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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Original data underlying the figures and plain text versions of tables are available at github.com/jonmound/MD2023data.
For access to the github repository containing the Leeds dynamo code, please contact the corresponding author.
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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.
The authors declare no competing interests.
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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.
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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