Effects of Spatial Variation of Thermal Electrons onWhistler-Mode Waves in Magnetosphere

Abstract

A ray-tracing method is developed to evaluate the wave growth/dampingand specifically propagation trajectories of the magnetosphericallyreflected Whistler-mode waves. The methodology is valid for weak wavegrowth/damping when plasma is comprised of a cold electron populationand a hot electron population, together with background neutralizingions, e.g. protons. The effect of anisotropic thermal electrons on thepropagation of Whistler-mode waves is studied in detail. Numericalresults are obtained for a realistic spatial variation model of plasmapopulation, including the cold electron density distribution, and thethermal electron density and temperature distribution. It is found that,analogous to the case of the typical cold plasma approximation, theoverall ray path of Whistler-mode waves is insensitive to the thermalelectron density and temperature anisotropy, and the ray path reflectswhere wave frequency is below or comparable to the local lower hybridresonance frequency flhr. However, the wave growth is expected tobe influenced by the thermal electron population. The results present afirst detailed verification for the validity of the typical cold plasmaapproximation for the propagation of Whistler-mode waves and may accountfor the observation that the Whistler-mode waves tend to propagate on aparticular magnetic shell L where the wave frequency is comparable toflhr.