Phases of nonlinear double tearing modes are studied numerically. The first two phases lead to the formation and growth of magnetic islands and are followed by a fast reconnection phase to complete the process, driven by a process of neighboring magnetic separatrices merging and magnetic islands coupling. The fast growth can be understood as a result of the island interaction equivalent to a steadily inward flux boundary driven. Resistivity dependences for various phases are studied and shown by scaling analysis for the first time. It is found that after an early Sweet-Parker phase with a ${\eta }^{1/2}$-scale, a slow nonlinear phase in a Rutherford regime with a ${\eta }^1$-scale is followed by the fast reconnection phase with a ${\eta }^{1/5}$-scale.

• Received 12 January 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.185004