Numerical experiments on dynamo action in sheared and rotating turbulence

TL;DR

This paper demonstrates through numerical simulations that nonhelical turbulence combined with shear can generate large-scale magnetic fields, with growth rates proportional to shear and applicable in various astrophysical contexts.

Contribution

The study extends previous shear dynamo results to shearing boxes with Keplerian rotation and explores the mechanism's operation across different dynamo thresholds.

Findings

Magnetic fields grow exponentially at scales larger than the forcing scale.

Growth rate of magnetic field is proportional to shear rate S.

Shear dynamo operates both below and above fluctuation dynamo threshold.

Abstract

Numerical simulations of forced turbulence in elongated shearing boxes are carried out to demonstrate that a nonhelical turbulence in conjunction with a linear shear can give rise to a mean-field dynamo. Exponential growth of magnetic field at scales larger than the outer (forcing) scale of the turbulence is found. Over a range of values of the shearing rate S spanning approximately two orders of magnitude, the growth rate of the magnetic field is proportional to the imposed shear, gamma ~ S, while the characteristic spatial scale of the field is l_b ~ S^(-1/2). The effect is quite general: earlier results for the nonrotating case by Yousef et al. 2008 (PRL 100, 184501) are extended to shearing boxes with Keplerian rotation; it is also shown that the shear dynamo mechanism operates both below and above the threshold for the fluctuation dynamo. The apparently generic nature of the shear dynamo effect makes it an attractive object of study for the purpose of understanding the generation of magnetic fields in astrophysical systems.