Core-mantle coupling and polar motion

Greff-Lefftz, M.; Legros, H.

doi:10.1016/0031-9201(95)03025-r

Cited by 32 publications

(37 citation statements)

References 11 publications

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“…(1987), Hulot et a/. (1996) and Greff-Lefftz & Legros (1995), see also Hide (1989), with findings in general agreement with those of the present study. If ps is the dynamic pressure associated with core motions u =us = ( u s , us, w,) in the free stream just below the viscous boundary layer at the CMB (u being the Eulerian flow velocity relative to a reference frame fixed to the solid Earth), and the CMB is the locus of points where the distance from the Earth's centre of mass is r = c + h(0, $) (c being the for a perfectly rigid mantle [see eqs (2) and (3) and Hide (1989)l.…”

Section: T H E T O P O G R a P H I C T O R Q U E E X E R T E D By T Hsupporting

confidence: 95%

“…Our findings (see below) are in general agreement with those of independent and parallel studies by Hulot, Le Huy & Le Moue1 (1996) and Greff-Lefftz & Legros (1995) extending earlier work by Hinderer et a/. (1987), in which effects of irregular latitude-dependent topography as well as electromagnetic coupling and induced changes in the inertia tensor of the imperfectly rigid mantle are also investigated.…”

Section: Introductionsupporting

confidence: 93%

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Topographic Core-Mantle Coupling and Polar Motion On Decadal Time-Scales

Hide

Boggs

Dickey

et al. 1996

Geophysical Journal International

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S U M M A R YAssociated with non-steady magnetohydrodynamic (MHD) flow in the liquid metallic core of the Earth, with typical relative speeds of a fraction of a millimetre per second, are fluctuations in dynamic pressure of about lo3 N m-2. Acting on the non-spherical core-mantle boundary (CMB), these pressure fluctuations give rise to a fluctuating net topographic torque L i ( t ) (i = 1 , 2 , 3 t w h e r e t denotes time--on the overlying solid mantle. Geophysicists now accept the proposal by one of us (RH) that L , ( t ) makes a significant and possibly dominant contribution to the total torque LT( t ) on the mantle produced directly or indirectly by core motions. Other contributions are the 'gravitational' torque associated with fluctuating density gradients in the core, the 'electromagnetic' torque associated with Lorentz forces in the weakly electrically conducting lower mantle, and the 'viscous' torque associated with shearing motions in the boundary layer just below the CMB. The axial component L ; ( t ) of LT(t) contributes to the observed fluctuations in the length of the day CLOD, an inverse measure of the angular speed of rotation of the solid Earth (mantle, crust and cryosphere)], and the equatorial components ( L f ( t ) , L;(t)) = L*(t) contribute to the observed polar motion, as determined from measurements of changes in the Earth's rotation axis relative to its figure axis.In earlier phases of a continuing programme of research based on a method for determining Li( t ) from geophysical data (proposed independently about ten years ago by Hide and Le Mouel), it was shown that longitude-dependent irregular CMB topography no higher than about 0.5 km could give rise to values of L 3 ( t ) sufficient to account for the observed magnitude of LOD fluctuations on decadal time-scales. Here, we report an investigation of the equatorial components (L,(t), L 2 ( t ) ) = L ( t ) of L i ( t ) taking into account just one topographic feature of the CMB-albeit possibly the most pronounced-namely the axisymmetric equatorial bulge, with an equatorial radius exceeding the polar radius by 9.5 kO.

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Section: T H E T O P O G R a P H I C T O R Q U E E X E R T E D By T Hsupporting

confidence: 95%

Section: Introductionsupporting

confidence: 93%

Topographic Core-Mantle Coupling and Polar Motion On Decadal Time-Scales

Hide

Boggs

Dickey

et al. 1996

Geophysical Journal International

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“…But electromagnetic coupling between the core and mantle appears to be two to three orders of magnitude too weak (Greff-Lefftz and Legros, 1995) and topographic coupling appears to be too weak by a factor of three to ten (Asari et al, 2006;Dumberry, 2010;Greff-Lefftz and Legros, 1995;Hide et al, 1996;Hulot et al, 1996). But electromagnetic coupling between the core and mantle appears to be two to three orders of magnitude too weak (Greff-Lefftz and Legros, 1995) and topographic coupling appears to be too weak by a factor of three to ten (Asari et al, 2006;Dumberry, 2010;Greff-Lefftz and Legros, 1995;Hide et al, 1996;Hulot et al, 1996).…”

Section: True Polar Wander and Glacial Isostatic Adjustmentmentioning

confidence: 94%

Earth Rotation Variations – Long Period

Gross

2015

Treatise on Geophysics

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“…Among these the TG assumption has been most commonly employed, in which the Lorentz force is eliminated from the horizontal force balance at the core surface, resulting in TG equilibrium. It has been adopted not only to model the core flow but also to compute the fluid pressure [Chulliat and Hulot, 2000;Chambodut et al, 2007] and the accompanying pressure torque acting on the topography of the mantle's bottom [e.g., Jault and Le Mouël, 1989;Greff-Lefftz and Legros, 1995]. However, the TG assumption is known to have theoretical and observational difficulties.…”

Section: Introductionmentioning

confidence: 99%

Radial vorticity constraint in core flow modeling

Asari

Lesur

2011

J. Geophys. Res.

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[1] We present a new method for estimating core surface flows by relaxing the tangentially geostrophic (TG) constraint. Ageostrophic flows are allowed if they are consistent with the radial component of the vorticity equation under assumptions of the magnetostrophic force balance and an insulating mantle. We thus derive a tangentially magnetostrophic (TM) constraint for flows in the spherical harmonic domain and implement it in a least squares inversion of GRIMM-2, a recently proposed core field model, for temporally continuous core flow models (2000.0-2010.0). Comparing the flows calculated using the TG and TM constraints, we show that the number of degrees of freedom for the poloidal flows is notably increased by admitting ageostrophic flows compatible with the TM constraint. We find a significantly improved fit to the GRIMM-2 secular variation (SV) by including zonal poloidal flow in TM flow models. Correlations between the predicted and observed length-of-day variations are equally good under the TG and TM constraints. In addition, we estimate flow models by imposing the TM constraint together with other dynamical constraints: either purely toroidal (PT) flow or helical flow constraint. For the PT case we cannot find any flow which explains the observed SV, while for the helical case the SV can be fitted. The poor compatibility between the TM and PT constraints seems to arise from the absence of zonal poloidal flows. The PT flow assumption is likely to be negated when the radial magnetostrophic vorticity balance is taken into account, even if otherwise consistent with magnetic observations.

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Core-mantle coupling and polar motion

Cited by 32 publications

References 11 publications

Topographic Core-Mantle Coupling and Polar Motion On Decadal Time-Scales

Topographic Core-Mantle Coupling and Polar Motion On Decadal Time-Scales

Earth Rotation Variations – Long Period

Radial vorticity constraint in core flow modeling

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