Ion acceleration and reflection on magnetotail antidipolarization fronts

Abstract:

Antidipolarization fronts (ADFs), tailward-moving structures in the Earth's magnetotail with sharp decreases in the magnetic $B_z$ component, are similar to mirror images of earthward-propagating dipolarization fronts (DFs) generated on the other side of the reconnection site. In this paper, we use ARTEMIS observations and numerical simulations to study the role of ADFs in accelerating and reflecting ions in the ambient plasma sheet. Both case and statistical observations show that the first signature one would observe before ADF arrival is the tailward-streaming, energy-dispersed ions; after about 1 minute, the ion fluxes are enhanced significantly with the peak direction shifted duskward, and then the peak direction gradually shifts back to the tailward direction until the ADF arrives. All these signatures are reproduced by our simulation model of ion acceleration and reflection on ADFs. We further examine typical trajectories of plasma sheet ions before and after their ADF reflection, to understand these seemingly complicated ion signatures as well as their similarities and differences with those characteristics ahead of DFs.