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Torsional oscillations and the magnetic field within the Earth's core

Nature volume 388, pages 760–763 (1997)Cite this article

Abstract

An estimate of the magnitude and geometry of the magnetic field within the Earth's core would be valuable for understanding the dynamics of the liquid outer core and for constraining numerical models of the geodynamo. The magnetic field down to the core–mantle boundary can be estimated from surface observations by assuming that the mantle is an insulator1, but such estimates cannot be further extrapolated into the conducting core itself. The magnetic field within the core has therefore remained largely unconstrained. Here we construct a simple picture of part of the magnetic field within the core by first showing that the fluid flow at the surface of the core is consistent with the presence of two large waves—‘torsional oscillations’ of the type that have been proposed to explain the temporal variation of the magnetic field at the core–mantle boundary. We then use the structure of these waves to calculate a one-dimensional map of the part of the magnetic field that points away from the rotation axis. These results may help distinguish between the different dynamic states proposed for outer-core flow2,3,4,5 and provide a test for recent numerical models of the geodynamo6,7,8,9.

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Figure 1: The axisymmetric, equatorially symmetric part of the velocity of the surface of the core (with the steady part removed) from AD1900 to AD1990.
Figure 2: The amplitudes and phases of the two fitted waves (see Fig. 1 legend).
Figure 3: r.m.s. Bs(s), averaged on axial cylinders, for various wave/friction model inversions.
Figure 4: Cartoon of a possible cross-section of poloidal magnetic field through the core.

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Acknowledgements

This work was supported by NASA, the NSF and a National Science Foundation graduate research fellowship (S.Z.).

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  1. Department of Earth and Planetary Sciences, Harvard University, Cambridge, 02138, Massachusetts, USA

    Stephen Zatman & Jeremy Bloxham

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Correspondence to Stephen Zatman.

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Zatman, S., Bloxham, J. Torsional oscillations and the magnetic field within the Earth's core. Nature 388, 760–763 (1997). https://doi.org/10.1038/41987

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