  There has been much debate in recent decades as to what fraction of ionizing
photons from star forming regions in the Galactic disk escape into the halo.
The recent detection of the Magellanic Stream in optical line emission at the
CTIO 4m and the AAT 3.9m telescopes may now provide the strongest evidence that
at least some of the radiation escapes the disk completely. We present a simple
model to demonstrate that, while the distance to the Magellanic Stream is
uncertain, the observed emission measures (${\cal E}_m \approx 0.5 - 1$
cm$^{-6}$ pc) are most plausibly explained by photoionization due to hot, young
stars. This model requires that the mean Lyman-limit opacity perpendicular to
the disk is $\tauLL \approx 3$, and the covering fraction of the resolved
clouds is close to unity. Alternative sources (e.g. shock, halo, LMC or
metagalactic radiation) contribute negligible ionizing flux.
