3.5. Larger scale view - ASS or BSS
Large-scale magnetic field orientations in the disk a spiral galaxy can follow either an AxiSymmetric (ASS) shape or a BiSymmetric (BSS) shape, or a mixture of both with a preponderance of one of the two shapes. If one travels along a 360° circle at a constant radius from the galactic center, an ASS magnetic field points in the same direction (clockwise and inward for 360°, say), while a BSS magnetic field points in one direction in half of the circular orbit (clockwise and inward for 180°, say) then points in the opposite direction in the second half of the orbit (counterclockwise and outward for 180°, say) - (e.g., Fig. 1 in Vallée 1992).
Figure 10 shows the two main shapes observed for the distribution of the global magnetic field lines. Each arrow indicates the local direction of the global magnetic field, following the prediction of theoretical models at each map point (e.g., Fig. 5 in Ruzmaikin et al. 1988; Fig. 1 in Moss 1995). In actuality, the magnetic field lines are continuous, going inward or outward all the way from or to the proximity of the galactic center (there are no magnetic monopoles).
Figure 10. Two sketches show the global magnetic field lines (arrows) as observed in nearby spiral galaxies, along two concentric circles (dashes) aroud the galactic nucleus. Two spiral arms of stars are shown (continuous curves with stars). The spiral-type loci of the middle interarm areas ars shown (dots). (a) ASS magnetic field shape. (b) BSS magnetic field shape.