Does the interstellar magnetic field follow the Chandrasekhar - Fermi law ?

We carried out 1.25 pc resolution MHD simulations of the ISM – including the large scale galactic fountain – by fully tracking the time-dependent evolution of the magnetic field and the formation of shock compressed regions in a supernova-driven ISM. The simulations show that large scale gas streams emerge, driven by SN explosions, which are responsible for the formation and destruction of shocked compressed layers with lifetimes up to some 15 Myr. The T≤ 10 3 K gas is distributed in filaments which tend to show a preferred orientation due to the anisotropy of the flow induced by the galactic magnetic field. The simulations also show that the magnetic field has a high variability, it is largely uncorrelated with the density and it is driven by inertial motions. The latter is consistent with the fact that ram pressure dominates the flow for 10 2 10 6 K thermal pressure dominates, while for T≤ 10 2 K (stable branch) magnetic pressure takes over.