GROWTH AND DAMPING OF OBLIQUE ELECTROMAGNETIC ION-CYCLOTRON WAVES IN THE EARTHS MAGNETOSPHERE

The dispersive properties of oblique electromagnetic ion cyclotron (EM IC) waves are examined for conditions in the Earth's outer magnetosphe re (L = 7) when the energetic particle distribution has a high-energy tail modeled by a generalized Lorentzian distribution. For small wave normal angles psi to the ambient magnetic field, the wave growth and d amping rates for a generalized Lorentzian distribution are smaller tha n those for a Maxwellian distribution (typically by a factor of 2), wh ile for larger wave normal angles (psi greater than or similar to 60 d egrees) the corresponding differences in the growth and damping rates are relatively small. For both the generalized Lorentzian and Maxwelli an distributions, maximum wave growth due to hot proton temperature an isotropy occurs for parallel propagation, but significant wave growth can occur for wave normal angles \psi\ less than or similar to 30 degr ees. Unstable waves produced near the magnetic equator are expected to be damped at higher latitudes as a result of cyclotron damping by the rmal helium (He+) ions near the bi-ion frequency or near the second ha rmonic of the helium gyrofrequency. A new physical process identified in this study is the excitation of high-frequency oblique (psi similar to 50 degrees-60 degrees) EMIC waves due to second harmonic resonance with hot anisotropic protons. This leads to significant wave growth a t frequencies above the maximum unstable frequency for parallel propag ating waves.