2.1.4. Radio Faraday Rotation
Using the Faraday Rotation method, a novel technique to study the magnetic field in our Galaxy was proposed by Vallée (1983b), consisting of measuring the linear polarization (Stokes Q and U) of extragalactic radio sources located in a selected celestial zone of a few dozens of degrees of angular extent at a time. Radio observations were done between 2 cm and 21 cm, using such telescopes as the 27 km Very Large Array in New Mexico, USA, the 46m Algonquin Radio Observatory in Ontario, Canada, the 100m Effelsberg Radio Telescope near Bonn in Germany, and the 3 km Westerbork Synthesis Radio Telescope in the Netherlands,
Selected celestial zones are often centered on specific large-scale features, such as a ring. Polarization observations were made of quasars and compact galaxies shining through the ring, to compare with polarization observations of other quasars and compact galaxies adjacent to the ring, in order to study the variation in RM with respect to angular distance from the ring center.
The first region thus chosen was the galactic anticenter (Vallée, 1983b); the next was the celestial zone encompassing the Gum nebula (Bignell & Vallée, 1983); the third was the Gemini area (MacLeod et al., 1984). The fourth was the area around the cluster of galaxies Abell 2319 (Broten et al., 1986), the fifth area encompassed the central nucleus of our galaxy (Bignell et al., 1988), and the sixth area enclosed the Orion-Eridanus region (MacLeod et al., 1988; Vallée et al., 1988a). This technique has facilitated the investigation of localized effects or RM contributions from a given celestial zone on the passage of the radiation through our Galaxy.