Investigating the source of near‐relativistic and relativistic electrons in Earth's inner radiation belt

Abstract

Using observations from NASA's Van Allen Probes, we study the role of sudden particle enhancements at low L shells (SPELLS) as a source of inner radiation belt electrons. SPELLS events are characterized by electron intensity enhancements of approximately an order of magnitude or more in less than 1 day at L < 3. During quiet and average geomagnetic conditions, the phase space density radial distributions for fixed first and second adiabatic invariants are peaked at 2 < L < 3 for electrons ranging in energy from ~50 keV to ~1 MeV, indicating that slow inward radial diffusion is not the dominant source of inner belt electrons under quiet/average conditions. During SPELLS events, the evolution of electron distributions reveals an enhancement of phase space density that can exceed 3 orders of magnitude in the slot region and continues into the inner radiation belt, which is evidence that these events are an important—and potentially dominant—source of inner belt electrons. Electron fluxes from September 2012 through February 2016 reveal that SPELLS occur frequently (~2.5/month at 200 keV), but the number of observed events decreases exponentially with increasing electron energy for ≥100 keV. After SPELLS events, the slot region reforms due to slow energy‐dependent decay over several day time scales, consistent with losses due to interactions with plasmaspheric hiss. Combined, these results indicate that the peaked phase space density distributions in the inner electron radiation belt result from an “on/off,” geomagnetic‐activity‐dependent source from higher radial distances.

Van Allen Probes Measurements of Energetic Particle Deep Penetration Into the Low L Region (L < 4) During the Storm on 8 April 2016

“…Even for ~100 keV electrons, the flux enhancements still happened within ~2 h at L ~ 3–6. This type of fast penetration of electrons into the low L region is not likely due to the conventional radial diffusion process, which is expected to be slower than the electron drift period (~2.6 h for 100 keV electrons at L = 3 and ~12.7 h for 30 keV electrons at L = 2), as has also been pointed out by previous studies (e.g., Su et al, ; Turner et al, 2017).…”

“…Specifically, the deep penetration of energetic particles into the low L region has been studied previously (e.g., Baker et al, ; Blake et al, ; Califf et al, ; Li et al, ; Reeves et al, ; Su, Selesnick, & Blake, ; Turner et al, , ; Zhao et al, ; Zhao & Li, , ). Some of the proposed mechanisms to explain the deep penetration of energetic particles include inward radial diffusion, shock‐induced acceleration and transport, convection of plasma sheet particles, substorm‐related injections, and transport of trapped inner magnetospheric energetic particles by enhanced convection electric field.…”

Diffusive Transport of Several Hundred keV Electrons in the Earth's Slot Region

“…In the slot region, plasmaspheric hiss, magnetosonic waves, and VLF transmitters are commonly observed and are the major factors to cause energetic electron flux decay. The sudden particle enhancement at low L shells is a dominant source of lower energy particles on a short timescale (Turner et al, ). At several hundred keV and for a 10 day timescale, our study suggests that the radial diffusion and pitch angle scattering loss could dominate the evolution of electrons in the slot region after the initial enhancement during intense storms.…”

“…A slot region with relatively lower particle fluxes forms between the two belts due to pitch angle scattering (Imhof et al, ; Lyons & Thorne, ). Although the two belts are usually well distinguished, energetic electrons are occasionally transported across the slot region following the enhancement in the outer belt during disturbed periods (Claudepierre et al, ; Reeves et al, ; Turner et al, ). The spatiotemporal evolution of energetic electrons in the slot region is energy‐dependent.…”

Variation of Radiation Belt Electron Flux During CME‐ and CIR‐Driven Geomagnetic Storms: Van Allen Probes Observations

“…For 33 magnetic storms that resulted in enhanced electron flux and 29 storms that did not, O'Brien et al (2001) conducted a cross-correlation analysis for solar wind and geomagnetic activity data for outer radiation belt electrons at geosynchronous orbit for 0.3-1.5 and >2-MeV energies. Recently, some studies are dedicated to electron flux variations using National Aeronautics and Space Administration's Van Allen Probes [Baker et al, 2014;Kilpua et al, 2015;Li et al, 2015;Shprits et al, 2017;Turner et al, 2015Turner et al, , 2016Zhao et al, 2017]; however, the causes for the different response of the electron flux are still unclear. Anderson et al (2015) used LANL 1.8-to 3.5-MeV electron flux data and studied 342 magnetic storms with Dst > -50 nT.…”