  Rotation measures of pulsars and extragalactic point sources have been known
to reveal large-scale antisymmetries in the Faraday rotation sky with respect
to the Galactic plane and halo that have been interpreted as signatures of the
mean magnetic field in the Galactic halo. We describe Faraday rotation
measurements of the diffuse Galactic polarized radio emission over a large
region in the northern Galactic hemisphere. Through application of Rotation
Measure Synthesis we achieve sensitive Faraday rotation maps with high angular
resolution, capable of revealing fine-scale structures of about 1 deg in the
Faraday rotation sky. Our analysis suggests that the observed antisymmetry in
the Faraday rotation sky at b > 0 deg is dominated by the magnetic field around
a local HI bubble at a distance of approx. 100 pc, and not by the magnetic
field of the Galactic halo. We derive physical properties of the magnetic field
of this shell, which we find to be 20 - 34 uG strong. It is clear that the
diffuse polarized radio emission contains important information about the local
magneto-ionic medium, which cannot yet be derived from Faraday rotation
measures of extragalactic sources or pulsars alone.
