Ion dynamics in a hybrid simulation of magnetotail reconnection

Abstract

The 2 1/2‐dimensional hybrid simulations are carried out to investigate self‐consistent ion dynamics in time‐dependent magnetic reconnection. The system dimensions 25 RE × 5 RE are able to accommodate large‐scale structures associated with magnetotail reconnection. The simulations are started with two ion populations: hot ions forming the thin current sheet self‐consistent with the initial equilibrium two‐dimensional tail‐like magnetic configuration and cold lobe ions with initially uniform density and temperature. The evolution of the initially stable system is triggered by superposition of a spatially localized resistivity. We find ion acceleration and formation of non‐Maxwellian multicomponent ion distributions as predicted by test particle studies. Fine features, such as layers of counterstreaming ions, are found in the distributions of the initially cold lobe ions. The hot current sheet “source,” however, gives rather smeared “output distributions” due to significant spatial overlapping of ion populations with different histories. We find that ions, which are strongly accelerated in the close vicinity of the reconnection site, contribute little to the equilibrium current before reconnection is initiated. Ions, which significantly contribute to the equilibrium current, are either ejected earthward or carried away inside the released plasmoid without significant energy gain during a single “impulsive” interaction with the reconnection fields. Multiple encounters of the acceleration region and quasi‐Fermi acceleration of ions trapped between the mirroring earthward boundary and the growing normal magnetic field are found. The simulations also show effective heating of initially cold background ions in the plasma sheet behind the released plasmoid. The tailward speed of energetic ions flowing around the plasmoid along reconnected lobe field lines exceeds the speed of plasmoid. The ions which contribute to these beams are part of the hot population and initially reside in the current sheet. These results match spacecraft observations prior to plasmoid arrival.