Current singularities: Drivers of impulsive reconnection

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Abstract
Reconnection in nature is generically not quasi-steady. Most often, it is impulsive or bursty, characterized not only by a fast growth rate but a rapid change in the time-derivative of the growth rate. New results, obtained by asymptotic analyses and high-resolution numerical simulations [using Adaptive Mesh Refinement] of the Hall magnetohydrodynamics (MHD) or two-fluid equations, are presented. Within the framework of Hall MHD, a two-dimensional collisionless reconnection model is considered in which electron inertia provides the mechanism for breaking field lines, and the electron pressure gradient plays a crucial role in controlling magnetic island dynamics. Current singularities tend to form in finite time and drive fast and impulsive reconnection. In the presence of resistivity, the tendency for current singularity formation slows down, but the reconnection rate continues to accelerate to produce large magnetic islands that eventually become of the order of the system size, quenching near-explosive growth. By a combination of analysis and simulations, the scaling of the reconnection rate in the nonlinear regime is studied, and its dependence on the electron and the ion skin depth, plasma beta, and system size is determined.