Statistically Determining the Spatial Extent of Relativistic Electron Precipitation Events Using 2‐s Polar‐Orbiting Satellite Data

Abstract

Within Earth's magnetosphere lie the Van Allen radiation belts, surrounding the Earth with energetic, charged particles. Under typical geomagnetic conditions, there are two such belts-a relatively stable inner belt, and a dynamic outer belt composed of electrons and low-energy protons, typically confined within 3 ≤ L ≤ 8 (e.g., Millan & Baker, 2012;Millan & Thorne, 2007). Outer belt electrons with relativistic energies often remain trapped by Earth's magnetic field for less than a day before either escaping across the magnetopause or precipitating into Earth's atmosphere (Thorne et al., 2005). Precipitating electrons can degrade shortwave radio signals (Evans & Greer, 2000), damage satellite instrumentation (Horne et al., 2013), and produce compounds capable of destroying mesospheric and stratospheric ozone (e.g., Brasseur & Solomon, 2005;Randall et al., 2005). Quantifying the size of the region over which relativistic electron precipitation (REP) occurs will improve understanding of the dynamics in the outer radiation belt, informing future radiation belt and climate models.If a particle's equatorial pitch angle is small enough that its mirror point lies within Earth's atmosphere (<∼100 km above the surface), the particle will likely collide with atmospheric particles and precipitate (Millan & Baker, 2012). Particles are defined to be in the bounce loss cone (BLC) if they will enter the atmosphere within a single bounce period (Selesnick, 2006). Mechanisms such as wave-particle interactions can cause pitch angle scattering, changing affected particle pitch angles, and causing particles that were previously trapped to instead enter the BLC (Millan & Thorne, 2007). Waves capable of pitch angle scattering relativistic electrons include whistler-mode plasmaspheric hiss and chorus, and electromagnetic ion-cyclotron (EMIC) waves (

Relativistic Electron Precipitation Near Midnight: Drivers, Distribution, and Properties

“…Similar to previous studies (e.g., Gasque et al., 2021), the observed wave‐driven REP occurs on small radial scales, typically <0.3 L , likely because the regions where wave‐particle interactions drive efficient pitch angle scattering are just as localized. Additionally, we also showed that pre‐midnight wave‐driven REP events tend to exhibit a maximum radial spatial scale of ∼0.7 L , while the ones detected in the post‐midnight sector are typically <∼0.5 L .…”

“…Similar to previous studies (e.g., Gasque et al, 2021), the observed wave-driven REP occurs on small radial scales, typically <0.3 L, likely because the regions where wave-particle interactions drive efficient pitch angle scattering are just as localized. Additionally, we also showed that pre-midnight wave-driven REP events tend to exhibit a maximum radial spatial scale of ∼0.7 L, while the ones detected in the post-midnight sector are typically <∼0.5 L. This is an interesting result that requires further understanding of the wave properties and wave-electron interactions that could justify why the precipitation extent seems to be asymmetric with respect to midnight.…”

“…the wave-driven REP events overall occur on localized scales (average ∼0.25 L, standard deviation ∼0.14), consistent with previous results (Capannolo et al, 2021;Gasque et al, 2021).…”

“…A recent study using high‐resolution POES data by Gasque et al. (2021) indeed clarifies that the midnight REP events with wider spatial extent (Δ L ∼1–2.5) reported by Shekhar et al. (2017) based on 16 s resolution POES data also exhibit localized scales.…”

“…Overall, REP occurs on localized spatial scales of an order of a few tenths of L-shells, independently of MLT. A recent study using high-resolution POES data by Gasque et al (2021) indeed clarifies that the midnight REP events with wider spatial extent (ΔL ∼1-2.5) reported by Shekhar et al (2017) based on 16 s resolution POES data also exhibit localized scales. Additionally, some works studied the association of REP with proton precipitation as a proxy for electromagnetic ion cyclotron (EMIC) wave activity, or with in situ or ground-based EMIC waves (e.g., Capannolo et al, 2021;Carson et al, 2012;Hendry et al, 2016).…”