Energetic ions in dipolarization events

We investigate ion acceleration in dipolarization events in the magnetotail, using the electromagnetic fields of an MHD simulation of magnetotail reconnection and flow bursts as basis for test particle tracing. The simulation results are compared with “Time History of Events and Macroscale Interactions during Substorms” observations. We provide quantitative answers to the relative importance of source regions and source energies. Flux decreases at proton energies up to 10–20 keV are found to be due to sources of lobe or plasma sheet boundary layer particles that enter the near tail via reconnection. Flux increases result from both thermal and suprathermal ion sources. Comparable numbers of accelerated protons enter the acceleration region via cross‐tail drift from the dawn flanks of the near‐tail plasma sheet and via reconnection of field lines extending into the more distant tail. We also demonstrate the presence of earthward plasma flow and accelerated suprathermal ions ahead of a dipolarization front. The flow acceleration stems from a net Lorentz force, resulting from reduced pressure gradients within a pressure pile‐up region ahead of the front. Suprathermal precursor ions result from, typically multiple reflections at the front. Low‐energy ions also become accelerated due to inertial drift in the direction of the small precursor electric field. Key Points Drift entry and reconnection entry into acceleration region are both important Both thermal and suprathermal sources contribute to flux increases Shifted isotropic precursor velocity distribution consistent with MHD flow