Ion velocity distributions in dipolarization events: Beams in the vicinity of the plasma sheet boundary

Using combined MHD/test particle simulations, we further explore characteristic ion velocity distributions in relation to magnetotail reconnection and dipolarization events, focusing on distributions at and near the plasma sheet boundary layer (PSBL). Simulated distributions right at the boundary are characterized by a single earthward beam, as discussed earlier (Birn et al., 2015). However, farther inside, the distributions consist of multiple beams parallel and anti-parallel to the magnetic field, remarkably similar to recent Magnetospheric Multi-Scale (MMS) observations. The simulations provide insight into the mechanisms: the lowest earthward beam results from direct acceleration at an earthward propagating dipolarization front (DF), with a return beam at somewhat higher energy. A higher-energy earthward beam results from dual acceleration, first near the reconnection site and then at the DF, again with a corresponding return beam resulting from mirroring closer to Earth. Multiple acceleration at the X-line or the propagating DF with intermediate bounces may produce even higher-energy beams. Particles contributing to the lower-energy beams are found to originate from the PSBL with thermal source energies, increasing with increasing beam energy. In contrast, the highest-energy beams consist mostly of particles that have entered the acceleration region via cross-tail drift with source energies in the supra-thermal range.