DISRUPTION OF CORONAL MAGNETIC-FIELD ARCADES

The ideal and resistive properties of isolated large-scale coronal mag netic arcades are studied using axisymmetric solutions of the time-dep endent magnetohydrodynamic (MHD) equations in spherical geometry. We e xamine how flares and coronal mass ejections may be initiated by sudde n disruptions of the magnetic field. The evolution of coronal arcades in response to applied shearing photospheric flows indicates that disr uptive behavior can occur beyond a critical shear. The disruption can be traced to ideal MHD magnetic nonequilibrium. The magnetic field exp ands outward in a process that opens the field lines and produces a ta ngential discontinuity in the magnetic field. In the presence of plasm a resistivity, the resulting current sheet is the site of rapid reconn ection, leading to an impulsive release of magnetic energy, fast flows , and the ejection of a plasmoid. We relate these results to previous studies of force-free fields and to the properties of the '' open-fiel d '' configuration. We show that the field lines in an arcade are forc ed open when the magnetic energy approaches (but is still below) the o pen-field energy, creating a partially open field in which most of the field lines extend away from the solar surface. Preliminary applicati on of this model to helmet streamers indicates that it is relevant to the initiation of coronal mass ejections.