Coherent nonhelical shear dynamos driven by magnetic fluctuations at low Reynolds numbers

TL;DR

This paper investigates nonhelical shear dynamos driven by magnetic fluctuations at low Reynolds numbers, highlighting the magnetic shear-current effect as a mechanism for large-scale magnetic field generation, with implications for accretion disk turbulence.

Contribution

It demonstrates the viability of the magnetic shear-current effect at low Reynolds numbers and explores the relationship between coherent and incoherent dynamo mechanisms, including the role of rotation.

Findings

Magnetic shear-current effect can drive large-scale dynamo at low Reynolds numbers.

Rotation influences the dominance of coherent versus incoherent dynamo mechanisms.

New insights into kinematic nonhelical shear dynamo behavior are provided.

Abstract

Nonhelical shear dynamos are studied with a particular focus on the possibility of coherent dynamo action. The primary results -- serving as a follow up to the results of Squire & Bhattacharjee [arXiv:1506.04109 (2015)] -- pertain to the "magnetic shear-current effect" as a viable mechanism to drive large-scale magnetic field generation. This effect raises the interesting possibility that the saturated state of the small-scale dynamo could drive large-scale dynamo action, and is likely to be important in the unstratified regions of accretion disk turbulence. In this paper, the effect is studied at low Reynolds numbers, removing the complications of small-scale dynamo excitation and aiding analysis by enabling the use of quasi-linear statistical simulation methods. In addition to the magnetically driven dynamo, new results on the kinematic nonhelical shear dynamo are presented. These illustrate the relationship between coherent and incoherent driving in such dynamos, demonstrating the importance of rotation in determining the relative dominance of each mechanism.