Constraints on the coupling at the core-mantle and inner core boundaries inferred from nutation observations

Abstract

SUMMARY We present an inversion of nutation observations in terms of parameters characterizing the Earth’s interior properties. We use a Bayesian inversion in the time-domain, allowing us to take fully into account non-linearities in the nutation model and to reduce the loss of information occurring in frequency-domain inversions. Among the parameters we retrieve are two complex parameters, K CMB and K ICB, referred to as ‘coupling constants’, characterizing the mechanical coupling at the core–mantle boundary (CMB) and the inner core boundary (ICB), respectively. Based on a joint inversion of nutation observations provided by different analysis centres, we find Im(K CMB) = (−1.78 ± 0.02) 10 −5 ,R e(K ICB) = (1.01 ± 0.02) 10 −3 and Im(K ICB) = (−1.09 ± 0.03) 10 −3 (where the errors correspond to 99.7 per cent confidence intervals). While our value of Im(K CMB) is similar to previous estimates, our new values of Re(K ICB) and Im(K ICB) are significantly different. This is mainly because of the different inversion strategy that we use and also because of the lengthier record of observation available. We show that, based on existing coupling models, neither viscous nor electromagnetic coupling alone can explain our new values of Re(K ICB) and Im(K ICB). A combination of these two mechanisms is required and necessitates a radial magnetic field at the ICB of total rms strength between 6 and 7 mT and a kinematic viscosity of the fluid core at the ICB should be between 10 and 30 m 2 s −1 , depending on the exact partition between viscous and electromagnetic coupling.