Discussion of the Electromotive Force Terms in the Model of Parker-unstable Galactic Disks with Cosmic Rays and Shear

TL;DR

This paper investigates the electromotive force in a 3D galactic disk model with cosmic rays and shear, revealing that nonlinear dynamo approximations are unsuitable due to lack of scale separation.

Contribution

It critically evaluates existing nonlinear dynamo EMF prescriptions against a detailed numerical model of Parker instability with cosmic rays.

Findings

Modeled EMF is fully nonlinear.

Nonlinear dynamo approximations are not applicable.

Scale separation conditions are not met.

Abstract

We analyze the electromotive force (EMF) terms and basic assumptions of the linear and nonlinear dynamo theories in our three-dimensional (3D) numerical model of the Parker instability with cosmic rays and shear in a galactic disk. We also apply the well known prescriptions of the EMF obtained by the nonlinear dynamo theory (Blackman & Field 2002 and Kleeorin et al. 2003) to check if the EMF reconstructed from their prescriptions corresponds to the EMF obtained directly from our numerical models. We show that our modeled EMF is fully nonlinear and it is not possible to apply any of the considered nonlinear dynamo approximations due to the fact that the conditions for the scale separation are not fulfilled.