Two-dimensional particle-in-cell simulations of magnetic reconnection and kinetic physics in the Earth's quasi-parallel bow shock

Recent space observations by Magnetospheric Multiscale (MMS) of the Earth's bow shock showed a large number of current sheets in the shock transition region (foreshock and ramp) as well as the downstream region, and some current sheets show signatures of magnetic reconnection (jets, Hall fields, and energized particles). It is important to understand the detailed physics of particle energization in reconnection sites, and how much they contribute to the total heating and acceleration in shocks.

We discuss kinetic physics in magnetic reconnection in shocks, based on numerical results by two-dimensional particle-in-cell simulations. We study quasi-parallel shocks using parameters relevant to the Earth's bow shock. In the foreshock region, electromagnetic instabilities due to multiple components of plasma (incident and reflected particles) generate winding magnetic field lines, which eventually grow to multiple current sheets in the ramp. Some current sheets show magnetic field reversals across the current, where magnetic reconnection can occur. In addition, during the shock reformation process, the formation of current sheets is intensified, and a large number of reconnection sites are observed in the shock transition region. There exist both small-scale (electron scale) magnetic islands and large-scale (ion scale) islands, and small-scale island regions show electron-only reconnection, in which electrons are significantly accelerated but no ion jets form. In contrast, ion-scale island regions show both electron and ion jets, and both species are accelerated and heated.

We present results of wave analyses, in the far upstream region where instabilities start to generate linear waves, and the shock ramp, where large-amplitude waves form due to non-linear effects. We discuss how current sheets form during the evolution of instabilities. Also, performing particle trancing in simulations, we discuss particle acceleration and heating in reconnection in shocks, and we investigate contributions of reconnection to the total energization in shocks, based on individual particle histories and spatiotemporal evolution of particle distribution functions. We compare simulation results with space measurements by MMS in the Earth's quasi-parallel bow shock.