Models of the large-scale corona. II. Magnetic connectivity and open flux variation

In this paper the changing connectivity of the coronal magnetic field during the formation and ejection of magnetic flux ropes is considered. Using recent simulations of the coronal field, it is shown that reconnection may occur both above and below the flux ropes. Those occurring above slowly strip away coronal arcades overlying the flux ropes and allow the flux ropes to be ejected. In contrast, those below help to push the flux ropes out. It is found that the reconnection occurring below each flux rope may result in significant skew being maintained within the coronal field above the PIL after the flux rope is ejected. In addition, after the eruption, as the coronal field closes down, the large-scale transport of open flux across the bipoles takes place through the process of "interchange reconnection.'' As a result, new photospheric domains of open flux are created within the centers of the bipoles, where field lines were previously closed. The net open flux in the simulation may be split into two distinct contributions. The first contribution is due to the nonpotential equilibrium coronal fields of the bipoles. The second contribution is a temporary enhancement to this during the ejection of the flux ropes, where previously closed field lines become open. It is shown that the nonpotential equilibrium contribution to the open flux is significantly higher than that due to a potential field deduced from the same photospheric boundary conditions. These results suggest that the nonpotential nature of coronal magnetic fields may affect the variation of the Sun's open flux during periods of high solar activity and should be considered in future simulations.