Onset of Fast Reconnection and its Nonlinear Stabilization in Laboratory and Space Plasmas

The onset of fast reconnection is widely studied in laboratory experiments, in situ satellite measurements in the Earth's magnetosphere, and solar flares. These observations place strong constraints on theory, which must explain not only a fast reconnection rate but also a sudden increase in the time-derivative of the reconnection rate. We will show that important features of such dynamics can be accounted for by Hall MHD (or two-fluid) reconnection models in one unifying framework. The theory also elucidates the role of diamagnetic drifts that can quench nonlinearly the onset of fast reconnection. Thus, the theory explains not only when reconnection is near-explosive, but also when it is not. We will compare the predictions of theory with data from tokamaks, laboratory experiments, and magnetospheric and solar observations. The problem takes on additional complexity when it is applied to large sytsems, which have been the subject of considerable interest recently. We have carried out a sequence of simulations using the same initial conditions for large systems using resistive and Hall MHD models, with resistivity and electron inertia providing the mechanism for breaking field lines. It is shown that the dynamics of thin current sheets in large sytsems is sensitive to the mechanism that breaks field lines, and that velocity shear along the thin current sheets plays an important role in controlling their geometry and stability. We address the implications of these results for fully kinetic simulations and observations of reconnection in large space and astrophysical plasmas.