Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Weak magnetic field changes over the Pacific due to high conductance in lowermost mantle

Nature Geoscience volume 13, pages 516–520 (2020)Cite this article

Subjects

Abstract

For the past few centuries, the temporal variation in Earth’s magnetic field in the Pacific region has been anomalously low. The reason for this is tied to large-scale flows in the liquid outer core near the core–mantle boundary, which are weaker under the Pacific and feature a planetary-scale gyre that is eccentric and broadly avoids this region. However, what regulates this type of flow morphology is unknown. Here we present results from a numerical model of the dynamics in Earth’s core that includes electromagnetic coupling with a non-uniform conducting layer at the base of the mantle. We show that when the conductance of this layer is higher under the Pacific than elsewhere, the larger electromagnetic drag force weakens the local core flows and deflects the flow of the planetary gyre away from the Pacific. The nature of the lowermost mantle conductance remains unclear, but stratified core fluid trapped within topographic undulations of the core–mantle boundary is a possible explanation.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Prices vary by article type

from$1.95

to$39.95

Learn more

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: The low geomagnetic secular variation in the Pacific.
Fig. 2: The westward, eccentric planetary gyre in Earth’s fluid core.
Fig. 3: Modification of core flows by a non-uniform EM drag at the CMB.
Fig. 4: Weak core flows from enhanced EM drag in the Pacific.

Data availability

The datasets generated as part of this study, together with the GMT scripts and data files necessary to reproduce all figures, are freely accessible on UAL Dataverse at https://doi.org/10.7939/DVN/TL8BP6 (ref. 60).

Code availability

All source codes used to generate the numerical simulations presented in this work are freely accessible on UAL Dataverse at https://doi.org/10.7939/DVN/TL8BP6 (ref. 60).

References

  1. Jones, C. A. in Treatise on Geophysics 1st edn, Vol. 8 (ed. Schubert, G.) Ch. 5 (Elsevier, 2007).

  2. Jackson, A. & Finlay, C. in Treatise on Geophysics 2nd edn, Vol. 5 (ed. Schubert, G.) Ch. 5 (Elsevier, 2015).

  3. Bloxham, J. & Gubbins, D. The secular variation of the Earth’s magnetic field. Nature 317, 777–781 (1985).

    Google Scholar 

  4. Jackson, A., Jonkers, A. R. T. & Walker, M. R. Four centuries of geomagnetic secular variation from historical records. Philos. Trans. R. Soc. Lond. A 358, 957–990 (2000).

    Google Scholar 

  5. Fisk, H. W. Isopors and isoporic movements. Intern. Geodetic Geophys. Union Sect. Terrest. Magnet. Elec. Bull. Stockholm 8, 280–292 (1931).

    Google Scholar 

  6. Fleming, J. A. Physics of the Earth, Vol. VIII: Terrestrial Magnetism and and Electricity (McGraw-Hill, 1939).

  7. Finlay, C. C., Jackson, A., Gillet, N. & Olsen, N. Core surface magnetic field evolution 2000–2010. Geophys. J. Int. 189, 761–781 (2012).

    Google Scholar 

  8. Finlay, C. C., Olsen, N., Kotsiaros, S., Gillet, N. & Tøffner-Clausen, L. Recent geomagnetic secular variation from swarm and ground observatories as estimated in the CHAOS-6 geomagnetic field model. Earth Planets Space 68, 112 (2016).

    Google Scholar 

  9. Cox, A. Analysis of present geomagnetic field for comparison with paleomagnetic results. J. Geomag. Geoelectr. 13, 101–112 (1962).

    Google Scholar 

  10. Runcorn, S. K. Polar path in geomagnetic reversals. Nature 356, 654–656 (1992).

    Google Scholar 

  11. Buffett, B. A. Constraints on magnetic energy and mantle conductivity from the forced nutations of the Earth. J. Geophys. Res. 97, 19581–19597 (1992).

    Google Scholar 

  12. Buffett, B. A., Mathews, P. M. & Herring, T. A. Modeling of nutation-precession: effects of electromagnetic coupling. J. Geophys. Res. 107, ETG 5-1 (2002).

    Google Scholar 

  13. Koot, L. & Dumberry, M. The role of the magnetic field morphology on the electromagnetic coupling for nutations. Geophys. J. Int. 195, 200–210 (2013).

    Google Scholar 

  14. Holme, R. in The Core-Mantle Boundary Region (eds Gurnis, M. et al.) 139–152 (AGU, 1998).

  15. Schaeffer, N. & Jault, D. Electrical conductivity of the lowermost mantle explains absorption of core torsional waves at the equator. Geophys. Res. Lett. 43, 4922–4928 (2016).

    Google Scholar 

  16. Vestine, E. H. & Kahle, A. B. The small amplitude of magnetic secular change in the Pacific area. J. Geophys. Res. 71, 527–530 (1966).

    Google Scholar 

  17. Doell, R. R. & Cox, A. Pacific geomagnetic secular variation. Science 171, 248–254 (1971).

    Google Scholar 

  18. Gillet, N., Jault, D. & Finlay, C. C. Planetary gyre, time-dependent eddies, torsional waves, and equatorial jets at the Earth’s core surface. J. Geophys. Res. Solid Earth 120, 3991–4013 (2015).

    Google Scholar 

  19. Pais, M. A. & Jault, D. Quasi-geostrophic flows responsible for the secular variation of the Earth’s magnetic field. Geophys. J. Int. 173, 421–443 (2008).

    Google Scholar 

  20. Aubert, J. Flow throughout the Earth’s core inverted from geomagnetic observations and numerical dynamo models. Geophys. J. Int. 192, 537–556 (2013).

    Google Scholar 

  21. Aubert, J. Earth’s core internal dynamics 1840–2010 imaged by inverse geodynamo modelling. Geophys. J. Int. 197, 1321–1334 (2014).

    Google Scholar 

  22. Aubert, J. Recent geomagnetic variations and the force balance in Earth’s core. Geophys. J. Int. 221, 378–393 (2020).

    Google Scholar 

  23. Schaeffer, N., Jault, D., Nataf, H.-C. & Fournier, A. Turbulent geodynamo simulations: a leap towards Earth’s core. Geophys. J. Int. 211, 1–29 (2017).

    Google Scholar 

  24. Runcorn, S. K. in Encyclopedia of Physics Vol. 47 (ed. Bartels, J.) 498–533 (Springer, 1956).

  25. Olson, P. & Christensen, U. R. The time-averaged magnetic field in numerical dynamos with non-uniform boundary heat flow. Geophys. J. Int. 151, 809–823 (2002).

    Google Scholar 

  26. Christensen, U. R. & Olson, P. Secular variation in numerical geodynamo models with lateral variations of boundary heat flow. Geophys. J. Int. 138, 39–54 (2003).

    Google Scholar 

  27. Aubert, J., Amit, H. & Hulot, G. Detecting thermal boundary control in surface flows from numerical dynamos. Phys. Earth Planet. Inter. 160, 143–156 (2007).

    Google Scholar 

  28. Aubert, J., Finlay, C. C. & Fournier, A. Bottom-up control of geomagnetic secular variation by Earth’s inner core. Nature 502, 219–223 (2013).

    Google Scholar 

  29. Labrosse, S. Thermal and compositional stratification of the inner core. C. R. Geosci. 346, 119–129 (2014).

    Google Scholar 

  30. More, C. & Dumberry, M. Convectively driven decadal zonal accelerations in Earth’s fluid core. Geophys. J. Int. 213, 434–446 (2018).

    Google Scholar 

  31. Dumberry, M. & Mound, J. E. Constraints on core-mantle electromagnetic coupling from torsional oscillation normal modes. J. Geophys. Res. 113, B03102 (2008).

    Google Scholar 

  32. Gillet, N., Jault, D., Canet, E. & Fournier, A. Fast torsional waves and strong magnetic field within the Earth’s core. Nature 465, 74–77 (2010).

    Google Scholar 

  33. Ohta, K. et al. The electrical conductivity of post-perovskite in Earth’s D″ layer. Science 320, 89–91 (2008).

    Google Scholar 

  34. Petford, N., Yuen, D., Rushmer, T., Brodholt, J. & Stackhouse, S. Shear-induced material transfer across the core-mantle boundary aided by the post-perovskite phase transition. Earth Planets Space 57, 459–464 (2005).

    Google Scholar 

  35. Kanda, R. V. S. & Stevenson, D. J. Suction mechanism for iron entrainment into the lower mantle. Geophys. Res. Lett. 33, L02310 (2006).

    Google Scholar 

  36. Otsuka, K. & Karato, S.-I. Deep penetration of molten iron into the mantle caused by a morphological instability. Nature 492, 243–247 (2012).

    Google Scholar 

  37. Dobson, D. P. & Brodholt, J. P. Subducted banded iron formations as a source of ultralow velocity zones at the core–mantle boundary. Nature 434, 371–374 (2005).

    Google Scholar 

  38. Labrosse, S., Hernlund, J. W. & Coltice, N. A crystallizing dense magma ocean at the base of the Earth’s mantle. Nature 450, 866–869 (2007).

    Google Scholar 

  39. Glane, S. & Buffett, B. A. Enhanced core–mantle coupling due to stratification at the top of the core. Front. Earth Sci. 6, 171 (2018).

    Google Scholar 

  40. Korte, M. & Constable, C. G. Continuous geomagnetic field models for the past 7 millennia II: CALS7K. Geochem. Geophys. Geosyst. 6, Q02H16 (2005).

    Google Scholar 

  41. Lawrence, K. P., Constable, C. G. & Johnson, C. L. Paleosecular variation and the average geomagnetic field at ± 20° latitude. Geochem. Geophys. Geosyst. 7, Q07007 (2006).

    Google Scholar 

  42. Holme, R. Electromagnetic core–mantle coupling III. Laterally varying mantle conductance. Phys. Earth Planet. Inter. 117, 329–344 (2000).

    Google Scholar 

  43. Buffett, B. A. & Creager, K. C. A comparison of geodetic and seismic estimate of inner-core rotation. Geophys. Res. Lett. 26, 1509–1512 (1999).

    Google Scholar 

  44. Buffett, B. A. Gravitational oscillations in the length of the day. Geophys. Res. Lett. 23, 2279–2282 (1996).

    Google Scholar 

  45. Dumberry, M. Geodynamic constraints on the steady and time-dependent inner core axial rotation. Geophys. J. Int. 170, 886–895 (2007).

    Google Scholar 

  46. Pichon, G., Aubert, J. & Fournier, A. Coupled dynamics of Earth’s geomagnetic westward drift and inner core super-rotation. Earth Planet. Sci. Lett. 437, 114–126 (2016).

    Google Scholar 

  47. Buffett, B. A. Geodynamic estimates of the viscosity of the Earth’s inner core. Nature 388, 571–573 (1997).

    Google Scholar 

  48. Cardin, P. & Olson, P. Chaotic thermal convection in a rapidly rotating spherical shell: consequences for flow in the outer core. Phys. Earth Planet. Inter. 82, 235–259 (1994).

    Google Scholar 

  49. Aubert, J., Gillet, N. & Cardin, P. Quasigeostrophic models of convection in rotating spherical shells. Geochem. Geophys. Geosyst. 4, 1052 (2003).

    Google Scholar 

  50. Gillet, N. & Jones, C. A. The quasi-geostrophic model for rapidly rotating spherical convection outside the tangent cylinder. J. Fluid Mech. 554, 343–369 (2006).

    Google Scholar 

  51. Buffett, B. A. & Christensen, U. R. Magnetic and viscous coupling at the core–mantle boundary: inferences from observations of the Earth’s nutations. Geophys. J. Int. 171, 145–152 (2007).

    Google Scholar 

  52. Jault, D. Axial invariance of rapidly varying diffusionless motions in the Earth’s core interior. Phys. Earth Planet. Inter. 166, 67–76 (2008).

    Google Scholar 

  53. Rochester, M. G. Geomagnetic westward drift and irregularities in the Earth’s rotation. Philos. Trans. R. Soc. Lond. A 252, 531–555 (1960).

    Google Scholar 

  54. Gillet, N., Jault, D. & Canet, E. Excitation of traveling torsional normal modes in an Earth’s core model. Geophys. J. Int. 210, 1503–1516 (2017).

    Google Scholar 

  55. Braginsky, S. I. Torsional magnetohydrodynamic vibrations in the Earth’s core and variations in day length. Geomag. Aeron. 10, 1–10 (1970).

    Google Scholar 

  56. Buffett, B. A. in The Core-Mantle Boundary Region (eds Gurnis, M. et al.) 153–165 (AGU, 1998).

  57. Aurnou, J. M., Brito, D. & Olson, P. L. Mechanics of inner core super-rotation. Geophys. Res. Lett. 23, 3401–3404 (1996).

    Google Scholar 

  58. Olson, P. & Aurnou, J. A polar vortex in the Earth’s core. Nature 402, 170–173 (1999).

    Google Scholar 

  59. Wessel, P., Smith, W. H. F., Scharroo, R., Luis, J. & Wobbe, F. Generic mapping tools: improved version released. EOS Trans. AGU 94, 409–410 (2013).

    Google Scholar 

  60. Dumberry, M. & More, C. Replication data for: Weak magnetic field changes over the Pacific due to high conductance in lowermost mantle. UAL Dataverse https://doi.org/10.7939/DVN/TL8BP6 (2020).

Download references

Acknowledgements

We thank N. Schaeffer for sharing his original numerical quasi-geostrophic code, which we extended over the course of this project, and N. Gillet for sharing his flow models. Figures were created using the GMT software59. Numerical simulations were performed on computing facilities provided by WestGrid and Compute/Calcul Canada. This work was supported by a Discovery grant from NSERC/CRSNG.

Author information

Authors and Affiliations

  1. Department of Physics, University of Alberta, Edmonton, Alberta, Canada

    Mathieu Dumberry & Colin More

Authors
  1. Mathieu Dumberry
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Colin More
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

M.D. designed the project and wrote the manuscript. The custom numerical codes were designed and written by both M.D. and C.M. The numerical experiments were carried out and analyzed by both M.D. and C.M.

Corresponding author

Correspondence to Mathieu Dumberry.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Peer review information Primary Handling Editors: Rebecca Neely; Melissa Plail.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 The lower magnetic field strength in the Pacific.

a, ∣Br∣ at the CMB over the time-period 1590-1990 from the gufm field model (ref. 4). The r.m.s. amplitude of ∣Br∣ in the Pacific (pink dashed circle) is 195.15 μT, the global average is 237.33 μT, for a ratio of 0.8223. b, Br at the CMB, in 2015 from the CHAOS-6 field model (ref. 8) truncated at spherical harmonic degree 16. The r.m.s. amplitude of ∣Br∣ over the Pacific (pink dashed circle) is 217.83 μT, the global average is 286.40 μT, for a ratio of 0.7606.

Extended Data Fig. 2 Equatorial planforms of the secular variation in QG model.

Snapshots of \(\frac{\partial a}{\partial t}\) (top row), \(\frac{\partial {b}_{s}}{\partial t}\) (middle row) and \(\frac{\partial {b}_{\phi }}{\partial t}\) (bottom row) from our quasi-geostrophic model for Ra = 5 × 108, Pm = 0.1 and different choices of χ. The time snapshots are the same as those for the vorticity planforms shown in Fig. 3 of the main text. The Pacific region is in the bottom section of each planform, delimited by a dashed green line in panels b, c and d. The color scales on the right are common to all 4 panels.

Supplementary information

Supplementary Information

Document containing Supplementary Information.

Source data

Source Data Fig. 4

Data points for Fig. 4.

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Dumberry, M., More, C. Weak magnetic field changes over the Pacific due to high conductance in lowermost mantle. Nat. Geosci. 13, 516–520 (2020). https://doi.org/10.1038/s41561-020-0589-y

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41561-020-0589-y

This article is cited by

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing