Quasi-geostrophic flows responsible for the secular variation of the Earth's magnetic field

Pais, Maria Alexandra; Jault, D.

doi:10.1111/j.1365-246x.2008.03741.x

Cited by 147 publications

(260 citation statements)

References 62 publications

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“…A similar equatorially-symmetric westwards flow close to the tangent cylinder has been identified in previous direct core-flow inversions e.g. [22,23], and is sometimes interpreted as an integral part of a planetary-scale gyre. …”

Section: Observationally-constrained High-latitude Flowmentioning

confidence: 76%

An accelerating high-latitude jet in Earth’s core

2016

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Observations of the change in Earth's magnetic field, the secular variation, provide information on the motion of liquid metal within the core that is responsible for its generation. The very latest high-resolution observations from ESA's Swarm satellite mission show intense field change at high-latitude localised in a distinctive circular daisy-chain configuration centred on the north geographic pole. Here we explain this feature with a localised, non-axisymmetric, westwards jet of 420 km width on the tangent cylinder, the cylinder of fluid within the core that is aligned with the rotation axis and tangent to the solid inner core.We find that the jet has increased in magnitude by a factor of three over the period 2000-2016 to about 40 km/yr, and is now much stronger than typical large-scale flows inferred for the core. The current accelerating phase may be a part of a longer term fluctuation of the jet causing both eastwards and westwards movement of magnetic features over historical

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Section: Observationally-constrained High-latitude Flowmentioning

confidence: 76%

An accelerating high-latitude jet in Earth’s core

2016

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“…k l n denotes the nth zero of the degree l − 1 spherical Bessel function, and in this particular calculation we let n = l. Recent studies have estimated flow models fitting SV models either at one standard deviation [Pais and Jault, 2008] or even more tightly [Holme and Olsen, 2006;Olsen and Mandea, 2008;Wardinski et al, 2008]. Here, we do not restrict ourselves to either of these cases, but study resulting models by varying the misfit level within one standard deviation.…”

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

confidence: 99%

“…Hence, a flow model estimated without accounting for this SV contribution can be affected by the aliasing. The SV misfit may even have to be no smaller than those of flow models based on magnetic models built with ground-based observation alone [Pais and Jault, 2008].…”

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

confidence: 99%

“…A significant error in flow imaging arises from the unmodeled SV due to the advection of the small-scale MF masked by the crustal field [Eymin and Hulot, 2005]. Instead of fitting the observed SV tightly, one needs to allow for a possible error from the unmodeled SV [Pais and Jault, 2008;Gillet et al, 2009]. The theoretical non-uniqueness problem in the flow modeling [e.g., Holme, 2007] is another setback which is even more serious.…”

Section: Introductionmentioning

confidence: 99%

“…These are typically based on dynamics relevant to core flow, such as tangentially geostrophic (TG) flow, purely toroidal (PT) flow and helical flow [see, e.g., Holme, 2007], as well as the recent quasigeostrophic flow [Pais and Jault, 2008]. 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.…”

Section: Introductionmentioning

confidence: 99%

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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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Bayesian inversion for the filtered flow at the Earth's core‐mantle boundary

2014

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The inverse problem of determining the flow at the Earth's core-mantle boundary according to an outer core magnetic field and secular variation model has been investigated through a Bayesian formalism. To circumvent the issue arising from the truncated nature of the available fields, we combined two modeling methods. In the first step, we applied a filter on the magnetic field to isolate its large scales by reducing the energy contained in its small scales, we then derived the dynamical equation, referred as filtered frozen flux equation, describing the spatiotemporal evolution of the filtered part of the field. In the second step, we proposed a statistical parametrization of the filtered magnetic field in order to account for both its remaining unresolved scales and its large-scale uncertainties. These two modeling techniques were then included in the Bayesian formulation of the inverse problem. To explore the complex posterior distribution of the velocity field resulting from this development, we numerically implemented an algorithm based on Markov chain Monte Carlo methods. After evaluating our approach on synthetic data and comparing it to previously introduced methods, we applied it to a magnetic field model derived from satellite data for the single epoch 2005.0. We could confirm the existence of specific features already observed in previous studies. In particular, we retrieved the planetary scale eccentric gyre characteristic of flow evaluated under the compressible quasi-geostrophy assumption although this hypothesis was not considered in our study. In addition, through the sampling of the velocity field posterior distribution, we could evaluate the reliability, at any spatial location and at any scale, of the flow we calculated. The flow uncertainties we determined are nevertheless conditioned by the choice of the prior constraints we applied to the velocity field.

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Quasi-geostrophic flows responsible for the secular variation of the Earth's magnetic field

Cited by 147 publications

References 62 publications

An accelerating high-latitude jet in Earth’s core

An accelerating high-latitude jet in Earth’s core

Radial vorticity constraint in core flow modeling

Bayesian inversion for the filtered flow at the Earth's core‐mantle boundary

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