NONEQUILIBRIUM IN SHEARED AXISYMMETRICAL FORCE-FREE MAGNETIC-FIELDS

The problem of the ideal evolution through a sequence of force-free co nfigurations of an axisymmetric magnetic held occupying the region out side a sphere is considered. The field is initially a potential arcade , and its footpoints on the boundary are slowly sheared by azimuthal s tationary motions. It is shown on general grounds that, after an initi al phase during which energy gets quietly stored, the field starts exp anding at an increasing rate, approaching very rapidly an open field ( possibly reached at finite time!)-which implies in general a breakdown of the quasi-static approximation at some stage. The possible importa nce of this result for our understanding of eruptive processes in the solar corona is emphasized.