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Pré-Publication, Document De Travail Année : 2016

Geodynamo simulations with vigorous convection and low viscosity

Nathanaël Schaeffer
Dominique Jault
Henri-Claude Nataf

Résumé

Direct numerical simulations of the magnetohydrodynamics in a spherical shell have been widely used to study the internal magnetic field of the Earth. However, despite continuing progress, the regime in which these simulations operate is in many aspects still far from what is expected in the Earth's core. This undermines the confidence in scaling the simulation outputs to actual planetary cores. In an attempt to reach further towards realistic regimes, we have produced a series of three geodynamo simulations with increasingly demanding parameters, lowering the viscosity while keeping a strong convective driving. The third and last simulation of our series has many parameters and diagnostics closer to the Earth's core than previous direct simulations. In particular the Ekman number E = 1e−7, the magnetic Prandtl number Pm = 0.1, the magnetic Reynolds number Rm > 500. The magnetic energy dominates the kinetic energy by a factor 12. We describe in details the mean fields, fluctuations, spectra, and dynamical balances, with an emphasis on spatio-temporal analysis. As the parameters become more and more Earth-like, we observe that ohmic heating becomes dominant in the energy dissipation budget. The cylinder tangent to the inner-core and aligned with the rotation axis appears as a sharp boundary between two regions of different dynamics. A strong zonal flow aligned with strong toroidal magnetic field occupies the inner region, where lighter fluid is trapped. This contrasts with the outer region, where the mean toroidal field is replaced by a strong poloidal field, which suppresses the mean zonal flow. There, we observe the emergence of large-scale eddies mostly invariant along the rotation axis, that drive a westward drift of the magnetic field near the equator. These slowly evolving and large scale flows are driven by the buoyancy of large-scale density anomalies, while faster fluctuations rely mostly on a balance between Laplace and Coriolis forces, even at large scales. Torsional waves are predominantly generated near the inner-core by Lorentz stress and propagate outward. The fields are very heterogeneous: regions with strong magnetic field and large scale flow lie close to regions with almost no magnetic field where small-scale convection occurs. We discuss our results, their relevance and possible implications for the Earth's core dynamics.
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Dates et versions

insu-01422187 , version 1 (24-12-2016)
insu-01422187 , version 2 (31-05-2017)
insu-01422187 , version 3 (14-06-2017)

Licence

Paternité - Pas d'utilisation commerciale - Pas de modification

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Nathanaël Schaeffer, Dominique Jault, Henri-Claude Nataf, Alexandre Fournier. Geodynamo simulations with vigorous convection and low viscosity. 2016. ⟨insu-01422187v1⟩
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