We study the polarization of 31 bright radio sources at four wavelengths with high sensitivity and at arcsec resolution. The objectives were to determine accurately the distribution of rotation measure (RM); to study the magnetic field structures in the faint regions of the sources; and to investigate the contribution of the Galactic foreground to the observed RM structures. The magnetic field lines follow the contours of total intensity and are either parallel or perpendicular to ridge lines. This behaviour can be explained by a model in which the field is aligned by the shear and compression of a supersonic jet excavating a cavity in a much denser medium. There tend to be larger variations in RM in the most luminous sources. At high Galactic latitudes the RM structure is on scales comparable with the beam (typically 5 arcsec). RM variations are usually larger on the brighter side of the source.
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Leahy et al. (1986) studied this question.