Magnetic reversals in a geodynamo model with a stably-stratified layer

TL;DR

This study uses numerical simulations to explore how a stably-stratified layer influences magnetic reversals in a geodynamo, revealing effects on transition thresholds, field strength, and reversal triggers.

Contribution

It demonstrates that a stably-stratified layer affects the dynamo transition, enhances magnetic field strength, and influences reversal dynamics through boundary effects and heat flux patterns.

Findings

Dipolar-multipolar transition shifts to higher Rayleigh numbers.

Stably-stratified layer increases magnetic field strength at the boundary.

Different heat flux patterns can trigger hemispheric dynamos and reversals.

Abstract

We study the process of magnetic reversals in the presence of a stably-stratified layer below the core-mantle boundary using direct numerical simulations of the incompressible magnetohydrodynamics equations under the Boussinesq approximation in a spherical shell. We show that the dipolar-multipolar transition shifts to larger Rayleigh numbers in the presence of a stably-stratified layer, and that the dipolar strength of the magnetic field at the core-mantle boundary increases due to the skin effect. By imposing an heterogeneous heat flux at the outer boundary, we break the equatorial symmetry of the flow, and show that different heat flux patterns can trigger different dynamo solutions, such as hemispheric dynamos and polarity reversals. Using kinematic dynamo simulations, we show that the stably-stratified layer leads to similar growth rates of the dipole and quadrupole components of the magnetic field, playing the role of a conducting boundary layer, favouring magnetic reversals, and a dynamics predicted by low-dimensional models.