Multiphase ISM simulations: comparing NIRVANA and ZEUS

Aims. Our aim is to compare the ZEUS and NIRVANA MHD codes in their application to multi-phase ISM simulations. A contrast in density of about 100 between the warm and cold ISM phases is typical, and discontinuities such as these can be difficult to handle numerically. It is plausible that the details of implementing a particular numerical method could affect the results of the simulation, particularly with regard to the mass and volume fractions of the warm, unstable, and cold gas. This work takes a step towards quantifying to what extent this may occur. Methods. Using the NIRVANA code, we simulate a multi-phase ISM with turbulence driven by the magnetorotational instability. We make comparisons to our previously published ZEUS models and assess the impact of the numerical method on quantities such as the mass and volume fractions in the ISM phases, turbulent velocities, and probability distribution functions of density, temperature, pressure, and magnetic field strength. We also compare models with which turbulence is driven by an artificial forcing function. Results. For the models that include turbulence driven by the magnetorotational instability, we find differences in the mass fractions that are typically on the order of 10%. This comparison, however, is complicated by the result that the saturation level of the MRI is different between the two codes. This suggests that, at the same saturation level, we would expect to find larger differences in the mass fractions, and this is indeed what we have found for our artificially forced models. For these runs we find differences in the cold mass fraction of approximately 30%, which is significant.