Destabilization of rotating flows with positive shear by azimuthal magnetic fields

TL;DR

This paper demonstrates that azimuthal magnetic fields can destabilize rotating flows with positive shear, which are traditionally considered stable, revealing new instability mechanisms relevant for magnetized fluid dynamics.

Contribution

It introduces a novel destabilization mechanism for positive shear flows via current-free azimuthal magnetic fields, extending understanding of flow stability in magnetohydrodynamics.

Findings

Positive shear flows can be destabilized by azimuthal magnetic fields.

Transition to kink-type instability occurs with homogeneous currents.

Potential observation in magnetized Taylor-Couette experiments.

Abstract

According to Rayleigh's criterion, rotating flows are linearly stable when their specific angular momentum increases radially outward. The celebrated magnetorotational instability opens a way to destabilize those flows, as long as the angular velocity is decreasing outward. Using a short-wavelength approximation we demonstrate that even flows with very steep positive shear can be destabilized by azimuthal magnetic fields which are current-free within the fluid. We illustrate the transition of this instability to a rotationally enhanced kink-type instability in case of a homogeneous current in the fluid, and discuss the prospects for observing it in a magnetized Taylor-Couette flow.