Evidence for magnetic reconnection initiated in the magnetosheath

We report Cluster spacecraft 1 observations of a reconnection exhaust embedded in the magnetosheath flow. The reconnection evidence consists of (1) accelerated plasma outflows, (2) interpenetrating ion beams, (3) reconnection inflows and (4) the associated tangential reconnection electric field. Furthermore, Hall magnetic fields were observed with no gaps in the middle of the exhaust. Together with the exhaust thickness being similar to 10 ion skin depths, this implies that the spacecraft crossed through the ion diffusion region. The dimensionless reconnection rate determined independently using the plasma and electric field measurements was similar to 0.07, implying fast reconnection. The same (but 36 times thicker) current sheet was observed upstream in the solar wind by the ACE and Wind spacecraft but without the reconnection signatures. These observations suggest that reconnection was initiated in the magnetosheath due to compression of the (non- reconnecting) solar wind current sheet at the bow shock and against the dayside magnetopause.