The effects of spiral arms on the multi-phase ISM
  Statistical parameters of the ISM driven by thermal energy injections from supernova explosions have been obtained from 3D, nonlinear, magnetohydrodynamic, shearing-box simulations for spiral arm and interarm regions. The density scale height obtained for the interarm regions is 50% larger than within the spiral arms because of the higher gas temperature. The filling factor of the hot gas is also significantly larger between the arms and depends sensitively on magnetic field strength. 
Anvar Shukurov, Graeme R. Sarson, Aake Nordlund, Boris Gudiksen, Axel   Brandenburg