Diffuse ionized gas in halos of spiral galaxies

The presence of gaseous halos in star forming disk galaxies is reviewed inthe context of a proposed disk-halo connection of the interstellar medium (ISM). Results from a new survey for H⁺ halos of edge-on galaxies are presented. The data confirm that thepresence of diffuse ionized gas (DIG) in the disk-halo interface of spiral galaxies is related to star formationprocesses in the underlying disk. A discussion allows us to establish a minimum energy release per unit area that is required to start the disk-halo mass exchange. By comparing some recent observational results for diagnostic emission lines with model predictions from photoionization we demonstratethat the origin and excitation of the ionized halo gas is still not completely understood and that a discussion gives important constraints formodels of the ISM. In comparison with similar findings for the Milky Way the need for an additional heating source is established. Special emphasis is given to some recent developments. In particular, newkinematical information for the DIG layer in NGC 5775 from ESO/VLT long-slitspectra is discussed in connection with the magnetic field structure in the halo of this object as deduced from VLA radio-continuum polarization data (Tüllmann et al., 2001). Finally, the rôle ofdust for the physical processes in the disk-halo interface is briefly addressed.