CIRCULATION OF MATTER AND EVOLUTION OF THE INTERNAL MAGNETIC-FIELD IN NEUTRON-STARS

Neutron stars may be in hydrostatic equilibrium only for very particular magnetic configurations with conservative magnetohydrodynamic forces. We consider the magnetohydrodynamic evolution of a neutron star with a non-zero external dipole field component. The ohmic dissipation should slightly change this equilibrium distribution of the currents, producing a slow circulation of matter which tends to maintain the magnetic configuration close to an equilibrium one. The evolution of a strong magnetic field (B congruent-to 10(12) - 10(13) G) under the influence of both ohmic dissipation and circulation is analysed in detail. The resulting field decay turns out to be crucially dependent on the thermal history of the neutron star. The time-scale of decay in the core may be rather short, particularly for old neutron stars with low surface temperatures T(s). For instance, this time-scale may be of the order of approximately 10(6) - 10(7) yr for T(s) approximately 5 x 10(5) - 10(6) K. The circulation accompanying field decay in the core is, in some aspects, like the classical meridional circulation in rotating stars. For example, the 'magnetic' circulation can partially mix the core matter. The velocity of circulation decreases with the age of the neutron star as a result of the decay of the magnetic field.