Modeling of nutation and precession: effects of electromagnetic coupling

Abstract

The presence of an internal magnetic field influences of the Earth's nutation through the effects of electromagnetic torques at the boundaries of the fluid core. We calculate the effect of electromagnetic torques on nutation by combining a solution for the full hydromagnetic response of the fluid core with the nutation theory of Mathews et al. [2002] . The coupling of the fluid outer core to the mantle and solid inner core is described by two complex constants, K^CMB and K^ICB, that characterize the electromagnetic torques at the core-mantle boundary (CMB) and the inner core boundary (ICB). Predictions for K^CMB and K^ICB are compared with estimates inferred from observations of the Earth's nutation. The estimate of K^CMB can be explained by the presence of a thin conducting layer at the base of the mantle with a total conductance of 10⁸ S. The overall root-mean-square (RMS) radial field at the CMB is 0.69 mT, which is partitioned into a dipole component (0.264 mT) and a nondipole component (0.64 mT). (The latter is represented using a uniform radial field.) The estimate of K^ICB can be explained with a mixture of dipole and nondipole components. The overall RMS field at the ICB is 7.17 mT, though smaller values are inferred when small adjustments are made to the dynamic ellipticity of the inner core and/or the fluid density at the boundary. The minimum RMS radial field required to explain the nutation observations is 4.6 mT.