Energy levels of the perturbation potential ℋ₁(R, φ) on the representative XY-plane. The levels for which the bar’s perturbations are dominating are emphasised, showing the zone of influence of the bar. The position of the Sun is shown by a blue cross. The blue spiral curves are the azimuthal minima of the potential ℋ₁(R, φ): outside the zone of influence of the bar; they correspond to the loci of the main spiral arms in our model. In calculations, we used the parameter’s values from Table 1.