Field lines that are potentially visible, for a number of neutron-star geometries. The geometry determines the LOS, which is characterized by the viewing angle θ_ob, and passes through a dipole field that is inclined to the spin axis by an angle α. These spin and magnetic axes are shown with thick black lines, and the NS coordinate lines are drawn in its rest frame. The frequency-color mapping follows the previous figures. Field lines with radio emission observable at 430 MHz are plotted in red, those observable at 1300 MHz in blue. Dots mark the emission points, at an altitude of 15R_NS for 430 MHz and at 5R_NS for 1300 MHz. The corresponding footpoint loops on the stellar surface are shown in Fig. 16. Left column: Side view with spin axis pointing vertically. Right column: Top view with spin axis pointing at the observer. Upper row: Almost aligned rotator, with our LOS passing close to the spin–magnetic axes. The emission points and field line footpoints form two almost concentric circles. Middle row: Inclined rotator with a LOS passing away from the magnetic axis, but close to the spin axis. The LOS samples a restricted range of longitudes and latitudes on the stellar surface, and the emission points and footpoints loops are completely separate for the two frequencies, meaning that the lower-frequency loop is not encircled by the higher-frequency one. Lower row: Inclined rotator with LOS passing away from the spin axis, but close to magnetic axis. The emission points and footpoints form concentric loops that are flattened on the side closer to the magnetic axis.