Nondiffusive Pitch-Angle Scattering of a Distribution of Energetic Particles by Coherent Whistler Waves

Abstract

Whether or not coherent magnetospheric whistler waves play important roles in the pitch-angle scattering of energetic particles is a crucial question in magnetospheric physics. The interaction of a thermal distribution of energetic particles with coherent whistler waves is thus investigated. The distribution is prescribed by the Maxwell-Jüttner distribution, which is a relativistic generalization of the Maxwell-Boltzmann distribution. Coherent whistler waves are modeled by circularly polarized waves propagating parallel to the background magnetic field. It is shown that for parameters relevant to magnetospheric chorus, a significant fraction (1–5%) of the energetic particle population undergoes drastic, nondiffusive pitch-angle scattering by coherent chorus. The scaling of this fraction with the wave amplitude may also explain the association of relativistic microbursts to large-amplitude chorus. A much improved condition for large pitch-angle scattering is presented that is related to, but may or may not include the exact resonance condition depending on the particle's initial conditions. The theory reveals a critical mechanism not contained in the widely used second-order trapping theory.