CIRCULATION OF MATTER AND EVOLUTION OF THE INTERNAL MAGNETIC-FIELD INNEUTRON-STARS

Neutron stars may be in hydrostatic equilibrium only for very particul ar magnetic configurations with conservative magnetohydrodynamic force s. We consider the magnetohydrodynamic evolution of a neutron star wit h a non-zero external dipole field component. The ohmic dissipation sh ould slightly change this equilibrium distribution of the currents, pr oducing a slow circulation of matter which tends to maintain the magne tic configuration close to an equilibrium one. The evolution of a stro ng magnetic field (B congruent-to 10(12) - 10(13) G) under the influen ce of both ohmic dissipation and circulation is analysed in detail. Th e resulting field decay turns out to be crucially dependent on the the rmal history of the neutron star. The time-scale of decay in the core may be rather short, particularly for old neutron stars with low surfa ce temperatures T(s). For instance, this time-scale may be of the orde r 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 st ars. For example, the 'magnetic' circulation can partially mix the cor e matter. The velocity of circulation decreases with the age of the ne utron star as a result of the decay of the magnetic field.