Abstract
A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers (10(4)< S < 10(8)) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfveacutenically fast formation of plasmoid (magnetic-island) chains. The plasmoid number scales as S(3/8) and the instability growth rate in the linear stage as S(1/4), in agreement with the theory by Loureiro et al. [Phys. Plasmas 14, 100703 (2007)]. In the nonlinear regime, plasmoids continue to grow faster than they are ejected and completely disrupt the reconnection layer. These results suggest that high-Lundquist-number reconnection is inherently time-dependent and hence call for a substantial revision of the standard Sweet-Parker quasistationary picture for S > 10(4).