Simple models of fluid flow at the core surface derived from geomagnetic field models

[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.

show abstract

“…Indeed, the decomposition (13) has been made in the same manner in previous studies [e.g., Bloxham, 1989;Wardinski et al, 2008].…”

Section: Radial Vorticity Equation and Tangentially Magnetostrophic Cmentioning

confidence: 99%

Radial vorticity constraint in core flow modeling

Asari

Lesur

2011

J. Geophys. Res.

View full text Add to dashboard Cite

show abstract

“…Voorhies' [1988;1991a;1992] models CVC1E6 and 08070.1 are barely distinguishable in these plots, reflecting the fact that the models were fit using a similar procedure starting with the 1980 DGRF as the initial condition. The most apparent differences between these two models and the model of Bloxham [1989] is as much variation between models fit to the same epoch by different workers as there is between models by a single worker of the flow for different epochs.…”

Section: Velocity and Pressure Fields In The Corementioning

confidence: 99%

“…Four of these models are shown in Figure 2. Figure 2a shows model IIa of Bloxham [1989], a geostrophic model determined assuming steady flow and using a smoothly varying model of the magnetic field at the CMB for the interval1975 -1980. This model is expressed in terms of spherical harmonics through degree and order 14.…”

Section: Velocity and Pressure Fields In The Corementioning

confidence: 99%

“…Much closer agreements with the magnitude of observed LOD changes were found in the case of combinations for model WL, which has irregular CMB topography< 1 km in amplitude, although the predicted changes are still a factor of 4 -8 to large. For the topography models used here, Bloxham's [1989] flow model predicts the wrong sign of LOD variation during 1975LOD variation during -1980. Voorhies' [1988] models predict the correct sign for the decade represented by models 06070.m.…”

Section: Predicted Torques and Length-of-day Variationsmentioning

confidence: 99%

See 1 more Smart Citation

Topographic Core-Mantle Coupling and Fluctuations in the Earth's Rotation

Hide¹,

Clayton

Hager

et al. 2013

Relating Geophysical Structures and Processes: The Jeffreys Volume

View full text Add to dashboard Cite

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 wavelength 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 0.5km (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. lNTRODUCfiONElectric currents generated in the Earth's liquid metallic core are responsible for the main geomagnetic field and its secular changes [see Jacobs, 1987ab;Melchior, 1986;Moffatt, 1978a] are produced by dynamo action involving irregular magnetohydrodynamic flow in the core. Concomitant dynamical stresses acting on the overlying mantle are invoked in the interpretation of the so-called "decadal" fluctuations in the rotation of the "solid Earth" (mantle, crust and cryosphere). Studies of these rotational manifestations of core motions bear directly on investigations of the structure, composition and dynamics of the Earth's deep interior

show abstract

“…Based on the frozen-flux approximation (ROBERTS and SCOTT, 1965), fluid motions near the core surface have been derived by many authors (WHALER, 1980;GUBBINS, 1982;LE MOUEL et al, 1985;VOORHIES, 1986;WHALER and CLARKE, 1988;BLOXHAM, 1989;LLOYD and GUBBINS, 1990;JACKSON and BLOXHAM, 1991) with additional constraints required to avoid the fundamental non-uniqueness (BACKUS, 1968). These studies are reviewed by BLOXHAM and JACKSON (1991).…”

Section: Introductionmentioning

confidence: 99%

Fluid Motion in the Earth's Core Derived from the Geomagnetic Field and Its Implications for the Geodynamo

Matsushima¹

1993

J. geomagn. geoelec

View full text Add to dashboard Cite

An attempt is made to derive fluid motion in the Earth's outer core from geomagnetic field data. We implicitly incorporate the energy source for the geodynamo in the prescribed radial dependence of poloidal velocity field. Then the Navier-Stokes equation for the toroidal constituent and the induction equation for the toroidal and the poloidal magnetic fields are solved so as to fit the magnetic field at the core-mantle boundary estimated through downward continuation on the assumption that the mantle is an insulator. The basic standpoint is that the large-scale magnetic field is maintained by induction processes associated with large-scale fluid motion within the core. We also impose the condition that time variations of the velocity and the magnetic fields are very slow. The generation balance in the induction equation, for the derived velocity and magnetic fields, suggests that the geodynamo is likely to be of a2w-type.

show abstract

Simple models of fluid flow at the core surface derived from geomagnetic field models

Cited by 88 publications

References 26 publications

Radial vorticity constraint in core flow modeling

Radial vorticity constraint in core flow modeling

Topographic Core-Mantle Coupling and Fluctuations in the Earth's Rotation

Fluid Motion in the Earth's Core Derived from the Geomagnetic Field and Its Implications for the Geodynamo

Contact Info

Product

Resources

About