The supernova-regulated ISM. II. The mean magnetic field

TL;DR

This study uses 3D MHD simulations to explore the origin, structure, and evolution of magnetic fields in the interstellar medium of spiral galaxies, revealing scale separation and steady-state growth of the mean magnetic field.

Contribution

It introduces a method to separate mean and fluctuating magnetic fields in simulations, demonstrating their growth, scale separation, and steady-state behavior in a galactic context.

Findings

Mean magnetic field reaches a steady state within 1.6 Gyr.

Distinct integral scales for mean and fluctuating fields (~0.7 kpc and 0.3 kpc).

Growth rates differ between mean and fluctuating magnetic components.

Abstract

The origin and structure of the magnetic fields in the interstellar medium of spiral galaxies is investigated with 3D, non-ideal, compressible MHD simulations, including stratification in the galactic gravity field, differential rotation and radiative cooling. A rectangular domain, 1x1x2 kpc^{3} in size, spans both sides of the galactic mid-plane. Supernova explosions drive transonic turbulence. A seed magnetic field grows exponentially to reach a statistically steady state within 1.6 Gyr. Following Germano (1992) we use volume averaging with a Gaussian kernel to separate magnetic field into a mean field and fluctuations. Such averaging does not satisfy all Reynolds rules, yet allows a formulation of mean-field theory. The mean field thus obtained varies in both space and time. Growth rates differ for the mean-field and fluctuating field and there is clear scale separation between the two elements, whose integral scales are about 0.7 kpc and 0.3 kpc, respectively.