Initial 1D gas density (in g cm⁻³) structure as a function of modified radius x₀ = 1 − R₀/r, with R₀ as the lower boundary radius. The cyan curve is corresponding FASTWIND density structure as described in Sect. 2.3. The red-dashed vertical line is the optical photosphere R_★. The shaded-purple region is the instability region where we provide the initial perturbations (see zoomed-in part).