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Tidal dissipation and the strength of the Earth’s internal magnetic field


Magnetic fields at the Earth’s surface represent only a fraction of the field inside the core1. The strength and structure of the internal field are poorly known2,3,4,5, yet the details are important for our understanding of the geodynamo. Here I obtain an indirect estimate for the field strength from measurements of tidal dissipation. Tidally driven flow in the Earth’s liquid core develops internal shear layers, which distort the internal magnetic field and generate electric currents. Ohmic losses damp the tidal motions and produce detectable signatures in the Earth’s nutations. Previously reported evidence of anomalous dissipation in nutations3,6 can be explained with a core-averaged field of 2.5 mT, eliminating the need for high fluid viscosity6 or a stronger magnetic field at the inner-core boundary3. Estimates for the internal field constrain the power required for the geodynamo7,8.

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Figure 1: Structure of flow and magnetic field perturbation in the fluid core when the inner core tilts out of alignment with the mantle.
Figure 2: Ohmic dissipation in the core due to internal shear layers.
Figure 3: Predicted dissipation as a function of magnetic field strength.

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Correspondence to Bruce A. Buffett.

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The file contains Supplementary Text comprising: Width of the Shear Layers and Influence of Electromagnetic Force on Inertial Waves. The file also contains Supplementary Figures 1-2 with legends and additional references. (PDF 154 kb)

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Buffett, B. Tidal dissipation and the strength of the Earth’s internal magnetic field. Nature 468, 952–954 (2010).

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