Inner Belt Wisp Precipitation Measured by ELFIN: Regimes of Energetic Electron Scattering by VLF Transmitter Waves.

Man‐made very low frequency (VLF) transmitter waves play a critical role in energetic electron scattering and precipitation from the inner radiation belt, a type of which is called wisp precipitation. Wisps exhibit dispersive energy‐versus‐L spectra due to the evolution of electron cyclotron resonance conditions with near‐monochromatic VLF transmitter waves. Here, we report on such observations of inner belt wisp precipitation events with full pitch angle resolution in the energy range of 50 to ∼500 keV as measured by Electron Loss and Fields Investigation (ELFIN) at L < ∼2 between March 2021 and April 2022. Statistical observations (82 events) reveal occasional (18 events) wisp precipitation events with local bounce‐loss‐cone electron flux enhancements, which provide new information compared with flux enhancements measured in previous studies only in the drift loss cone. Based on magnetic field and plasmaspheric density models, quasilinear theory, and detailed pitch angle distributions of wisps from ELFIN, we have estimated the wisp electron bounce‐averaged pitch angle diffusion coefficients to be of the order of 10−4 to 10−2 s−1. These are several orders of magnitude larger than the diffusion rates calculated from models using global statistical averages of VLF transmitter wave power. When using our estimated diffusion coefficients to deduce the associated local transmitter wave amplitudes near the equator, based on quasilinear calculations from a transmitter‐induced electron diffusion model, we find these wave amplitudes to be >1 mV/m. Although probable overestimates, such inferred wave amplitudes exceed the theoretical threshold amplitude for nonlinear interactions, strongly suggesting that it is necessary to include nonlinear effects for an accurate evaluation of energetic electron scattering by transmitter waves. Plain Language Summary: Man‐made ground‐based very low frequency (VLF) transmitter waves can scatter and precipitate energetic radiation belt electrons, which pose a threat to telecommunication and human assets in space. Therefore, understanding the controlling parameters and the efficiency of energetic electron scattering by VLF transmitter waves are of both theoretical and practical importance. Our paper reports statistical analyses of inner belt wisp precipitation events due to scattering by VLF transmitter waves as observed by the Electron Loss and Fields Investigation CubeSats. Occasional strong local wisp precipitation events have been found to occur at preferential longitudes east of the South Atlantic Anomaly in the Northern Hemisphere and were primarily observed in the morning magnetic local time. These local wisp precipitation events have been used to infer energetic electron diffusion rates and VLF transmitter wave amplitudes near the equator based on quasilinear diffusion theory. Such inferred wave amplitudes exceed the theoretical threshold amplitude for nonlinear interactions, strongly suggesting that it is necessary to include nonlinear effects for an accurate evaluation of energetic electron scattering by transmitter waves. Key Points: We report statistical observations of inner belt wisps with full pitch angle distributions measured by Electron Loss and Fields InvestigationLocal bounce‐loss‐cone wisp precipitation events allow inference of quasilinear diffusion rates and transmitter wave amplitudesInferred transmitter wave amplitudes exceed nonlinear interaction threshold amplitudes near the equator [ABSTRACT FROM AUTHOR]