Growth of a localized seed magnetic field in a turbulent medium

TL;DR

This study demonstrates that localized seed magnetic fields can be rapidly amplified and dispersed throughout a turbulent medium, suggesting astrophysical ejections can seed widespread magnetic fields efficiently.

Contribution

It provides the first numerical analysis of localized seed magnetic field amplification and dispersal in turbulence, extending understanding beyond uniform seed fields.

Findings

Turbulence efficiently amplifies localized seed magnetic fields.

Localized magnetic fields disperse and fill the system quickly.

Growth and diffusion are fast even in high magnetic Prandtl number turbulence.

Abstract

Turbulence dynamo deals with amplification of a seed magnetic field in a turbulent medium and has been studied mostly for uniform or spatially homogeneous seed magnetic fields. However, some astrophysical processes (e.g. jets from active galaxies, galactic winds, or ram-pressure stripping in galaxy clusters) can provide localized seed magnetic fields. In this paper, we numerically study amplification of localized seed magnetic fields in a turbulent medium. Throughout the paper, we assume that driving scale of turbulence is comparable to the size of the system. Our findings are as follows. First, turbulence can amplify a localized seed magnetic field very efficiently. The growth rate of magnetic energy density is as high as that for a uniform seed magnetic field. This result implies that a magnetic field ejected from an astrophysical object can be a viable source of magnetic field in a cluster. Second, the localized seed magnetic field disperses and fills the whole system very fast. If turbulence in a system (e.g. a galaxy cluster or a filament) is driven at large scales, we expect that it takes a few large-eddy turnover times for magnetic field to fill the whole system. Third, growth and turbulence diffusion of a localized seed magnetic field are also fast in high magnetic Prandtl number turbulence. Fourth, even in decaying turbulence, a localized seed magnetic field can ultimately fill the whole system. Although the dispersal rate of magnetic field is not fast in purely decaying turbulence, it can be enhanced by an additional forcing.