Magnetic dynamo action in helical turbulence

TL;DR

This paper explores how magnetic fields are amplified in helical turbulence, revealing that helicity increases the complexity of eigenmodes involved in dynamo action, challenging traditional models.

Contribution

It provides a numerical analysis of the Kazantsev-Kraichnan model for helical turbulence, showing the increased number of eigenmodes and limitations of the alpha model.

Findings

Helical turbulence increases the number of bound eigenmodes.

Unbound eigenmodes also contribute to large-scale magnetic fields.

Conventional alpha model captures only unbound modes, limiting its applicability.

Abstract

We investigate magnetic field amplification in a turbulent velocity field with nonzero helicity, in the framework of the kinematic Kazantsev-Kraichnan model. We present the numerical solution of the model for the practically important case of Kolmogorov distribution of velocity fluctuations, with a large magnetic Reynolds number. We find that in contrast to the nonhelical case where growing magnetic fields are described by a few bound eigenmodes concentrated inside the inertial interval of the velocity field, in the helical case the number of bound eigenmodes considerably increases; moreover, new unbound eigenmodes appear. Both bound and unbound eigenmodes contribute to the large-scale magnetic field. This indicates a limited applicability of the conventional alpha model of a large-scale dynamo action, which captures only unbound modes.