Growth and damping of oblique electromagnetic ion cyclotron waves in the Earth's magnetosphere

Abstract

The dispersive properties of oblique electromagnetic ion cyclotron (EMIC) waves are examined for conditions in the Earth's outer magnetosphere (L = 7) when the energetic particle distribution has a high‐energy tail modeled by a generalized Lorentzian distribution. For small wave normal angles ψ to the ambient magnetic field, the wave growth and damping rates for a generalized Lorentzian distribution are smaller than those for a Maxwellian distribution (typically by a factor of 2), while for larger wave normal angles (ψ ≳ 60°) the corresponding differences in the growth and damping rates are relatively small. For both the generalized Lorentzian and Maxwellian distributions, maximum wave growth due to hot proton temperature anisotropy occurs for parallel propagation, but significant wave growth can occur for wave normal angles |ψ| ≲ 30°. Unstable waves produced near the magnetic equator are expected to be damped at higher latitudes as a result of cyclotron damping by thermal helium (He+) ions near the bi‐ion frequency or near the second harmonic of the helium gyrofrequency. A new physical process identified in this study is the excitation of high‐frequency oblique (ψ ∼ 50°–60°) EMIC waves due to second harmonic resonance with hot anisotropic protons. This leads to significant wave growth at frequencies above the maximum unstable frequency for parallel propagating waves.