Fast reconnection in a two-stage process

Magnetic reconnection plays an essential role in the generation and evolution of astrophysical magnetic fields. The best tested and most robust reconnection theory is that of Parker and Sweet. According to this theory, the reconnection rate scales with magnetic diffusivity as lambda(Omega) ad lambda(Omega)(1/2). In the interstellar medium, the Parker-Sweet reconnection rate is far too slow to be of interest. Thus, a mechanism for fast reconnection seems to be required. We have studied the magnetic merging of two oppositely directed flux systems in weakly ionized, but highly conducting, compressible gas. In such systems, ambipolar diffusion steepens the magnetic pro le, leading to a thin current sheet. If the ion pressure is small enough and the recombination of ions is fast enough, the resulting rate of magnetic merging is fast and independent of lambdaOmega. Slow recombination or sufficiently large ion pressure leads to slower merging, which scales with lambda(Omega) as lambda(Omega)(1/2). We derive a criterion for distinguishing these two regimes and discuss applications to the weakly ionized ISM and to protoplanetary accretion disks.