Reconnection in the Interstellar Medium

We discuss the role of ambipolar diffusion for simple reconnection in apartially ionized gas, following the reconnection geometry of Parker and Sweet.When the recombination time is short the mobility and reconnection of themagnetic field is substantially enhanced as matter escapes from thereconnection region via ambipolar diffusion. Our analysis shows that in theinterstellar medium it is the recombination rate that usually limits the rateof reconnection. We show that heating effects can reduce this rate byincreasing the recombination time and raising the local ion pressure. In thecolder parts of the ISM, when temperatures are or the order of 100K or less, weobtain a significant enhancement over the usual Sweet-Parker rate, but only indense molecular clouds will the reconnection velocity exceed 0.001 times theAlfv\'en speed. The ratio of the ion orbital radius to the reconnection layerthickness is typically a few percent, except in dense molecular clouds where itcan approach unity. We briefly discuss prospects for obtaining much fasterreconnection speeds in astrophysical plasmas.Comment: 25 pages, 1 figure, submitted to Ap