Small-scale Dynamo in Supernova-driven Interstellar Turbulence

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. that did not include physical resistivity η. We vary η, as well as the numerical resolution and supernova rate, , to delineate the regime in which an SSD occurs. For a given we find convergence for SSD growth rate with resolution of a parsec. For , with the solar neighborhood rate, the critical resistivity below which an SSD occurs is , and this increases with the supernova rate. Across the modeled range of 0.5–4 pc resolution we find that for , the SSD saturates at about 5% of kinetic energy equipartition, independent of growth rate. In the range growth rate increases with . SSDs in the supernova-driven interstellar medium commonly exhibit erratic growth.

Non-equilibrium ionisation plasmas in the interstellar medium

“…in the Galactic Centre, there will be naturally less field tangling and ordered structures should be more common there. Moreover, dynamo simulations show that field amplification is mainly due to a small scale field dynamo (Gent et al 2020) and less to field tangling. Although flux freezing is always a good approximation in the ISM, as a low degree of ionisation is sufficient for a reasonable coupling of plasma to the field, the chaotic nature of the magnetic field, i.e.…”

Gravity Versus Magnetic Fields in Forming Molecular Clouds

“…The thermal energy peaks at a density of n ≈ 0.1 cm −3 and decreases at higher densities as temperatures drop. The magnetic energy is lower than the kinetic energy at all times, and is maintained at a fraction of 0.1 − 0.3 of the total kinetic energy, probably due to the saturation of the small-scale, turbulent dynamo in our simulation box (Balsara et al 2004;Meinecke et al 2014;Gent et al 2021). The small-scale dynamo drives flux growth at wavelengths close to the dissipation scale, which is determined in these models by the numerical resistivity.…”

Supernova induced processing of interstellar dust: impact of ISM gas density and gas turbulence

“…Each model is within a 3D periodic domain large enough to evolve the SN remnant beyond 1 Myr. The models have an equidistant grid resolution of 0.5 pc along each edge, sufficiently well resolved to obtain well converged solutions (Gent et al 2020(Gent et al , 2021 and also to exhibit instabilities resembling Vishniac-Ostriker-Bertschinger (VOB) overstability (Vishniac 1983;Vishniac et al 1985), Rayleigh-Taylor (RT) or potentially Richtmyer-Meshkov (RM) instability (Brouillette 2002).…”

“…Equations ( 4) and ( 5) include terms with 𝜁 𝐷 to provide momentum and energy conserving corrections for the artificial mass diffusion applying in Equation ( 3). Terms containing 𝜈 3 , 𝜒 3 and 𝜂 3 apply sixth-order hyperdiffusion, in which W (3) is the fifth order rate of strain tensor, to resolve grid-scale instabilities (see, e.g., Brandenburg & Sarson 2002;Haugen & Brandenburg 2004;Gent et al 2021), with mesh Reynolds number set to be 1 for each 𝛿𝑥. The incidence of SN denoted by 𝜎 occurs once at 𝑡 = 0.…”