Evolution of eruptive flares .1. Plasmoid dynamics in eruptive flares

We investigate the resistive processes of plasmoid dynamics in eruptive flares by performing 2.5-dimensional resistive MHD numerical simulations. We start with a linear force-free field arcade and impose the localized resistive perturbation on the symmetry axis of the arcade. Then the magnetic fields begin to dissipate, producing inflows toward this region. These inflows make the magnetic fields convex to the symmetry axis and hence a neutral point is formed on this axis, leading to a formation of a magnetic island around the symmetry axis. At the first stage, the magnetic island slowly rises by the upflow produced by the initial resistive perturbation. Then, once the anomalous resistivity sets in, the magnetic island begins to be accelerated. This acceleration stops after the fast MHD shock is formed at the bottom of the magnetic island, which implies that the upflow around the central part of the magnetic island is no longer strong. These three stages in the evolution of the plasmoid are confirmed to exist in the observational results. Moreover, a time lag between the start time when the magnetic island begins to be accelerated and the peak time of the neutral-point electric field can be explained by the inhibition of magnetic reconnection by the perpendicular magnetic field. We also study the difference of the initial rise motion of the plasmoid between the simulation results and the observational ones, and we conclude that, in actual situations, the initial resistive perturbation proceeds very weakly and at many positions inside the arcade.