Fluctuation dynamos in supersonic turbulence at $\mathrm{Pm}\gtrsim 1$

Abstract

Fluctuation dynamos provide a robust mechanism for amplifying weak seed magnetic fields in turbulent astrophysical plasmas. However, their behaviour in the highly compressible regimes characteristic of the interstellar medium remains incompletely understood. Using high-resolution 3D magnetohydrodynamic simulations of supersonic turbulence with rms Mach number ${\mathcal{M}}_{\mathrm{rms}}\approx 11$, we explore fluctuation dynamos across magnetic Prandtl numbers $\mathrm{Pm}=1-10$. At $\mathrm{Pm}=1$, dynamo growth is slower and saturates at lower magnetic-to-kinetic energy ratios, with amplification in the kinematic phase dominated by compression rather than line stretching. In contrast, at $\mathrm{Pm}=10$, vortical stretching emerges as the dominant mechanism, yielding faster growth, higher saturation levels, and stronger suppression of density--magnetic field correlations by magnetic pressure. This transition is reflected in the correlation coefficient between density and magnetic field strength, which is strongly positive at $\mathrm{Pm}=1$ but decreases significantly at higher $\mathrm{Pm}$. Across all runs, the ratio of velocity-to-magnetic integral scales is $\sim 3.4$, in the saturated phase, independent of $\mathrm{Pm}$, while the ratio of viscous to resistive dissipation scales increases with the increase in $\mathrm{Pm}$. Synthetic Faraday rotation measures reveal coherence lengths of $\sim$one-fourth to one-third of the forcing scale across the range of $\mathrm{Pm}$ explored. Using these coherence scales, we discuss the potential contribution of fluctuation dynamos to Faraday rotation expected from turbulent, gas-rich young disk galaxies.

Published in ApJ

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