  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 Alfven 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 beta and the shear angle of the
reconnecting field and discuss observational evidence for diamagnetic
stabilization at the magnetopause.
