Evolution of the magnetic field in magnetars

We use numerical MHD to look at the stability of a possible poloidal field in neutron stars ( Flowers & Ruderman 1977, ApJ, 215, 302), and follow its unstable evolution, which leads to the complete decay of the field. We then model a neutron star after the formation of a solid crust of high conductivity. As the initial magnetic field we use the stable "twisted torus" field which was the result of our earlier work (Braithwaite & Nordlund 2006, A& A, 450, 1077), since this field is likely to exist in the interior of the star at the time of crust formation. We follow the evolution of the field under the influence of diffusion, and find that large stresses build up in the crust, which will presumably lead to cracking. We put this forward as a model for outbursts in soft gamma repeaters.