  In order to examine the excitation and ionization mechanism of extraplanar
diffuse ionized gas (DIG) we have obtained optical longslit spectra of seven
edge-on spiral galaxies. In four objects the brightest emission lines can be
traced out to distances of typically 1.5 kpc above the disk. For NGC1963 and
NGC3044 line ratios such as [NII]6583/Ha or [SII]6717/Ha as well as
[OIII]5007/Hb could be measured for the halo DIG. This allows us to discuss the
DIG in the halo of these objects in the framework of diagnostic diagrams. For
these two objects, the line ratios of [OIII]5007/Hb decrease with increasing z,
different from the recently reported trend in NGC891 (Rand 1998). We find that
emission lines from the DIG in the disks are in good agreement with
photoionization models using a dilute radiation field. However, with increasing
z these models fail to predict the measured [OI]6300/Ha and HeI5876/Ha line
ratios for NGC1963. Diagnostic diagrams reveal for NGC1963 the need for a
second ionization mechanism of the halo DIG (besides photoionization). This
additional source could be shock ionization. The same diagrams demonstrate an
intermediate classification for NGC3044. Plots of [SII]/[NII] vs. emission
measure reveal significant changes towards the halo and seem to trace local
small scale density fluctuations of the extraplanar DIG.
