Turbulent geodynamo simulations: a leap towards Earth's core

Abstract

We present an attempt to reach realistic turbulent regime in direct numerical simulations of the geodynamo. We rely on a sequence of three convection-driven simulations in a rapidly rotating spherical shell. The most extreme case reaches towards the Earth's core regime by lowering viscosity (magnetic Prandtl number Pm=0.1) while maintaining vigorous convection (magnetic Reynolds number Rm>500) and rapid rotation (Ekman number E=1e-7), at the limit of what is feasible on today's supercomputers. A detailed and comprehensive analysis highlights several key features matching geomagnetic observations or dynamo theory predictions -- all present together in the same simulation -- but it also unveils interesting insights relevant for Earth's core dynamics.In this strong-field, dipole-dominated dynamo simulation, 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 Alfv{é}n 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 1 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.

48 pages, 28 figures, accepted for publication in GJI

References in corpus (5)

• Spherical convective dynamos in the rapidly rotating asymptotic regime
• Approaching a realistic force balance in geodynamo simulations
• Parameter dependences of convection driven dynamos in rotating spherical fluid shells
• Electrical conductivity of the lowermost mantle explains absorption of core torsional waves at the equator
• Variability modes in core flows inverted from geomagnetic field models

Cited by in corpus (85)

• Dynamo theories
• The Turbulent Dynamo
• Convective Lengthscale in Planetary Cores
• Force balance in numerical geodynamo simulations: a systematic study
• Time-dependent low latitude core flow and geomagnetic field acceleration pulses
• Dynamo-based limit to the extent of a stable layer atop Earth's core
• Approaching Earth's core conditions in high-resolution geodynamo simulations
• Precessing spherical shells: flows, dissipation, dynamo and the lunar core
• Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes
• Rapid Variations of Earth's Core Magnetic Field
• The landscape of Saturn's internal magnetic field from the Cassini Grand Finale
• Contributions to the geomagnetic secular variation from a reanalysis of core surface dynamics
• The interplay of fast waves and slow convection in geodynamo simulations nearing Earth's core conditions
• Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo
• Fast Quasi-Geostrophic Magneto-Coriolis Modes in the Earth's core
• Magnetic modeling of inflated low-mass stars using interior fields no larger than ~10 kilogauss
• Geomagnetic semblance and dipolar-multipolar transition in top-heavy double-diffusive geodynamo models
• Rotating double-diffusive convection in stably stratified planetary cores
• Deep convection-driven vortex formation on Jupiter and Saturn
• Relating force balances and flow length scales in geodynamo simulations
• Magnetic fields driven by tidal mixing in radiative stars
• Oscillatory thermal-inertial flows in liquid metal rotating convection
• Fossil field decay due to nonlinear tides in massive binaries
• Heat transport and convective velocities in compositionally-driven convection in neutron star and white dwarf interiors
• Turbulent kinematic dynamos in ellipsoids driven by mechanical forcing
• Magnetic effects on fields morphologies and reversals in geodynamo simulations
• Experimental study of the convection in a rotating tangent cylinder
• Mean zonal flows induced by weak mechanical forcings in rotating spheroids
• Systematic parameter study of dynamo bifurcations in geodynamo simulations
• Transition from multipolar to dipolar dynamos in stratified systems
• Pressure torque of torsional Alfvén modes acting on an ellipsoidal mantle
• Solenoidal force balances in numerical dynamos
• Geomagnetic dipole stability and zonal flows controlled by mantle heat flux heterogeneities
• Weak magnetic field changes over the Pacific due to high conductance in lowermost mantle
• Fingering convection in the stably-stratified layers of planetary cores
• Magnetohydrodynamics of stably stratified regions in planets and stars
• Strong large scale magnetic fields in rotating convection-driven dynamos: the important role of magnetic diffusion
• Core-surface kinematic control of polarity reversals in advanced geodynamo simulations
• Asymptotic behaviour of rotating convection-driven dynamos in the plane layer geometry
• Bridging the Rossby number gap in rapidly rotating thermal convection
• pizza: An open-source pseudo-spectral code for spherical quasi-geostrophic convection
• Lorentz force mediation of turbulent dynamo transitions
• Low inertia reversing geodynamos
• Three-dimensional solutions for the geostrophic flow in the Earth's core
• Convectively driven decadal zonal accelerations in Earth's fluid core
• Polar ionospheric currents and high temporal resolution geomagnetic field models
• Acoustic and inertial modes in planetary-like rotating ellipsoids
• Anelastic torsional oscillations in Jupiter's metallic hydrogen region
• Axisymmetric dynamo action produced by differential rotation, with anisotropic electrical conductivity and anisotropic magnetic permeability
• Can one use Earth's magnetic axial dipole field intensity to predict reversals?
• Comparison of Quasi-Geostrophic, Hybrid and 3D models of planetary core convection
• Magnetic quenching of the inverse cascade in rapidly rotating convective turbulence
• Solitary magnetostrophic Rossby waves in spherical shells
• Interplay between tidal flows and magnetic fields in nonlinear simulations of stellar and planetary convective envelopes
• Magnetic structure, dipole reversals, and 1/f noise in resistive MHD spherical dynamos
• Seven decades of exploring planetary interiors with rotating convection experiments
• Parallel-in-time integration of Kinematic Dynamos
• Penetration of boundary-driven flows into a rotating spherical thermally-stratified fluid
• Spatial And Temporal Changes Of The Geomagnetic Field: Insights From Forward And Inverse Core Field Models
• Magnetic Taylor-Proudman constraint explains the flows into the Tangent Cylinder
• The role of slow magnetostrophic waves in the formation of the axial dipole in planetary dynamos
• Kinematic dynamos in triaxial ellipsoids
• A playground for compressible natural convection with a nearly uniform density
• Co-estimation of core and lithospheric magnetic fields by a maximum entropy method
• 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
• Efficiency gains of a multi-scale integration method applied to a scale-separated model for rapidly rotating dynamos
• Fluid Dynamics of Earth's core: geodynamo, inner core dynamics, core formation
• Transition to chaos and magnetic field generation in rotating Rayleigh-Bénard convection
• Planetary Dynamos in Evolving Cold Gas Giants
• Dynamo saturation down to vanishing viscosity: strong-field and inertial scaling regimes
• Characterisation of hydromagnetic waves propagating over a steady, non-axisymmetric background magnetic field
• Weak branch and multimodal convection in rapidly rotating spheres at low Prandtl number
• Geophysical flows over topography, a playground for laboratory experiments
• Modeling and predicting the short term evolution of the Geomagnetic field
• The role of magnetic waves in tangent cylinder convection
• Did lunar tides sustain the early Earth's dynamo?
• Influence of Rotation on Fingering Convection in Planetary Cores
• Slow convection and fast rotation in crystallization-driven white dwarf dynamos
• On nonlinear transitions, minimal seeds and exact solutions for the geodynamo
• Exponential Integrators with Parallel-in-Time Rational Approximations for the Shallow-Water Equations on the Rotating Sphere
• Constraints on the magnetic field within a stratified outer core
• Effect of electromagnetic boundary conditions on the onset of small-scale dynamos driven by convection
• Convection in a Rapidly Rotating Spherical Shell: Newton's Method Using Implicit Coriolis Integration
• Regime transitions of dynamos driven by fingering double-diffusive convection