Other
Scientific paper
Jul 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989rspta.328..351h&link_type=abstract
Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, Volume 328, Issue 1599,
Other
29
Scientific paper
As arguments in favour of the notion that very slow convection in the highly viscous mantle is confined to the upper 700 km gradually weakened over the past 20 years, so geophysicists have increased their willingness to entertain the idea that significant horizontal variations in temperature and other structural parameters occur at all levels in the lower mantle. Concomitant density variations, including those caused by distortions in the shape of the core--mantle interface, would contribute substantially to long-wavelength features of the Earth's gravity field and also affect seismic travel times. The implied departures from axial symmetry in the thermal and mechanical boundary conditions thus imposed by deep mantle convection on the underlying low-viscosity liquid metallic core would affect not only spatial variations in the long-wavelength features of the main geomagnetic field (which is generated by dynamo action involving comparatively rapid chaotic magnetohydrodynamic flow in the core) but also temporal variations on all relevant timescales, from decades and centuries characteristic of the geomagnetic secular variation to tens of millions of years characteristic of changes in the frequency of polarity reversals. Core motions should influence the rotation of the `solid' Earth (mantle, crust and cryosphere), and in the absence of any quantitatively reasonable alternative line of attack, geophysicists have long supposed that irregular `decade' fluctuations in the length of the day of about 5 × 10-3 s must be manifestations of angular momentum exchange between the core and mantle produced by time-varying torques at the core--mantle interface. The stresses responsible for these torques comprise (a) tangential stresses produced by viscous forces in the thin Ekman--Hartmann boundary layer just below the interface and also by Lorentz forces associated with the interaction of electric currents in the weakly conducting lower mantle with the magnetic field there, and (b) normal stresses produced largely by dynamical pressure forces acting on irregular interface topography (i.e. departures in shape from axial symmetry). The hypothesis that topographic stresses might provide the main contribution to the torque was introduced by the author in the 1960s and the present paper gives details of his recently proposed method for using Earth rotation and other geophysical data in a new test of the hypothesis. The method provides a scheme for investigating the consistency of the hypothesis with various combinations of `models' of (a) motions in the outer reaches of the core based on geomagnetic secular variation data, and (b) core--mantle interface topography based on gravity and seismic data, thereby elucidating the validity of underlying assumptions about the dynamics and structure of the Earth's deep interior upon which the various `models' are based. The scheme is now being applied in a complementary study carried out in collaboration with R. W. Clayton, B. H. Hager, M. A. Spieth and C. V. Voorhies.
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