Shear dynamo problem: Quasilinear kinematic theory

TL;DR

This paper develops a non-perturbative quasilinear theory for large-scale dynamo action in turbulent shear flows, deriving equations that clarify the role of shear and turbulence in magnetic field evolution.

Contribution

It introduces a systematic, non-perturbative quasilinear framework for analyzing shear dynamo problems, emphasizing Galilean invariance and deriving integro-differential equations for mean magnetic fields.

Findings

Shear-current dynamo is essentially absent in non-helical turbulence.

Time evolution of cross-shear mean magnetic field components is independent of other components.

Large-scale non-helical dynamo action remains possible despite shear effects.

Abstract

Large-scale dynamo action due to turbulence in the presence of a linear shear flow is studied. Our treatment is quasilinear and kinematic but is non perturbative in the shear strength. We derive the integro-differential equation for the evolution of the mean magnetic field, by systematic use of the shearing coordinate transformation and the Galilean invariance of the linear shear flow. For non helical turbulence the time evolution of the cross-shear components of the mean field do not depend on any other components excepting themselves. This is valid for any Galilean-invariant velocity field, independent of its dynamics. Hence the shear-current assisted dynamo is essentially absent, although large-scale non helical dynamo action is not ruled out.