Fast dynamos in weakly ionized gases

TL;DR

This paper investigates how weakly ionized interstellar gases can support fast dynamo action at larger scales than previously thought, potentially explaining the large-scale magnetic fields observed in galaxies.

Contribution

It introduces the idea that ion-neutral decoupling scale, rather than resistive scale, limits magnetic field amplification in weakly ionized gases, offering a new perspective on galactic dynamo mechanisms.

Findings

Ion-neutral decoupling scale can be the limiting scale for dynamo action.

Magnetic power spectra in weakly ionized gases peak at larger scales.

This mechanism may explain large-scale galactic magnetic fields.

Abstract

The turnover of interstellar gas on $\sim 10^9$yr timescales argues for the continuous operation of a galactic dynamo. The conductivity of interstellar gas is so high that the dynamo must be "fast" - i.e. the magnetic field must be amplified at a rate nearly independent of the magnetic diffusivity. Yet, all the fast dynamos so far known - and all direct numerical simulations of interstellar dynamos - yield magnetic power spectra that peak at the resistive scale, while galactic magnetic fields have substantial power on large scales. In this paper we show that in weakly ionized gas the limiting scale may be the ion-neutral decoupling scale, which although still small is many orders of magnitude larger than the resistive scale.