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Large-scale magnetic fields at high Reynolds numbers in magnetohydrodynamic simulations

Science  25 Mar 2016:
Vol. 351, Issue 6280, pp. 1427-1430
DOI: 10.1126/science.aad1893

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Simulating turbulent solar magnetic fields

An accurate simulation of solar magnetic fields must reproduce both the large-scale dynamo and small-scale turbulence. Simply increasing the computational resolution does not always help, because it can prevent the 11-year solar cycle from emerging. Hotta et al. show how both large- and small-scale phenomena can be reproduced in some of the highest-resolution simulations yet available. This will improve our understanding of both the solar magnetic field and other turbulent magnetohydrodynamic systems.

Science, this issue p. 1427

Abstract

The 11-year solar magnetic cycle shows a high degree of coherence in spite of the turbulent nature of the solar convection zone. It has been found in recent high-resolution magnetohydrodynamics simulations that the maintenance of a large-scale coherent magnetic field is difficult with small viscosity and magnetic diffusivity (Embedded Imagesquare centimenters per second). We reproduced previous findings that indicate a reduction of the energy in the large-scale magnetic field for lower diffusivities and demonstrate the recovery of the global-scale magnetic field using unprecedentedly high resolution. We found an efficient small-scale dynamo that suppresses small-scale flows, which mimics the properties of large diffusivity. As a result, the global-scale magnetic field is maintained even in the regime of small diffusivities—that is, large Reynolds numbers.

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