Topographic Core-Mantle Coupling and Fluctuations in the Earth's Rotation

Abstract

Astronomically-determined irregular fluctuations in the Earth's rotation vector on decadal time scales can be used to estimate the fluctuating torque on the lower surface of the Earth's mantle produced by magnetohydrodynamic flow in the underlying liquid metallic core. A method has been proposed for testing the hypothesis that the torque is due primarily to fluctuating dynamic pressure forces acting on irregular topographic features of the core-mantle boundary and also on the equatorial bulge. The method exploits (a) geostrophically-constrained models of fluid motions in the upper reaches of the core based on geomagnetic secular variation data, and (b) patterns of the topography of the CMB based on the mantle flow models constrained by data from seismic tomography, determinations of long wavelength anomalies of the Earth's gravitational field and other geophysical and geodetic data According to the present study, the magnitude of the axial component of the torque implied by determinations of irregular changes in the length of the day is compatible with models of the Earth's deep interior characterized by the presence of irregular CMB topography of effective "height" no more than about 0.5km (about 6% of the equatorial bulge) and strong horizontal variations in the properties of the D" layer at the base of the mantle. The investigation is now being extended to cover a wider range of epochs and also the case of polar motion on decadal time scales produced by fluctuations in the equatorial components of the torque. lNTRODUCfiONElectric currents generated in the Earth's liquid metallic core are responsible for the main geomagnetic field and its secular changes [see Jacobs, 1987ab;Melchior, 1986;Moffatt, 1978a] are produced by dynamo action involving irregular magnetohydrodynamic flow in the core. Concomitant dynamical stresses acting on the overlying mantle are invoked in the interpretation of the so-called "decadal" fluctuations in the rotation of the "solid Earth" (mantle, crust and cryosphere). Studies of these rotational manifestations of core motions bear directly on investigations of the structure, composition and dynamics of the Earth's deep interior

On the theory of core-mantle coupling

“…The Lorentz force in (20a) is therefore negligible on the CMB and, as g is small, @ p c % À2 0 r o V cos sin on the CMB. Therefore (Speith et al 1986, Hide et al 1993, by (34d),…”

Broadband assessment of degree‐2 gravitational changes from GRACE and other estimates, 2002–2015

“…Decadal and long‐term LOD variations and a strong oscillation at period of near 6 years [ Chen et al ., , ; Mound and Buffett , ] are believed to be related core‐mantle coupling [ Hide et al ., ]. We remove these low‐frequency LOD variations using a high pass filter with a cutoff period of 2 years, to focus on Δ C 20 at seasonal and shorter timescales.…”

“…At periods less than a few years, nontidal EOP variations mainly arise from two sources: (1) surface load variations from barometric pressure, ocean bottom pressure, terrestrial water storage, and ice mass changes (polar ice sheets and mountain glaciers) and (2) angular momentum exchange between the solid Earth and its fluid envelope due to winds and ocean currents. At decadal and longer timescales, EOP variations are dominantly associated with mass redistribution related to tectonics, glacial isostatic adjustment (GIA), and contemporary ice melting and water storage redistribution and with angular momentum exchange between the mantle and core [e.g., Peltier , ; Hide et al ., ; Chen et al ., ].…”

Normal mode sensitivity to Earth’s D″ layer and topography on the core-mantle boundary: what we can and cannot see

“…This region is believed to act as a thermal boundary layer and is an important factor in controlling mantle convection. The D″ region also plays an important role in segregating chemical heterogeneities for long periods of time (Lay 2007; Garnero & McNamara 2008); it could have a profound effect on the evolution and growth of the inner core (Hide et al 1993; Lay 2007) and its observed anisotropy by regulating the core heat flux (Aubert et al 2008).…”