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The origin of geomagnetic jerks

Abstract

Geomagnetic jerks, which in the second half of the twentieth century occurred in 1969 (refs 1, 2), 1978 (refs 3, 4), 1991 (ref. 5) and 1999 (ref. 6), are abrupt changes in the second time-derivative (secular acceleration) of the Earth's magnetic field. Jerks separate periods of almost steady secular acceleration, so that the first time-derivative (secular variation) appears as a series of straight-line segments separated by geomagnetic jerks. The fact that they represent a reorganization of the secular variation implies that they are of internal origin (as has been established through spherical harmonic analysis7), and their short timescale implies that they are due to a change in the fluid flow at the surface of the Earth's core (as has also been established through mapping the time-varying flow at the core surface8). However, little is understood of their physical origin. Here we show that geomagnetic jerks can be explained by the combination of a steady flow and a simple time-varying, axisymmetric, equatorially symmetric, toroidal zonal flow. Such a flow is consistent with torsional oscillations in the Earth's core, which are simple oscillatory flows in the core that are expected on theoretical grounds9, and observed in both core flow models10 and numerical dynamo models11.

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Figure 1: The secular variation (first derivative) of the magnetic field observed at Niemegk observatory (left; Germany) and at Macquarie Island observatory (right; south of Australasia).
Figure 2: The time-dependent part of the flow model (after removing the time average) from 1957 to 2001.
Figure 3: The spatially averaged power of the flow model from 1957 to 2001, compared with the one-, two- and three-wave fits to the flow model.

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Acknowledgements

This work was supported by NSF and NASA. The authors thank A. Jackson and S. Macmillan for providing observatory data.

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Correspondence to Jeremy Bloxham.

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Bloxham, J., Zatman, S. & Dumberry, M. The origin of geomagnetic jerks. Nature 420, 65–68 (2002). https://doi.org/10.1038/nature01134

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