NONEQUILIBRIUM IN SHEARED AXISYMMETRIC FORCE-FREE MAGNETIC FIELDS

摘要

The problem of the ideal evolution through a sequence of force-free configurations of an axisymmetric magnetic field occupying the region outside a sphere is considered. The field is initially a potential arcade, and its footpoints on the boundary are slowly sheared by azimuthal stationary motions. It is shown on general grounds that, after an initial phase during which energy gets quietly stored, the field starts expanding 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 importance of this result for our understanding of eruptive processes in the solar corona is emphasized.