I. Effect of mantle convection on the geodynamo behaviour in numerical models
Thomas Frasson, Université Grenoble Alpes

Paleomagnetic evidence shows that the behaviour of the geodynamo has changed during geological times. These behaviour changes are visible through variations in the strength and stability of the magnetic dipole. Variations in the heat flux at the core-mantle boundary (CMB) due to mantle convection have been suggested as one possible mechanism capable of driving such a change of behaviour. 

Coupling mantle convection models and geodynamo models is crucial to understanding how the geodynamo can react to variations in mantle convection. This coupling can be studied by applying heterogeneous heat flux conditions at the top of the core in geodynamo models. Previous studies have notably shown that large-scale heat flux heterogeneities at the CMB can have a large impact on the stability of the magnetic dipole in viscous and moderately viscous dynamo models.

In order to better constrain the core-mantle coupling, we exploit numerical models of mantle convection and numerical geodynamo models. We use mantle convection models corrected for true polar wander (TPW) to obtain realistic CMB heat flux conditions at the top of the core in the reference frame useful for core dynamics. We then apply heterogeneous heat flux conditions at the top of the core in low viscosity geodynamo models. We show that an equatorial cooling of the core is the most efficient at destabilizing the magnetic dipole, while a polar cooling of the core tends to stabilize the dipole. Complex heat flux patterns tend to destabilize the magnetic dipole, except when it is dominated by polar cooling.

II. The Influence of Mantle Convection on Earth’s Geomagnetic Field Observables
Daniel Thallner, University of Florida

Studying the geomagnetic field and its variations over geological timescales provides insights into the geodynamo and deep Earth processes. Notably, a significant weak-field anomaly in the paleomagnetic record during the Precambrian-Cambrian transition suggests a shift from a thermally driven to a compositionally driven geodynamo upon inner core nucleation. However, a similar field behavior is indicated by weak geomagnetic field strengths observed during the Devonian period, that could be related to flow pattern changes in the outer core between small and large inner core regimes. These periods of weak field strength align with cyclic fluctuations observed in the long-term paleointensity record, which mirrors the time scale of convective overturn in Earth's mantle. While the mantle's cooling history dominates the core's heat transport, its full impact on the geodynamo is still not well understood.

To quantify the impact of mantle convection on the long-term geomagnetic field, we compared numerical geodynamo simulations with heterogeneous core-mantle boundary (CMB) heat flux. We conducted 270 numerical geodynamo simulations with CMB heat flux boundary conditions reflecting seismic tomography models of present-day Earth and plate-driven 3D global mantle convection models for both the present-day Earth and different states throughout a supercontinent cycle. By pairing these boundary conditions to simulations with identical geodynamo core parameters, we can directly compare the resulting geomagnetic fields at Earth's surface. Preliminary findings indicate that simulations with more detailed CMB flux from 3D mantle convection models show increased dipole-dominated geomagnetic fields with reduced secular variation and a lower likelihood of polarity reversals relative to their counterparts with CMB heat flux based on seismic tomography. This indicates that simulations using long wavelength spatial heterogeneities may overestimate the variability of the resulting magnetic field.

Contributors: Daniele Thallner, Courtney Sprain, Juliane Dannberg, Rene Gassmoeller, Richard Bono, Chris Davies, Domenico Meduri, Andy Biggin

1. webinar
2. 2024