The magnetohydrodynamic stability of coronal arcades. III - Sheared equilibrium fields

The ideal magnetohydrodynamic stability of solar coronal arcades where all the field lines are tied to the photosphere is examined. Two sets of photospheric boundary conditions are examined, and the first detailed quantitative comparison is presented. It is found that conditions where all components of the perturbation vanish at the photosphere are significantly more stable to interchange modes than those for which a displacement along the field lines is allowed there. Much stronger radial pressure gradients are needed to destabilize the former case. It is also found that three sample force-free fields are stable to all perturbations which were imposed. These results outline a pressing need for a more precise treatment of the transition region/corona boundary in stability problems.