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 Alfvén wave (KAW) instability driven by a fast electron beam is investigated in a finite-β plasma of Q < β < 1 (where β 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, and the maximum growth rate, γ ^m , occurs between 0.5 k_\perp ^u. Both the maximal growing perpendicular wave number k_\perp ^m and the maximal growth rate γ ^m depend sensitively on the velocity of electron beam v_b , and the most favorable beam velocity occurs between 8v_A < v_b < 10v_A . On the other hand, the excited KAWs are weakly dispersive with k ρ _i < 1 and have the maximum growth rate at relatively low perpendicular wave numbers in the range 0.3 for a beam velocity v_b < 10v_A . A possible application to the upward electron beams in the terrestrial magnetosphere is briefly discussed.