Warm plasma effects on electromagnetic ion cyclotron wave MeV electron interactions in the magnetosphere

The full kinetic linear dispersion relation in a warm plasma with He+ and O+ ions is used to estimate the minimum resonant electron energies required for resonant scattering of relativistic electrons by electromagnetic ion cyclotron waves. We find two significant differences from the cold‐plasma approximation: (1) waves can be excited inside the stop bands and at ion gyrofrequencies for relatively small wave numbers k < Ωp/vA and (2) short wavelengths with k > Ωp/vA experience strong cyclotron damping. We show that, in general, minimum resonant energy of electrons Emin depends only on the wave number k, magnetic field strength B, and plasma mass density ρ and depends on the wave frequency ω only implicitly, via the dispersion relation. Formulae for Emin as function of ω based on cold‐plasma approximation predict the lowest energy loss where ω → since in this approximation k → ∞ at these frequencies. We show this inference is incorrect and that kinetic effects mean that the ion gyrofrequencies are no longer necessarily preferential for low energy loss. The lowest values of Emin are obtained where the dispersion supports the largest wave numbers k and in the regions of the largest mass densities ρ and the lowest magnetic fields B. For realistic magnetospheric conditions Emin ∼ 2 MeV and can only drop to ∼500 keV inside dense plasmaspheric plumes, with plasma density of the order of 500 cm−3, or during plasmaspheric expansions to high L shells (L ∼ 7). Key Points Short wavelength EMIC waves are needed to scatter electrons efficiently EMIC waves at helium gyrofrequency have small wave numbers EMIC waves may scatter electrons under 2 MeV only in plasmaspheric plumes