ENERGY-REQUIREMENTS FOR OPENING THE SOLAR CORONA

Eruptive solar events like flares and coronal mass ejections are thought to involve the release o energy stored in nonpotential magnetic fields. If this is the case, then the magnetic field configuration must change as the eruptive event occurs. One possible change is the opening of field lines that were originally closed-a change that is observed in many coronagraph sequences of coronal mass ejections and that may also be inferred from the observed post-event closing of magnetic structures in Yohkoh soft X-ray images. This work calculates the energy needed to open, both partially and fully, a simple dipolar magnetic field in spherical geometry appropriate to large-scale solar eruptions. Results show that the energy increases rapidly as more field lines are opened-very nearly as the cube of the fraction of the total flux to be opened. The energy needed to open the field is compared with the maximum magnetic free energy that can be built up by shearing a force-free field with compatible magnetic topology, and the results show that it is energetically possible to open nearly all the coronal flux through shearing. This suggests that energy stored in sheared magnetic fields may be sufficient to power those eruptive events that do not result in a complete opening of the field. We also present evidence for equilibrium loss in the sequences of solutions for sheared coronal fields.