Excitation of Upper-Hybrid and Whistler-Mode Waves by Electron Velocity Ring Distribution

Abstract

The magnetospheres of the Earth and other magnetized planets are replete with high-frequency fluctuations, which are sometimes accompanied by multiple-harmonic electron cyclotron waves, and lower frequency waves of the whistler-mode type. Such waves are presumed to be excited by energetic electrons trapped in the dipolar magnetic field, the so-called loss-cone electrons, the electron ring distribution being a highly idealized example. The present paper investigates the stability of electron ring distribution with respect to the excitation of quasi-electrostatic upper-hybrid wave instability as well as the quasi-electromagnetic whistler mode instability that operates near electron cyclotron frequency. By employing a two-dimensional particle-in-cell numerical simulation, it is demonstrated that the relatively early dynamics is dominated by the upper-hybrid wave instability, but over a longer time period it is the whistler mode instability that ultimately determines the final relaxed state. The simulation results are interpreted with the quasilinear theoretical framework.