  As part of a large study to map the distribution of star formation across
galactic disks, we have obtained deep Halpha images of the nearby Sculptor
Group spirals NGC 247 and NGC 7793. These images are of sufficiently high
quality that they allow identification and analysis of diffuse Halpha emission
at surface brightness levels ranging from those of extremely low density HII
regions to those of the local Galactic disk diffuse emission. This paper
presents a study of the large scale distribution and global energetics of
diffuse ionized gas (DIG) in these galaxies and investigates the association
between DIG and discrete HII regions. Our results support the hypothesis that
the DIG is photoionized by Lyc photons which leak out of traditional HII
regions, and suggest that the local HI column density plays a role in
regulating the amount of leakage which can occur. This interpretation has
profound implications for the derivation of star-formation rates based on
Halpha emission--line fluxes since HII region counts alone will lead to
significant underestimates of the true rate. The contribution of the diffuse
Halpha component to the total Halpha emission, ie. the diffuse fraction, in
these galaxies is found to be similar to values found in other disk galaxies
with differing Hubble types and star formation rates. The constancy of the
diffuse fraction is rather unexpected and implies that the overall fraction of
photons which can leak out of HII regions and ionize the ISM over large scales
is relatively invariant from one galaxy to another.
