On the Energy‐Dependent Deep (L < 3.5) Penetration of Radiation Belt Electrons.

Deep penetration of outer radiation belt electrons to low L (<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lower L. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lower L to match the observations of storm time values of E‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthal E‐fields contribute preferentially to the deep penetration of lower‐energy electrons. Plain Language Summary: Electrons trapped by the Earth's magnetic field gather in two regions known as the Van Allen radiation belts. It is well reported that electrons can be transported radially inward from the outer radiation belt during geomagnetically active times. More specifically, low energy (100 s of keV) electrons can be moved radially deeper than higher energy (∼1 MeV) electrons. Previous studies suggested that enhanced convection electric fields could contribute to the earthward transport of low energy (<200 keV) electrons. However, the mechanism which leads to different efficiencies of electron transport at different energies has not been quantified. This study expands the traditional radial diffusion model with an empirically determined convection term and shows that the net convection velocity increases for lower energy electrons. For the first time, we quantitatively modeled the energy‐dependent penetration of radiation belt electrons in a wide energy range (10 s of keV to 2 MeV) in the presence of enhanced large‐scale electric fields, during two geomagnetic storm events observed by the Van Allen Probes mission. Key Points: Convective radial transport of storm‐time enhanced large‐scale E‐fields is an efficient inward transport mechanism of 10–100 s keV electronsThe energy‐dependent electron penetration can be explained by the relation between the timescales of electron drift and large‐scale E‐fieldsA radial diffusion‐convection model is developed to reproduce the storm‐time penetration of lower energy electrons to lower L [ABSTRACT FROM AUTHOR]