GEOMAGNETISM, EARTH ROTATION AND THE ELECTRICAL-CONDUCTIVITY OF THE LOWER MANTLE

A new approximation is derived for the electromagnetic torque acting o n an electrically conducting mantle as a result of fluid flow in the c ore. The torque considered is that associated with the toroidal field induced in the mantle by advection of poloidal field (which is assumed to be frozen into an infinitely conducting liquid core) by the flow o f liquid at the top of the core. The expression relies on the assumpti on that the mantle is an insulator apart from a 'thin' (with respect t o the core radius) layer of finite conductance adjacent to the core-ma ntle boundary. This allows the toroidal field scalar in the mantle to be expressed as a first-order Taylor approximation. The time-dependent torque calculated at a sequence of epochs this century is compared wi th the torque which has previously been inferred from astronomical obs ervations of the length of day. Although the initial results appear un promising, a significant correlation exists when poorly determined com ponents of the velocity field, which contribute substantially to varia tions in the calculated torque, are ignored. By regression analysis th e conductance of the assumed thin layer is determined as 6.7 x 10(8) S and the lag of the electromagnetic torque behind the astronomical tor que as 6 years, which is interpreted as the delay time for electromagn etic signals through the mantle. Finally, the implications of these re sults for the conductivity of the lower mantle are discussed. They imp ly that the bottom few hundred kilometres of the mantle probably have a conductivity of a few hundred to a few thousand siemens per metre; p reviously published lower-mantle conductivity models are examined in l ight of this conclusion. The close correlation of the variations in th e calculated electromagnetic torque, which depend primarily on fluctua tions in the core velocity field, with the length of day torque provid es independent evidence that core flow is not steady on the decade tim e-scale.