Abstract

In 1977, Flowers and Ruderman described a perturbation that destabilized a purely dipolar magnetic field in a star. They considered the effect of cutting the star in half along a plane parallel to the symmetry axis and rotating each half 90 degrees in opposite directions, which would cause the energy of the magnetic field in the exterior of the star to be greatly reduced, just as it happens with a pair of aligned magnets. We formally solve for the energy of the external magnetic field and check that it decreases monotonously along the entire rotation. We also describe the instability using perturbation theory, and see that it happens due to the work done by the interaction of the magnetic field with surface currents. Finally, we consider the stabilizing effect of adding a toroidal field by studying the internal energy perturbation when the rotation is not done along a sharp cut, but with a continuous displacement field that switches the direction of rotation across a region of small but finite width. Using these results, we estimate the relative strengths of the toroidal and poloidal field needed to make the star stable to this displacement and see that the energy of the toroidal field required for this is much smaller than the energy of the poloidal field.