Expected RM variation for an edge-on view of a simplified axisymmetric disk magnetic field (left panel) and a simplified bisymmetric disk magnetic field (right panel) with trailing spiral arms, which are shaded red for the approaching side and blue for the receding side. The black arrows show the direction of the magnetic field vectors. The direction of the rotation of the disk is given by the bold black arrow below the four disks. Panel a: upper part shows the face-on view of the two different disk fields. In the case of an axisymmetric disk field (left panel) all vectors are pointing inwards or outwards; here just one configuration is shown. In case of the bisymmetric magnetic field (right panel) the vectors from one side of the galaxy are pointing inwards and on the other side of the galaxy outwards (or the other way around). In the second row, the expected RM behavior is plotted along the major axis of each galaxy model for an observer watching the system edge-on. The red curve represents the RM values from the approaching side, the blue curve from the receding side. Panel b: upper part shows again the face-on view of the two different disk fields as above (left panel: axisymmetric disk field, right panel: bisymmetric magnetic field). In a simplified approach, a radial reversal is assumed at half of the disk radius. The last row shows the expected and simplified RM value variation in case of a radial field reversal at half the radius of the disk.