Particle-in-cell simulations of collisionless magnetic reconnection are performed to study asymmetric reconnection in which an outflow is blocked by a hard wall while leaving sufficiently large room for the outflow of the opposite direction. This condition leads to a slow, roughly constant motion of the diffusion region away from the wall, the so-called “$X$-line retreat.” The typical retreat speed is $\sim 0.1$ times the Alfvén speed. At the diffusion region, ion flow pattern shows strong asymmetry and the ion stagnation point and the $X$ line are not collocated. A surprise, however, is that the reconnection rate remains the same unaffected by the retreat motion.

• Received 15 August 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.205004