GEOPHYSICAL MONOGRAPH SERIES, VOL. 76, PP. 107-120, 1993
Topographic core-mantle coupling and fluctuations in the Earth's rotation
Robert Hooke Institute, The Observatory, Clarendon Laboratory, Parks Road Oxford 0X1 3PU, England, U.K.
Seismological Laboratory, California Institute of Technology Pasadena, CA 91125. U.S.A.
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
Jet Propulsion Laboratory, California Institute of Technology Pasadena, CA 91109, U.S.A.
Geodynamics Branch Code 921, Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.
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 wave-length 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 0.5 km (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.
Citation: Hide, R.,