HR: 12:05h
AN: G51C-08 INVITED     [PDF]
TI: Inner Core Tilt and Polar Motion: Probing the Dynamics Deep Inside the Earth
AU: * Dumberry, M
EM: dumberry@geophysics.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Ma 02138 United States
AU: Bloxham, J
EM: bloxham@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Ma 02138 United States
AB: A tilted inner core permits exchange of angular momentum between the core and the mantle through gravitational and pressure torques and, as a result, changes in the direction of Earth's axis of rotation with respect to the mantle. Some of the observed variations in the direction of Earth's rotation could then be caused by equatorial torques on the inner core which tilt the latter out of its alignment with the mantle. In this work, we investigate whether such a scenario could explain the decade polar motion known as the Markowitz wobble. We show that a decade polar motion of the same amplitude as the observed Markowitz wobble requires a torque of $10^{20}$ N m which tilts the inner core by 0.07 degrees. This result critically depends on the viscosity of the inner core; for a viscosity less than $5 \times 10^{17}$ Pa s, larger torques are required. A torque of $10^{20}$ N m with decadal periodicity can perhaps be produced by electromagnetic coupling between the inner core and a component of the flow in the outer core known as torsional oscillations, provided that the radial magnetic field at the inner core boundary is on the order of 3 to 4 mT and satisfies certain geometrical constraints. The resulting polar motion thus produced is eccentric and polarized, in agreement with the observations. Our model suggests that equatorial torques at the inner core boundary might also excite the Chandler wobble, provided shorter wavelength torsional oscillations with higher natural frequencies have enough power or provided there exists another physical mechanism that can generate a large torque at a 14 month period.
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 1239 Rotational variations
DE: 1507 Core processes (8115)
DE: 1510 Dynamo theories
SC: Geodesy [G]
MN: 2003 Fall Meeting