Visualizing the Invisible using Polarisation Observations

TL;DR

This paper discusses how polarization observations, specifically Stokes Q and U parameters, can reveal hidden features of the interstellar medium and magnetic fields in the galaxy, beyond traditional intensity measurements.

Contribution

It highlights the importance of polarization data in observing the interstellar medium and reviews Canada's leading role in this research area.

Findings

Polarization imaging reveals magnetic field perturbations at various scales.

Stokes Q and U provide information not accessible through total intensity measurements.

Canadian studies significantly advance cosmic magnetic field understanding.

Abstract

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.