The Orientation of the Reconnection X-line

We propose a criterion for identifying the orientation of the X-line when two regions of plasma with arbitrary densities, temperatures, and magnetic fields undergo reconnection. The X-line points in the direction that maximizes the (suitably-defined) Alfv'en speed characterizing the reconnection outflow. For many situations a good approximation is that the X-line bisects the angle formed by the magnetic fields.