Magnetic Reconnection in a Quasi-Parallel Shock: Two-Dimentional Local Particle-in-Cell Simulation

Magnetic reconnection in a quasi-parallel bow shock is investigated with two-dimensional local particle-in-cell simulations. In the shock transition and downstream regions, large amplitude magnetic fluctuations exist, and abundant current sheets form. In some current sheets, reconnection occurs, and ion-scale and electron-scale magnetic islands are generated. In electron-scale island regions, only electron outflow jets are observed, producing a quadrupolar out-of-plane magnetic field pattern, while in ion-scale islands, both ions and electrons are involved and energized in reconnection. Normalized reconnection rates are obtained to be between around 0.1 to 0.2, and particle acceleration signatures are seen in distribution functions. Plain Language Summary In the Earth's bow shock, instabilities generate winding magnetic field lines and turbulence in the shock transition region, location where magnetic field, density, and temperature rapidly increase and the bulk flow speed rapidly decreases into the downstream. Many current sheets exist in these regions, provoking magnetic reconnection to occur, which has been observed by space observations. We investigate a quasi-parallel shock (i.e., where the shock normal angle is less than 45 degrees) by means of two-dimensional, fully kinetic (both electrons and ions are particles) simulation. Plasma parameters are defined as relevant to the Earth's bow shock. In a simulation where the shock angle is 25 degrees, many ion-scale and electron-scale magnetic islands are generated due to magnetic reconnection. In electron-scale small island regions, only electron jets are generated in reconnection, and no ion jets exist. A quadrupolar magnetic field pattern forms due to the electron jets, and fast reconnection rates are observed. In ion-scale magnetic island regions, both ions and electrons are involved in fast reconnection. Accelerated particles due to magnetic reconnection are found in particle distribution functions, and reconnection in shock may be a viable injection mechanism for energetic particle generation through shock acceleration.