Growth of obliquely propagating ion cyclotron waves in the magnetosphere

Abstract

We present linear growth rate calculations of electromagnetic ion cyclotron waves driven by hot anisotropic protons in a magnetized plasma. The angle between the wave vector and the magnetic field is arbitrary. This introduces Landau damping by the thermal electrons, an effect not present for strictly parallel propagation. The model contains concentrations of O+ and He+ as well as H+, with densities and temperatures typical of ion cyclotron wave generation regions inside the plasmasphere around L = 3. The behavior of growth regions in k space is studied. Although the growth rates peak at θ = 0°, they do not depend strongly on θ for θ<30°. At large wave normal angles, weak peaks in wave growth are found with much smaller growth rates than at 0°. Electron Landau damping of ion cyclotron waves maximizes at very oblique wave normal angles where the growth rate can be reduced significantly, but has little or no effect in the region of k space (θ<45°) where the waves are growing the fastest.