Kinematic dynamo in spherical Couette flow

TL;DR

This paper numerically investigates how flow dynamics in spherical Couette systems influence magnetic field generation, highlighting the roles of Rossby waves, differential rotation, and boundary conditions in dynamo action.

Contribution

It provides a comprehensive analysis of the factors affecting kinematic dynamo action in spherical Couette flow, including rotation rates, boundary conditions, and aspect ratios, with new insights into Rossby wave contributions.

Findings

Rossby waves are crucial for dynamo action.

Differential rotation and Rossby waves jointly facilitate dynamo.

Conducting boundaries and larger aspect ratios promote dynamo onset.

Abstract

We investigate numerically kinematic dynamos driven by flow of electrically conducting fluid in the shell between two concentric differentially rotating spheres, a configuration normally referred to as spherical Couette flow. We compare between axisymmetric (2D) and fully three dimensional flows, between low and high global rotation rates, between prograde and retrograde differential rotations, between weak and strong nonlinear inertial forces, between insulating and conducting boundaries, and between two aspect ratios. The main results are as follows. Azimuthally drifting Rossby waves arising from the destabilisation of the Stewartson shear layer are crucial to dynamo action. Differential rotation and helical Rossby waves combine to contribute to the spherical Couette dynamo. At a slow global rotation rate, the direction of differential rotation plays an important role in the dynamo because of different patterns of Rossby waves in prograde and retrograde flows. At a rapid global rotation rate, stronger flow supercriticality (namely the difference between the differential rotation rate of the flow and its critical value for the onset of nonaxisymmetric instability) facilitates the onset of dynamo action. A conducting magnetic boundary condition and a larger aspect ratio both favour dynamo action.