Journal of geomagnetism and geoelectricity

Introduction

I. A. Eltayeb

1991 Volume 43 Issue 2 Pages 77
Published: 1991
Released on J-STAGE: August 11, 2011

DOIhttps://doi.org/10.5636/jgg.43.77

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The Nature and Consequences of Thermal Interactions Twixt Core and Mantle

David E. LOPER

1991 Volume 43 Issue 2 Pages 79-91
Published: 1991
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.43.79

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It is now well accepted that only a part of the heat flux from the Earth's interior is due to radioactive decay; a significant portion of the flux is due to secular cooling of the Earth (STACEY, 1980; JACKSON and POLLACK, 1984; CHRISTENSEN, 1985; STACEY and LOPER, 1988), and it follows directly that the core and mantle are cooling, albeit at differing rates (STACEY and LOPER, 1984). The cooling of the core, and the concomitant freezing of the inner core, is believed to be responsible for the operation of a gravitationally powered geomagnetic dynamo. The flux of heat from the core is convected upward through the mantle via plumes, whose surface expression is hotspots such as that at Hawaii. In this brief review, current ideas on the structure and dynamics of the core and mantle will be discussed and related to surface features and processes that result from the thermal interaction between core and mantle.

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On Viscosity Estimates for the Earth's Fluid Outer Core and Core-Mantle Coupling

L. Ian LUMS, Keith D. ALDRIDGE

1991 Volume 43 Issue 2 Pages 93-110
Published: 1991
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.43.93

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The kinematic viscosity of the Earth's fluid core is uncertain because estimates inferred from studies of liquid metals and from real-Earth measurements differ significantly. The role of dynamic effects is invoked in an attempt to reconcile this conflict. The implications of these additional effects for the coupling between a fluid and its container are discussed with respect to the nearly-diurnal free wobble and the free core nutation, which depend on the properties of the fluid core near the core-mantle boundary.

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Exchange of Angular Momentum between the Core and the Mantle

D. JAULT, J. L. LE MOUËL

1991 Volume 43 Issue 2 Pages 111-129
Published: 1991
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.43.111

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Exchanges of angular momentum between the core and the mantle responsible of the so-called decade variations in the length of the day (l.o.d.) are reexamined. It is proposed that, for the relevant time constants, the changes in the core angular momentum are carried by cylindrical annuli rigidly rotating around the Earth's rotation axis. Then the possible coupling mechanisms are discussed. The electromagnetic torque is first computed using a full expansion of the flow at the core-mantle boundary as inferred from the secular variation of the geomagnetic field. Its time changes are estimated and compared to the time changes of the torque responsible for the l.o.d. time changes. The topographic torque, i.e. the torque exerted on the non-axisymmetric core-mantle boundary by the pressure field linked with the flow at the core mantle boundary (CMB), is then shown to be too large by two orders of magnitude if the amplitude of the topography is as large as proposed by seismologists. A mechanism is presented which can reduce this torque down to values compatible with l.o.d. data.

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Coupling an Inviscid Core to an Electrically Insulating Mantle

Coerte V. VOORHIES

1991 Volume 43 Issue 2 Pages 131-156
Published: 1991
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.43.131

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The torque exerted on the mantle is examined with emphasis on purely mechanical topographic or pressure core-mantle coupling. Pressure perturbations accompanying tangentially geostrophic, frozen-flux core flow acting on core-mantle boundary topography can exert an “enormous” torque on the mantle if thermal core-mantle interactions drive such flow. Conditions for growing or changing a dipole and correlating its fluctuations with those in the length of the day are analyzed.

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The Response of a Compressible, Non-Homogeneous Earth to Internal Loading

Theory

V. DEHANT, J. M. WAHR

1991 Volume 43 Issue 2 Pages 157-178
Published: 1991
Released on J-STAGE: September 14, 2010

DOIhttps://doi.org/10.5636/jgg.43.157

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Convective flow inside the mantle will cause topography at the Core Mantle Boundary (CMB). The amplitude and shape of the topography depend on details of the flow pattern, and on the Earth's rheology. The topography can be modeled by using seismic tomography results. When lateral variations density are inferred, they are used as internal loads in a viscous Earth. So far, all models have assumed the Earth is incompressible and composed of homogeneous layers.
In this paper, we describe a formalism for computing mantle flow, geoid perturbations, and boundary deformation (at the surface or at the CMB) due to internal loading by lateral density heterogeneities for a spherically-symmetric Earth that is compressible and that has radially dependent viscosity and density. We assume the Earth is neutrally stable below the lithosphere. The solution vector, as well as the density anomalies, are expanded as sums of spherical harmonics with coefficients that depend on radius. Green's functions, responses to a unit mass load, are computed for each order of the spherical harmonics. These functions can be convolved with tomography results inside the mantle to get the geoid or the CMB topography, for example.

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