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 thin 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 transverse 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 can calculate the anomalous resistivity directly and show that the resistivity is significant. 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.