Séminaire ISTerre

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Investigating regional heterogeneity at the core-mantle boundary and its impact on outer core flow

vendredi 6 janvier 2023 - 14h00

Hannah Rogers - ISTerre
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Currently, there is relatively poor understanding of how the magnetic field is affected by heterogeneity on the core-mantle boundary (CMB) despite increasing evidence of heterogeneity from geochemical, seismic tomography, geomagnetic, and geodynamics studies. An example of heterogeneity are the Large Low seismic Velocity Provinces (LLVPs) that have been identified at the base of the mantle. LLVPs have been proposed to indicate topographic features, thermal variation, regional stratification, or chemical exchange between core and mantle, all of which would affect the flow at the surface of the core and the magnetic field it generates. If spherical harmonic separations are conducted to investigate heterogeneity, a boundary is enforced at the region edge and large amounts of ringing are generated. One way to investigate regional potential fields on a sphere is to use spherical Slepian functions which convert spherical harmonic data into a new localized basis. These functions are constructed by maximizing the power of a function within a specified region where it is orthogonal in both the region and the complementary part of the sphere. This allows for the preferential ranking and selection of a subsection of the functions to be constrained in the region of interest. Spherical Slepian functions are the only functions that achieve a regional separation expressed as a finite sum of spherical harmonics, and in an easily computable framework, depending only on the geometry of the region and maximum spherical harmonic degree. The key contribution of this work is using spherical Slepian functions during the inversions for SV at the core-mantle boundary. Slepian functions provide a novel application of geomagnetic virtual observatory datasets, which allows 20 years of magnetic data from Swarm, CHAMP, and Cryosat to be treated as an even spread of data points in time and space, similar to ground observatory data. We apply altitude-cognizant Slepian functions to core surface SV models by directly inverting from satellite virtual observatory secular variation data into regions of interest. We successfully demonstrate the technique and current short comings by investigating the influence of LLVPs on the movement of liquid iron at the core surface over time. Unwanted spatial leakage is generated at the region edges in the separated SV but to less of an extent than when using spherical harmonic separations.

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Equipe organisatrice : Géodynamo

Salle Dolomieu, Maison des Géosciences, 38400 Saint Martin d'Hères

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