It is commonly believed that the magnetic field threading a neutron star provides the ultimate mechanism (on top of fluid viscosity) for enforcing long-term corotation between the slowly spun down solid crust and the liquid core. We show that this ar
gument fails for axisymmetric magnetic fields with closed field lines in the core, the commonly used `twisted torus field being the most prominent example. The failure of such magnetic fields to enforce global crust-core corotation leads to the development of a persistent spin lag between the core region occupied by the closed field lines and the rest of the crust and core. We discuss the repercussions of this spin lag for the evolution of the magnetic field, suggesting that, in order for a neutron star to settle to a stable state of crust-core corotation, the bulk of the toroidal field component should be deposited into the crust soon after the neutron stars birth.
Traditionally, the subject of hydromagnetic equilibrium in neutron stars has been addressed in the context of standard magnetohydrodynamics, with matter obeying a barotropic equation of state. In this paper we take a step towards a more realistic tre
atment of the problem by considering neutron stars with interior superfluid components. In this multifluid model stratification associated with a varying matter composition (the relative proton to neutron density fraction) enters as a natural ingredient, leading to a non-barotropic system. After formulating the hydromagnetic equilibrium of superfluid/superconducting neutron stars as a perturbation problem, we focus on the particular case of a three-fluid system consisting of superfluid neutrons and normal protons and electrons. We determine the equilibrium structure of dipolar magnetic fields with a mixed poloidal-toroidal composition. We find that, with respect to barotropic models, stratification has the generic effect of leading to equilibria with a higher fraction of magnetic energy stored in the toroidal component. However, even in models with strong stratification the poloidal and toroidal components are comparable, with the former contributing the bulk of the magnetic energy.
We consider the pinning of superfluid (neutron) vortices to magnetic fluxtubes associated with a type II (proton) superconductor in neutron star cores. We demonstrate that core pinning affects the spin-down of the system significantly, and discuss im
plications for regular radio pulsars and magnetars. We find that magnetars are likely to be in the pinning regime, while most radio pulsars are not. This suggests that the currently inferred magnetic field for magnetars may be overestimated. We also obtain a new timescale for the magnetic field evolution which could be associated with the observed activity in magnetars, provided that the field has a strong toroidal component.
