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Gyres, jets and waves in the Earth’s core

Nature Reviews Earth & Environment volume 4, pages 377–392 (2023)Cite this article

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Abstract

Turbulent motions of liquid metal in Earth’s  outer core generate the geomagnetic field. Magnetic field observations from low-Earth-orbit satellites, together with advanced numerical simulations, indicate that present-day core motions are dominated by a planetary-scale gyre, a jet in the northern polar region and waves involving the magnetic field. In this Review, we explore the dynamics of core gyres, jets and waves and discuss their impact on the magnetism and rotation of the Earth. The planetary gyre is anticyclonic, offset from the rotation axis towards low latitudes under the Atlantic hemisphere and involves flow speeds of 15–50 km yr−1 that are fastest in a focused westward jet under the Bering Strait. A quasi-geostrophic, Magnetic–Archimedes–Coriolis force balance is thought to control the dynamics of the planetary gyre and high latitude jet. Waves in the core flow with periods ~7 years have been detected at low latitudes, that are consistent with an interplay among magnetic, Coriolis and inertial effects. The arrival of wave energy at the core surface accounts for many of the characteristics of interannual geomagnetic field variations. Fluctuations in outer core flow patterns, including the planetary gyre, account for decadal changes in Earth’s length of day, while interannual changes are well explained by wave processes. Systematic investigations of core–mantle coupling mechanisms in models that include wave dynamics promise new insights on poorly constrained physical properties, including deep mantle conductivity. Long-term satellite monitoring of changes in the Earth’s magnetic field is essential if further progress is to be made in understanding core dynamics, as the high-resolution magnetic record remains short compared with the timescales of waves and convection in the core.

Key points

  • Since 1999, satellite observations have provided a reliable global picture of how Earth’s magnetic field is changing on interannual-to-decadal timescales. The most intense changes are found at mid-to-low latitudes under the Atlantic hemisphere and under Alaska and Siberia at high northern latitudes.

  • Global knowledge of geomagnetic field changes, together with an understanding of the motional induction process in the core, enables the general circulation of liquid metal in the outer core to be inferred.

  • Key features of the core flow include a planetary-scale, eccentric, anticyclonic gyre with an intense jet-like concentration under the Bering Strait and waves at low latitudes.

  • Numerical simulations of core dynamics are now approaching conditions relevant to Earth. These demonstrate that a combination of core convection and hydromagnetic waves can account for the observed field variations.

  • Recorded changes in the length of day on interannual and decadal periods over the past century are well explained by changes in the axisymmetric part of the core flow inferred from geomagnetic observations.

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Fig. 1: Observed core surface magnetic field and flow in 2020.
Fig. 2: The planetary gyre.
Fig. 3: Evolution of the high latitude jet.
Fig. 4: Observed and modelled waves in the core.
Fig. 5: Observed changes in length of day and predictions from core flows.

Data availability

Swarm satellite magnetic field data are available from https://earth.esa.int/web/guest/swarm/data-access and https://vires.services/. Ground observatory magnetic data are available from ftp://ftp.nerc-murchison.ac.uk/geomag/Swarm/AUX_OBS/hour/.

Code availability

The CHAOS-7 field model and its updates are at http://www.spacecenter.dk/files/magnetic-models/CHAOS-7/.A Python package for using the CHAOS model is available at https://pypi.org/project/chaosmagpy/. The flow models presented here, and the Python codes used to calculate them, are available from https://geodyn.univ-grenoble-alpes.fr/. The numerical code and simulation data for the core dynamics simulations presented here are available from Julien Aubert (aubert@ipgp.fr) upon reasonable request. Data and additional video files from the simulation presented are also available at https://4d-earth-swarm.univ-grenoble-alpes.fr/dataand https://www.ipgp.fr/~aubert/4dearth.

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Acknowledgements

The authors thank the European Space Agency (ESA) for the prompt availability of Swarm L1b data. The staff of the geomagnetic observatories and INTERMAGNET are thanked for supplying high-quality observatory data. This work was supported by the ESA under the framework of EO Science for Society, through contract 4000127193/19/NL/IA (Swarm+4D Deep Earth: Core). This work has also been partially supported by the French Spatial Agency (CNES) in the context of the Swarm mission of the ESA. The authors also thank the Isaac Newton Institute for Mathematical Sciences, Cambridge, for support and hospitality during the programme DYT2, during which final work on this paper was carried out, supported by the EPSRC grant no. EP/R014604/1.

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  1. DTU Space, Technical University of Denmark, Kongens Lyngby, Denmark

    Christopher C. Finlay

  2. Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, UGE, Grenoble, France

    Nicolas Gillet & Dominique Jault

  3. Institut de physique du globe de Paris, CNRS, Université Paris Cité, Paris, France

    Julien Aubert

  4. School of Earth and Environment, University of Leeds, Leeds, UK

    Philip W. Livermore

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Glossary

Alfvén waves

Waves arising in an electrically conducting fluid owing to fluid inertia and magnetic (Lorentz) forces.

Frozen-flux approximation

Under this approximation, changes in the magnetic field are produced by advection and stretching of a moving conductor and magnetic diffusion effects are neglected.

High latitude jet

A localized region of high fluid velocity located under Alaska and Siberia that is associated with a distinctive pattern of magnetic field change at high northern latitudes.

Hydromagnetic waves

Waves that can occur in electrically conducting fluids in the presence of a strong magnetic field with properties dependent on the force balance in the system; examples include Alfvén waves, Magneto–Coriolis and Magneto–Archimedes–Coriolis waves.

Inner core tangent cylinder

An imaginary cylinder parallel to the rotation axis of the Earth and just touching the inner core in the equatorial plane that acts as a natural dynamical barrier to flows.

Magneto–Archimedes–Coriolis

(MAC). A dynamical balance between magnetic (Lorentz), Archimedes (buoyancy) and Coriolis forces that is thought to be important in the core on decade and longer timescales.

Magneto–Coriolis (MC) waves

Waves in rapidly rotating, electrically conducting fluids where the force balance is between magnetic and Coriolis effects, with inertia having a negligible role; also sometimes called magnetostrophic waves.

Magnetohydrodynamic

Combination of hydrodynamics, as described by the Navier–Stokes equation, and electrodynamics under the quasi-static approximation as described by the magnetic induction equation; also sometimes called hydromagnetics.

Planetary gyre

The basic anticyclonic (westward) circulation of the liquid metal in the outer core that is of planetary scale, offset from the rotation axis towards low latitudes under the Atlantic hemisphere and largely equatorially symmetric albeit with some localized departures.

Quasi-geostrophic

(QG). An approximate leading order balance in the Navier–Stokes equation between the Coriolis force and the pressure gradient that occurs in rapidly rotating fluids and leads to approximately columnar flow structures.

Swarm satellite mission

Trio of low-Earth-orbit satellites launched by the European Space Agency in 2013 to survey the magnetic field of the Earth.

Torsional waves

Special Alfvén waves that can occur in rapidly rotating fluids that are axisymmetric and equatorially symmetric and propagate in the cylindrical radial direction.

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Finlay, C.C., Gillet, N., Aubert, J. et al. Gyres, jets and waves in the Earth’s core. Nat Rev Earth Environ 4, 377–392 (2023). https://doi.org/10.1038/s43017-023-00425-w

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