  In 1977, Flowers and Ruderman described a perturbation that destabilises a
purely dipolar magnetic field in a fluid star. They considered the effect of
cutting the star in half along a plane containing the symmetry axis and
rotating each half by $90\degr$ 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 stabilising
effect of adding a toroidal field by studying the potential 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
stabilisation is much smaller than the energy of the poloidal field. We also
show that, contrary to a common argument, the Flowers-Ruderman instability
cannot be applied many times in a row to reduce the external magnetic energy
indefinitely.
