EFFECTS OF PRESSURE AND RESISTIVITY ON THE AMBIPOLAR DIFFUSION SINGULARITY - TOO LITTLE, TOO LATE

Ambipolar diffusion, or ion-neutral drift, can lead to steepening of t he magnetic field profile and even to the formation of a singularity i n the current density. These results are based on an approximate treat ment of ambipolar drift in which the ion pressure is assumed vanishing ly small and the frictional coupling is assumed to be very strong, so that the medium can be treated as a single fluid. This steepening, if it really occurs, must act to facilitate magnetic reconnection in the interstellar medium, and so could have important consequences for the structure and evolution of the galactic magnetic held on both global a nd local scales. In actuality, the formation of a singularity must be prevented by physical effects omitted by the strong coupling approxima tion, In this paper we solve the coupled equations for charged and neu tral fluids in a simple slab geometry, which was previously shown to e volve to a singularity in the strong coupling approximation. We show t hat both ion pressure and resistivity play a role in removing the sing ularity, but that, for parameters characteristic of the interstellar m edium, the peak current density is nearly independent of ion pressure and scales inversely with resistivity. The current gradient length sca le, however, does depend on ion pressure. In the end, effects outside the fluid approximation, such as the finite ion gyroradius, impose the strictest limit, on the evolution of the magnetic profile.