Geodynamo simulations with vigorous convection and low viscosity

Nathanaël Schaeffer (1) , Dominique Jault (1) , Henri-Claude Nataf (1) , Alexandre Fournier (2)

1 Géodynamo
2 IPGP - Institut de Physique du Globe de Paris

We analyze a suite of three convection-driven dynamo simulations in a rapidly rotating spherical shell. These three simulations form a sequence along which the viscosity is decreased, while maintaining a large magnetic Reynolds number (Rm > 500). Our most extreme case is characterized by an Ekman number E = 1e-7 and magnetic Prandtl number Pm = 0.1. In this strong-field, dipole-dominated dynamo, the magnetic energy is one order of magnitude larger than the kinetic energy. The spatial distribution of magnetic intensity is highly heterogeneous, and a stark dynamical contrast exists between the interior and the exterior of the tangent cylinder (the cylinder parallel to the axis of rotation that circumscribes the inner core). In the interior, the magnetic field is strongest, and is associated with a vigorous twisted polar vortex, whose dynamics may occasionally lead to the formation of a reverse polar flux patch at the surface of the shell. Furthermore, the strong magnetic field also allows accumulation of light material within the tangent cylinder, leading to stable stratification there. Torsional Alfvén waves are frequently triggered in the vicinity of the tangent cylinder and propagate towards the equator. Outside the tangent cylinder, the magnetic field inhibits the growth of zonal winds and the kinetic energy is mostly non-zonal. Spatio-temporal analysis indicates that the low-frequency, non-zonal flow is quite geostrophic (columnar) and predominantly large-scale: an m=1 eddy spontaneously emerges in our most extreme simulations, without any heterogeneous boundary forcing. Our spatio-temporal analysis further reveals that (i) the low-frequency, large-scale flow is governed by a balance between Coriolis and buoyancy forces – magnetic field and flow tend to align, minimizing the Lorentz force; (ii) the high-frequency flow obeys a balance between magnetic and Coriolis forces; (iii) the convective plumes mostly live at an intermediate scale, whose dynamics is driven by a 3-term MAC balance – involving Coriolis, Lorentz and buoyancy forces. However, small-scale (E^{1/3}) quasi-geostrophic convection is still observed in the regions of low magnetic intensity.

Domaines: Géophysique [physics.geo-ph]; Dynamique des Fluides [physics.flu-dyn]; Modélisation et simulation

Nathanaël Schaeffer, Dominique Jault, Henri-Claude Nataf, Alexandre Fournier. Geodynamo simulations with vigorous convection and low viscosity. 2017. ⟨insu-01422187v2⟩