Transient nonlinear pitch angle scattering of energetic electrons by coherent VLF wave packets in the magnetosphere

Abstract

With the aid of adiabatic theory, a study is made of the transient nonlinear pitch angle scattering experienced by energetic electrons under the influence of coherent VLF wave packets in the magnetosphere. Finite wave packets of both fixed and variable frequency are considered, as are wide ranges of L shell, wave frequency, and wave amplitude. Our results indicate that large mean pitch angle changes can be induced in the portion of the energetic population that undergoes a nonlinear cyclotron resonance interaction with the wave packet. Two classes of nonlinearly interacting particles are involved, the nonlinearly resonant particles trapped in the wave and the nonlinearly resonant nontrapped particles. It is shown that the mean pitch angle change of these two classes of particles are of opposite sign, but that the mean pitch angle change of the ensemble can still be significant. In the case of fixed‐frequency wave packets on magnetic shells near L=4, trapped particles typically experience pitch angle changes of 5° or more for moderate wave amplitudes (∼10 mγ). In the case of variable frequency wave packets, similar changes can be produced with much lower wave amplitudes (∼2 mγ). We conclude that the utilization of transient nonlinear pitch angle scattering effects offers interesting options for energetic particle control during ground‐ or space‐based VLF wave‐injection experiments.