1. Fast Reconnection in High-Lundquist- Number Plasmas Due to Secondary Tearing Instabilities A.Bhattacharjee, Y.-M. Huang, H. Yang, and B. Rogers Center for Integrated Computation and Analysis of Reconnection and Turbulence, University of New Hampshire and Dartmouth College

2. Onset of fast reconnection in large, high- Lundquist-number systems The scaling of reconnection in large systems is a problem of great interest where the dimensionless ratio of characteristic dissipation scale to system size is a very small number. What are the mechanisms for the onset of fast reconnection in such systems? Does the answer depend on the mechanism that breaks field lines? One criterion has emerged from Hall MHD (or two-fluid) models, and has been tested carefully in laboratory experiments (MRX at PPPL, VTF at MIT). The criterion is: (Ma and Bhattacharjee 1996, Cassak et al. 2005) or in the presence of a guide field (Aydemir 1992)

3. Onset of fast reconnection in large, high- Lundquist-number systems (continued) Here we demonstrate here that within the framework of resistive MHD, extended current sheets form in large systems that become spontaneously unstable to fast, super- Alfvenic tearing instabilities, and produce a time- dependent, nonlinear, impulsive reconnection regime in which the reconnection rate exceeds Sweet-Parker by an order of magnitude (without the intervention of Hall MHD effects).

5. Super-Alfvenic Tearing Instability of Extended Thin Current Sheets Harris sheet Dispersion relation (Coppi, et al. 1976) Thin current sheets of aspect ratio Fastest growing instability For Sweet-Parker sheets (Louriero et al., 2007),,

8. Summary Onset of fast reconnection, mediated by the dynamics of thin current sheets, in high-Lundquist-number laboratory and space plasmas. Two mechanisms: Hall MHD, seen in theory and laboratory experiments of moderate size, when the Sweet-Parker width falls below the ion skin depth. This onset can be nonlinearly stabilized due to diamagnetic effects. Fast, secondary tearing instabilities of thin current sheets in large systems, substantially exceeding Sweet-Parker rates within the realm of resistive MHD (without invoking Hall current and/or electron pressure tensor effects). Which is the dominant effect?