Influence of viscoelastic coupling on the axial rotation of the Earth and its fluid core

S U M M A R YThe influence of the viscomagnetic coupling at the core-mantle boundary (CMB) on the differential rotation of the core and non-tidal acceleration induced by the Pleistocenic deglaciation is investigated for stratified mantle models with steadystate or transient rheologies. For a realistic modelling of the viscomagnetic coupling, the time-dependent viscoelastic topography at the CMB induces a westward drift of the core with respect to the mantle that can be correlated to the zonal component of the secular drift of the geomagnetic field. Starting from a reference model based on the Maxwell rheology and a viscosity profile inferred from J2 data and non-tidal acceleration, with 1021 P a s and 5 x 1021 P a s for upper and lower mantle respectively, we study first the effects of the hardening of the upper mantle in the transition zone between 420 and 670 km depth. This hardening is responsible for a 30 per cent reduction in the westward drift with respect to a uniform upper mantle. The impact of a viscosity decrease on the top of the lower mantle is considered next. The softening of the lower mantle beneath the 670 km discontinuity counteracts the effects of the transition zone, enhancing the differential rotation of the core. The D" layer is also implemented in order to analyse the influence of the decoupling at the bottom of the mantle, responsible for faster relaxation of the topography. If lower mantle viscosity is increased beyond the threshold of 10"Pas we obtain a differential rotation of the core in the opposite direction or an eastward drift. We also analyse the impact of a Burgers rheology for the lower mantle to simulate transient effects. For this rheology, steady-state viscosities in the lower mantle higher than Pa s, in agreement with estimates from long-wavelength geoid anomalies and recent findings from true polar wander, allow a westward drift of the core.Non-tidal acceleration is generally less affected by rheological variations than the differential rotation. Changes in the non-tidal acceleration induced by the various rheological models are, on the other hand, comparable with the error bounds in the observed values.

Section: U(b T )mentioning

confidence: 45%

Section: Discussionmentioning

confidence: 64%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Mantle Rheology, Viscomagnetic Coupling At the Core-Mantle Boundary and Differential Rotation of the Core Induced By Pleistocenic Deglaciation

Lefftz

Sabadini

Legros

1994

“…In addition, we solve for the resulting perturbations in the whole planet, instead of in the mantle only. The earlier work of Lefftz & Legros (1992) is also noteworthy, showing that variations in the topography at the CMB, for instance due to mantle convection, can produce significant changes in the gravity field. Secondly, the changes in the elliptical component of the gravity field resulting from variations in rotation rate of the mantle (and inner core) is also not a novel idea.…”

Section: Introductionmentioning

confidence: 83%

Variations in the Earth's gravity field caused by torsional oscillations in the core

Dumberry¹,

Bloxham²

2004

S U M M A R YWe investigate whether a component of the flow in the Earth's fluid core, namely torsional oscillations, could be detected in gravity field data at the surface and whether it could explain some of the observed time variations in the elliptical part of the gravity field (J 2 ). Torsional oscillations are azimuthal oscillations of rigid coaxial cylindrical surfaces and have typical periods of decades. This type of fluid motion supports geostrophic pressure gradients, which produce deformations of the core-mantle boundary. Because of the density discontinuity between the core and the mantle, such deformations produce changes in the gravity field that, because of the flow geometry, are both axisymmetric and symmetric about the equator. Torsional oscillations are thus expected to produce time variations in the zonal harmonics of even degree in the gravity field. Similarly, the changes in the rotation rates of the mantle and inner core that occur to balance the change in angular momentum carried by the torsional oscillations also produce zonal variations in gravity. We have built a model to calculate the changes in the gravity field and in the rotation rates of the mantle and inner core produced by torsional oscillations. We show that the changes in the rotation rate of the inner core produce changes in J 2 that are a few orders of magnitude too small to be observed. The amplitudes of the changes in J 2 from torsional oscillations are 10 times smaller than the temporal changes that are observed to occur about a linear secular trend. However, provided the mechanism responsible for these changes in J 2 is identified and that this contribution is removed from the data, it may be possible in the future to detect the lowest harmonic degrees of the torsional oscillations in the gravity field data. We also show that torsional oscillations have contributed to the linear secular change in J 2 by about −0.75 × 10 −12 per year in the last 20 years. Finally, the associated change in the vertical ground motion at the surface of the Earth that is predicted by our mechanism is of the order of 0.2 mm, which is too small to be detected with the current precision in measurements.

“…At lower harmonic degrees within the effects of present day mass variation, there is the additional issue of separating the core effect (e.g. Lefftz & Legros 1992; Fang et al . 1996; Greiner‐Mai et al .…”

Section: Discussionmentioning

confidence: 99%

Vertical deformation and absolute gravity

Fang¹,

Hager²

2001

Summary Crustal deformation in the Greenland and Antarctic areas is strongly influenced by both postglacial rebound and contemporary mass redistribution. We explore the relationship between the displacement field and the gravitational disturbance for a viscoelastic Maxwell Earth with an arbitrary radial viscosity profile. We seek to determine whether the effects of viscous relaxation in the memory of surface mass change can be separated from the effects of present day mass variation by combined measurements of vertical displacement and absolute gravity when the viscosity profile in the Earth's interior is unknown. Our conclusion is positive. Specifically, the non‐elastic effects can be reduced substantially by combined measurements of displacement and gravity change for a Maxwell viscoelastic Earth regardless of its radial viscosity profile. The underlying physics has nothing to do with the mathematical structure of viscous relaxation modes. Rather, it is due to the fact that the non‐elastic response of a Maxwell Earth is nearly incompressible.