1999A&A...350..447D

Polarimetric data from the 2.695GHz Effelsberg survey of the Galactic Plane have been reduced, resulting in images of the Stokes Q and U emission components over Galactic longitudes given by 74°≥l≥4.9deg, at a resolution of 4.3'. This extends the earlier work of Junkes et al. (1987a) out to the latitude limit of the survey observations (b=±5°). Patchy, diffuse polarised emission is seen to extend to high latitudes (|b|=5°), with generally brighter polarisation detected above |b|>2°. The distributions of both the polarised intensities and polarisation position-angles are shown, as a function of Galactic longitude, and compared with similar data from the southern Galactic Plane. These data will assist in higher resolution studies of the Galactic magnetic field and the magneto-ionic properties of the interstellar medium. Between longitudes of 20°-45°, bright patches of polarised emission are detected, which exhibit quasi-periodic variations of the polarisation angles with scale-sizes of approximately 7°. Both the characteristics and the possible origins of these structures are discussed. This bright emission appears to originate from the Sagittarius spiral arm, between distances of approximately 2.5 and 8kpc. An anticorrelation between polarised intensities and regions of enhanced HI gas density (lying at kinematic distances of 2-2.5kpc) is found. Possible mechanisms are suggested to account for this anticorrelation, arising through the action of a Faraday ``screen'' associated with the Sagittarius arm. The structured, quasi-periodic nature of the polarisation position-angles over this region of the Plane most likely results from a peculiar magnetic field geometry associated with the Sagittarius arm. With scale-sizes of between 200 and 600pc, such magnetic field geometries may be produced by a mechanism similar to the Parker instability.