Points with error bars show (from left to right) the observed values of PI at 6.2 cm, RM between 3.6 cm and 6.2 cm and φ_obs respectively, for the sectors described in Sect. 5. From top to bottom, the results for the rings with mean radius 0.15 kpc, 0.25 kpc, 0.35 kpc and 0.45 kpc are presented. The lines are no fits but show the model-output (see Sect. 7.2) using B_sp = 1.0, B_ver = 0.7, φ = 30.0°, c_pi = 20.0, c_rm = 150.0, i = 43°, and PA = 70° for all rings. The red dashed line shows RM_fg; the expected rotation measure of the Galactic foreground, by which the model was shifted.