Electron acceleration mechanisms in magnetic reconnection and flux ropes in the Earth's quasi-parallel bow shock

Recent shock observations by NASA’s Magnetospheric Multiscale (MMS) have revealed numerous reconnecting current sheets in the Earth’s bow shock. Magnetic reconnection can produce energetic particles, but the role of reconnection in shock heating and acceleration remains to be investigated.



To understand electron acceleration and heating in reconnection in shocks, we perform 2D particle-in-cell (PIC) simulations of quasi-parallel shocks. In a shock with its Alfven Mach number around 10, reconnecting current sheets show both ion-coupled reconnection and electron-only reconnection. The electron temperature increases significantly in ion-scale flux ropes. The energy spectrum in the shock transition region shows a non-thermal power-law component.



Tracing electron trajectories in the PIC simulation, we identified five energization mechanisms. Fermi acceleration in contracting islands, acceleration in a moving flux rope, a new type of betatron acceleration (“island betatron acceleration”), and two conventional shock acceleration mechanisms (parallel electric field, and shock drift acceleration).



Energization in a moving flux rope and island betatron acceleration account for the higher energy part in the electron spectrum. Fermi acceleration and the conventional shock acceleration mechanisms produce the lower to medium energy part of the spectrum. We conclude that reconnection plays a major role to energize electrons in the shock transition region.