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

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 mu = 10-50 MeV/G, K = 0.14 G(1/2)Re 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 mu = 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 similar to 2-3 events/month. For protons, only similar to 1 event/month was observed for mu = 10 MeV/G, and much fewer events were identified for mu > 20 MeV/G. Leveraging dual-Probe configurations, fast electron deep penetrations at L < 4 are revealed: similar to 70% of electron deep penetration events occurred within similar to 9 hr; similar to 8%-13% occurred even within 3 hr, with lower-mu electrons penetrating faster than higher-mu 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.