Magnetic effects on fields morphologies and reversals in geodynamo simulations. (arXiv:2007.05530v1 [physics.flu-dyn])

<a href="http://arxiv.org/find/physics/1/au:+Menu_M/0/1/0/all/0/1">Mélissa D. Menu</a>, <a href="http://arxiv.org/find/physics/1/au:+Petitdemange_L/0/1/0/all/0/1">Ludovic Petitdemange</a>, <a href="http://arxiv.org/find/physics/1/au:+Galtier_S/0/1/0/all/0/1">Sébastien Galtier</a>

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.

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.

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