Inner core tilt and Earth's rotation

A tilted inner core permits exchange of angular momentum between the core and the mantle and, as a result, changes in the direction of Earth's axis of rotation with respect to the mantle. In previous studies, I have developed models to calculate the amplitude of the polar motion that results when an equatorial torque tilts the inner core out of its alignment with the mantle. This was to test whether a combination of electromagnetic and gravitational torques on the inner core could reproduce the observed decadal variation in polar motion known as the Markowitz wobble (Figure 1, below). Though the conclusion is that it is likely not the case, these models can also be used to place useful constraints on many internal quantities for which we have no direct observations, including the magnetic field and core flows near the inner core boundary, the viscosity of the inner core and density heterogeneities in the mantle.

I also investigated the effect of elastic deformation inside the inner core on the period of the inner core wobble, a normal mode of nutation of Earth similar to the Chandler wobble, but involving the inner core (Figure 2).

Figure 1: Trajectory of the axis of Earth's rotation with respect to a fixed point at the surface, viewed from above the North pole. m1 is in the direction of Greenwich. Units are milliarcseconds (mas); 30 mas corresponds approximately to 1 meter at the Earth's surface. Figure taken from Dumberry, GJI, 2008.

Figure 2: Geometry of the inner core wobble: the figure axis of the inner core is offset from the figure axis of the rest of Earth. As a function of time, the tilted inner core precesses in a prograde direction with a period of approximately 7.5 years. Figure taken from Dumberry, GJI, 2009.

Some of my papers on this topic