Suppression of a kinematic dynamo by large shear

TL;DR

This study numerically investigates how large-scale shear flows influence the efficiency of a kinematic dynamo driven by small-scale helical flows in a spherical shell, revealing that shear can suppress dynamo action.

Contribution

It demonstrates that large shear flows can significantly suppress dynamo efficiency, challenging the notion that shear universally promotes dynamo action.

Findings

Large shear flows suppress dynamo efficiency.

Small-scale helical flows can independently sustain dynamo action.

Shear's effect depends on flow configuration and strength.

Abstract

We numerically solve the magnetic induction equation in a spherical shell geometry, with a kinematically prescribed axisymmetric flow that consists of a superposition of a small-scale helical flow and a large-scale shear flow. The small-scale flow is chosen to be a local analog of the classical Roberts cells, consisting of strongly helical vortex rolls. The large-scale flow is a shearing motion in either the radial or the latitudinal directions. In the absence of large-scale shear, the small-scale flow is an efficient dynamo, in agreement with previous results. Adding increasingly large shear flows strongly suppresses the dynamo efficiency, indicating that shear is not always a favourable ingredient in dynamo action.