Precipitation of Relativistic Electrons Under Resonant Interaction With Electromagnetic Ion Cyclotron Wave Packets

We use numerical simulations to study the resonant interaction of relativistic electrons with rising-frequency electromagnetic ion cyclotron (EMIC) wave packets in the H+ band. We find that precipitating fluxes are formed by quasi-linear interaction and several nonlinear interaction regimes having opposite effects. In particular, the direct influence of Lorentz force on the particle phase (force bunching) decreases precipitation for particles with low equatorial pitch angles (up to 15-25.) and can even block it completely. Four other nonlinear regimes are possible: nonlinear shift of the resonance point, which can cause pitch angle drift in both directions; phase bunching that slightly increases pitch angle for untrapped particles; directed scattering that strongly decreases pitch angle for untrapped particles; and particle trapping by the wave field that decreases pitch angle. The evolution of the equatorial pitch angle distribution during several passes of particles through the wave packet is studied. The precipitating fluxes are evaluated and compared with theoretical estimates. We show that strong diffusion limit is maintained for a certain range of energies by a wave packet with realistic amplitude and frequency drift. In this case, the quasi-linear theory strongly underestimates the precipitating flux. With increasing energy, the precipitating fluxes decrease and become close to the quasi-linear estimates.