Toy-model for suppression of azimuthal asymmetry due to impact-released debris. The frame in the left shows the distribution of impact velocities in a series of dynamical simulations, performed with τ_dyn = 0.50, ϵ_n = 0.5, for r_h = 0.72–0.98. Assuming ρ = 450 kg m⁻³, this corresponds to Saturnocentric distances 110–150 000 km. Shaded regions indicate the frequency of impacts with v_n greater than 0.5 and 1.0 cm s⁻¹ : their relative amount increases rapidly, while the mean of impact velocity is nearly constant with distance. The modelled A₄₀ amplitudes are shown in the right frame with a dashed line, while the grey points denote the HST measurements. The red and green curves indicate toy-models with different values for the parameters v_thresh and Y: the insert in the left frame displays the amount of free debris as a function of distance following from Eq. (25), while the curves in the right indicate A₄₀ amplitudes in photometric models where the uniform debris population has been added.