Reconnection in the Interstellar Medium

We discuss the role of ambipolar diffusion for simple reconnection in a partially ionized gas, following the reconnection geometry of Parker and Sweet. When the recombination time is short, the mobility and reconnection of the magnetic field is substantially enhanced as matter escapes from the reconnection region via ambipolar diffusion. Our analysis shows that in the interstellar medium it is the recombination rate that usually limits the rate of reconnection. Consequently, the typical reconnection velocity in the interstellar medium is ~(η/τrecomb)1/2(1 + 2xβ)1/2(xβ)-1, where η is the ohmic resistivity, x is the ionization fraction, and β is the ratio of gas pressure to magnetic pressure. We show that heating effects can reduce this speed by increasing the recombination time and raising the local ion pressure. In the colder parts of the interstellar medium (ISM), when temperatures are ~102 K or less, we obtain a significant enhancement over the usual Sweet-Parker rate, but only in dense molecular clouds will the reconnection velocity exceed 10-3 times the Alfvén speed. In all cases the ion-neutral drag has a negligible effect on the overall speed of reconnection, in spite of the fact that the typical ion-neutral collision time is usually shorter than all other relevant timescales. The ratio of the ion orbital radius to the reconnection layer thickness is typically a few percent, except in dense molecular clouds where it can approach unity. We briefly discuss prospects for obtaining much faster reconnection speeds in astrophysical plasmas.