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

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">M&#xe9;lissa 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">S&#xe9;bastien 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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