On core surface flows inferred from satellite magnetic data

Eymin, Céline; Hulot, Gauthier

doi:10.1016/j.pepi.2005.06.009

Cited by 87 publications

(117 citation statements)

References 39 publications

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“…Possibly, Reynolds stress effects (Heimpel et al 2005) and thermal wind effects are now at work here. The high latitude retrograde flow found close to the outer boundary in the dipole dominated cases is also a typical feature in core flow models derived from geomagnetic secular variation data (Eymin and Hulot 2005). Aurnou et al (2003) and Aubert (2005) show that the amplitude of these flows can be scaled to Earth-like values based on the heat flux through the outer boundary as discussed in Sect.…”

Section: Flow Structurementioning

confidence: 58%

Theory and Modeling of Planetary Dynamos

Wicht

Tilgner

2010

Space Sci Rev

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Numerical dynamo models are increasingly successful in modeling many features of the geomagnetic field. Moreover, they have proven to be a useful tool for understanding how the observations connect to the dynamo mechanism. More recently, dynamo simulations have also ventured to explain the surprising diversity of planetary fields found in our solar system. Here, we describe the underlying model equations, concentrating on the Boussinesq approximations, briefly discuss the numerical methods, and give an overview of existing model variations. We explain how the solutions depend on the model parameters and introduce the primary dynamo regimes. Of particular interest is the dependence on the Ekman number which is many orders of magnitude too large in the models for numerical reasons. We show that a minor change in the solution seems to happen at E = 3×10 −6 whose significance, however, needs to be explored in the future. We also review three topics that have been a focus of recent research: field reversal mechanisms, torsional oscillations, and the influence of Earth's thermal mantle structure on the dynamo. Finally we discuss the possibility of tidally or precession driven planetary dynamos.

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Section: Flow Structurementioning

confidence: 58%

Theory and Modeling of Planetary Dynamos

Wicht

Tilgner

2010

Space Sci Rev

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“…The large-scale SV may also be attributed to "nonmodeled advection," i.e., advection of the small-scale MF truncated and neglected in the flow modeling [Eymin and Hulot, 2005]. Hence, a flow model estimated without accounting for this SV contribution can be affected by the aliasing.…”

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

confidence: 99%

“…Also plotted in Figure 3 are the spectra of the free decay (used to form C _ b in equation (19)) and the non-modeled advection of the truncated small-scale MF. The latter is derived following the protocol of Eymin and Hulot [2005]. It is revealed that the non-modeled advection is not as significant as the free decay, which is not the case for the models of Pais and Jault [2008].…”

Section: Core Flow Models Estimated With Tm Constraintmentioning

confidence: 99%

“…Meanwhile, there still remain obstacles in better resolving the core surface flow. 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].…”

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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“…First, the modelled SV at large spatial scales can result from the advection of unmodelled MF at small spatial scales (Eymin and Hulot, 2005;Pais and Jault, 2008). The second origin is diffusion.…”

Section: Introductionmentioning

confidence: 99%

Geomagnetic secular variation violating the frozen-flux condition at the core surface

2010

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We consider a method to extract the part of a given geomagnetic secular variation (SV) model that is not consistent with a frozen-flux condition. This condition is usually derived from the diffusionless radial induction equation at the core-mantle boundary (CMB), and is defined explicitly in the spatial domain: radial flux changes within closed null-flux curves at the core surface are not allowed at any instant. We study here this condition in the spherical harmonic (SH) domain, relying on the SH expansion of the diffusionless equation. SV models at a certain epoch are separated into advective and non-advective parts. The advective (resp. non-advective) part satisfies (resp. does not satisfy) the frozen-flux condition redefined in the SH domain. We show that this separation is not unique. In this work, we achieve a unique separation by assuming the orthogonality of the two parts in terms of the radial SV energy at the CMB. From the recent geomagnetic models, GRIMM and CM4, we find that the non-advective part shows up mainly in the small reverse patches of the radial magnetic field at the CMB. However, non-advective behaviors are also observed outside these patches. As far as no restriction is imposed on core flow configuration, time variations of the non-advective part are not correlated to those of the SV models. However, if the flow is restricted to be tangentially geostrophic, time variations of the SV models have to be partly non-advective. Furthermore, for this flow configuration, the secular decrease of the axial dipole has to be largely non-advective.

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On core surface flows inferred from satellite magnetic data

Cited by 87 publications

References 39 publications

Theory and Modeling of Planetary Dynamos

Theory and Modeling of Planetary Dynamos

Radial vorticity constraint in core flow modeling

Geomagnetic secular variation violating the frozen-flux condition at the core surface

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