MMS Observations of Super Thin Electron-Scale Current Sheets in the Earth's Magnetotail

Flapping of the current sheet (CS) associated with the propagation of high-velocity bulk flows allowed to observe the super thin current sheet (STCS) in the magnetotail plasma sheet (PS). During the interval of interest, 111 crossings of the CS neutral plane were detected. In 95 crossings, the STCSs with a current density J >= 20 nA/m(2) were observed. A half-thickness (L-STCS) of the STCSs was about a few electron gyroradii or less. In a number of the STCSs the parameter of adiabaticity (kappa(e)) was <1.0 for suprathermal electron population (>1 keV). Our analysis has shown that the electric current in such STCSs is carried by unmagnetized electrons (kappa(e) < 1), and the stress balance is supported by the off-diagonal terms of their pressure tensor. In this sense, the underlying physics of the formation of STCSs at electron scales by unmagnetized electrons is similar to the mechanism of ion-scale thin CS formation by the quasi-adiabatic ions. The low-energy population of magnetized electrons is also crucially important since it keeps the STCS stable and allows their observation as a quasi-stationary structure. We compare the observed half-thickness of the STCS with that predicted by a new kinetic theory (lambda) considering the coupling between ion-scale TCS and electron-scale STCS. We found a reasonable agreement between both values: L-STCS similar to (0.3-1)lambda. Further improvement in the theory taking into account the dynamics of unmagnetized electrons may provide better agreement with observations.