Is there a polarization horizon?
  Modern radio spectrometers make measurement of polarized intensity as a
function of Faraday depth possible. I investigate the effect of depolarization
along a model line of sight. I model sightlines with two components informed by
observations: a diffuse interstellar medium with a lognormal electron density
distribution and a narrow, denser component simulating a spiral arm or H~{\sc
ii} region, all with synchrotron-emitting gas mixed in. I then calculate the
polarized intensity from 300-1800~MHz and calculate the resulting Faraday depth
spectrum. The idealized synthetic observations show far more Faraday complexity
than is observed in Global Magneto-Ionic Medium Survey observations. In a model
with a very nearby H~{\sc ii} region observed at low frequencies, most of the
effects of a "depolarization wall" are evident: the H~{\sc ii} region
depolarizes background emission and less (but not zero) information from beyond
the H~{\sc ii} region reaches the observer. In other cases, the effects are not
so clear, as significant amounts of information reach the observer even through
significant depolarization, and it is not clear that low-frequency observations
sample largely different volumes of the interstellar medium than high-frequency
observations. The observed Faraday depth can be randomized such that it does
not always have any correlation with the true Faraday depth.
