MMS Observations of Oscillating Energy Conversion and Electron Vorticity in an Electron-Scale Layer within a Southward Magnetopause Reconnection Exhaust

The MMS satellites traversed a ∼6 d_i-wide and ∼500 km/s southward reconnection exhaust at the dayside magnetopause on 6 December 2015 and ∼29 d_i from the associated X-line region. A narrow ∼0.26–0.34 d_i layer of enhanced ±3.5 nW/m³ oscillating energy conversion perpendicular to the magnetic field resides in this exhaust. It contained two regions of diverging in-plane electric fields in general agreement with two clockwise electron flow vortices and a proposed increase of the electron vorticity ∇ × V_e. The layer developed sunward of a unipolar Hall magnetic field for a duskward B_M/B_L ∼ 0.9 guide field. Each electron flow vortex supported a local ∆B_M ∼ 10 nT strengthening of this Hall field. The presence of this electron-scale layer in a southward exhaust for a duskward guide field is consistent with a two-dimensional simulation of a similar structure that evolved from an X-line into a northward exhaust for a similar strength dawnward guide field.