Ion Acceleration at Thin Current Sheets far Downstream of the Earth’s bow Shock.

Abstract:

Collisionless shocks, have been studied for many decades and considered the prime accelerator of particles at the Earth’s environment and some other astrophysical settings. The shock transition region is easy to identify by spacecraft observations. It marks the location at which most of the kinetic solar wind energy is partitioned on wave energy, particle acceleration, and heating. Recent publications provided evidence that reconnecting current sheets at the shock ramp region participate in the thermalization of the solar wind plasma. Thin current sheets are generated far downstream and reconnection may occur further downstream as well. The occurrence varies between single to multiple current sheets.

We have studied several crossings of the magnetosheath by the MMS spacecraft, characterising and quantifying the occurrence and consequences of current sheets in terms of plasma heating and ion acceleration far downstream of the shock. These observations also suggest that current sheet formation, downstream ion acceleration, and heating and turbulence occurs likely as a consequence of solar wind input. These observations also provide evidence that under certain plasma conditions these current sheets are an important part of the thermalisation of the magnetosheath plasma as it propagates further downstream, toward the magnetopause. We investigated in detail the energy and velocity distributions and well as the fields to identify acceleration mechanisms.