Selection of axial dipole from a seed magnetic field in rapidly rotating dynamo models

TL;DR

This paper explores how the Lorentz force influences magnetic polarity selection in rapidly rotating spherical shell dynamo models, revealing its significant effect even at low Elsasser numbers and emphasizing the dynamic growth of magnetic fields.

Contribution

It demonstrates the dynamic role of the Lorentz force in magnetic field growth and saturation, contrasting with previous kinematic-focused studies, through nonlinear and kinematic simulations.

Findings

Lorentz force affects flow before saturation

Magnetic field growth is a dynamic effect

Significant Lorentz force effects at Elsasser numbers 0.3-0.4

Abstract

In this study, we investigate preferences of dipolar magnetic structure from a seed magnetic field in the rapidly rotating spherical shell dynamo models. In this study, we set up a realistic model to show the effect of the Lorentz force in the polarity selection. The important results that has come out from our study is that the magnetic field acts on the flow much before the saturation. Our study suggests that the growth of the magnetic field is not a kinematic effect as one might think off, rather a dynamic effect. This dynamic effect grows as the field generated with time and finally brings the saturation to the dynamo action. Previous studies show that Lorentz force effect the flow when Elsasser number more or less 1 and the studies were focused on the saturation by looking at the time-averaged quantities. However, in this study, we show a clear effect of the Lorentz force even at Elsasser number of $0.3-0.4$. To show the effect of the Lorentz force, we did two different simulations, one is a nonlinear model and another is kinematic model and shows that how a magnetic field can change the flow structure and by doing that the generated field changes, while this kind of behavior is not observed in kinematic dynamo models. This study shows a scale dependent behaviour of the kinetic helicity at two different spectral range.