  We investigate the properties of the Galactic ISM by applying Faraday
tomography to a radio polarization data set in the direction of the Galactic
anti-centre. We address the problem of missing large-scale structure in our
data, and show that this does not play an important role for the results we
present. The main peak of the Faraday depth spectra in our data set is not
measurably resolved for about 8% of the lines of sight. An unresolved peak
indicates a separation between the regions with Faraday rotation and
synchrotron emission. However, cosmic rays pervade the ISM, and synchrotron
emission would therefore also be produced where there is Faraday rotation. We
suggest that the orientation of the magnetic field can separate the two
effects. By modelling the thermal electron contribution to the Faraday depth,
we map the strength of the magnetic field component along the line of sight.
Polarized point sources in our data set have rotation measures that are
comparable to the Faraday depths of the diffuse emission in our data. Our
Faraday depth maps show narrow canals of low polarized intensity. We conclude
that depolarization over the telescope beam produces at least some of these
canals. Finally, we investigate the properties of one conspicuous region in
this data set and argue that it is created by a decrease in line-of-sight
depolarization compared to its surroundings.
