Nonlinear pitch angle scattering of relativistic electrons by EMIC waves in the inner magnetosphere

Abstract

We derive the second-order resonance condition for interaction between a relativistic electron and a coherent Electromagnetic Ion Cyclotron (EMIC) wave with a variable frequency. We perform test particle simulations of relativistic electrons interacting with EMIC waves with a fixed frequency and a rising-tone frequency such as EMIC triggered emissions observed in the inner magnetosphere. Trapping of resonant electrons leads to rapid and efficient pitch angle scattering of relativistic electrons, resulting in bursty precipitation of relativistic electrons. The efficiency of the pitch angle scattering depends on the gradient of the magnetic field, the frequency sweep rate, and the wave amplitude. The effective wave trapping occurs for a wide range of pitch angles from 10 to 60 degrees. The most effective pitch angle scattering takes place for the case of a rising-tone emission with an enhanced magnetic field gradient. Since the efficiency of pitch angle scattering also depends on the wave amplitude, resonant electrons may not be scattered into the loss cone in a single passage through the wave packet. However, repeated interactions with a series of wave packets result in scattering of relativistic electrons into the loss cone.