Subcritical dynamos in shear flows

TL;DR

This paper explores subcritical dynamo mechanisms in shear flows, linking hydrodynamic turbulence transition to magnetic field generation, with implications for astrophysical objects like accretion disks.

Contribution

It introduces the concept of subcritical dynamo action in shear flows and discusses its relevance to astrophysical turbulence and magnetic field generation.

Findings

Subcritical dynamo processes can drive turbulence in high-Reynolds number shear flows.

Nonlinear hydrodynamic transition to turbulence shares similarities with dynamo action.

Subcritical dynamos may generate large-scale magnetic fields and sustain small-scale dynamos.

Abstract

Identifying generic physical mechanisms responsible for the generation of magnetic fields and turbulence in differentially rotating flows is fundamental to understand the dynamics of astrophysical objects such as accretion disks and stars. In this paper, we discuss the concept of subcritical dynamo action and its hydrodynamic analogue exemplified by the process of nonlinear transition to turbulence in non-rotating wall-bounded shear flows. To illustrate this idea, we describe some recent results on nonlinear hydrodynamic transition to turbulence and nonlinear dynamo action in rotating shear flows pertaining to the problem of turbulent angular momentum transport in accretion disks. We argue that this concept is very generic and should be applicable to many astrophysical problems involving a shear flow and non-axisymmetric instabilities of shear-induced axisymmetric toroidal velocity or magnetic fields, such as Kelvin-Helmholtz, magnetorotational, Tayler or global magnetoshear instabilities. In the light of several recent numerical results, we finally suggest that, similarly to a standard linear instability, subcritical MHD dynamo processes in high-Reynolds number shear flows could act as a large-scale driving mechanism of turbulent flows that would in turn generate an independent small-scale dynamo.