Core flow inversion tested with numerical dynamo models

Rau, Steffen; Christensen, Ulrich R.; Jackson, Andrew; Wicht, Johannes

doi:10.1046/j.1365-246x.2000.00097.x

Cited by 63 publications

(82 citation statements)

References 41 publications

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“…12 can be associated with such regions of flux expulsion. This does not imply that magnetic diffusion is absent in these regions, it simply may indicate that diffusion is fit by frozen-flux flow, although [38] found this scenario unlikely. To obtain realistic estimates of diffusive effects is demanding.…”

Section: Uncertainties Of the Flow Inversionmentioning

confidence: 89%

The 2003 geomagnetic jerk and its relation to the core surface flows

Wardinski

Holme

Asari

et al. 2008

Earth and Planetary Science Letters

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In this paper we examine the core surface flow obtained by an inversion of a continuous model of the geomagnetic field and its temporal variation using the diffusion-less induction equation. The continuous CHAOS model is derived from satellite data up to spherical harmonic degree 14 and covers the period 1999 to 2006. The CHAOS secular variation, when downward continued to the core surface, shows stripe-like features, which can be attributed to spherical harmonic degree 12 and higher. These contributions are removed by applying a tapering method, and the resulting tapered model is then inverted for the core surface flow. Satellite-based field models have a high spatial resolution; however, their temporal resolution is limited. In order to enhance the temporal resolution of the flow, we additionally constrain the flow to fit the secular variation from ground-based observatory data.A range of solutions, subject to different constraints, are computed, two flow hypotheses being considered: purely toroidal flow and tangentially geostrophic flow.We show that both flow types provide similar results; however, the purely toroidal flow provides a better fit to the secular variation in the equatorial region than Finally, we compare observed changes in the length-of-day and the predictions from the flow solutions.

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Section: Uncertainties Of the Flow Inversionmentioning

confidence: 89%

The 2003 geomagnetic jerk and its relation to the core surface flows

Wardinski

Holme

Asari

et al. 2008

Earth and Planetary Science Letters

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show abstract

“…Last but not least, velocity fields obtained from solving selfconsistent three-dimensional numerical dynamos are largely geostrophic near the CMB. This has been observed even in those cases when, inside the core, the Lorentz force exceeds the Coriolis force by almost one order of magnitude [Rau et al, 2000]. Such numerical results have, nonetheless, to be considered with some skepticism, since numerical simulations are still being run in unrealistic parameter regimes [e.g., Dormy et al, 2000].…”

Section: Introductionmentioning

confidence: 99%

Nonuniqueness of inverted core‐mantle boundary flows and deviations from tangential geostrophy

2004

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[1] The task of finding a flow at the top of the core that fits geomagnetic observations at the Earth's surface is an underdetermined problem. Weighted regularized least squares together with the tangential geostrophy hypothesis strongly constrain the flow, inhibiting both medium-to small-scale flow features from emerging and low-latitude currents from crossing the geographic equator. In this study, we solve the inverse problem in the frozen flux approximation, using a weak regularization and alleviating the tangential geostrophy constraint. Two classes of solutions are found (I and II), both fitting the data within the errors but nonetheless showing very distinct features. Using, as a further criterion, the ability of the flow solutions to reproduce the decade variations of the length of day does not allow us, in general, to give a clear preference to one class over another. For typical flows from the whole set of solutions described above, we identify the regions on the core-mantle boundary (CMB) where tangential geostrophy is violated, and we quantify those deviations. Assuming that they are due to Lorentz forces acting on the fluid, we compute preliminary charts of electrical density currents (J) on the CMB, consistent with the frozen flux and insulating mantle hypotheses. The most pronounced features in these charts are concentrated over the hemisphere roughly centered beneath the Atlantic Ocean. In light of thermodynamical arguments, class II flows seem closer to real surface core flows. We compute an upper limit J RMS % 0.4 A/m 2 for geomagnetically ''visible'' density currents related to these flows.

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“…The global correlations between the non-zonal radial shear of the flow ∂u nz h /∂r to the non-zonal tangential flow itself u nz h /R at the top of the free stream for cases T 1lq, T 0, T 1, T 1hp and Y 1 2 are well above the 95% significance level, which is about 0.2 for the resolution of the numerical dynamos in this study (Rau et al, 2000). When only high-latitudes are considered, the correlations are high for all cases.…”

Section: Model Validationmentioning

confidence: 50%

A simple model for mantle-driven flow at the top of Earth’s core

et al. 2008

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We derive a model for the steady fluid flow at the top of Earth's core driven by thermal coupling with the heterogeneous lower mantle. The model uses a thermal wind balance for the core flow, and assumes a proportionality between the horizontal density gradients at the top of the core and horizontal gradients in seismic shear velocity in the lowermost mantle. It also assumes a proportionality between the core fluid velocity and its radial shear. This last assumption is validated by comparison with numerical models of mantle-driven core flow, including self-sustaining dynamo (supercritical) models and non-magnetic convection (subcritical) models. The numerical dynamo models show that thermal winds with correlated velocity and radial shear dominate the boundary-driven large-scale flow at the top of the core. We then compare the thermal wind flow predicted by mantle heterogeneity with the 150 year time-average flow obtained from inverting the historical geomagnetic secular variation, focusing on the non-zonal components of the flows because of their sensitivity to the boundary heterogeneity. Comparing magnitudes provides an estimate of the ratio of lower mantle seismic anomalies to core density anomalies. Comparing patterns shows that the thermal wind model and the time-average geomagnetic flow have comparable length scales and exhibit some important similarities, including an anticlockwise vortex below the southern Indian and Atlantic Oceans, and another anticlockwise vortex below Asia, suggesting these parts of the non-zonal core flow could be thermally controlled by the mantle. In other regions, however, the two flows do not match well, and some possible reasons for the dissimilarity between the predicted and observed core flow are identified. We propose that better agreement could be obtained using core flows derived from geomagnetic secular variation over longer time periods.

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Core flow inversion tested with numerical dynamo models

Cited by 63 publications

References 41 publications

The 2003 geomagnetic jerk and its relation to the core surface flows

The 2003 geomagnetic jerk and its relation to the core surface flows

Nonuniqueness of inverted core‐mantle boundary flows and deviations from tangential geostrophy

A simple model for mantle-driven flow at the top of Earth’s core

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