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Published June 2020 | Published
Journal Article Open

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


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, non‐diffusive 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.

Additional Information

© 2020 American Geophysical Union. Version of Record online: 17 June 2020. Accepted manuscript online: 11 June 2020; Manuscript accepted: 05 May 2020; Manuscript revised: 30 April 2020; Manuscript received: 10 January 2020. The authors thank Mackenzie Wooten for reproducing some of the initial algebra. This material is based upon work supported by the Air Force Office of Scientific Research under award No. FA9550-17-1-0023 and by the NSF Division of Atmospheric and Geospace Sciences under award No. 1914599. The associated scripts that generate data and plots for this paper are available online (https://doi.org/10.22002/D1.1333).

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