Amplification of Hydromagnetic Waves in the Magnetosphere by a Cyclotron Instability Process with Applications to the Theory of Hydromagnetic Whistlers

摘要

Cyclotron instability is an instability process caused by resonance between a circularly polarized electromagnetic wave and a stream of charged particles. In this paper, cyclotron instabilities which take place in magnetospheric hydromagnetic waves coupled with a stream of either electrons or protons are investigated. The physical nature of the instabilities is examined based upon the general theory of growing waves given by Sturrock, who showed that there are two kinds of instabilities, convective and nonconvective. The growth time of an instability is computed for a typical example representing an actual situation in the magnetosphere. It can be of the order of ten seconds, if ions are resonponsible for the instability in an ion resonance mode of hydromagnetic waves. An attempt is made to apply the theory to hydromagnetic whistlers, where some kind of amplification process seems to take place fairly often.