An accelerating high-latitude jet in Earth’s core

Abstract

Observations of the change in Earth’s magnetic field—the secular variation—provide information about the motion of liquid metal within the core that is responsible for the magnetic field’s generation. High-resolution observations from the European Space Agency’s Swarm satellite mission show intense field change at high latitude, localized in a distinctive circular daisy-chain configuration centred on the north geographic pole. Here we show that this feature can be explained by a localized, non-axisymmetric, westward jet of 420 km width on the tangent cylinder, the cylinder of fluid within the core that is aligned with the rotation axis and tangent to the solid inner core. We find that the jet has increased in magnitude by a factor of three over the period 2000–2016 to about 40 km yr−1, and is now much stronger than typical large-scale flows inferred for the core. We suggest that the current accelerating phase may be part of a longer-term fluctuation of the jet causing both eastward and westward movement of magnetic features over historical periods, and may contribute to recent changes in torsional-wave activity and the rotation direction of the inner core.

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Figure 1: Polar projection view of the radial component of the main field and secular variation at the CMB from the observation-based field reconstruction CHAOS-6 at epoch 2015.
Figure 2: Northern polar view of the flow speed and direction at the CMB of the best-fitting high-latitude jet with M = 1 at epoch 2015.
Figure 3: Quantification of fit of the simple jet model.
Figure 4: A superposition of the flow direction and magnitude (arrows) and the azimuthal gradient of the radial field (in μT per °) at the CMB using CHAOS-6 at epoch 2015; together these combine to produce the SV when azimuthal advection dominates.
Figure 5: Time dependence of the jet and drift of high-latitude flux patches.

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Acknowledgements

Swarm data used in the construction of the magnetic field models were provided by the European Space Agency. The support of the CHAMP mission by the German Aerospace Center (DLR) and the Federal Ministry of Education and Research is gratefully acknowledged. The staff of the geomagnetic observatories and INTERMAGNET are thanked for supplying high-quality observatory data. The deep-Earth research group within the School of Earth and Environment, University of Leeds, is thanked for comments and discussion on an early version of this manuscript. The figures were produced using the Python package Matplotlib33. The authors would like to thank R. Holme and I. Wardinski for constructive comments that helped improve the manuscript. P.W.L. was partially supported by the NERC grant NE/G014043/1.

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All authors contributed to the design and rationale of this work. C.C.F. provided and commented on the observational field models; P.W.L. and R.H. devised the numerical scheme. P.W.L. performed the calculations and wrote the paper, on which all authors commented.

Corresponding author

Correspondence to Philip W. Livermore.

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The authors declare no competing financial interests.

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Livermore, P., Hollerbach, R. & Finlay, C. An accelerating high-latitude jet in Earth’s core. Nature Geosci 10, 62–68 (2017). https://doi.org/10.1038/ngeo2859

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