Refractive effects of plasma density gradients on magnetospheric upper hybrid emissions

Large amplitude (greater than or equal to10 mV/m) electrostatic plasma waves near the upper hybrid frequency have been observed from 00 to 500 magnetic latitude during satellite plasmapause crossings. The author presents results of theoretical wave growth rate and convective amplification calculations which indicate how these intense upper hybrid (IUH) events can occur over a wide range of latitudes. The effects of wave refraction are shown to be crucial to the proper calculation of wave amplification. A three dimensional numerical ray tracing code has been employed to calculate wave trajectories and growth. The author finds upper hybrid wave growth for an IUH event at 100 MLAT during which an electron distribution function with a loss cone feature was measured along with wave data. For propagation in a dipole magnetic field no significant wave growth is found. This is due to rapid refraction of the wave vector component parallel to the local field lines. The author shows that a parallel density gradient is necessary to counteract this refractive effect and give significant amplification. By mapping the observed distribution function to other latitudes while conserving the electrons' magnetic moments, the author finds similar results except at the equator. There the long magnetic field gradient scale lengths enable large amplitudes to be attained without requiring density variations. The results of the UH ray tracing analysis are related to generation mechanisms of the magnetospheric continuum radiation