CRUSTAL MAGNETIC-FIELD DECAY AND NEUTRON-STAR COOLING

Calculations of ohmic decay of the magnetic fields which are initially confined to the surface layers of the crust with the densities rho less-than-or-equal-to rho0 = 4.10(11) g/cm3 are presented. It is shown that the neutron star cooling can effectively increase the conductivity of the crust sigma and the ohmic decay timescale tau(B). The scenario of the crustal field evolution is as follows. At the initial stage the field decays comparatively quickly: it can decrease by a factor of 10-100 during the first approximately 10(6)-10(7) yrs depending on the initial field location. At this stage the conductivity is determined by the electron-phonon Scattering. After about 10(6)-10(7) yrs sigma in the crust is determined by the scattering on impurities and may be sufficiently high. At the late stage, due to the higher conductivity the ohmic decay time tau(B) is large and can increase during the evolution because of the magnetic field diffusion into the deeper layers of the crust where sigma is higher. Our calculations how that the crustal field decays very slowly even in the case of the field confined to the outer regions of the crust. For instance, if the electric currents were initially concentrated within the layers with rho approximately (2-4) . 10(11) g/cm3, then the surface field damps by a factor of 10(2)-10(3) after approximately 10(10) yrs.