What is the condition for fast magnetic reconnection?

The magnetic reconnection driven by magnetic buoyancy instability is studied using two-dimensional MHD simulations for two resistivity models; uniform resistivity, and anomalous resistivity. It is found that the reconnection rate is not uniquely determined by the driving process but strongly dependent on the resistivity model, i.e., the local plasma condition near the neutral point. The uniform resistivity case becomes steady Sweet-Parker type, while the anomalous resistivity case tends to nonsteady Petschek type. In the latter case, the reconnection rate increases with increasing threshold of anomalous resistivity. The formation of magnetic islands (plasmoids) and their subsequent ejection from the current sheet is found to be a key physical process leading to fast reconnection.