Kinetic Theory of Compressed Neutral Sheets

We present a 2D Vlasov equilibrium of current sheets incorporating the effect of plasma compression. The theory includes a magnetic guide field, both external and self-consistent. When the scale-length is compressed below the ion gyro-diameter a perpendicular ambipolar electric field is generated that cannot be transformed away. This electric field causes unequal electron and ion E×B drifts (due to gyro-averaging), resulting in an enhancement of the current density in the Hall direction. The electric field generates sheared flows near the center of the current sheet which can drive lower-hybrid waves giving rise to anomalous dissipation and breaking the frozen-in condition where the impact to magnetic reconnection is optimal. As Mahajan and Hazeltine [doi:10.1063/1.873939] showed, sheared flows are accompanied by non-gyrotropic particle distributions, which the model naturally accounts for. The electric field also generates off-diagonal components of the pressure tensor which along with the anomalous dissipation impacts reconnection. The theory suggests that the compression drives the physics in current layers via the ambipolar electric field. We compare the results to observations of thin current sheets using MMS data.