Observational evidence for an elongated (>50 ion skin depths) electron diffusion region during fast magnetic reconnection

We report observational evidence for an elongated electron reconnection diffusion region during fast reconnection. The Cluster in-situ observations in a magnetosheath reconnecting current sheet reveal a broad current layer (width around 12 ion skin depths) supporting the reversal of the reconnecting magnetic field with an embedded intense outflow current that implies a super-Alfvenic electron outflow jet with a transverse scale of ~8 electron skin depths. The oblique trajectory of the spacecraft through the reconnection layer allows the deduction of a fast reconnection rate of 9% of the upstream Alfven speed as well as the minimum extent of the super-Alfvenic electron outflow jet of 50 ion skin depths downstream from the X-line. The deduced reconnection rate is consistent with the directly measured rate of 8% based on the tangential electric field and the inflow plasma velocity. These observations confirm and extend the surprising finding from recent large-scale full particle simulations of the presence of an elongated (> 20 ion skin depths) electron diffusion region even during fast reconnection [Shay et al., 2007; Karimabadi et al., 2007].