The analysis of a new, large sample of extragalactic source rotation measures (RMs) provides firm evidence for large-scale prevailing magnetic fields in the Galaxy. Several regions of strong galactic RM are identified, and their boundaries in (l, b) are well delineated for the first time. The largest feature dominates the RM sky; it is centered southward of the galactic plane at l = 90 deg and extends as far as b = 40 deg. It is shown that low RMs at high galactic latitudes are well correlated in sign over large angular scales (about 30 deg), and that they are influenced by the larger-scale prevailing magnetic fields. Various models of the prevailing magnetic field in the plane of the Galaxy are discussed; a bisymmetric spiral field model is proposed in which the prevailing magnetic field sense reverses at or near the Sagittarius arm. The effect of galactic loop I appears to cause a weak, large-scale modulation of the RMs at positive galactic latitudes.
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Simard‐Normandin et al. (1980) studied this question.