The role of the magnetic field morphology on the electromagnetic coupling for nutations

Abstract

Observations of the Earth's nutations provide constraints on the mechanical coupling at the core–mantle and inner core boundaries. An important physical mechanism that could be responsible for the observed dissipation is the electromagnetic (EM) coupling, to which this paper is devoted. Previous studies assumed that the main feature of the magnetic field that affects the EM coupling is its overall strength, its morphology being considered unimportant. In particular, these studies rely on the hypothesis that the contribution to the torque from all the non-dipolar components of the field can be approximated by the contribution that a uniform radial field with the same strength would have. In this study, we compute the EM torque for more realistic configurations of the magnetic field at the core boundaries and thereby assess the role of its spatial distribution on the strength of the EM torque. For field strengths typical of the core–mantle boundary (CMB), we show that the spatial distribution affects weakly the strength of the torque, with the approximation by a uniform field leading to an overestimation of the torque magnitude by ∼15–20 per cent. However, for field strengths typical of the inner core boundary (ICB), the morphology of the field has a more significant influence on the EM torque and the approximation by a uniform field overestimates the torque by ∼30–40 per cent. Assuming that EM coupling is responsible for the observed dissipation, we infer constraints on the strength of the radial magnetic field at both the CMB and ICB. We show how the unknown morphology of the magnetic field induces uncertainties on the estimated field strength at the ICB, which can take values anywhere in the range of ∼9–16 mT. These very large values suggest that EM coupling at the ICB cannot be the only mechanism responsible for the observed dissipation.