Earth Planets Space, Vol. 53 (No. 6), pp. 423-429, 2001
T. G. Forbes
Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, U.S.A.
(Received July 20, 2000; Revised November 27, 2000; Accepted November 27, 2000)
Abstract: It is not always appreciated that Petschek's reconnection mechanism is a particular solution of the MHD equations which applies only when special conditions are met. Specifically, it requires that the flow into the reconnection region be set up spontaneously without external forcing. This condition is satisfied when reconnection in a simple current sheet is initiated by enhancing the resistivity in a localized region. Such a process disrupts the current sheet and launches slow-mode waves which steepen into nearly switch-off shocks of the type predicted by Petschek. As these shocks propagate outwards, the current sheet reforms at the original point of the disturbance, and a quasi-steady Petschek-like configuration is set up. Syrovatskii-like configurations which force reconnection by driving a flow toward an initially current-free, orthogonal x-point are less likely to satisfy the conditions required for Petschek-type reconnection.