Small-Scale Dynamo in Supernova-Driven Interstellar Turbulence

TL;DR

This study uses realistic supernova-driven turbulence simulations to demonstrate the operation of a small-scale dynamo in the interstellar medium, identifying critical resistivity thresholds and saturation levels.

Contribution

It provides the first detailed analysis of SSD behavior in supernova-driven turbulence, including effects of resistivity, resolution, and supernova rate, confirming SSD action in realistic conditions.

Findings

SSD growth is consistent with dynamo action, not magnetic field tangling.

Critical resistivity for SSD onset is between 0.001 and 0.005 kpc^{-1} km s^{-1}.

SSD saturates at about 5% of kinetic energy, regardless of growth rate.

Abstract

Magnetic fields grow quickly even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium of galaxies. Many studies have focused on idealized turbulent driving of the SSD. Here we simulate more realistic supernova-driven turbulence to determine whether it can drive an SSD. Magnetic field growth occurring in our models appears inconsistent with simple tangling of magnetic fields, but consistent with SSD action, reproducing and confirming models by Balsara et al. (2004) that did not include physical resistivity $\eta$. We vary $\eta$, as well as the numerical resolution and supernova rate, $\dot\sigma$, to delineate the regime in which an SSD occurs. For a given $\dot\sigma$ we find convergence for SSD growth rate with resolution of a parsec. For $\dot\sigma\simeq\dot\sigma_{\rm sn}$, with $\dot\sigma_{\rm sn}$ the solar neighbourhood rate, the critical resistivity below which an SSD occurs is $0.005>\eta_{\rm crit}>0.001\,\rm kpc^{-1}\,\rm km s^{-1}$, and this increases with the supernova rate. Across the modelled range of 0.5--4 pc resolution we find that for $\eta<\eta_{\rm crit}$, the SSD saturates at about 5% of kinetic energy equipartition, independent of growth rate. In the range $0.2\dot\sigma_{\rm sn}\leq \dot\sigma\leq8\dot\sigma_{\rm sn}$ growth rate increases with $\dot\sigma$. SSDs in the supernova-driven interstellar medium commonly exhibit erratic growth.