Approaching Earth's core conditions in high-resolution geodynamo simulations

Abstract

The geodynamo features a broad separation between the large scale at which Earth's magnetic field is sustained against ohmic dissipation and the small scales of the turbulent and electrically conducting underlying fluid flow in the outer core. Here, the properties of this scale separation are analysed using high-resolution numerical simulations that approach closer to Earth's core conditions than earlier models. The new simulations are obtained by increasing the resolution and gradually relaxing the hyperdiffusive approximation of previously published low-resolution cases. This upsizing process does not perturb the previously obtained large-scale, leading-order quasi-geostrophic (QG), and first-order magneto-Archimedes-Coriolis (MAC) force balances. As Earth's core conditions are approached in the upsized simulations, kinetic energy spectra feature a gradually broadening self-similar, power-law spectral range extending over more than a decade in length scale. In this range, the spectral energy density profile of vorticity is shown to be approximately flat between the large scale at which the magnetic field draws its energy from convection through the QG-MAC force balance and the small scale at which this energy is dissipated. The resulting velocity and density anomaly planforms in the physical space consist in large-scale columnar sheets and plumes, respectively co-existing with small-scale vorticity filaments and density anomaly ramifications. In contrast, magnetic field planforms keep their large-scale structure after upsizing. The diagnostic outputs of the upsized simulations are more consistent with the asymptotic QG-MAC theory than those of the low-resolution cases that they originate from, but still feature small residual deviations that may call for further theoretical refinements to account for the structuring constraints of the magnetic field on the flow.

arXiv comment31 pages, 13 figures, accepted in Geophysical Journal International

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Cited by in corpus (15)

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• Rapid Variations of Earth's Core Magnetic Field
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• Relating force balances and flow length scales in geodynamo simulations
• Oscillatory thermal-inertial flows in liquid metal rotating convection
• Core-surface kinematic control of polarity reversals in advanced geodynamo simulations
• Direct numerical simulations of optimal thermal convection in rotating plane layer dynamos
• Low inertia reversing geodynamos
• Can one use Earth's magnetic axial dipole field intensity to predict reversals?
• Kinematic dynamos in triaxial ellipsoids
• Parallel finite volume simulation of the spherical shell dynamo with pseudo-vacuum magnetic boundary conditions
• An assessment of implicit-explicit time integrators for the pseudo-spectral approximation of Boussinesq thermal convection in an annulus
• Dynamic regimes in planetary cores: τ-{\ell} diagrams
• Characterisation of hydromagnetic waves propagating over a steady, non-axisymmetric background magnetic field
• Constraints on the magnetic field within a stratified outer core