The results show that the kinetic resonant interaction of beam electrons is the driving source for KAW instability, unlike the case driven by a fast ion beam, where both the kinetic resonant interaction of beam ions and the return-current are the driving source for the KAW instability. KAW instability has a nonzero growth rate in the range of the perpendicular wave number, 0 < k(perpendicular to) < k(perpendicular to)(u), and the maximum growth rate, gamma(m), occurs between 0.5k(perpendicular to)(u) < k(perpendicular to)(m) < 0.8k(perpendicular to)(u). Both the maximal growing perpendicular wave number k(perpendicular to)(m) and the maximal growth rate gamma(m) depend sensitively on the velocity of electron beam upsilon(b), and the most favorable beam velocity occurs between 8 upsilon(A) < upsilon(b) < 10 upsilon(A). On the other hand, the excited KAWs are weakly dispersive with k(perpendicular to) rho(i) < 1 and have the maximum growth rate at relatively low perpendicular wave numbers in the range 0.3 < k(perpendicular to)(m) rho(i) < 0.6 for a beam velocity upsilon(b) < 10 upsilon(A). A possible application to the upward electron beams in the terrestrial magnetosphere is briefly discussed.