Compressed Current Sheets in the Magnetotail: 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 and non-ideal current sheets (i.e. current sheets that cannot be explained by the Harris equilibrium model) in detail and assess their role in magnetic reconnection. We use MMS satellite data to analyze kinetic-scale structures and dynamics associated with compressed current sheets. Our analysis shows that a transverse ambipolar 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. Our kinetic equilibrium model shows that as a large scale Harris current sheet is compressed, an ambipolar electric field forms and produces velocity shear near the magnetic null, indicating that velocity shear-driven waves can arise in the center of compressed current sheets. The presence and location of shear-driven waves at the center of current sheets is notable for a couple of reasons. First, 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. Second, using MMS wave data we can calculate the anomalous resistivity directly and show that the resistivity is significant, particularly at the magnetic null. Finally, we show that the electron distribution function is non-gyrotropic, which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. Our kinetic equilibrium shows that such non-gyrotropic distribution functions can be generated by a quasi-static electric field and does not necessarily arise from wave induced effects.