Ion Acceleration at Injection Fronts in the Inner Magnetosphere

During geomagnetic storms a large volume of ions are transported from the magnetotail deep into the inner magnetosphere leading to ion acceleration to the energies of tens to hundreds keV. Energized ions become the dominant source of plasma pressure in the inner magnetosphere. Hot plasma pressure drives large electrical currents which determine global electrodynamics and coupling of the inner magnetosphere-ionosphere system. Recent analysis of ion measurements from the RBSPICE experiment of the Van Allen Probes mission showed that the buildup of plasma pressure in the inner magnetosphere largely occurs in the form of localized discrete injections similar to dipolarization fronts observed in the magnetotail. According to previous studies, in the magnetotail ions can be rapidly energized to ~100 keV in the process of nonlinear trapping enabled by magnetic field reconnection and/or an electrostatic field ahead of dipolarization fronts. It is not clear whether similar processes can operate in the inner magnetosphere where the ambient magnetic field is much higher and the propagation speeds of injection fronts are much lower. The goal of this paper is to investigate the mechanisms of ion energization at injection fronts in the inner magnetosphere with the use three-dimensional test-particle simulations and the comparison with ion measurements at RBSPICE. For this purpose we construct an analytical model of the electric and magnetic field perturbations associated with the injection fronts which are superimposed onto the ambient magnetic field. The model reproduces characteristic properties of injection fronts derived from spacecraft measurements and particle-in-cell kinetic simulations.