Left: polarized emission at λ6.2 cm (VLA+Effelsberg) at 25′′ resolution in polar coordinates (azimuthal angle in degrees, measured counterclockwise from the north-eastern major axis in the galaxy plane, and ln of radius in arcminutes). The range of azimuthal angles is plotted twice for better visibility of the spiral arms. Right: same for the polarized emission at λ20.1 cm (VLA) at 51′′ resolution. Numbers refer to the spiral arms listed in Table 4.