Test-Particle Simulations of Linear and Nonlinear Interactions Between a 2-D Whistler-Mode Wave Packet and Radiation Belt Electrons
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Using test-particle simulations, we describe (in 2-D) the interaction of an ensemble of electrons with a discrete whistler-mode wave packet, which propagates with a moderate wave-normal angle with respect to the background magnetic field. We evaluate both the average transport coefficients used in quasi-linear diffusion transport and also the full nonlinear wave-particle interactions. The magnitude of the calculated diffusion coefficients is found to increase proportionally to the wave amplitude squared, in agreement with quasi-linear diffusion theory. Cyclotron resonance of counterstreaming electrons (n = 1 harmonic number) is more important for pitch angle scattering than Landau resonance. Landau resonance of costreaming electrons (n = 0) is comparable to cyclotron resonance for energy diffusion and advection. Strong acceleration of high pitch angle, costreaming electrons arises in nonlinear wave-particle interactions with high-amplitude waves. In our simulations with zero parallel electric field, the energy source for electron acceleration is the wave's perpendicular electric field, in both the cyclotron and Landau resonances. The Landau resonance can happen even with zero parallel electric field, if the wave packet propagates with a finite wave-normal angle. This resonance between the particle's azimuthal velocity and the wave fields leads to trapping and substantial parallel acceleration.