Letter | Published:

Magnetic reversals from planetary dynamo waves

Nature volume 539, pages 551554 (24 November 2016) | Download Citation

Abstract

A striking feature of many natural dynamos is their ability to undergo polarity reversals1,2. The best documented example is Earth’s magnetic field, which has reversed hundreds of times during its history3,4. The origin of geomagnetic polarity reversals lies in a magnetohydrodynamic process that takes place in Earth’s core, but the precise mechanism is debated5. The majority of numerical geodynamo simulations that exhibit reversals operate in a regime in which the viscosity of the fluid remains important, and in which the dynamo mechanism primarily involves stretching and twisting of field lines by columnar convection6. Here we present an example of another class of reversing-geodynamo model, which operates in a regime of comparatively low viscosity and high magnetic diffusivity. This class does not fit into the paradigm of reversal regimes that are dictated by the value of the local Rossby number (the ratio of advection to Coriolis force)7,8. Instead, stretching of the magnetic field by a strong shear in the east–west flow near the imaginary cylinder just touching the inner core and parallel to the axis of rotation is crucial to the reversal mechanism in our models, which involves a process akin to kinematic dynamo waves9,10. Because our results are relevant in a regime of low viscosity and high magnetic diffusivity, and with geophysically appropriate boundary conditions, this form of dynamo wave may also be involved in geomagnetic reversals.

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References

  1. 1.

    Field Generation in Electrically Conducting Fluids (Cambridge Univ. Press, 1978)

  2. 2.

    Cosmical Magnetic fields: Their Origin and Their Activity (Oxford Univ. Press, 1979)

  3. 3.

    Reversals of the Earth’s Magnetic Field (Cambridge Univ. Press, 1994)

  4. 4.

    & Geomagnetic polarity transitions. Rev. Geophys. 37, 201–226 (1999)

  5. 5.

    & Mechanisms for magnetic field reversals. Philos. Trans. R. Soc. Lond. A 368, 1595–1605 (2010)

  6. 6.

    , & Dipole collapse and reversal precursors in a numerical dynamo. Phys. Earth Planet. Inter. 173, 121–140 (2009)

  7. 7.

    & Scaling properties of convection-driven dynamos in rotating spherical shells and application to planetary magnetic fields. Geophys. J. Int . 166, 97–114 (2006)

  8. 8.

    & Magnetic reversal frequency scaling in dynamos with thermochemical convection. Phys. Earth Planet. Inter. 229, 122–133 (2014)

  9. 9.

    Hydromagnetic dynamo models. Astrophys. J. 122, 293–314 (1955)

  10. 10.

    & Parameter dependences of convection-driven dynamos in rotating spherical fluid shells. Geophys. Astrophys. Fluid Dyn. 100, 341–361 (2006)

  11. 11.

    & in Treatise on Geophysics 2nd edn (ed. ) Vol. 8, 245–277 (Elsevier, 2015)

  12. 12.

    & On the genesis of the Earth’s magnetism. Rep. Prog. Phys. 76, 096801 (2013)

  13. 13.

    , & Periodic reversals of magnetic field generated by thermal convection in a rotating spherical shell. J. Phys. Soc. Jpn 66, 2194–2201 (1997)

  14. 14.

    Reversal models from dynamo calculations. Philos. Trans. R. Soc. Lond. A 358, 921–942 (2000)

  15. 15.

    , & Polarity reversals from paleomagnetic observations and numerical dynamo simulations. Space Sci. Rev . 155, 293–335 (2010)

  16. 16.

    & A detailed study of the polarity reversal mechanism in a numerical dynamo model. Geochem. Geophys. Geosyst. 5, Q03H10 (2004)

  17. 17.

    Geodynamo simulations—how realistic are they? Annu. Rev. Earth Planet. Sci. 30, 237–257 (2002)

  18. 18.

    & Toroidal flux oscillation as possible cause of geomagnetic excursions and reversals. Phys. Earth Planet. Inter. 168, 237–243 (2008)

  19. 19.

    Kinematic dynamo models. Philos. Trans. R. Soc. Lond. A 272, 663–698 (1972)

  20. 20.

    & Three-dimensional dynamo waves in a sphere. Geophys. Astrophys. Fluid Dyn. 96, 481–498 (2002)

  21. 21.

    & Generation of a strong magnetic field using uniform heat flux at the surface of the core. Nat. Geosci. 2, 802–805 (2009)

  22. 22.

    & in The Earth’s Magnetic Interior (eds et al.) 117–129 (Springer, 2011)

  23. 23.

    & Symmetry and stability of the geomagnetic field. Geophys. Res. Lett. 33, L21311 (2006)

  24. 24.

    , , & Simple mechanism for reversals of Earth’s magnetic field. Phys. Rev. Lett. 102, 144503 (2009)

  25. 25.

    , & Dipolar versus multipolar dynamos: the influence of the background density stratification. Astron. Astrophys. 546, A19 (2012)

  26. 26.

    A dynamo model of Jupiter’s magnetic field. Icarus 241, 148–159 (2014)

  27. 27.

    , & Hemispherical Parker waves driven by thermal shear in planetary dynamos. Europhys Lett . 104, 49001 (2013)

  28. 28.

    & Stellar dynamos and cycles from numerical simulations of convection. Astrophys. J. 775, 69 (2013)

  29. 29.

    & From stable dipolar towards reversing numerical dynamos. Phys. Earth Planet. Inter. 131, 29–45 (2002)

  30. 30.

    & Dipole moment scaling for convection-driven planetary dynamos. Earth Planet. Sci. Lett. 250, 561–571 (2006)

  31. 31.

    , & Dipole collapse and dynamo waves in global direct numerical simulations. Astrophys. J. 752, 121 (2012)

  32. 32.

    & Transition between viscous dipolar and inertial multipolar dynamos. Geophys. Res. Lett. 41, 7115–7120 (2014)

  33. 33.

    Mechanism for geomagnetic polarity reversals. Nature 326, 167–169 (1987)

  34. 34.

    Numerical Investigations of Rotating MHD in a Spherical Shell. PhD thesis, ETH Zürich , 24–26 (2014);

  35. 35.

    , & Thermal core–mantle interaction: exploring regimes for locked dynamo action. Phys. Earth Planet. Inter. 165, 83–92 (2007)

  36. 36.

    et al. A spherical shell numerical dynamo benchmark with pseudo-vacuum magnetic boundary conditions. Geophys. J. Int. 196, 712–723 (2014)

  37. 37.

    An investigation of reversing numerical dynamos driven by either differential or volumetric heating. Phys. Earth Planet. Inter. 176, 69–82 (2009)

  38. 38.

    & Effects of buoyancy and rotation on the polarity reversal frequency of gravitationally driven numerical dynamos. Geophys. J. Int . 178, 1337–1350 (2009)

  39. 39.

    , & A numerical study on magnetic polarity transition in an MHD dynamo model. Earth Planets Space 59, 665–673 (2007)

  40. 40.

    & Eccentricity of the geomagnetic dipole caused by lopsided inner core growth. Nat. Geosci. 5, 565–569 (2012)

  41. 41.

    , & in Geomagnetic Field Variations (eds et al.) 107–158 (Springer, 2009)

  42. 42.

    , & The magnetic structure of convection-driven numerical dynamos. Geophys. J. Int . 172, 945–956 (2008)

  43. 43.

    , & The role of buoyancy in polarity reversals of the geodynamo. Geophys. J. Int. 199, 1698–1708 (2014)

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Acknowledgements

We acknowledge the computational resources provided by the Centro Svizzero di Calcolo Scientifico (CSCS) under the project s577. We are grateful to J. M. Favre from CSCS for assistance with high-performance visualization. We thank A. Willis for developing the original dynamo code that was used for the calculations reported here, and P. Marti for subsequent optimizations for the CSCS Cray. This work was partially supported by ERC grant no. 247303 (MFECE) to A.J. and by the Danish Council for Independent Research (DFF) grant no. 4002-00366 to C.C.F.

Author information

Affiliations

  1. Institute of Geophysics, ETH Zurich, 8092 Zurich, Switzerland

    • Andrey Sheyko
    •  & Andrew Jackson
  2. Division of Geomagnetism, DTU Space, Technical University of Denmark, 2800 Kongens Lyngby, Denmark

    • Christopher C. Finlay

Authors

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Contributions

A.S. carried out the numerical geodynamo simulations and analysed the runs, C.C.F. drafted the manuscript and participated in the analysis of the results. All authors contributed equally to the design of the study, discussed the results, and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Andrey Sheyko.

Reviewer Information Nature thanks P. Olson and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data

Supplementary information

Videos

  1. 1.

    Radial magnetic field at the surface of the liquid core

    The radial magnetic field Br at the core-mantle boundary (CMB) is plotted in Hammer projection. Time marks are in magnetic diffusion units and values correspond to abscissas in Fig. 2. Frames are on average one thousand timesteps apart.

  2. 2.

    Longitude-averaged azimuthal magnetic field in the meridional section

    The azimuthal magnetic field is averaged in longitude to give Bj. One can see bands of the magnetic field with alternating sign appearing at the equator and travelling polewards. Time marks are in magnetic diffusion units and values correspond to abscissas in Fig. 2. Frames are on average one thousand timesteps apart.

  3. 3.

    Radial magnetic field above the equatorial plane

    The radial magnetic field Br in the plane parallel to equatorial in the middle of the shell between inner and outer boundaries, i.e. at z = (ri + ro)/2. One can see regularly altering directions of the field Br. Time marks are in magnetic diffusion units and values correspond to abscissas in Fig. 2. Frames are on average one thousand timesteps apart.

  4. 4.

    Longitude-averaged azimuthal velocity field in the meridional section

    The azimuthal velocity field u' is averaged in longitude. The prograde flow evident in the time-averaged Fig. 4 appears here in the form of prograde columns existing much of the time in the southern hemisphere. Time marks are in magnetic diffusion units and values correspond to abscissas in Fig. 2. Frames are on average one thousand timesteps apart.

  5. 5.

    Azimuthal velocity field above the equatorial plane

    The azimuthal velocity field uφ in the plane parallel to equatorial in the middle of the shell between inner and outer boundaries, i.e. at z = (ri + ro)/2. One can clearly see the mean westward drift of the flow. Time marks are in magnetic diffusion units and values correspond to abscissas in Fig. 2. Frames are on average one thousand timesteps apart.

  6. 6.

    Temperature on the surface of the liquid core

    The temperature at the core-mantle boundary is in Hammer projection. The southern hemisphere turned out to be always colder during the reversals. Time marks are in magnetic diffusion units and values correspond to abscissas in Fig. 2. Frames are on averageone thousand timesteps apart.

  7. 7.

    Radial magnetic field at the Earth’s surface

    The radial component of the magnetic field Br at the radius corresponding to the Earth’s surface is plotted in Hammer projection. The mantle is considered to be insulating, and first 13 spherical harmonics of the poloidal field on the core-mantle boundary are upward continued to the Earth’s surface. Time marks are in magnetic diffusion units and values correspond to abscissas in Fig. 2. Frames are on average one thousand timesteps apart.

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https://doi.org/10.1038/nature19842

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