CURRENT SHEET FORMATION IN THE INTERSTELLAR-MEDIUM

There is phenomenological evidence that magnetic reconnection operates in the interstellar medium, and magnetic reconnection is also necessa ry for the operation of a galactic dynamo. The extremely long ohmic di ffusion times of magnetic fields in typical interstellar structures su ggest that reconnection occurs in two stages, with thin current layers that have relatively short resistive decay times forming by magnetohy drodynamical processes first, followed by reconnection of the fields i n the layers. We propose that ambipolar drift can lead to the formatio n of these thin sheets in weakly ionized interstellar gas and can deli neate the parameter regime in which this occurs by means of a numerica l model: we find that the magnetic field cannot be too large and the m edium cannot be too diffusive. Both limits are imposed by the requirem ent that the field be wound up about 1 time by the eddy.