Current Sheet Formation in the Interstellar Medium

There is phenomenological evidence that magnetic reconnection operates in the interstellar medium, and magnetic reconnection is also necessary for the operation of a galactic dynamo. The extremely long ohmic di†usion times of magnetic Ðelds in typical interstellar structures suggest that reconnection occurs in two stages, with thin current layers that have relatively short resistive decay times forming by magnetohydrodynamical processes Ðrst, followed by reconnection of the Ðelds in the layers. We propose that ambipolar drift can lead to the formation of these thin sheets in weakly ionized interstellar gas and can delineate the parameter regime in which this occurs by means of a numerical model : we Ðnd that the magnetic Ðeld cannot be too large and the medium cannot be too di†usive. Both limits are imposed by the requirement that the Ðeld be wound up about 1 time by the eddy. Subject headings : di†usion È ISM: magnetic Ðelds È MHD