Dynamo action driven by precessional turbulence

TL;DR

This paper investigates how precessional turbulence can efficiently generate magnetic fields through a dynamo mechanism, analyzing the spectral contributions of vortices, inertial waves, and shear across different scales.

Contribution

It provides a detailed spectral analysis of precession-driven dynamo action, revealing the scale-dependent roles of vortices, inertial waves, and shear in magnetic field growth.

Findings

Dynamo growth rate increases with Poincaré number and magnetic Prandtl number.

Dynamo operates across a broad range of scales with a transition from vortex-driven to multi-mechanism driving.

Critical Poincaré number for dynamo onset decreases as magnetic Prandtl number increases.

Abstract

We reveal and analyze an efficient magnetic dynamo action due to precession-driven hydrodynamic turbulence in the local model of a precessional flow, focusing on the kinematic stage of this dynamo. The growth rate of magnetic field monotonically increases with Poincar\'{e} number, $\rm Po$, characterizing precession strength, and magnetic Prandtl number, $\rm Pm$, equal to the ratio of viscosity to resistivity, for the considered ranges of these parameters. The critical ${\rm Po}_c$ for the dynamo onset decreases with increasing $\rm Pm$. To understand the scale-by-scale evolution (growth) of the precession dynamo and its driving processes, we perform spectral analysis by calculating the spectra of magnetic energy and of different terms in the induction equation in Fourier space. To this end, we decompose the velocity field of precession-driven turbulence into 2D vortical and 3D inertial wave modes. It is shown that the dynamo operates across a broad range of scales and exhibits a remarkable transition from a primarily vortex-driven regime at lower $\rm Po$ to a more complex regime at higher $\rm Po$ where it is driven jointly by vortices, inertial waves and the shear of the background precessional flow. The vortices and shear drive the dynamo mostly at large scales, comparable to the flow system size, and at intermediate scales, while at smaller scales it is mainly driven by inertial waves. This study can be important not only for understanding the magnetic dynamo action in precession-driven flows, but also in a general context of flows where vortices emerge and govern the flow dynamics and evolution.