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Longitudinal structure of Earth’s magnetic field controlled by lower mantle heat flow

Nature Geoscience (2023)Cite this article

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Abstract

Thermal interactions between Earth’s core and mantle provide the power that maintains the geomagnetic field. However, the effect of these interactions and, in particular, the thermochemical piles at the base of the mantle on magnetic field behaviour remains uncertain. Here we present numerical dynamo simulations with strong lateral variations in heat flow imposed at the core–mantle boundary to reproduce conditions within Earth and indicate how the mantle controls core dynamics. Comparing these simulations to recent global magnetic field models, based on observational data spanning tens of thousands of years, they successfully reproduce the morphology and secular variation of Earth’s modern field and the inferred large-scale flow structure at the top of the core. These simulations reveal that the long-term geomagnetic signatures of thermal core–mantle interactions are evident in the longitudinal structure of the geomagnetic field as equatorial patches of reverse flux, rather than the high-latitude patches suggested by less Earth-like simulations. Comparison of these simulations with the field models also suggests that the amplitude of the present-day longitudinal hemispheric imbalance in secular variation is anomalously large, indicating our present-day geomagnetic field may be unusual.

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Fig. 1: Time-averaged magnetic fields for our simulations.
Fig. 2: Comparison of our Ra = 2,000 and q* = 2.3 simulation with gufm1.
Fig. 3: The longitudinal variation in Pi.
Fig. 4: Time-averaged radial magnetic field at the CMB in the equatorial regions of the simulations.

Data availability

Original data underlying the figures and plain text versions of tables are available at github.com/jonmound/MD2023data.

Code availability

For access to the github repository containing the Leeds dynamo code, please contact the corresponding author.

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Acknowledgements

We thank C. Constable for useful discussion that improved this work. C.J.D. acknowledges funding via a Natural Environment Research Council Pushing the Frontiers award, reference NE/V010867/1. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. Figures were produced using Matplotlib61 and Cartopy62. This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) and ARC2, part of the High Performance Computing facilities at the University of Leeds, UK.

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Authors and Affiliations

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

    Jonathan E. Mound & Christopher J. Davies

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Contributions

J.E.M. and C.J.D. both conceived of the study, carried out and analysed the simulations and co-wrote the paper.

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Correspondence to Jonathan E. Mound.

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Nature Geoscience thanks Nathanaël Schaeffer, Andreas Nilsson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Louise Hawkins, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Time-averaged magnetic fields.

Time-averaged magnetic fields. Maps are for GGF100k (a) and our simulations with Ra = 6000 and q* = 2.3, 5.0 (b,c). The radial component of the magnetic field on the CMB truncated at spherical harmonic degree and order 4. Note that the simulations use a non-dimensional scale.

Extended Data Fig. 2 Evolution of the contributions to magnetic field and secular variation semblance over our simulations.

Evolution of the contributions to magnetic field and secular variation semblance over our simulations. The χ2 contribution from the O/E, Z/NZ, AD/NAD, FCF, and Hsv measures are given by the orange, green, red, purple, and brown filled areas, respectively. The black solid line highlights the sum of the four compliance criteria for the magnetic field geometry and the grey horizontal lines indicate the values below which this total compliance is considered excellent, good, or marginal in comparison with Earth as derived from gufm1. Values to the right of each panel indicate the percentage of 400-year windows that fall in each compliance band. Simulations have Ra = 2000 (panels a,b,c) or Ra = 6000 (panels d,e,f) and q* = 0.0 (a,d), q* = 2.3 (b,e), or q* = 5.0 (c,f).

Extended Data Fig. 3 Time-averaged radial magnetic field at the core–mantle boundary in the equatorial regions of the simulations.

Time-averaged radial magnetic field at the core–mantle boundary in the equatorial regions of the simulations. Runs are characterised by Ra = 6000 and q* = 2.3, 5 (a,b). Both plots use the same colour scale for the (non-dimensional) magnetic field strength and are truncated at spherical harmonic degree and order 8.

Extended Data Fig. 4 Time-averaged values of the paleosecular variation index from our simulations.

Time-averaged values of the paleosecular variation index from our simulations. Runs are characterised by Ra = 2000 and q* = 0, 2.3, 5 (a,c,e); Ra = 6000 and q* = 2.3, 5 (d,f). The final map is GGF100k (b). All plots use the same colour scale.

Extended Data Table 1 Dynamo Parameters that Vary Between Runs
Full size table
Extended Data Table 2 Geomagnetic Field Measures
Full size table

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Mound, J.E., Davies, C.J. Longitudinal structure of Earth’s magnetic field controlled by lower mantle heat flow. Nat. Geosci. (2023). https://doi.org/10.1038/s41561-023-01148-9

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