  We model the non-linear ideal magnetohydrodynamics of poloidal magnetic
fields in neutron stars in general relativity assuming a polytropic equation of
state. We identify familiar hydromagnetic modes, in particular the
'sausage/varicose' mode and 'kink' instability inherent to poloidal magnetic
fields. The evolution is dominated by the kink instability, which causes a
cataclysmic reconfiguration of the magnetic field. The system subsequently
evolves to new, non-axisymmetric, quasi-equilibrium end-states. The existence
of this branch of stable quasi-equilibria may have consequences for magnetar
physics, including flare generation mechanisms and interpretations of
quasi-periodic oscillations.
