The dual role of shear in large-scale dynamos

TL;DR

This paper explores how shear influences large-scale dynamos, particularly in reducing magnetic quenching and aiding turbulence, with new simulations confirming phase shifts that could diagnose dynamo activity.

Contribution

It provides new insights into shear's dual role in large-scale dynamos, confirming estimates for magnetic diffusivity reduction and presenting simulations of phase shifts in magnetic fields.

Findings

Shear reduces magnetic diffusivity by a factor of 5 at high magnetic Reynolds numbers.

A 3pi/4 phase shift between poloidal and toroidal fields is confirmed in simulations.

Shear-driven turbulence and large-scale fields are emphasized as key in dynamo processes.

Abstract

The role of shear in alleviating catastrophic quenching by shedding small-scale magnetic helicity through fluxes along contours of constant shear is discussed. The level of quenching of the dynamo effect depends on the quenched value of the turbulent magnetic diffusivity. Earlier estimates that might have suffered from the force-free degeneracy of Beltrami fields are now confirmed for shear flows where this degeneracy is lifted. For a dynamo that is saturated near equipartition field strength those estimates result in a 5-fold decrease of the magnetic diffusivity as the magnetic Reynolds number based on the wavenumber of the energy-carrying eddies is increased from 2 to 600. Finally, the role of shear in driving turbulence and large-scale fields by the magneto-rotational instability is emphasized. New simulations are presented and the 3pi/4 phase shift between poloidal and toroidal fields is confirmed. It is suggested that this phase shift might be a useful diagnostic tool in identifying mean-field dynamo action in simulations and to distinguish this from other scenarios invoking magnetic buoyancy as a means to explain migration away from the midplane.