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.
