The surface-density modulation factor λ_Σ obtained from Equation (27) (λ_Σ,OL) for ﹩{\dot{M}}_{{\rm{in}}}=1{0}^{-7}\,{M}_{\odot }\,{{\rm{yr}}}^{-1}﹩ (left) and 10⁻⁶ M_⊙ yr⁻¹ (right) at t = 8 kyr. The trajectories of the logarithmic spiral arms inferred from Figure 7 are overplotted as dotted–dashed curves. The logarithmic spiral arms and the locations of crescent-like structures closely follow the peaks of λ_Σ,OL, supporting their origin in the disk eccentricity excited by the accretion streamer.