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.