Statistical Analysis of the Differential Deep Penetration of Energetic Electrons and Protons into the Low L Region (L < 4)

Abstract

Deep penetration of energetic electrons (10s–100s of keV) to low L‐shells (L < 4), as an important source of inner belt electrons, is commonly observed during geomagnetically active times. However, such deep penetration is not observed as frequently for similar energy protons, for which underlying mechanisms are not fully understood. To study their differential deep penetration, we conducted a statistical analysis using phase space densities (PSDs) of µ = 10–50 MeV/G, K = 0.14 G1/2Re electrons and protons from multiyear Van Allen Probes observations. The results suggest systematic differences in electron and proton deep penetration: electron PSD enhancements at low L‐shells occur more frequently, deeply, and faster than protons. For µ = 10–50 MeV/G electrons, the occurrence rate of deep penetration events (defined as daily‐averaged PSD enhanced by at least a factor of 2 within a day at L < 4) is ∼2–3 events/month. For protons, only ∼1 event/month was observed for µ = 10 MeV/G, and much fewer events were identified for µ > 20 MeV/G. Leveraging dual‐Probe configurations, fast electron deep penetrations at L < 4 are revealed: ∼70% of electron deep penetration events occurred within ∼9 hr; ∼8%–13% occurred even within 3 hr, with lower‐µ electrons penetrating faster than higher‐µ electrons. These results suggest nondiffusive radial transport as the main mechanism of electron deep penetrations. In comparison, proton deep penetration happens at a slower pace. Statistics also show that the electron PSD radial gradient is much steeper than protons prior to deep penetration events, which can be responsible for these differential behaviors of electron and proton deep penetrations.

First Results From REPTile‐2 Measurements Onboard CIRBE

“…Figure 2 demonstrates variations of radiation belt electrons across a wide range in energy: 0.25–6 MeV and L : 1–7. Some of these features have been reported before in different formats, such as energy‐dependent deep penetration of electrons in which very few multi‐MeV electrons were observed below L = 2.6 (Baker et al., 2014; Hogan et al., 2021; X. Li, Baker, et al., 2017; X. Li et al., 1993, 2021; O’Brien et al., 2023; Zhao & Li, 2013a) while less energetic (∼1 MeV) electrons are often seen penetrating into the slot region and sometimes even into the inner belt ( L < 2) (Baker et al., 2004; Claudepierre et al., 2019; Kim et al., 2016; Zhao & Li, 2013b; Zhao et al., 2023; Zheng et al., 2006). Dispersionless injections have mostly been studied using observations near the equatorial plane (Gabrielse et al., 2016; X. Li et al., 1998; Sarris et al., 2002).…”

Long‐Term Variations of Energetic Electrons Scattered by Signals From the North West Cape Transmitter

“…Zhao et al. (2013) reported that 100's keV electron flux would enhance in the inner belt even during moderate storms. Selesnick et al.…”

“…Based on the discrepant characteristic of energy dependent increases of flux, we can conclude that the #4 flux increases are related to electron penetration during a geomagnetic storm and the #5 flux increases are produced by NWC transmitter signals. It is evident that storms are just moderate for #1 and #4 flux increases, with minimum Dst ≈ −50 nT, which suggests that strong injections of 100's keV electrons in the inner belt can happen during moderate or weak storms (Zhao et al., 2023; Zhao & Li, 2013).…”

“…However, these electrons precipitate within a drift period, thus, the same phenomena are not observed in dayside panels (Figures 6e1 and 6f1). Injections are only observed for <200 keV electrons because electrons with lower energies can penetrate into the inner belt more easily (e.g., Zhao et al., 2023).…”

“…Similarly, the #2 flux increases are also related to the large geomagnetic storm at the end of 2006. Moreover, #2 increases are observed when the NWC station is at dayside It is evident that storms are just moderate for #1 and #4 flux increases, with minimum Dst ≈ 50 nT, which suggests that strong injections of 100's keV electrons in the inner belt can happen during moderate or weak storms (Zhao et al, 2023;Zhao & Li, 2013).…”