FAST PLASMOID FORMATION IN DOUBLE ARCADES

The quasi-static evolution of a double arcade system is investigated. It has been proposed (Mikic, Barnes, & Schnack) that a periodic array of arcades in two dimensions can develop an ideal MHD instability when footpoint shear exceeds a critical value. Biskamp & Welter found similar results but argued that the instability is an artifact of the symmetry imposed on the system and that two nonsymmetric neighboring arcades can only develop slow plasmoid formation. We have simulated the evolution of two neighboring nonsymmetric arcades with footpoint shearing profiles which can change in time. In particular, we consider the scenario in which one arcade is sheared more strongly than the other, followed by shearing the other arcade more strongly ("asymmetry switching"). We find that rapid forced reconnection can take place repeatedly, forming a series of plasmoids. In our simulations, we have used "soft walls" by introducing unsheared magnetic field between the arcades and simulation box. This is intended to simulate possible influences of nearby fluxes surrounding magnetized structures in the corona, and we find that it reduces the effects of the simulation box. We find that a two arcade system laterally confined by surrounding fluxes can develop instability with nonsymmetric footpoint shear but that the instability is significantly slower in comparison with the case in which the system is confined by a " hard box " or periodic boundary conditions. The fast plasmoid formation process due to asymmetry switching is not due to the presence of simulation walls. The behavior of double arcades with asymmetry switching is described using equilibrium bifurcation diagrams with footpoint shear and degree of asymmetry, as two independent control parameters.