Earth's nutations

The nutations are daily variations in the Earth's rotation caused by the torque that the Moon, the Sun and, to a lesser extent, other planets exert on the equatorial bulge (Figure 1). In this motion, the mantle, fluid core and inner core are in slight differential rotation. This is both because they would, in isolation, react differently to the imposed torque, and also because the forced nutations excite free rotational modes which involve differential rotation. The overall nutational response of the Earth depends then on the coupling between its internal regions. The forcing responsible for the nutation is known very precisely, and the response of the Earth is also observed with great precision. Physical models of the internal coupling must be capable of reproducing the observed nutations when subjected to the known external torques. Nutations thus provide an opportunity to gain knowledge on physics taking place deep inside our planet and inaccessible by direct observations.

With former Postdoctoral Fellow Laurence Koot (now at Royal Obersvatory of Belgium), we are pursuing our on-going work on nutations. This involves developing a new model of electromagnetic coupling at the core-mantle and inner core boundaries (Figure 2), as well as developing a model for anelastic deformations taking place within the inner core.

Figure 1: Left: Variations in space of the Earth's rotation vector in space as a result of torques from the Sun, Moon and other planets. Right: Actual data.

Figure 2: Electromagnetic coupling between the fluid core and solid inner core (left) and between the fluid core and mantle (right). Differential motion at boundaries deform magnetic field lines which results in a torque.


Some of my papers on this topic.

Koot, L. and Dumberry, M., 2013, The role of the magnetic field morphology on the electromagnetic coupling for nutations, Geophys. J. Int., 195, 200-210. (link)

Dumberry, M. and Koot, L., 2012, A global model of electromagnetic coupling for Earth nutations, Geophys. J. Int., 191, 530-544. (link)

Koot, L. and Dumberry, M., 2011, Viscosity of the Earth's inner core: Constraints from nutation observations, Earth Planet. Sci. Lett., 308, 343-349. (link)

Koot, L., Dumberry, M., Rivoldini, A., de Viron, O and Dehant, V., 2010, Constraints on the coupling at the core-mantle and inner core boundaries inferred from nutation observations, Geophys. J. Int., 182, 1279-1294. (pdf)

Dumberry, M., 2009, Influence of elastic deformations on the inner core wobble, Geophys. J. Int., 178, 57-64. (pdf)