Magnetic effects on fields morphologies and reversals in geodynamo simulations

TL;DR

This paper investigates how magnetic forces influence the morphology and reversals of magnetic fields in geodynamo simulations, emphasizing the dominant role of the Lorentz force in maintaining dipolar magnetic fields.

Contribution

It demonstrates that the Lorentz force's dominance alters previous understanding of dynamo regimes and highlights the importance of force balance at different spatial scales in geodynamo models.

Findings

Strong Lorentz forces sustain dipolar magnetic fields.

Inertia does not cause dipole collapse when Lorentz and Coriolis forces are dominant.

Spatial force balance varies with length scale, affecting magnetic field morphology.

Abstract

The dynamo effect is the most popular candidate to explain the non-primordial magnetic fields of astrophysical objects. Although many systematic studies of parameters have already been made to determine the different dynamical regimes explored by direct numerical geodynamo simulations, it is only recently that the regime corresponding to the outer core of the Earth characterized by a balance of forces between the Coriolis and Lorentz forces is accessible numerically. In most previous studies, the Lorentz force played a relatively minor role. For example, they have shown that a purely hydrodynamic parameter (the local Rossby number $Ro_\ell$ determines the stability domain of dynamos dominated by the axial dipole (dipolar dynamos). In this study, we show that this result cannot hold when the Lorentz force becomes dominant. We model turbulent geodynamo simulations with a strong Lorentz force by varying the important parameters over several orders of magnitude. This method enables us to question previous results and to argue on the applications of numerical dynamos in order to better understand the geodynamo problem. Strong dipolar fields considerably affect the kinetic energy distribution of convective motions which enables the maintenance of this field configuration. The relative importance of each force depends on the spatial length scale, whereas $Ro_\ell$ is a global output parameter which ignores the spatial dependency. We show that inertia does not induce a dipole collapse as long as the Lorentz and the Coriolis forces remain dominant at large length scales.