Spatial evolution of magnetic reconnection diffusion region structures with distance from the X-line

We report Magnetospheric Multiscale (MMS) four-spacecraft observations of a thin reconnecting current sheet with weakly asymmetric inflow conditions and a guide field of approximately twice the reconnecting magnetic field. The event was observed at the interface of interlinked magnetic field lines at the flank magnetopause when the maximum spacecraft separation was 370 km and the spacecraft covered ~1.7 ion inertial lengths (di) in the reconnection outflow direction. The ion-scale spacecraft separation made it possible to observe the transition from electron-only super-ion-Alfvénic outflow near the electron diffusion region (EDR) to the emergence of sub-Alfvénic ion outflow in the ion diffusion region (IDR). The EDR to IDR evolution over a distance less than 2 di also shows the transition from a near-linear reconnecting magnetic field reversal to a more bifurcated current sheet, as well as significant decreases in the parallel electric field and dissipation. Both the ion and electron heating in this diffusion region event were similar to previously reported heating in the far downstream exhausts. The dimensionless reconnection rate, obtained four different ways, was in the range of 0.13-0.27. This event reveals the rapid spatial evolution of the plasma and electromagnetic fields through the EDR to IDR transition region.