The Success of MMS in Characterizing Electron Diffusion Regions During Reconnection in the Earth’s Magnetosphere.

A major science goal of the MMS mission is “to reveal, for the first time, the small-scale three-dimensional structure and dynamics of the elusively thin and fast-moving electron diffusion region”. The mission has been hugely successful in accomplishing this goal, where remarkable and detailed data have been obtained for a large number of electron diffusion regions (EDRs) encounters. In this talk I will review results for three of the perhaps most studied EDRs [1,2,3]. First, in the dayside magnetopause strong density asymmetries yield crescent shaped features in the electron velocity distributions [1] over a range along the topological separator [4], whereas within the EDR the frozen-in-law is broken by dynamics of oblique electron beams [5]. In the Earth magnetotail, perfectly symmetric and anti-parallel reconnection has been observed [2], which permitted a direct evaluation of the off-diagonal electron pressure stress elements responsible for reconnection [6]. Reconnection with a weak guide-field has also been observed [3], which together with trapped electron dynamics and associated electron pressure anisotropy impact the Regime-type of the EDR [7,8]