The nature of Petschek-type reconnection

It is not always appreciated that Petschek's reconnection mechanism is a pa rticular solution of the MHD equations which applies only when special cond itions are met. Specifically, it requires that the flow into the reconnecti on region be set up spontaneously without external forcing, This condition is satisfied when reconnection in a simple cur-rent sheet is initiated by e nhancing the resistivity in a localized region. Such a process disrupts the current sheet and launches slow-mode waves which steepen into nearly switc h-off shocks of the type predicted by Petschek. As these shocks propagate o utwards, the current sheet reforms at the original point of the disturbance , and a quasisteady Petschek-like configuration is set up. Syrovatskii-like configurations which force reconnection by driving a flow toward an initia lly current-free, orthogonal x-point are less likely to satisfy the conditi ons required for Petschek-type reconnection.