How Alfv\'en waves set the large scale structure of magnetic reconnection.

A PIC simulation of anti-parallel reconnection shows the formation of the out-of-plane or Hall magnetic field that extends hundreds of inertial lengths from the X-line.This structure is generated by field-aligned electron currents that flow outside the magnetic separatrices when ion and electrons decouple on length scales less than $d_{i}$. We observe that this Hall field propagates from the X-point to far downstream into the exhaust along the magnetic field lines at Alfv\'enic speed.Thus the propagation of this large scale reconnection structure can be associated with a Alfv\'en wave generated in the inner electron diffusion region, specifically near the X-line.