J.E. Mound and B.A. Buffett
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada
Variations in the length-of-day (LOD) occur on many timescales, many of which have been correlated with angular momentum exchanges between the mantle and atmosphere, oceans or core. However an LOD variation with a six-to-seven year period has been observed for which there is at present no explanation. Due to the absence of atmospheric excitation at this period core-mantle interaction is a likely source of this LOD oscillation. We develop a theoretical model for free oscillations in the rotation of the core-mantle system where a combination of gravitational and electromagnetic couplings allow angular momentum to be transferred between the solid inner core, fluid outer core and mantle of the Earth. The model includes the effects of gravitational coupling between the inner core and mantle, electromagnetic coupling across the ICB and CMB, torsional oscillations in the fluid core and relaxation of the inner-core shape. We show that excitation of a free oscillation arising due to gravitational coupling between the aspherical inner core and mantle can explain the observed LOD signal. The LOD signal associated with this free oscillation depends on physical properties of the core-mantle system such as magnetic field strength, inner core boundary (ICB) and core mantle boundary (CMB) ellipticity and relaxation time of the inner-core shape. We find that observation of this free oscillation enables us to place constraints on some of these parameters, particularly the equatorial ICB and CMB ellipticity which are controlled by the overlying mantle convection. Our results thus have implications for models of mantle flow. In addition we predict a small variation in surface gravity associated with the inner core rotation which may be observable by the GRACE satellite mission.