Geomagnetic dipole stability and zonal flows controlled by mantle heat flux heterogeneities

Abstract

This work aims at acquiring a more complete understanding of how lateral heterogeneities of the CMB heat flux affect the geodynamo while other relevant parameters are pushed towards realistic values. For this purpose, we ran geodynamo simulations with degree 1 and 2 spherical harmonic patterns of heat flux at the CMB. Several geodynamo models are used, ranging from standard numerical dynamos to more extreme parameters, including strong field cases and turbulent cases. We show that heat flux heterogeneities with amplitudes compatible with our knowledge of mantle convection history can favour multipolar dynamos. The multipolar transition is associated with a disruption of westward flows either through eastward thermal winds or through a loss of equatorial symmetry. Strong field dynamo models are found to have larger westward flows and are less sensitive to heat flux heterogeneities. Furthermore, we find that the dipolar fraction of the magnetic field correlates with $M_{Za}^{\ast }=\frac{{\Lambda }_{Za}}{R{m}_{Za}^2}$ where ${\Lambda }_{Za}$ is the zonal antisymmetric Elsasser number and $R{m}_{Za}$ is the zonal antisymmetric magnetic Reynolds number. Importantly, $M_{Za}^{\ast }$ estimated for the Earth's core is consistent with a reversing dipolar magnetic field. Within the range of $M_{Za}^{\ast }$ susceptible to reversals, breaking the equatorial symmetry or forcing eastward zonal flows through an equatorial cooling of the core consistently triggers reversals or a transition towards multipolar dynamos in our simulations. Our results support that time variations of heat-flux heterogeneities driven by mantle convection through Earth's history are capable of inducing the significant variations in the reversal frequency observed in the palaeomagnetic record.

arXiv comment24 pages, 13+ figures, 4 appendices

References in corpus (22)

• Turbulent geodynamo simulations: a leap towards Earth's core
• Efficient Spherical Harmonic Transforms aimed at pseudo-spectral numerical simulations
• Spherical convective dynamos in the rapidly rotating asymptotic regime
• A simple mechanism for the reversals of Earth's magnetic field
• Dipole Collapse and Dynamo Waves in Global Direct Numerical Simulations
• Zonal flow regimes in rotating anelastic spherical shells: an application to giant planets
• Dipolar versus multipolar dynamos: the influence of the background density stratification
• Strong-Field Spherical Dynamos
• Convective Lengthscale in Planetary Cores
• Bistability and hysteresis of dipolar dynamos generated by turbulent convection in rotating spherical shells
• Approaching a realistic force balance in geodynamo simulations
• A rare event algorithm links transitions in turbulent flows with activated nucleations
• Dynamo regimes and transitions in the VKS experiment
• Geomagnetic semblance and dipolar-multipolar transition in top-heavy double-diffusive geodynamo models
• Rotating double-diffusive convection in stably stratified planetary cores
• Relating force balances and flow length scales in geodynamo simulations
• A Gaussian Model for Simulated Geomagnetic Field Reversals
• Magnetic effects on fields morphologies and reversals in geodynamo simulations
• Systematic parameter study of dynamo bifurcations in geodynamo simulations
• Remarks on some typical assumptions in dynamo theory
• Core flows and heat transfer induced by inhomogeneous cooling with sub- and supercritical convection
• Reversal and amplification of zonal flows by boundary enforced thermal wind

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