Geomagnetic evidence for fluid upwelling at the core-mantle boundary

Whaler, Kathy

doi:10.1111/j.1365-246x.1986.tb03844.x

Cited by 95 publications

(76 citation statements)

References 23 publications

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“…The SV data were inverted for toroidal and poloidal flow using the linear relationship between SV and flow spherical harmonic coefficients. The relation is through the Gaunt/Elsasser matrix (H) whose elements depend on the main field coefficients which change with time (Whaler, 1986). The main field, SV and flow coefficients are truncated at degree and order n max = 14, and thus we have assumed that only large scale flows are responsible for the large scale SV.…”

Section: Flow Modellingmentioning

confidence: 99%

Forecasting secular variation using core flows

Beggan

Whaler

2010

Earth Planet Sp

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Over the past ten years satellite measurements in combination with data from ground-based observatories have allowed very detailed models of the secular variation (SV) of the Earth's magnetic field to be constructed. However, forecasting the change of the main field still remains a challenge, primarily because the core processes controlling SV are not sufficiently well understood. Hence, most forecasts do not appeal to any physical modelling constraints but use, for example, polynomial extrapolation from previous measurements. We attempt to apply a physical model to forecast the average SV during 2010-2015 by developing a core flow model. This steady flow model, derived from SV data during 2004.5 to 2009.5, generates a set of Gauss SV coefficients which are used to advect the large scale magnetic field forwards in time. Although this model has not been submitted as a candidate for IGRF-11, we present our SV prediction model and compare it to other candidate IGRF-11 SV models. In addition, we examine the use of the Ensemble Kalman filter to optimally assimilate field models derived from (1) forecast methods and (2) noisy data measurements. Such a scenario might conceivably arise if high quality satellite data with global coverage are not available for a significant period of time. We show that the overall misfit of the assimilated model to the actual field can be lower than the individual misfits of the input models, provided the uncertainties of each model are reasonably well known.

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Section: Flow Modellingmentioning

confidence: 99%

Forecasting secular variation using core flows

Beggan

Whaler

2010

Earth Planet Sp

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“…Recent studies on determining fluid motion at the top of the core (LIRE et al,1986;WHALER, 1986;VOORHIES, 1986) are all based on the frozen flux hypothesis, which results from the assumption that the conductivity of the core is so large that we can assume it to be infinite. BACKUS (1968) derived the necessary condition for this hypothesis, which is that the radial flux must be conserved through patches on the CMB bounded by curves of zero radial component of the core field (null flux lines).…”

Section: Pattern Of the Field At The Core Mantle Boundarymentioning

confidence: 99%

Rates of change of the Earth's magnetic field measured by recent analyses.

Harrison

Huang

1990

J. geomagn. geoelec

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The availability of accurate representations of the Earth's magnetic field for the past 25 years allows us to study the typical rates of change of the field as a function of the degree of the spherical harmonic. It is shown that there is a general increase in the relative rate of change as the degree of harmonic is increased. The analysis was only carried out to degree eight harmonics, because knowledge of the rate of change of the higher degrees is poor. If the projected rate of change continues in the same manner for higher degrees, then it is predicted that harmonics of, say, degree 14 will change at a rate of about 6% per year. This will mean that it should be easy to determine whether harmonics higher than degree fourteen originate in the core or whether they are caused by crustal magnetization. Although westward drift of the field does not necessarily tell us about motions of the core fluid at the core mantle boundary, it is probably true that these motions depend on the same parameters as those used to determine the westward drift. Since the westward drift depends on the longitudinal gradient of the component of field under consideration, it generally depends more on the higher orders of harmonic within each degree than on the lower orders, all other things being equal. In fact, not all other things are equal, because it is shown that in general the power in the higher orders of harmonic is less than the power in the lower orders. Another important factor in determining longitudinal motions is that it is the field at the core mantle boundary which is important, not the field at the Earth's surface. This results in the higher degrees being relatively more important than at the surface of the Earth. But because of the longitudinal differentiation, it turns out that in order to determine a major portion of the longitudinal drift it is necessary to have information about harmonics up to about degree 20. Therefore, with knowledge only of degrees eight or less, we really know rather little about longitudinal motions. Evidence for secular acceleration was found. Not surprisingly, the average rate of secular acceleration

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“…where the column vector _ b contains the SV Gauss coefficients, m the SH coefficients of the toroidal and poloidal scalar functions of the core surface flow u H , and A _ b is the matrix whose elements are linear functions of the MF Gauss coefficients [Whaler, 1986]. It is worth remarking that, by virtue of the identity of equations (3) and (7) in terms of their analytical forms, the RV equation can be expressed in the SH domain as…”

Section: Core Flow Inversion With the Tm Constraint In The Spherical mentioning

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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Geomagnetic evidence for fluid upwelling at the core-mantle boundary

Cited by 95 publications

References 23 publications

Forecasting secular variation using core flows

Forecasting secular variation using core flows

Rates of change of the Earth's magnetic field measured by recent analyses.

Radial vorticity constraint in core flow modeling

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