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We use dispersion and scattering observations of pulsars, masers, and extragalactic sources to probe the Galactic distribution of ionized gas. The vast majority of the existing observations has been toward the inner Galaxy. Consequently, the radial scale length of the ionized Galactic disk, which appears to extend well beyond the solar circle, remains largely unconstrained. The possible existence of ionized material at large distances, e.g. 50~kpc, is suggested by the appearance of the H$\,${\sc i} disks of nearby galaxies and models of low-redshift quasar absorption systems and Galactic ``fountains.''
We are using the VLBA to conduct an angular broadening study of the Galactic anticenter. We have observed twelve low-latitude extragalactic sources. Seven of the sources are at $b < 1\deg$; if the ionized Galactic disk extends to large distances, the lines of sight to these sources will probe considerable path lengths through the ionized material and result in substantial angular broadening. The remaining five sources are at $b < 10\deg$; the lines of sight to these sources will be sensitive to flaring or warping of the ionized component of the disk and to possible deviations between the neutral and ionized disks.
Fringes were detected for nine of the twelve sources. We present sample images and use the angular sizes of these sources to constrain the scale length of the ionized Galactic disk.
