Oblique whistler-mode waves in the presence of electron beams

Abstract

The problems of oblique whistler-mode propagation, instability and damping for hot anisotropic plasma in the presence of electron beams are considered using the quasi-longitudinal and quasi-electrostatic approximations. For the quasi-longitudinal approximation, the solution of the real part of the dispersion equation is presented as a result of a perturbation of the whistler-mode refractive index in a cold, dense plasma N 0. This enables us to make a direct comparison between different perturbations of N 0 due to electron finite density, temperature, anisotropy and electron beams. For the quasi-electrostatic approximation, the solution is found in the case when the net electron current due to electron beams is either equal to zero or close to zero. In the case of quasi-longitudinal whistler-mode propagation in the vicinity of the magnetopause, the corrections to N 0 due to electron beams are generally small and can be neglected. For quasi-electrostatic whistler-mode propagation in the same region of the magnetosphere the contribution of electron beams is more significant but does not result in qualitative changes of the basic properties of the waves. For the values of plasma parameters observed near the magnetopause by AMPTE-UKS we can expect only parallel or almost parallel whistler-mode waves to be unstable at frequencies ω below the half electron gyrofrequency Ω. This prediction of the theory is compatible with actual simultaneous observation of the wave spectrum in the magnetopause region. Wave observations at ω >; 0.5Ω could be explained if we would assume larger values for the anisotropy of the electron distribution, or that the waves can penetrate from a region of stronger magnetic field. The first possibility is considered in detail and it shows that these waves can be unstable both within the quasi-longitudinal and quasi-electrostatic approximations. The conditions for instability for quasi-electrostatic waves are more stringent than those for quasilongitudinal ones.