Electron Distribution Functions in the Separatrix Layer inside the Diffusion Region

Abstract:

Abundant kinetic processes occur in the separatrix layer (SL) that separates the reconnection inflow and exhaust. Electron behavior in the SL, such as violation of the frozen-in condition and enhancement of non-gyrotropy, are distinct from those in the inflow and exhaust, but may be closely related to similar features in the electron diffusion region (EDR). Here we report distinct types of electron distribution functions (EDFs) in the SL within the ion diffusion region at various reconnection evolution stages from 2D PIC simulation and Cluster observation results. In the inflow-side of the SL, the EDFs exhibit a temperature anisotropy as in the inflow region, and pronounced field-aligned electron beams propagate towards the X-line with higher energies than the main inflow population. Electrons with high perpendicular velocities are only found in the outflow-side of the SL at early times, while at later times, electrons in the inflow-side of the SL also acquire high perpendicular energies. In the outflow side of the SL, apart from the separatrix, a long narrow belt-shaped region of enhanced non-gyrotropy extends from the EDR edge to beyond the ion diffusion region. The region of enhanced non-gyrotropy marks the boundary for how far away from the mid-plane accelerated electrons may propagate along reconnected field lines. Between the separatrix and the region of enhanced non-gyrotropy, EDFs are similar to those in the inflow-side of the SL. EDFs in the outflow-side of the non-gyrotropy region show signatures of electrons with high perpendicular energies from the EDR, and are more isotropic than those in inflow-side of the non-gyrotropy region. The distinct features in EDFs provide guidance to infer satellite locations with respect to the separatrix and to study electron heating across the separatrix.