Reconnection in the Interstellar Medium

Abstract

We discuss the role of ambipolar diffusion for simple reconnection in a\rpartially ionized gas, following the reconnection geometry of Parker and\rSweet. When the recombination time is short, the mobility and\rreconnection of the magnetic field is substantially enhanced as matter\rescapes from the reconnection region via ambipolar diffusion. Our\ranalysis shows that in the interstellar medium it is the recombination\rrate that usually limits the rate of reconnection. Consequently, the\rtypical reconnection velocity in the interstellar medium is\r~(eta/tau_recomb)^1/2(1 + 2xbeta)^1/2(xbeta)^-1, where eta is the ohmic\rresistivity, x is the ionization fraction, and beta is the ratio of gas\rpressure to magnetic pressure. We show that heating effects can reduce\rthis speed by increasing the recombination time and raising the local\rion pressure. In the colder parts of the interstellar medium (ISM), when\rtemperatures are ~10^2 K or less, we obtain a significant enhancement\rover the usual Sweet-Parker rate, but only in dense molecular clouds\rwill the reconnection velocity exceed 10^-3 times the Alfvén\rspeed. In all cases the ion-neutral drag has a negligible effect on the\roverall speed of reconnection, in spite of the fact that the typical\rion-neutral collision time is usually shorter than all other relevant\rtimescales. The ratio of the ion orbital radius to the reconnection\rlayer thickness is typically a few percent, except in dense molecular\rclouds where it can approach unity. We briefly discuss prospects for\robtaining much faster reconnection speeds in astrophysical plasmas.