Diamagnetic suppression of component magnetic reconnection at the magnetopause

We present particle‐in‐cell simulations of collisionless magnetic reconnection in a system (like the magnetopause) with a large density asymmetry across the current layer. In the presence of an ambient component of the magnetic field perpendicular to the reconnection plane the gradient creates a diamagnetic drift that advects the X‐line with the electron diamagnetic velocity. When the relative drift between the ions and electrons is of the order the Alfvén speed the large scale outflows from the X‐line necessary for fast reconnection cannot develop and the reconnection is suppressed. We discuss how these effects vary with both the plasma β and the shear angle of the reconnecting field and discuss observational evidence for diamagnetic stabilization at the magnetopause.