Visualizing the Invisible using Polarisation Observations
  An electromagnetic wave can be uniquely characterized by the four Stokes
parameters: I, Q, U, and V. Typical observations in astronomy rely solely on
total intensity measurements of the incoming radiation (Stokes I). However, a
significant amount of information both about the emitting region and the
propagation path is carried in the remaining Stokes parameters. These data
provide a means to observe parts of the interstellar medium which remain
invisible in Stokes I, at any wavelength. For example, when an electromagnetic
wave propagates through a region containing free electrons and a magnetic
field, the plane of polarisation of the wave will rotate - an effect recorded
only in Stokes Q and U. The interstellar medium of the Galaxy is such a region,
containing free electrons (observed as HII) and a magnetic field of a few
microgauss. By imaging in Stokes Q and U we are able to observe signatures of
magnetic field perturbations from the small scale (tens of pc) to the large
scale (kpc). In this paper, we review the status of Canadian polarisation
studies of cosmic magnetic fields and discuss the leading role Canada is
playing in polarsation studies around the world.
