Time scales separation for dynamo action

TL;DR

This paper explores how time-dependent flow modulations can induce magnetic field amplification in astrophysical dynamos through parametric instability, even in configurations that are not naturally dynamo-active.

Contribution

It demonstrates that low-frequency flow perturbations can trigger exponential magnetic field growth via parametric instability, revealing a new mechanism for dynamo action.

Findings

Low-frequency flow perturbations can induce exponential magnetic field growth.

Parametric instability driven by shear flows is key to the amplification.

Implications for natural dynamos suggest new pathways for magnetic field generation.

Abstract

The study of dynamo action in astrophysical objects classically involves two timescales: the slow diffusive one and the fast advective one. We investigate the possibility of field amplification on an intermediate timescale associated with time dependent modulations of the flow. We consider a simple steady configuration for which dynamo action is not realised. We study the effect of time dependent perturbations of the flow. We show that some vanishing low frequency perturbations can yield exponential growth of the magnetic field on the typical time scale of oscillation. The dynamo mechanism relies here on a parametric instability associated with transient amplification by shear flows. Consequences on natural dynamos are discussed.