  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. While the distance
to the Magellanic Stream is uncertain, the observed H-alpha emission is most
plausibly explained by photoionization due to hot, young stars. Our model
requires that the mean Lyman-limit opacity perpendicular to the disk is tau
\approx 2.8 (f_esc = 6%), assuming the covering fraction of resolved clouds is
close to unity. Within the context of this model, it now becomes possible to
determine distances to high velocity clouds, and the 3D orientation of the
Magellanic Stream. Here, we discuss complications of the model (e.g., porosity,
topology), future tests, ongoing improvements, and the importance of H-alpha
limb brightening from surface ionization. More speculatively, we propose a
direct experiment for locating an HVC in 6-dimensional phase space above the
Galactic plane.
