Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

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

Nature volume 465, pages 74–77 (2010)Cite this article

Subjects

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.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on SpringerLink
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

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.

Similar content being viewed by others

References

  1. Aubert, J., Labrosse, S. & Poitou, C. Modelling the palaeo-evolution of the geodynamo. Geophys. J. Int. 179, 1414–1428 (2009)

    Article  ADS  Google Scholar 

  2. Roberts, P. H., Yu, Z. J. & Russel, C. T. On the 60-year signal from the core. Geophys. Astrophys. Fluid Dyn. 101, 11–35 (2007)

    Article  ADS  Google Scholar 

  3. Zatman, S. & Bloxham, J. Torsional oscillations and the magnetic field within the Earth’s core. Nature 388, 760–763 (1997)

    Article  ADS  Google Scholar 

  4. Buffett, B. A., Mound, J. & Jackson, A. Inversion of torsional oscillations for the structure and dynamics of Earth's core. Geophys. J. Int. 177, 878–890 (2009)

    Article  ADS  Google Scholar 

  5. Abarca del Rio, R., Gambis, D. & Salstein, D. A. Interannual signals in length of day and atmospheric angular momentum. Ann. Geophys. 18, 347–364 (2000)

    Article  ADS  Google Scholar 

  6. Gillet, N., Pais, M. A. & Jault, D. Ensemble inversion of time-dependent core flow models. Geochem. Geophys. Geosyst. 10 Q06004 10.1029/2008GC002290 (2009)

    Article  ADS  Google Scholar 

  7. Pais, M. A. & Jault, D. Quasi-geostrophic flows responsible for the secular variation of the Earth's magnetic field. Geophys. J. Int. 173, 421–443 (2008)

    Article  ADS  CAS  Google Scholar 

  8. Jault, D. Axial invariance of rapidly varying diffusionless motions in the Earth's core interior. Phys. Earth Planet. Inter. 166, 67–76 (2008)

    Article  ADS  Google Scholar 

  9. Alfvén, H. Existence of electromagnetic-hydrodynamic waves. Nature 150, 405–406 (1942)

    Article  ADS  Google Scholar 

  10. Jault, D., Gire, C. & Le Mouël, J.-L. Westward drift, core motions and exchanges of angular momentum between core and mantle. Nature 333, 353–356 (1988)

    Article  ADS  Google Scholar 

  11. Jackson, A., Bloxham, J. & Gubbins, D. in Dynamics of the Earth's Deep Interior and Earth Rotation (eds LeMouël, J.-L. Smylie, D. E. & Herring, T. A.) Vol. 72 97–107 (American Geophysical Union, 1993)

    Google Scholar 

  12. Jackson, A., Jonkers, A. R. T. & Walker, M. R. Four centuries of geomagnetic secular variation from historical records. Phil. Trans. R. Soc. Lond. A 358, 957–990 (2000)

    Article  ADS  CAS  Google Scholar 

  13. Gross, R. S. A combined length-of-day series spanning 1832–1977: LUNAR97. Phys. Earth Planet. Inter. 123, 65–76 (2001)

    Article  ADS  Google Scholar 

  14. Jackson, A. Time-dependency of tangentially geostrophic core surface motions. Phys. Earth Planet. Inter. 103, 293–311 (1997)

    Article  ADS  Google Scholar 

  15. Pais, M. A., Oliveira, O. & Nogueira, F. Nonuniqueness of inverted core-mantle boundary flows and deviation from tangential geostrophy. J. Geophys. Res. 109 B08105 10.1029/2004JB003012 (2004)

    Article  ADS  Google Scholar 

  16. Bloxham, J., Zatman, S. & Dumberry, M. The origin of geomagnetic jerks. Nature 420, 65–68 (2002)

    Article  ADS  CAS  Google Scholar 

  17. Mound, J. E. & Buffett, B. Detection of a gravitational oscillation in length of day. Earth Planet. Sci. Lett. 243, 383–389 (2006)

    Article  ADS  CAS  Google Scholar 

  18. Jordi, C., Morrison, L. V., Rosen, R. D., Salstein, D. A. & Rosselló, G. Fluctuations in the Earth's rotation since 1830 from high-resolution astronomical data. Geophys. J. Int. 117, 811–818 (1994)

    Article  ADS  Google Scholar 

  19. Braginsky, S. I. Torsional magnetohydrodynamic vibrations in the Earth's core and variations in day length. Geomagn. Aeron. 10, 1–8 (1970)

    ADS  Google Scholar 

  20. Fournier, A., Eymin, C. & Alboussière, T. A case for variational geomagnetic data assimilation: insights from a one-dimensional, nonlinear and sparsely observed MHD system. Nonlinear Process. Geophys. 14, 1–18 (2007)

    Article  Google Scholar 

  21. Canet, E., Fournier, A. & Jault, D. Forward and adjoint quasi-geostrophic models of the geomagnetic secular variation. J. Geophys. Res. 114 B11101 10.1029/2008JB006189 (2009)

    Article  ADS  Google Scholar 

  22. Buffett, B. A., Mathews, P. M. & Herring, T. A. Modeling of nutation and precession: effects of electromagnetic coupling. J. Geophys. Res. 107 (B4). 2070 10.1029/2000JB000056 (2002)

    Article  ADS  Google Scholar 

  23. Jackson, A. & Livermore, P. On Ohmic heating in the Earth's core. I: Nutation constraints. Geophys. J. Int. 10.1111/j.1365–246X.2008.04008.x (2009)

  24. Dumberry, M. & Mound, J. E. Constraints on core-mantle electromagnetic coupling from torsional oscillation normal modes. J. Geophys. Res. 113 10.1029/2007JB005135 (2008)

  25. Ohta, K. et al. Electrical conductivities of pyrolitic mantle and MORB materials up to the lowermost mantle conditions. Earth Planet. Sci. Lett. 289, 497–502 (2010)

    Article  ADS  CAS  Google Scholar 

  26. Zhang, J., Richards, P. G. & Schaff, D. P. Wide-scale detection of earthquake waveform doublets and further evidence for inner core super-rotation. Geophys. J. Int. 174, 993–1006 (2008)

    Article  ADS  Google Scholar 

  27. Taylor, J. B. The magneto-hydrodynamics of a rotating fluid and the Earth's dynamo problem. Proc. R. Soc. Lond. A 274, 274–283 (1963)

    Article  ADS  Google Scholar 

  28. Rhines, P. B. Waves and turbulence on a beta-plane. J. Fluid Mech. 122, 417–443 (1975)

    Article  ADS  Google Scholar 

  29. Olsen, N. & Mandea, M. Rapidly changing flows in the Earth's core. Nature Geosci. 1, 390–394 (2008)

    Article  ADS  CAS  Google Scholar 

  30. Friis-Christensen, E., Lühr, H. & Hulot, G. Swarm: a constellation to study the Earth's magnetic field. Earth Planets Space 58, 351–358 (2006)

    Article  ADS  Google Scholar 

Download references

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.

Author information

Authors and Affiliations

  1. Laboratoire de Géophysique Interne et Tectonophysique, CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble cedex 9, France,

    Nicolas Gillet, Dominique Jault & Elisabeth Canet

  2. Equipe de Géomagnétisme, Institut de Physique du Globe de Paris, Université Paris-Diderot, CNRS, 4 place Jussieu, F-75252 Paris cedex 5, France,

    Alexandre Fournier

Authors
  1. Nicolas Gillet
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Dominique Jault
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Elisabeth Canet
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Alexandre Fournier
    View author publications

    You can also search for this author in PubMed Google Scholar

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.

Corresponding author

Correspondence to Nicolas Gillet.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Data, Supplementary Figures 1-3 with legends and References. (PDF 202 kb)

PowerPoint slides

Rights and permissions

Reprints and permissions

About this article

Cite this article

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

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/nature09010

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing