Excitation of Ion Cyclotron Waves by Ion and Electron Beams in Compensated-current System

Abstract

Abstract Ion cyclotron waves (ICWs) can play important roles in the energization of plasma particles. Charged particle beams are ubiquitous in space, and astrophysical plasmas and can effectively lead to the generation of ICWs. Based on linear kinetic theory, we consider the excitation of ICWs by ion and electron beams in a compensated-current system. We also investigate the competition between reactive and kinetic instabilities. The results show that ion and electron beams both are capable of generating ICWs. For ICWs driven by ion beams, there is a critical beam velocity, v bi c , and critical wavenumber, k z c , for a fixed beam density; the reactive instability dominates the growth of ICWs when the ion-beam velocity and the wavenumber , and the maximal growth rate is reached at for a given . For the slow ion beams with , the kinetic instability can provide important growth rates of ICWs. On the other hand, ICWs driven by electron beams are excited only by the reactive instability, but require a critical velocity, (the Alfvén velocity). In addition, the comparison between the approximate analytical results based on the kinetic theory and the exact numerical calculation based on the fluid model demonstrates that the reactive instabilities can well agree quantitatively with the numerical results by the fluid model. Finally, some possible applications of the present results to ICWs observed in the solar wind are briefly discussed.