Geomagnetic fluctuations reveal stable stratification at the top of the Earth’s core

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Abstract

Modern observations of the geomagnetic field reveal fluctuations1,2,3 with a dominant period of about 60 years. These fluctuations are probably a result of waves in the liquid core4, although the precise nature of the waves is uncertain. Common suggestions include a type of magnetic wave, known as a torsional oscillation5, but recent studies6 favour periods that are too short to account for a 60-year fluctuation. Another possibility involves MAC waves7, which arise from the interplay between magnetic, Archimedes and Coriolis forces. Waves with a suitable period can emerge when the top of the core is stably stratified. Here I show that MAC waves provide a good description of time-dependent zonal flow at the top of the core8, as inferred from geomagnetic secular variation9. The same wave motion can also account for unexplained fluctuations in the dipole field10. Both of these independent predictions require a 140-kilometre-thick stratified layer with a buoyancy frequency comparable to the Earth’s rotation rate. Such a stratified layer could have a thermal origin11,12, implying a core heat flow of about 13 terawatts. Alternatively, the layer could result from chemical stratification13,14. In either case, the existence of a stratified layer at the top of the core obscures the nature of flow deeper in the core, where the magnetic field is continually regenerated.

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Figure 1: Schematic illustration of the wave motion.
Figure 2: A representative wave inside the stratified layer.
Figure 3: Estimate of surface core flow and prediction due to MAC waves.
Figure 4: Fluctuations in the dipole field due to MAC waves.

Change history

  • 26 March 2014

    Minor edits were made to the symbols in the keys of Fig. 4.

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Acknowledgements

A. Jackson and I. Wardinski provided models of surface core flow and magnetic field. Comments and suggestions from R. Holme substantially improved the final text. This work was supported in part by the US National Science Foundation (EAR-1045277).

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Correspondence to Bruce Buffett.

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The author declares no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Azimuthal velocity Vϕ on a meridional cross-section.

Wave motion in the stratified layer at the top of the core induces geostrophic flow in the underlying fluid. The geostrophic nature of the deeper flow is inferred from the axial independence of the velocity.

Extended Data Figure 2 Radial velocity along a radial transect at colatitude θ = 40°.

Radial motion is confined to the stratified layer and vanishes in the nearly geostrophic interior.

Extended Data Table 1 Period, quality factor and relative amplitude of the eight gravest modes

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Buffett, B. Geomagnetic fluctuations reveal stable stratification at the top of the Earth’s core. Nature 507, 484–487 (2014). https://doi.org/10.1038/nature13122

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