Particle Trapping and Acceleration at Dipolarization Fronts: High-Resolution MHD and Test-Particle Simulations

Abstract:

Much of plasma heating and transport from the magnetotail into the inner magnetosphere occurs in the form of mesoscale discrete injections associated with sharp dipolarizations of magnetic field (dipolarization fronts). In this study we investigate the mechanisms of electron and ion acceleration at dipolarization fronts in a high-resolution global magnetospheric MHD model (LFM). We use large-scale three-dimensional test-particle simulations (CHIMP) to address the following science questions: 1) what are the characteristic scales of dipolarization regions that can stably trap ions? 2) what role does the trapping play in particle transport and acceleration? 3) how does it depend on particle mass, energy, charge state and distance from Earth? 4) to what extent particle acceleration is adiabatic? High-resolution LFM was run using idealized solar wind conditions with fixed nominal values of density and velocity and a southward IMF component of -5 nT. To simulate particle interaction with dipolarization fronts, a large ensemble of test particles distributed in energy, pitch-angle, and gyrophase was initialized inside one of the LFM dipolarization channels in the magnetotail. Full Lorentz ion trajectories were then computed over the course of the front inward propagation from the distance of 17 to 6 Earth radii. A large fraction of particles with different initial energies stayed in phase with the front over the entire distance. The effect of magnetic trapping at different mass, energy, and charge states was elucidated with a correlation of the the guiding center and the ExB drift velocities.