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Formation of current coils in geodynamo simulations

Abstract

Computer simulations have been playing an important role in the development of our understanding of the geodynamo1,2,3, but direct numerical simulation of the geodynamo with a realistic parameter regime is still beyond the power of today’s supercomputers. Difficulties in simulating the geodynamo arise from the extreme conditions of the core, which are characterized by very large or very small values of the non-dimensional parameters of the system. Among them, the Ekman number, E, has been adopted as a barometer of the distance of simulations from real core conditions, in which E is of the order of 10-15. Following the initial computer simulations of the geodynamo4,5, the Ekman number achieved has been steadily decreasing, with recent geodynamo simulations6,7,8 performed with E of the order of 10-6. Here we present a geodynamo simulation with an Ekman number of the order of 10-7—the highest-resolution simulation yet achieved, making use of 4,096 processors of the Earth Simulator. We have found that both the convection flow and magnetic field structures are qualitatively different from those found in larger-Ekman-number dynamos. The convection takes the form of sheet plumes or radial sheet jets9, rather than the columnar cell structures10 that are usually found. We have found that this sheet plume convection is an effective dynamo and the generated current is organized as a set of coils in the shape of helical springs or at times as a torus.

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Figure 1: Equatorial and meridional cross-sections of the axial component of the vorticity, ωz .
Figure 2: Sheet plumes observed in experiment and simulation.
Figure 3: Current field structure near the inner core at t = 430.
Figure 4: Detailed structure of two typical current coils (blue) with their associated magnetic field lines (pink).

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Acknowledgements

We thank P. Olson and J. Aurnou for pointing out the close connection between the sheet plume structure observed in this simulation and the experiments of ref. 9. We thank I. Sumita for a detailed explanation of his laboratory experiments and comments, and for providing Fig. 2a. We thank N. Ohno for helping with the visualization. This work was supported by KAKENHI (17540404) and The Mitsubishi Foundation.

Author Contributions A.K. was involved in project planning, simulation code development, simulation runs, data analysis and manuscript preparation. T.M. was involved in a part of the simulation runs, data analysis and manuscript preparation. T.S. was involved in project planning and manuscript preparation.

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Correspondence to Akira Kageyama.

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Supplementary Figures

The file contains Supplementary Figures 1-2 and Legends. Supplementary Figure 1 shows an equatorial cross section of the convection velocity at Ekman number E=2.3e-7. Supplementary Figure 2 shows the current field at E=2.6e-6. Characteristic feature of the current field lines of helical coils is less distinctive in this case. (PDF 1230 kb)

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Kageyama, A., Miyagoshi, T. & Sato, T. Formation of current coils in geodynamo simulations. Nature 454, 1106–1109 (2008). https://doi.org/10.1038/nature07227

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