Inner surface density profiles (in physical units) predicted with the fractional changes in the right panel of Figure 2, after N = 30 orbits have passed at R_g, adopting an idealized, Mestel disk configuration with a rotational velocity of V_c = 180 km s⁻¹ and setting of R_g = 5 kpc. The spiral is assumed to have a pitch angle i_p = 20°, and a radial wavelength λ_r = 2π/k = 3 kpc. The total spiral duration is just over 5 Gyr. The grayscale of the lines is as in Figure 2. The initial Mestel disk is shown as a dashed line. The adopted spiral properties are shown in the legend.