The disk of the Milky Way contains free electrons and magnetic fields which contribute significantly to the energetics of the interstellar medium1. The concentrations of electrons and magnetic fields are too low to be detected by direct methods, but may be investigated using Faraday rotation, a wavelength-dependent shift in linear polarization angle induced by a magneto-ionic medium2. Structures in polarization angle arising from Faraday rotation have been detected recently at long radio wavelengths3. These structures are disorganized and filamentary, probably arising from interstellar gas in the vicinity of the Sun. Here we report a more distant, highly ordered Faraday-rotation structure of elliptical shape, with its long axis parallel to the plane of the Galaxy. The feature appears to be located in an inter-arm region of the Milky Way, between the spiral arm containing the Sun and the next outer (Perseus) spiral arm. Within the elliptical region, small-scale structure which characterizes the turbulence seen in adjacent regions of the interstellar medium is absent. The origin of this magneto-ionic feature is uncertain, but it must arise from an organization of the magnetic-field and electron-density distributions on a scale of the order of 50 parsecs (165 light years).
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We thank M. Normandeau (Univ. Calgary; now Univ. California at Berkeley) for the acquisition and initial processing of the data, and A. G. Willis (DRAO) for some later processing. The Dominion Radio Astrophysical Observatory is operated as a national facility by the National Research Council of Canada. The Canadian Galactic Plane Survey is a Canadian project with international partners, and is supported by the Natural Sciences and Engineering Research Council of Canada.
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Gray, A., Landecker, T., Dewdney, P. et al. A large-scale, interstellar Faraday-rotation feature of unknown origin. Nature 393, 660–662 (1998). https://doi.org/10.1038/31413
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