Magnetic Diffusion in Star Formation
  Magnetic diffusion plays a vital role in star formation. We trace its
influence from interstellar cloud scales down to star-disk scales. On both
scales, we find that magnetic diffusion can be significantly enhanced by the
buildup of strong gradients in magnetic field structure. Large scale nonlinear
flows can create compressed cloud layers within which ambipolar diffusion
occurs rapidly. However, in the flux-freezing limit that may be applicable to
photoionized molecular cloud envelopes, supersonic motions can persist for long
times if driven by an externally generated magnetic field that corresponds to a
subcritical mass-to-flux ratio. In the case of protostellar accretion, rapid
magnetic diffusion (through Ohmic dissipation with additional support from
ambipolar diffusion) near the protostar causes dramatic magnetic flux loss. By
doing so, it also allows the formation of a centrifugal disk, thereby avoiding
the magnetic braking catastrophe.
