On the short-scale spatial variability of electron inflows in electron-only magnetic reconnection in the turbulent magnetosheath observed by MMS

Abstract:

Magnetic reconnection in current sheets is an energy conversion process that rapidly converts magnetic energy into particle energy. In turbulent plasmas, which contain a large number of intense current sheets, reconnection has long been suggested to play an important role in the dissipation of turbulence energy at kinetic scales. It was recently discovered by MMS that reconnection does indeed occur in some of the turbulent magnetosheath current sheets, but surprisingly, the process only involves electrons, with no ion coupling. The lack of ion coupling was attributed to the small-scale sizes of the current sheets. We have examined in detail the electron inflow properties of an electron-only reconnection event observed by all 4 MMS spacecraft. Even though the maximum MMS inter-spacecraft separation for the event was only 7 electron skin depth (d_e), the amplitudes of the inflows differed by as much as a factor of two among the spacecraft. Furthermore, the two inflow velocities were often highly asymmetric. We compared the observations with a 3D kinetic simulation of electron-only reconnection and found that the asymmetries in the inflows could be the result of the spacecraft trajecting through different positions in the out-of-plane (X line) direction, even though the spacecraft separations were only a few d_e apart. This finding suggests new underlying 3D properties of electron-only reconnection and could be used as an observable signature to identify electron-only reconnection.