Formation of the inner electron radiation belt by enhanced large‐scale electric fields

Abstract

A two‐dimensional bounce‐averaged test particle code was developed to examine trapped electron trajectories during geomagnetic storms with the assumption of conservation of the first and second adiabatic invariants. The March 2013 storm was selected as an example because the geomagnetic activity Kp index sharply increased from 2 + to 7− at 6:00 UT on 17 March. Electron measurements with energies between 37 and 460 keV from the Magnetic Electron Ion Spectrometer (MagEIS) instrument onboard Van Allen Probes (VAP) are used as initial conditions prior to the storm onset and served to validate test particle simulations during the storm. Simulation results help to interpret the observed electron injection as nondiffusive radial transport over a short distance in the inner belt and slot region based on various electric field models, although the quantitative comparisons are not precise. We show that electron drift trajectories are sensitive to the selection of electric field models. Moreover, our simulation results suggest that the actual field strength of penetration electric fields during this storm is stronger than any existing electric field model, particularly for L ≤ 2.

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).…”

“…Several physical mechanisms have been proposed to explain the deep penetration of protons and electrons into the low L region (e.g., Califf et al, ; Korth et al, ; Li et al, ; Lyons & Thorne, ; Reeves et al, ; Ripoll et al, ; Selesnick, Su, & Blake, ; Su et al, ; Turner et al, , 2017; Zhao & Li, ). The most prevalent mechanisms include shock‐induced energization, substorm injections, inward radial diffusion, convection of plasma sheet particles, and transport of trapped energetic particles by enhanced large‐scale convection electric field.…”

“…Su et al () simulated the rapid flux enhancements in the low L region using dipole magnetic field and various electric field models during deep penetration events. They concluded that though the results are very sensitive to electric field models, an enhanced large‐scale convection electric field can transport preexisting energetic electrons in the inner magnetosphere into lower L region and significantly increase relativistic electron fluxes in the low L region.…”

“…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.…”

Typical values of the electric drift E × B/B2 in the inner radiation belt and slot region as determined from Van Allen Probe measurements

“…The analysis can be performed around the magnetic equator [e.g., Volland, 1973;Stern, 1975;Maynard and Chen, 1975;Gussenhoven et al, 1981;Matsui et al, 2004Matsui et al, , 2013 or in the ionosphere [e.g., Richmond et al, 1980] and then extended to the entire inner magnetosphere via mapping along equipotential field lines (in a quasi-steady state, it is commonly assumed that the conductivity along magnetic field lines is so high that field-aligned potential drops are negligible [e.g., Mozer, 1976]). The analysis can be performed around the magnetic equator [e.g., Volland, 1973;Stern, 1975;Maynard and Chen, 1975;Gussenhoven et al, 1981;Matsui et al, 2004Matsui et al, , 2013 or in the ionosphere [e.g., Richmond et al, 1980] and then extended to the entire inner magnetosphere via mapping along equipotential field lines (in a quasi-steady state, it is commonly assumed that the conductivity along magnetic field lines is so high that field-aligned potential drops are negligible [e.g., Mozer, 1976]).…”