Saturated-State Turbulence and Structure from Thermal and   Magnetorotational Instability in the ISM: Three-Dimensional Numerical   Simulations
  This paper reports on 3D numerical simulations of dynamics and thermodynamics in the diffuse ISM. Our models are local, account for sheared galactic rotation, magnetic fields, and realistic cooling, and resolve scales ~1-200 pc. This combination permits the study of quasi-steady turbulence in a cloudy medium representing the warm/cold atomic ISM. Turbulence is driven by the magnetorotational instability (MRI); our models are the first to study the saturated state of MRI under strongly inhomogeneous conditions, with cloud/intercloud contrasts of about 100. For volume-averaged densities n=0.25 - 4 cm^3, the saturated-state velocity dispersion ranges from 8-1 km/s, scaling as n^-0.77. The MRI is therefore likely quite important in driving turbulence in low-density ISM regions, both at high latitudes in the inner Galaxy,and throughout the far outer Galaxy. The MRI may be key to suppressing star formation at large radii in spiral galaxies, where without MRI-driven turbulence, a thin, cold layer could form and become gravitationally instable. Our presentation also includes analysis of thermal structure (temperature and density PDFs are broadened and include extreme departures from equilibrium, but retain the classical two-phase bimodal character); results on the distribution of clump masses (the mass spectrum peaks at about 100 solar masses); comparisons of saturated-state MRI scalings with single-phase simulation results (we find B^2 is independent of n); and examples of synthetic HI line profile maps (showing that physical clumps are not easily distinguished in velocity components, and vice versa). 
Robert A. Piontek, Eve C. Ostriker