  We numerically study the evolution of magnetic fields and fluid flows in a
thin spherical shell. We take the initial field to be a latitudinally confined,
predominantly toroidal flux tube. For purely toroidal, untwisted flux tubes, we
recover previously known radial-shredding instabilities, and show further that
in the nonlinear regime these instabilities can very effectively destroy the
original field. For twisted flux tubes, including also a poloidal component,
there are several possibilities, including the suppression of the
radial-shredding instability, but also a more directly induced evolution,
brought about because twisted flux tubes in general are not equilibrium
solutions of the governing equations.

Solar and Stellar Astrophysics