Role of magnetic flux distribution in coronal energy storage

Magnetic fields in the solar corona are most likely the dominant source of energy that powers coronal mass ejections (CMEs). Such energy must be above and beyond that of a potential (current-free) magnetic field, and thus the pre-CME coronal magnetic field should contain significant electric currents. Given the diffuse nature of the corona, the coronal magnetic field is likely to be largely force free, implying that electric currents are closely aligned with the field itself. In this work we explore such force-free fields, in the spherical geometry appropriate to the solar corona, with the aim of understanding how the magnetic flux distribution at the coronal base affects the storage of magnetic energy. We find that energy storage is enhanced when a region of strong potential field overlies a nonpotential field whose footpoints are confined to low solar latitudes. Furthermore, those flux distributions consistent with strong overlying potential fields may enable larger energy buildup, when examined in the context of limits imposed by the scalar virial theorem and the Aly-Sturrock theorem. Finally, we demonstrate the existence of force-free fields containing detached flux ropes, with energies that lie above the Aly-Sturrock limit.