Magnetic-energy release from a zero-net-current layer

Abstract

The storage of magnetic energy in many natural systems is driven by localized convection. Since the initial state can often be described as a potential field, the stress arising from subsequent magnetic twisting is characterized by vanishing net-current flow. Such a layering scheme provides the lowest global energy excess for a given local field twist. In this paper we investigate the nonlinear release of this stored energy by the resistive magnetic-tearing instability. Our aim is to discover whether the evolution of this dynamic reconnection process is modified by the finite width and restoring force of the energizing field reversal. We use a force-free magnetic-field model with the sheared reversing component varying as tanh y sech Ky. We study the unstable nonlinear evolution by a 2·5-dimensional resistive magnetohydrodynamic simulation in a slab geometry. Almost all of the perpendicular (with respect to the ignorable coordinate) stored magnetic energy is released for some cases in this model. Although the magnetic reconnection is due to resistivity, the resulting closed flux surfaces evolve to a lower-energy state, i.e. to a nearly circular one, by mostly ideal MHD. Thus an elongated flux surface drives the plasma flow, and the flow speed is comparable to the Alfvén speed. The excess or stored magnetic energy is thereby released. In general, however, these processes take place only after reconnection occurs, and thus the release of energy is limited by the reconnection rate. The evolution of our new current/field system, which is more realistic as a model of a solar flare, exhibits an energy-release rate similar to that of more conventional field configurations. This rate can be increased, however, by the effects of an assumed anomalous resistivity enhancement.