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 di‐wide and ∼500 km/s southward reconnection exhaust at the dayside magnetopause on 6 December 2015 and ∼29 di from the associated X‐line region. A narrow ∼0.26–0.34 di 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 ∇ × Ve. The layer developed sunward of a unipolar Hall magnetic field for a duskward BM/BL ∼ 0.9 guide field. Each electron flow vortex supported a local ∆BM ∼ 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. Plain Language Summary: Magnetic reconnection is a critical process that drives large‐scale plasma convection in the Earth's magnetosphere. In this letter, we report new direct MMS observations for the presence of two electron flow vortices within a sub‐ion scale energy conversion channel that resides in a dayside magnetopause reconnection exhaust. Each of the two electron‐scale flow vortices is shown to contribute a significant fraction toward the large‐scale Hall magnetic field. Key Points: Electron‐scale 0.3 di layer of oscillating energy conversion and electron vorticity detected in 6 di wide reconnection jet 30 di from EDRTwo regions of in‐plane diverging electric field in energy conversion layer consistent with two measured clockwise electron flow vorticesOut‐of‐plane magnetic field increased ∼10 nT from each passing electron vortex immediately sunward of 50 nT unipolar Hall magnetic field [ABSTRACT FROM AUTHOR]