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    Scaling laws of resistive magnetohydrodynamic reconnection in the high-Lundquist-number, plasmoid-unstable regime

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    The Sweet-Parker layer in a system that exceeds a critical value of the Lundquist number (SS) is unstable to the plasmoid instability. In this paper, a numerical scaling study has been done with an island coalescing system driven by a low level of random noise. In the early stage, a primary Sweet-Parker layer forms between the two coalescing islands. The primary Sweet-Parker layer breaks into multiple plasmoids and even thinner current sheets through multiple levels of cascading if the Lundquist number is greater than a critical value Sc≃4Γ—104S_{c}\simeq4\times10^{4}. As a result of the plasmoid instability, the system realizes a fast nonlinear reconnection rate that is nearly independent of SS, and is only weakly dependent on the level of noise. The number of plasmoids in the linear regime is found to scales as S3/8S^{3/8}, as predicted by an earlier asymptotic analysis (Loureiro \emph{et al.}, Phys. Plasmas \textbf{14}, 100703 (2007)). In the nonlinear regime, the number of plasmoids follows a steeper scaling, and is proportional to SS. The thickness and length of current sheets are found to scale as Sβˆ’1S^{-1}, and the local current densities of current sheets scale as Sβˆ’1S^{-1}. Heuristic arguments are given in support of theses scaling relations.Comment: Submitted to Phys. Plasma
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