Electron acceleration and heating in magnetic reconnection in the shock turbulence in the Earth's bow shock

Recent space observations by NASA's Magnetospheric Multiscale (MMS) mission revealed that the turbulence in the Earth's bow shock and the magnetosheath drives magnetic reconnection and dissipates energy from magnetic field to thermal energy. Electron-only reconnection occurs in small-thickness (less than ion kinetic scale) current sheets in the shock turbulence, generating electron jets, while ions are passing through these regions. In these electron-only reconnection sites, electrons can be accelerated and heated effectively, because of large electric fields produced by the electron convection and the particle kinetic effects. Using two-dimensional particle-in-cell simulations, we investigate the shock transition region of the Earth's quasi-parallel bow shock. Performing statistical analysis of electron-only reconnection sites, we show that the reconnection electric field in electron-only reconnection is much greater than that predicted in the standard ion-coupled reconnection. Because of fast electron flows, reaching the order of the electron Alfven speed, strong electric fields are generated in reconnection regions. MMS observations in the Earth's magnetosheath show strong reconnection electric fields consistent with the prediction in electron-only reconnection. We perform particle tracing in the shock simulation to understand how electrons are energized in the shock driven reconnection. Electron temperatures in the transition region are increasing with time as many reconnection sites are generated, and the energy distribution function shows a power-law with nonthermal particles. Reconnection sites evolve dynamically, and within a fraction of ion cyclotron period, some electrons are energized very rapidly, mainly due to the perpendicular electric field, reflecting between multiple reconnection X lines or trapped in magnetic islands. We discuss the implications of reconnection for the shock electron heating and acceleration.