Magnetic effects on fields morphologies and reversals in geodynamo simulations

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) 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 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.

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

“…Gillet & Jones 2006). Here the situation differs likely because of the larger P m, which enables a stronger magnetic field (see Menu et al 2020). At the dominant lengthscale , the ratio F i /F L is around 1 for the multipolar model, while it is less than 0.5 for the dipolar one.…”

“…This bound differs from the original threshold of f dip = 0.35 considered by Christensen & Aubert (2006), but it is found to better separate the two types of dynamo models contained in our dataset. Note that the same bound of 0.5 was recently chosen by Menu et al (2020) in their study. The magnetic field amplitude is measured by the Elsasser number…”

“…The early numerical dataset admittedly contained a majority of dynamos operating at large Ekman numbers, and relatively low magnetic Prandtl number P m. In the dynamos studied by Soderlund et al ( 2012), the convective flow was not dramatically altered by the presence of a self-sustained magnetic field. Since then, a large number of simulations have been published with lower values of E and comparatively larger values of P m (Yadav et al 2016;Schaeffer et al 2017;Schwaiger et al 2019;Menu et al 2020). For those strong-field dynamos, in the sense of a ratio of bulk magnetic energy to bulk kinetic energy larger than one, the magnetic field has a significant impact on the flow, and on the dipolar-multipolar transition.…”

“…For those strong-field dynamos, in the sense of a ratio of bulk magnetic energy to bulk kinetic energy larger than one, the magnetic field has a significant impact on the flow, and on the dipolar-multipolar transition. Menu et al (2020) reported simulations with a prevailing Lorentz force, that remain dipolar way beyond the supposedly critical RoL 0.12 value. Given that Earth hosts a strong-field dynamo, it is then worth investigating whether the competition between Lorentz force and inertia may actually lead to the transition.…”

“…The relevance of this force ratio for sustaining the dipolar field has already been put forward by Menu et al (2020), using models with a purely thermal forcing and P r = 1. By considering turbulent simulations with large P m, they show that strong Lorentz forces at large scale prevent the collapse of the dipole by inertia.…”