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Fast torsional waves and strong magnetic field within the Earth’s core

Abstract

The magnetic field inside the Earth’s fluid and electrically conducting outer core cannot be directly probed. The root-mean-squared (r.m.s.) intensity for the resolved part of the radial magnetic field at the core–mantle boundary is 0.3 mT, but further assumptions are needed to infer the strength of the field inside the core. Recent diagnostics obtained from numerical geodynamo models1 indicate that the magnitude of the dipole field at the surface of a fluid dynamo is about ten times weaker than the r.m.s. field strength in its interior, which would yield an intensity of the order of several millitesla within the Earth’s core. However, a 60-year signal found in the variation in the length of day2 has long been associated with magneto-hydrodynamic torsional waves carried by a much weaker internal field3,4. According to these studies, the r.m.s. strength of the field in the cylindrical radial direction (calculated for all length scales) is only 0.2 mT, a figure even smaller than the r.m.s. strength of the large-scale (spherical harmonic degree n ≤ 13) field visible at the core–mantle boundary. Here we reconcile numerical geodynamo models with studies of geostrophic motions in the Earth’s core that rely on geomagnetic data. From an ensemble inversion of core flow models, we find a torsional wave recurring every six years, the angular momentum of which accounts well for both the phase and the amplitude of the six-year signal for change in length of day detected over the second half of the twentieth century5. It takes about four years for the wave to propagate throughout the fluid outer core, and this travel time translates into a slowness for Alfvén waves that corresponds to a r.m.s. field strength in the cylindrical radial direction of approximately 2 mT. Assuming isotropy, this yields a r.m.s. field strength of 4 mT inside the Earth’s core.

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Figure 1: High coherence is found not only on long timescales, but also on an approximately six-year period.
Figure 2: The six-year ΔLOD signal is carried by geostrophic wave-like patterns travelling from the inner core to the outer core Equator.
Figure 3: Torsional Alfvén waves can account for the six-year geostrophic oscillation.
Figure 4: The magnetic field exceeds 2–3 mT in most of the fluid domain outside the tangent cylinder, except towards the Equator, where it reaches values close to what is observed at the CMB.

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Acknowledgements

We thank M. Dumberry and P. Roberts for discussions. We also thank A. Jackson, who helped improve the quality of the manuscript. N.G. was funded by a grant from the French Agence Nationale de la Recherche, Research program VS-QG (grant number BLAN06-2.155316). The IPGP contribution number for A.F. is 2618. The study has also been supported in part by the French Centre National d'Etudes Spatiales.

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Contributions

The ensemble core flow inversions, data analysis and error covariances were carried out by NG. The ΔLOD time series analysis was performed by N.G. and D.J. E.C. and A.F. developed the torsional wave variational data assimilation code; runs were performed by N.G. and E.C. The manuscript was mainly written by N.G. and D.J., with a little help from A.F. D.J. played an important part in the discussion of the theoretical background.

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Correspondence to Nicolas Gillet.

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The authors declare no competing financial interests.

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Gillet, N., Jault, D., Canet, E. et al. Fast torsional waves and strong magnetic field within the Earth’s core. Nature 465, 74–77 (2010). https://doi.org/10.1038/nature09010

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