Geomagnetic semblance and dipolar-multipolar transition in top-heavy double-diffusive geodynamo models

Abstract

Convection in the liquid outer core of the Earth is driven by thermal and chemical perturbations. The main purpose of this study is to examine the impact of double-diffusive convection on magnetic field generation by means of 3D global geodynamo models, in the so-called "top-heavy" regime of double-diffusive convection, when both thermal and compositional background gradients are destabilizing. Using a linear eigensolver, we begin by confirming that, compared to the standard single-diffusive configuration, the onset of convection is facilitated by the addition of a second buoyancy source. We next carry out a systematic parameter survey by performing $79$ numerical dynamo simulations. We show that a good agreement between simulated magnetic fields and the geomagnetic field can be attained for any partitioning of the convective input power between its thermal and chemical components. On the contrary, the transition between dipole-dominated and multipolar dynamos is found to strongly depend on the nature of the buoyancy forcing. Classical parameters expected to govern this transition, such as the local Rossby number or the degree of equatorial symmetry of the flow, fail to capture the dipole breakdown. A scale-dependent analysis of the force balance instead reveals that the transition occurs when the ratio of inertial to Lorentz forces at the dominant length scale reaches $0.5$, regardless of the partitioning of the buoyancy power. The ratio of integrated kinetic to magnetic energy $E_k/E_m$ provides a reasonable proxy of this force ratio. Given that $E_k/E_m\ll 1$ in the Earth's core, the geodynamo is expected to operate far from the dipole-multipole transition. It hence appears that the occurrence of geomagnetic reversals is unlikely related to dramatic and punctual changes of the amplitude of inertial forces in the Earth's core, and that another mechanism must be sought.

References in corpus (26)

• Turbulent geodynamo simulations: a leap towards Earth's core
• Efficient Spherical Harmonic Transforms aimed at pseudo-spectral numerical simulations
• A Comparison of Turbulent Thermal Convection Between Conditions of Constant Temperature and Constant Flux
• Transport properties for liquid silicon-oxygen-iron mixtures at Earth's core conditions
• Spherical convective dynamos in the rapidly rotating asymptotic regime
• Scaling regimes in spherical shell rotating convection
• A simple mechanism for the reversals of Earth's magnetic field
• Scaling Laws for Planetary Dynamos
• Approaching a realistic force balance in geodynamo simulations
• Reconciliation of experiments and theory on transport properties of iron and the geodynamo
• Force balance in numerical geodynamo simulations: a systematic study
• Turbulent Rayleigh-Bénard convection in spherical shells
• Subcritical thermal convection of liquid metals in a rapidly rotating sphere
• Scaling laws in spherical shell dynamos with free-slip boundaries
• Transition between viscous dipolar and inertial multipolar dynamos
• A hemispherical dynamo model : Implications for the Martian crustal magnetization
• Why dynamos are prone to reversals
• Rotating double-diffusive convection in stably stratified planetary cores
• Relating force balances and flow length scales in geodynamo simulations
• A New Model for Mixing By Double-Diffusive Convection (Semi-Convection). III. Thermal and Compositional Transport Through Non-Layered ODDC
• Magnetic effects on fields morphologies and reversals in geodynamo simulations
• Regimes of thermo-compositional convection and related dynamos in rotating spherical shells
• Double-diffusive convection in a rotating cylindrical annulus with conical caps
• The onset of thermo-compositional convection in rotating spherical shells
• pizza: An open-source pseudo-spectral code for spherical quasi-geostrophic convection
• Implicit-Explicit Runge-Kutta schemes for hyperbolic systems and kinetic equations in the diffusion limit

Cited by in corpus (7)

• Transition from multipolar to dipolar dynamos in stratified systems
• Geomagnetic dipole stability and zonal flows controlled by mantle heat flux heterogeneities
• Fingering convection in the stably-stratified layers of planetary cores
• Core-surface kinematic control of polarity reversals in advanced geodynamo simulations
• Unlocking planetesimal magnetic field histories: a refined, versatile model for thermal evolution and dynamo generation
• An assessment of implicit-explicit time integrators for the pseudo-spectral approximation of Boussinesq thermal convection in an annulus
• Regime transitions of dynamos driven by fingering double-diffusive convection