Understanding large-scale dynamos in unstratified rotating shear flows

TL;DR

This paper investigates how large-scale dynamos operate and saturate in unstratified, magnetorotationally driven turbulence using simulations and new analytical methods, revealing the roles of shear, velocity fluctuations, and Lorentz forces.

Contribution

It provides a detailed analysis of nonhelical mean-field dynamo mechanisms in unstratified flows, overcoming previous analytical challenges and illustrating the dynamo process schematically.

Findings

Shear amplifies azimuthal magnetic fields from radial ones.

Velocity fluctuations regenerate radial magnetic fields.

Lorentz and Coriolis forces influence dynamo saturation.

Abstract

We combine simulations with new analyses that overcome previous pitfalls to explicate how nonhelical mean-field dynamos grow and saturate in unstratified, magnetorotationally driven turbulence. Shear of the mean radial magnetic field amplifies the azimuthal component. Radial fields are regenerated by velocity fluctuations that induce shear of radial magnetic fluctuations, followed by Lorentz and Coriolis forces that source a negative off-diagonal component in the turbulent diffusivity tensor. We present a simple schematic to illustrate this dynamo growth. A different part of the Lorentz force forms a third-order correlator in the mean electromotive force that saturates the dynamo.