MMS Observations of Compressed Current Sheets: Importance of the Ambipolar Electric Field

Micro-scale features are now being resolved by NASA's Magnetospheric Multi-Scale (MMS) mission, which means for the first time, we are able to investigate gyro-scale current sheets in detail and assess their role in magnetic reconnection. Our analysis of kinetic-scale structures and dynamics associated with compressed current sheets in MMS data shows that a perpendicular ambipolar electric field is localized to the region of lower hybrid fluctuations and the pressure gradient is comparatively small, leading to the interpretation that E×B velocity shear is the underlying fluctuation driving mechanism. The presence and location of shear-driven waves at the center of current sheets is notable because laboratory experiments and PIC simulations have shown that shear-driven lower hybrid fluctuations are capable of producing significant anomalous cross-field transport and resistivity, which can trigger magnetic reconnection. We estimate terms in generalized Ohm's Law, including anomalous resistivity, viscosity, and off-diagonal pressure tensor, directly from data and show the terms are comparable. Finally, we show that the electron distribution function is non-gyrotropic (generated by a quasi-static electric field), which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur.