Structure and Dynamics of a Compressed Current Sheet in the Earth's Magnetotail*

When a current sheet with a reversed magnetic field configuration is compressed to kinetic scales during geomagnetically active periods, in situ observations show evidence of intense lower hybrid wave activity, magnetic reconnection, and substorm onset. The waves are often attributed to the density gradient driven lower hybrid drift instability. However, studies have shown that velocity-shear can intensify due to plasma compression and drive broadband turbulence peaking at the lower hybrid frequency. Nonetheless, velocity shear-driven waves have largely been overlooked in relation to compressed current sheets in the magnetotail despite evidence that they play important roles in ion heating, acceleration, transport, and other anomalous dissipation processes. We use Magnetospheric Multi-Scale satellite data to analyze kinetic scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse electric field is localized to the region of lower hybrid fluctuations and the pressure gradient in this region is comparatively small, leading to the interpretation that compression of the current sheet and the resulting velocity shear is the underlying fluctuation driving mechanism. Additionally, a new kinetic equilibrium model shows an ambipolar electric field forms self-consistently and intensifies as a large scale Harris current sheet is compressed. This produces velocity shear in the current sheet near the magnetic null, indicating that velocity shear-driven waves can arise in thinning current sheets and provide anomalous dissipation to trigger the reconnection process. In addition, the distribution function becomes non-gyrotropic, which explains the observation of crescent shaped distributions, temperature gradients, and the formation of substructures in the current sheet such as embedded and bifurcated current sheets.