Sensitive Dependence of Ultrarelativistic Electron Precipitation on EMIC Wave Frequency

Electromagnetic ion cyclotron (EMIC) waves have long been considered to cause atmospheric precipitation loss of relativistic electrons, the quantitative evaluation of which is critical for understanding the radiation belt electron flux dropouts. In this study, we report test particle calculation results on precipitation loss rates of 5 MeV electrons with initial pitch angles of 4–10° at the equator by He + band EMIC waves. We find a sensitive dependence of the precipitation loss rates on the wave frequency in that a preferred wave frequency (maximum loss frequency) exists at which the loss rate peaks predominantly and away from which it declines substantially. According to a cold plasma dispersion relation, a small difference in wave frequency just below the He + gyrofrequency corresponds to a significant difference in wavenumber. Different frequencies lead to phase trapping of different extents, which determines the significance of loss cone scattering at the exit latitude from the wave zone. At frequencies below the maximum loss frequency, phase trapping causes large advective pitch angle changes, prohibiting loss cone scattering. At the maximum loss frequency, the phase‐trapping effect is optimized such that diffusion is dominant over advection and large enough to allow effective loss cone scattering. At frequencies above the maximum loss frequency, the phase‐trapping effect disappears, and the resulting (both diffusive and advective) scattering is too weak to drive loss cone scattering unless the initial pitch angle is close enough to the loss cone angle.