Radially Dependent Large Scale Dynamos in Global Cylindrical Shear Flows and the Local Cartesian Limit

TL;DR

This paper investigates large-scale magnetic field generation in cylindrical shear flows and their local Cartesian limits, revealing new mechanisms for dynamo growth driven by non-axisymmetric fluctuations and shear, with implications for astrophysical disks.

Contribution

It introduces a novel understanding of large-scale dynamo mechanisms in cylindrical and Cartesian shear flows, emphasizing the role of non-axisymmetric fluctuations and shear without relying on the Omega effect.

Findings

Global cylindrical simulations show MRI-driven large-scale dynamo growth with radially alternating fields.

Analytic theory explains dynamo growth via fluctuation-induced EMF sustained by azimuthal shear.

Large-scale dynamo can be sustained in linear shear flows with non-axisymmetric fluctuations, even without helicity.

Abstract

For cylindrical differentially rotating plasmas, we study large-scale magnetic field generation from finite amplitude non-axisymmetric perturbations by comparing numerical simulations with quasi-linear analytic theory. When initiated with a vertical magnetic field of either zero or finite net flux, our global cylindrical simulations exhibit the magnetorotational instability (MRI) and large scale dynamo growth of radially alternating mean fields, averaged over height and azimuth. This dynamo growth is explained by our analytic calculations of a non-axisymmetric fluctuation-induced EMF that is sustained by azimuthal shear of the fluctuating fields. The standard "Omega effect" (shear of the mean field by differential rotation) is unimportant. For the MRI case, we express the large-scale dynamo field as a function of differential rotation. The resulting radially alternating large-scale fields may have implications for angular momentum transport in disks and corona. To connect with previous work on large scale dynamos with local linear shear and identify the minimum conditions needed for large scale field growth, we also solve our equations in local Cartesian coordinates. We find that large scale dynamo growth in a linear shear flow without rotation can be sustained by shear plus non-axisymmetric fluctuations -- even if not helical, a seemingly previously unidentified distinction. The linear shear flow dynamo emerges as a more restricted version of our more general new global cylindrical calculations.