Electromagnetic wave propagation and instabilities in the magnetosphere

Abstract

Growth rates for both the RH- and LH-modes of an EM wave propagating along a magnetic field through an isotropic loss-cone plasma have been obtained. It is found that growing modes can exist, and are found to depend critically on the mirror ratioR, and the specific details of the distribution function of the energetic component. To study the energetic-particle distribution observed at low energies by satellites within the magnetosphere, an isotropic double-humped loss-cone velocity distribution is then studied with a view to determining whether the secondary hump can introduce an instability not present for monotonic distribution. It is found that such a distribution can be unstable in a mirror geometry if the energetic component is sufficiently monoenergetic. Within the magnetosphere, nearly monoenergetic fluxes are observed, peaking in the energy range 1–10 keV, depending on the McIlwain parameterL. It is possible that the initial injection of monoenergetic particles may have been much more sharply peaked than the one presently observed, and, as a result of wave-particle interactions, subsequently relaxed to the presently observed distribution. It is seen here that the EM waves within the magnetosphere can contribute to the relaxation of such an initial injection.