Dispersed Relativistic Electron Precipitation Patterns Between the Ion and Electron Isotropy Boundaries

Abstract

Relativistic electron precipitation to the Earth's atmosphere is an important loss mechanism of inner magnetosphere electrons, contributing significantly to the dynamics of the radiation belts. Such precipitation may be driven by electron resonant scattering by middle‐latitude whistler‐mode waves at dawn to noon; by electromagnetic ion cyclotron (EMIC) waves at dusk; or by curvature scattering at the isotropy boundary (at the inner edge of the electron plasma sheet anywhere on the nightside, from dusk to dawn). Using low‐altitude ELFIN and near‐equatorial THEMIS measurements, we report on a new type of relativistic electron precipitation that shares some properties with the traditional curvature scattering mechanism (occurring on the nightside and often having a clear energy/L‐shell dispersion). However, it is less common than the typical electron isotropy boundary and it is observed most often during substorms. It is seen equatorward of (and well separated from) the electron isotropy boundary and around or poleward of the ion isotropy boundary (the inner edge of the ion plasma sheet). It may be due to one or more of the following mechanisms: EMIC waves in the presence of a specific radial profile of the cold plasma density; a regional suppression of the magnetic field enhancing curvature scattering locally; and/or electron resonant scattering by kinetic Alfvén waves.

Energetic Particle Precipitation in Sub‐Auroral Polarization Streams

“…An additional feature of this precipitation pattern is that it locates well equatorward from the isotropic <100 keV electron precipitation from the plasma sheet and shows signatures of energy/L-shell dispersion (contours of constant precipitating-to-trapped flux ratio show an energy increase with L-shell increase within [58.5°, 60°], see panels (d, e)). ELFIN detected multiple such events during strong substorms (see Artemyev et al, 2023, for discussions of the energy/L-shell dispersion in EMIC-driven precipitation), the preferable condition for the SAPS formation, confirming that the event from Figures 2 and 3 is not unique, but rather a typical SAPSassociated electron precipitation pattern.…”

Inclusion of Nonresonant Effects Into Quasi‐Linear Diffusion Rates for Electron Scattering by Electromagnetic Ion Cyclotron Waves

“…In contrast, alternate scattering mechanisms of relativistic electrons, principally those by whistler-mode waves and magnetic field line curvature, are not expected to exhibit a similar low energy cutoff in the range between 50 and 1,000 keV. To clarify, we note that 50 keV is the minimum energy of the lowest energy channel of the ELFIN measurements, but otherwise not particularly significant (see details of ELFIN observations of EMIC-driven precipitation events and their comparison with other scattering mechanisms in Angelopoulos et al, 2023;Artemyev et al, 2023;Grach et al, 2022). Statistics of direct measurements of the energy cutoff of precipitating electrons have shown that EMIC waves often drive subrelativistic precipitation well below the theoretical minimum resonance energy for the typical frequency of the observed waves.…”

Field Line Curvature (FLC) Scattering in the Dayside Off‐Equatorial Minima Regions

“…The simulation results indicate that the process of FLC scattering exerts on energetic and heavy ions on the nightside, and the ions precipitation driven by FLC scattering mainly occurs in the outer region (L > 4.5) on the nightside. Besides, many previous studies related to FLC scattering mainly focus on the nightside of the terrestrial magnetosphere (Artemyev et al, 2023;Cao et al, 2023;Capannolo et al, 2022;Chen et al, 2023;Dubyagin et al, 2018;Gilson et al, 2012;Ma et al, 2022;Wilkins et al, 2023;Yue et al, 2014;Zhu et al, 2021), which is attributed to the occurrence of the magnetic minimum and the most apparent field line stretching in this region.…”