Schematic sketch of the toy model (left) illustrating the origin of the characteristic shark-fin spiral arm profile (right). Black lines are LOSs in the Galactic plane converging toward the observer, blue lines mark the inner and outer “tangents” of the spiral arm, and the green line is the LOS through the center of the arm. The arms have an arbitrary density of n = 1 a.u. where as the interarm regions remain empty. The arms follow a logarithmic spiral where as the direction of the magnetic field B is purely toroidal. The distinct LOSs and B define the angle ϑ. Colored dots indicate the intersection of the different LOSs and the arm borders. The diagram on the right shows a re-constructed signal along the longitude l. The dashed black line is calculated assuming that the signal strength is proportional to the length of the portion of the LOSs which lies within the spiral arm. The dashed line is weighted by cos ϑ. Note that the peak in the observed signal corresponds to the inner tangent (blue), and not to the central LOS (green) as usually assumed.