Previous models had to introduce additional heating sources in order to increase the temperature and the line ratios. Our models demonstrate that the rate of extra heating is reduced in comparision with previous estimates. The intrinsic increase of the gas temperature is due to the progressive hardening of the radiation going through the gas layer. The trend in the observed line ratios is modeled without additional heating. However, high values in particular of [NII]/Hα and [SII]/Hα still need extra heating. On the other hand the main cooling line [OII] puts strong constrains on this.

An important issue is the incorporation of other phases of the ISM, in particular interfaces which could boost some line ratios up to the observed values. We are thus able to reproduce with our models many observed line ratios of the DIG in several edge-on galaxies satisfactorily.