The Small-scale Dynamo in a Multiphase Supernova-driven Medium

Funding Information: F.A.G. and M.J.K.-L. acknowledge support from the Academy of Finland ReSoLVE Centre of Excellence (grant 307411), the Ministry of Education and Culture Global Programme USA Pilot 9758121, and the ERC under the EU’s Horizon 2020 research and innovation program (Project UniSDyn, grant 818665) and generous computational resources from CSC—IT Center for Science, Finland, under Grand Challenge GDYNS Project 2001062. M.-M.M.L. was partly supported by US NSF grant AST18-15461. We acknowledge the constructive criticism of the anonymous referee, which helped us to improve the paper. Publisher Copyright: © 2023. The Author(s). Published by the American Astronomical Society.Magnetic fields grow quickly, even at early cosmological times, suggesting the action of a small-scale dynamo (SSD) in the interstellar medium (ISM) of galaxies. Many studies have focused on idealized, isotropic, homogeneous, turbulent driving of the SSD. Here we analyze more realistic simulations of supernova-driven turbulence to understand how it drives an SSD. We find that SSD growth rates are intermittently variable as a result of the evolving multiphase ISM structure. Rapid growth in the magnetic field typically occurs in hot gas, with the highest overall growth rates occurring when the fractional volume of hot gas is large. SSD growth rates correlate most strongly with vorticity and fluid Reynolds number, which also both correlate strongly with gas temperature. Rotational energy exceeds irrotational energy in all phases, but particularly in the hot phase while SSD growth is most rapid. Supernova rate does not significantly affect the ISM average kinetic energy density. Rather, higher temperatures associated with high supernova rates tend to increase SSD growth rates. SSD saturates with total magnetic energy density around 5% of equipartition to kinetic energy density, increasing slightly with magnetic Prandtl number. While magnetic energy density in the hot gas can exceed that of the other phases when SSD grows most rapidly, it saturates below 5% of equipartition with kinetic energy in the hot gas, while in the cold gas it attains 100%. Fast, intermittent growth of the magnetic field appears to be a characteristic behavior of supernova-driven, multiphase turbulence.Peer reviewe