Fluctuation dynamos in supersonic turbulence at Pm ≳ 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 numberM 11rms , we explore fluctuation dynamos across magnetic Prandtl numbers Pm = 1–10. At 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 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 Pm = 1 but decreases significantly at higher Pm. Across all runs, the ratio of velocity-to-magnetic integral scales is ∼3.4, in the saturated phase, independent of Pm, while the ratio of viscous to resistive dissipation scales increases with the increase in Pm. Synthetic Faraday rotation measures reveal coherence lengths of ∼one-fourth to one-third of the forcing scale across the range of 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.