A NUMERICAL STUDY OF THE PREEJECTION, MAGNETICALLY-SHEARED CORONA AS A FREE-BOUNDARY PROBLEM

A class of magnetostatic equilibria with axial symmetry outside a unit sphere in the presence of plasma pressure and an r(-2) gravitational field is constructed. The structure contains a localized current-carry ing region confined by a background bipolar potential field, and the s hape of the region changes subject to the variation of the electric cu rrent. The continuity requirement for the magnetic field and plasma pr essures at the outer boundary of the cavity defines a free boundary pr oblem, which is solved numerically using a spectral boundary scheme. T he model is then used to study the expansion of the current-carrying r egion, caused by the buildup of magnetic shear, against the background confining field. The magnetic shear in our model is induced by the lo ading of an azimuthal field, accompanied by a depletion of plasma dens ity. We show that due to the additional effect of confinement by the d ense surrounding plasma, the energy of the magnetic field can exceed t he energy of its associated open field, presumably a necessary conditi on for the onset of coronal mass ejections. (However, the plasma beta of the confining fluid is higher than that in the outer boundary of a realistic helmet-streamer structure.) Furthermore, under the assumptio n that coronal mass ejections are driven by magnetic buoyancy, the res ult from our model study lends further support to the notion of a susp ended magnetic flux rape in the low-density cavity of a helmet-streame r as a promising pre-ejection configuration.