Dynamical evolution of a solar coronal magnetic field arcade

Abstract

We present calculations of the long-term dynamical evolution of a solar coronal magnetic field arcade which is subjected to shearing photospheric flows. The evolution is obtained by numerical solution of a subset of the resistive magnetohydrodynamic equations. For a simplified model of the bipolar magnetic field observed in the solar corona, we find that photospheric flow produces a slow evolution of the magnetic field with a buildup of magnetic energy. For certain photospheric shear profiles, the field configuration produced is linearly unstable to an ideal magnetohydrodynamic mode when the shear exceeds a critical value. The nonlin-ear evolution of this instability shows the spontaneous formation of current sheets. Reconnection of the magnetic field produces a rapid release of magnetic energy. The major fraction of the energy is dissipated resistívely, while a small fraction is converted into kinetic energy of an ejected plasmoid. The relevance of these results to two-ribbon flares is discussed.