Supernova-regulated ISM -- V. Space- and time-correlations

Abstract

We apply correlation analysis to random fields in numerical simulations of the supernova-driven interstellar medium (ISM) with the magnetic field produced by dynamo action. We solve the thermo-magnetohydrodynamic (MHD) equations in a shearing, Cartesian box representing a local region of the ISM, subject to thermal and kinetic energy injection by supernova explosions, and parameterized optically-thin radiative cooling. We consider the cold, warm\, and hot phases of the ISM separately; the analysis mostly considers the warm gas, which occupies the bulk of the domain. Various physical variables have different correlation lengths in the warm phase: $40 pc$, $50 pc$, and $60 pc$ for random magnetic field, density, and velocity, respectively, in the midplane. The correlation time of the random velocity is comparable to the eddy turnover time, about $10^7 yr$, although it may be shorter in regions with higher star formation rate. The random magnetic field is anisotropic, with the standard deviations of its components $b_x/b_y/b_z$ having approximate ratios $0.5/0.6/0.6$ in the midplane. The anisotropy is attributed to the global velocity shear from galactic differential rotation, and locally inhomogeneous outflow to the galactic halo. The correlation length of Faraday depth along the $z$-axis, $120 pc$, is greater than for electron density, $60 - 90 pc$, and vertical magnetic field, $60 pc$. Such comparisons may be sensitive to the orientation of the line of sight. Uncertainties of the structure functions of synchrotron intensity rapidly increase with the scale. This feature is hidden in power spectrum analysis, which can undermine the usefulness of power spectra for detailed studies of interstellar turbulence.