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The geodynamo features a broad separation between the large scale at whichEarth's magnetic field is sustained against ohmic dissipation and the smallscales of the turbulent and electrically conducting underlying fluid flow inthe outer core. Here, the properties of this scale separation are analysedusing high-resolution numerical simulations that approach closer to Earth'score conditions than earlier models. The new simulations are obtained byincreasing the resolution and gradually relaxing the hyperdiffusiveapproximation of previously published low-resolution cases. This upsizingprocess does not perturb the previously obtained large-scale, leading-orderquasi-geostrophic (QG), and first-order magneto-Archimedes-Coriolis (MAC) forcebalances. As Earth's core conditions are approached in the upsized simulations,kinetic energy spectra feature a gradually broadening self-similar, power-lawspectral range extending over more than a decade in length scale. In thisrange, the spectral energy density profile of vorticity is shown to beapproximately flat between the large scale at which the magnetic field drawsits energy from convection through the QG-MAC force balance and the small scaleat which this energy is dissipated. The resulting velocity and density anomalyplanforms in the physical space consist in large-scale columnar sheets andplumes, respectively co-existing with small-scale vorticity filaments anddensity anomaly ramifications. In contrast, magnetic field planforms keep theirlarge-scale structure after upsizing. The diagnostic outputs of the upsizedsimulations are more consistent with the asymptotic QG-MAC theory than those ofthe low-resolution cases that they originate from, but still feature smallresidual deviations that may call for further theoretical refinements toaccount for the structuring constraints of the magnetic field on the flow.

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