Three-Dimensional Rotational Dynamics and Coupling of the Core-Mantle System

Investigation of the properties and dynamics of the deep interior of the Earth necessarily relies on indirect observations. Seismological studies provide information on the physical properties of the deep Earth and its boundaries and in some cases repeat observations can reveal the dynamics of the deep Earth; for example, the observed 'super-rotation' of the solid inner core. Study of geomagnetic secular variation provides insight on the nature of fluid flow at the surface of the fluid core, and indirect information on the physical properties of the deep Earth. The planet's rotational dynamics provide an additional source of information on the Earth's deep interior and developing a more complete understanding of planetary rotational dynamics is the goal of this project. Variation in the Earth's rotation involves changes in both the rate of rotation (observed as a change in the length of day and correlated with so-called torsional oscillation flow in the fluid core) and the orientation of the rotation axis with respect to the celestial reference frame (periodic fluctuations arising from gravitational interaction with the sun, moon and planets are referred to as nutations). Both length-of-day variations and nutations involve angular momentum exchange between the mantle, outer core and inner core. The strength of the coupling between these regions depends on physical properties of the Earth such as the strength and geometry of the magnetic field within the core, the electrical conductivity of the core and lower mantle, and the viscosities of the outer and inner cores. Although both length-of-day variations and nutations involve similar dynamic effects and provide complementary evidence on the nature of the deep Earth, previous work has tended to analyse these phenomena separately. The first goal of this project is to refine and harmonise the theoretical descriptions of core-mantle coupling in models of nutation and of length-of-day variation. In so doing, we will take advantage of theoretical advances that have occurred to improve one of the types of model, but that have not yet been applied to the other. For example, the theory of viscous coupling in nutation models is more fully developed than that in models of length-of-day. On the other hand, recent work has led to improved descriptions of the geometry of the magnetic field in length-of-day models, and an appreciation for the importance of a commonly neglected effect that adds to ohmic dissipation at the core-mantle boundary. Using the updated models we will reanalyse the existing rotation data sets to obtain improved estimates of the physical properties of the deep Earth. In the final stage of this project we will develop a single model that can self-consistently describe both nutations and torsional oscillations. This would allow for joint inversion of the independent data sets, providing further improvements in the constraints on the physical properties of the deep Earth. The joint model will be used to investigate the dynamics of cross-coupling within the system, including the possibility that torsional oscillations excite an observed decadal-period variation in the orientation of the rotation axis known as the Markowitz Wobble, and a proposed correlation between the timing of phase jumps in the Chandler Wobble (which has a period of 433 days) and so-called geomagnetic jerks (which have also been linked to torsional oscillations). The new information that we gain concerning the physical properties and short time scale dynamics of the Earth's core-mantle system will be useful for testing numerical models of the geodynamo process that is responsible for generation of the Earth's magnetic field.