Abstract
Motion of the rotation axis of the Earth contains decadal variations with amplitudes on the order of 10 mas. The origin of these decadal polar motions is unknown. A class of rotational normal modes of the core–mantle system termed torsional oscillations are known to affect the length of day (LOD) at decadal periods and have also been suggested as a possible excitation source for the observed decadal polar motion. Torsional oscillations involve relative motion between the outer core and the surrounding solid bodies, producing electromagnetic torques at the inner-core boundary (ICB) and core–mantle boundary (CMB). It has been proposed that the ICB torque can explain the excitation of the approximately 30-yr-period polar motion termed the Markowitz wobble. This paper uses the results of a torsional oscillation model to calculate the torques generated at Markowitz and other decadal periods and finds, in contrast to previous results, that electromagnetic torques at the ICB can not explain the observed polar motion. This article has been accepted for publication in Geophysical Journal International ©: 2004 RAS Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Highlights
Variations in the rotation of the Earth occur over a wide range of timescales
This paper focuses on an observed set of polar motions with decadal periods
The values for the magnetic field components at the inner-core boundary (ICB) are chosen to match Dumberry & Bloxham (2002); both the ICB and core–mantle boundary (CMB) dipole field strengths are in agreement with constraints obtained from analyses of tidally forced nutation series (Mathews et al 2002)
Summary
Variations in the rotation of the Earth occur over a wide range of timescales. Both the orientation and magnitude of the planetary rotation vector are known to vary on timescales ranging from days to millions of years. When the forcing is well known, as is the case for lunisolar tides, the observed nutations of the planet can be used to constrain physical parameters such as the dynamic ellipticity of the core and of the whole Earth When external forcings alone can not produce the observed rotational variations, an excitation within the Earth system (including the core, oceans and atmosphere) must exist and constraints can be placed on the dynamics of these regions
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