Effects of Nonresonant Instability and Magnetic Reconnection on Ion Heating and Acceleration in Quasi-parallel Shock Waves

To understand the ion injection for diffusive shock acceleration in space and astrophysical shock waves, such as in planetary bow shocks and supernova remnant shocks, ion heating and acceleration in high-Alfvén-Mach-number (MA) quasi-parallel shock waves are studied by means of full particle-in-cell (PIC) simulations and theory. We perform 2D and 3D PIC simulations in the regime of MA ≥ 10, where shock-driven turbulence contains a number of current sheets and magnetic reconnection sites. The nonresonant ion–ion beam mode grows in the shock transition region, which creates current sheets, and some of them drive magnetic reconnection. The ion temperature in magnetic islands produced by ion-coupled reconnection becomes significantly higher than those in the surrounding regions. This temperature enhancement is due to two physical reasons: the reduction of the number density of the incident ions in the nonresonant wave, and the energization and heating in the magnetic islands. Ions that enter magnetic islands are energized by the Hall electric field pointing toward the island center. Ions are also energized by the motional electric field produced in the outer regions of the islands and the current sheets. The energy increase rate of ions by these energization mechanisms is much larger than that of the conventional shock drift acceleration. These energetic ions are unmagnetized, and they escape from the shock transition region toward the upstream region; therefore, ions in shocks with reconnecting current sheets can be injected into diffusive shock acceleration more efficiently than those in laminar shock waves.