Observation of double layer in the separatrix region during magnetic reconnection

We present in situ observation of double layer (DL) and associated electron measurement in the subspin time resolution in the separatrix region during reconnection for the first time. The DL is inferred to propagate away from the X line at a velocity of about ion acoustic speed and the parallel electric field carried by the DL can reach -20 mV/m. The electron displays a beam distribution inside the DL and streams toward the X line with a local electron Alfvén velocity. A series of electron holes moving toward the X line are observed in the wake of the DL. The identification of multiple similar DLs indicates that they are persistently produced and therefore might play an important role in energy conversion during reconnection. The observation suggests that energy dissipation during reconnection can occur in any region where the DL can reach.