The Alfv,n edge in asymmetric reconnection

We show that in the case of magnetic reconnection where the Alfv,n velocity is much higher in the plasma on one side of the current sheet than the other, an Alfv,n edge is formed. This edge is located between the electron and ion edges on the high Alfv,n velocity side of the current sheet. The Alfv,n edge forms because the Alfv,n wave generated near the X-line will propagate faster than the accelerated ions forming the ion edge. We discuss possible generation mechanism and the polarization of the Alfv,n wave in the case when higher Alfv,n speed is due to larger magnetic field and smaller plasma density, as in the case of magnetopause reconnection. The Alfv,n wave can be generated due to Hall dynamics near the X-line. The Alfv,n wave pulse has a unipolar electric field and the parallel current will be such that the outer current on the high magnetic field side is flowing away from the X-line. Understanding Alfv,n edges is important for understanding the separatrix regions at the boundaries of reconnection jets. We present an example of Alfv,n edge observed by the Cluster spacecraft at the magnetopause.