Dynamo generated by the centrifugal instability

TL;DR

This paper introduces a new magnetic field amplification mechanism driven by centrifugal instability in a spherical shell, demonstrating subcritical dynamo behavior influenced by rotation and flow instabilities.

Contribution

It presents a novel dynamo scenario involving spherical Taylor-Couette vortices and provides a low-dimensional model for subcritical dynamo bifurcations.

Findings

Critical magnetic Reynolds number saturates at high Reynolds numbers.

Global rotation can significantly lower the dynamo threshold.

Numerical results align with a simple dynamical system model.

Abstract

We present a new scenario for magnetic field amplification where an electrically conducting fluid is confined in a differentially rotating, spherical shell with thin aspect-ratio. When the angular momentum sufficiently decreases outwards, an hydrodynamic instability develops in the equatorial region, characterised by pairs of counter-rotating toroidal vortices similar to those observed in cylindrical Couette flow. These spherical Taylor-Couette vortices generate a subcritical dynamo magnetic field dominated by non-axisymmetric components. We show that the critical magnetic Reynolds number seems to reach a constant value at large Reynolds number and that the global rotation can strongly decrease the dynamo onset. Our numerical results are understood within the framework of a simple dynamical system, and we propose a low-dimensional model for subcritical dynamo bifurcations. Implications for both laboratory dynamos and astrophysical magnetic fields are finally discussed.