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Shear-driven dynamo waves at high magnetic Reynolds number

Abstract

Astrophysical magnetic fields often display remarkable organization, despite being generated by dynamo action driven by turbulent flows at high conductivity1,2. An example is the eleven-year solar cycle, which shows spatial coherence over the entire solar surface3,4,5. The difficulty in understanding the emergence of this large-scale organization is that whereas at low conductivity (measured by the magnetic Reynolds number, Rm) dynamo fields are well organized, at high Rm their structure is dominated by rapidly varying small-scale fluctuations. This arises because the smallest scales have the highest rate of strain, and can amplify magnetic field most efficiently. Therefore most of the effort to find flows whose large-scale dynamo properties persist at high Rm has been frustrated. Here we report high-resolution simulations of a dynamo that can generate organized fields at high Rm; indeed, the generation mechanism, which involves the interaction between helical flows and shear, only becomes effective at large Rm. The shear does not enhance generation at large scales, as is commonly thought; instead it reduces generation at small scales. The solution consists of propagating dynamo waves, whose existence was postulated more than 60 years ago6 and which have since been used to model the solar cycle7.

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Figure 1: Small-scale dynamo solution.
Figure 2: Diagrams of activity versus time.
Figure 3: Effect of the shear.
Figure 4: Evidence for dynamo waves.

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Acknowledgements

This work was supported in part by the Science and Technology Facilities Council (STFC) and by the Center for Magnetic Self-Organisation (sponsored by the National Science Foundation) at the University of Chicago. Computations were performed on the STFC-supported UKMHD consortium cluster (DiRAC) at the University of Leeds.

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The authors contributed equally to all aspects (theoretical and numerical) of the paper.

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Correspondence to S. M. Tobias.

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Tobias, S., Cattaneo, F. Shear-driven dynamo waves at high magnetic Reynolds number. Nature 497, 463–465 (2013). https://doi.org/10.1038/nature12177

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