Structural Analysis of the Pre-Eruptive Magnetic Field for the May 12, 1997 CME Event*

A simple analytical model of the coronal magnetic field prior to the CME eruption on May 12, 1997 is developed in the current-free approximation. The magnetic field is constructed by superimposing a large-scale background field and a localized bipole field to model the active region. The background field is determined from the normal component of the observed photospheric magnetic field averaged over the longitude of the Sun. The influence of the solar wind is taken into account by imposing a source-surface boundary condition that makes the field radial at a specified radius. The field of the active region is modeled with the help of a subphotospheric dipole whose strength, location, and orientation are optimized to fit the magnetic field obtained from an MDI magnetogram. A corresponding force-free magnetic field is developed then by shearing and twisting the potential configuration. The structure of the potential and force-free configurations is analyzed and related to the characteristics of the observed eruption. *Research supported by NASA and the Center for Integrated Space Weather Modeling (an NSF Science and Technology Center).