Differentiating Between the Leading Processes for Electron Radiation Belt Acceleration

Abstract

Many spacecraft fly within or through a natural and variable particle accelerator powered by the coupling between the magnetosphere and the solar wind: the Earth’s radiation belts. Determining the dominant pathways to plasma energization is a central challenge for radiation belt science and space weather alike. Inward radial transport from an external source was originally thought to be the most important acceleration process occurring in the radiation belts. Yet, when modeling relied on a radial diffusion equation including electron lifetimes, notable discrepancies in model-observation comparisons highlighted a need for improvement. Works by Professor Richard M. Thorne and others showed that energetic (hundreds of keV) electrons interacting with whistler-mode chorus waves could be efficiently accelerated to very high energies. The same principles were soon transposed to understand radiation belt dynamics at Jupiter and Saturn. These results led to a paradigm shift in our understanding of radiation belt acceleration, supported by observations of a growing peak in the radial profile of the phase space density for the most energetic electrons of the Earth’s outer belt. Yet, quantifying the importance of local acceleration at the gyroscale, versus large-scale acceleration associated with radial transport, remains controversial due to various sources of uncertainty. The objective of this review is to provide context to understand the variety of challenges associated with differentiating between the two main radiation belt acceleration processes: radial transport and local acceleration. Challenges range from electron flux measurement analysis to radiation belt modeling based on a three-dimensional Fokker-Planck equation. We also provide recommendations to inform future research on radiation belt radial transport and local acceleration.

Can strong substorm-associated MeV electron injections be an important cause of large radiation belt enhancements?

“…The present study employs the new hourly snapshot technique to examine the significance of relativistic electron injections for the radiation belt enhancement event where ~2 MeV fluxes increased by ~3 orders of magnitude within a period of 7.5 h. The Van Allen Probes were in relatively good positions to analyze electron responses during and between 4 well-separated, intense substorms (S1 to S4). Our analysis approach overcomes the limitation of rapid radial transport not being accurately represented by current diffusion-driven radiation belt models (Lejosne et al, 2022) and reveals:…”

“…We note that Figure 4B does not cover the entire outer belt, since no adequate data is available for determination of the purely spatial PSD for higher L-regions where there might be a local peak (Boyd et al, 2018). It is noteworthy, however, that in general, for various technical and physical reasons, a local PSD peak is not conclusive evidence for local acceleration (Lejosne et al, 2022). For instance, depending on the preexisting electron spectra and flux radial gradients before injections or large flux drops at higher L's after injections, we may have locally peaked PSD even for injection-driven enhancements (e.g., Sergeev et al, 1998;Green et al, 2004;Xiong et al, 2022).…”

Outer Radiation Belt Flux and Phase Space Density Response to Sheath Regions: Van Allen Probes and GPS Observations

“…During geoeffective sheaths, enhancements are seen near L * ∼ 4. Likely both local acceleration and inward radial diffusion are contributing to the enhancements (e.g., Allison et al., 2019; Lejosne et al., 2022). Geoeffective sheaths are typically associated with strong substorm activity (e.g., Kalliokoski et al., 2020; Pulkkinen et al., 2007), which in turn excites chorus waves that can efficiently accelerate electrons to >1 MeV energies (Allison & Shprits, 2020; Allison et al., 2021; Jaynes et al., 2015; Miyoshi et al., 2013).…”

Outer Radiation Belt Flux and Phase Space Density Response to Sheath Regions: Van Allen Probes and GPS Observations

“…During geoeffective sheaths, enhancements are seen near L * ∼ 4. Likely both local acceleration and inward radial diffusion are contributing to the enhancements (e.g., Allison et al, 2019;Lejosne et al, 2022). Geoeffective sheaths are typically associated with strong substorm activity (e.g., Pulkkinen et al, 2007;Kalliokoski et al, 2020), which in turn excites chorus waves that can efficiently accelerate electrons to > 1 MeV energies (Miyoshi et al, 2013;Jaynes et al, 2015;Allison et al, 2021).…”