Magnetic fields in spiral galaxies

Radio polarization observations have revealed large-scale magnetic fields in spiral galaxies. The average total field strength most probably increases with the rate of star formation. The uniform field generally follows the orientation of the optical spiral arms, but is often strongest outside the arms. Long magnetic-field filaments are seen, sometimes up to a 30 kpc length. The field seems to be anchored in large gas clouds and is inflated out of the disk, e.g. by a galactic wind. The field in radio halos around galaxies is highly uniform in limited regions, resembling the structure of the solar corona. The detection of Faraday rotation in spiral galaxies excludes the existence of large amounts of antimatter. The distribution of Faraday rotation in the disks shows two different large-scale structures of the interstellar field; axisymmetric-spiral and bisymmetric-spiral, which are interpreted as two modes of the galactic dynamo driven by differential rotation