Illustration of two simulations, representing strong (r_h = 0.80; left) and intermediate strength of self-gravity (r_h = 0.58; right). The former case leads to prominent wake structure, the latter to overstable oscillations with weak embedded wakes. The cartoon in the uppermost row illustrates how such systems would appear at different ring longitudes, when viewed from B = 12° elevation. Orbital motion is counter-clockwise in the figure. The next row shows the normalised density autocorrelation function from these experiments: in the left the shading, from white to black corresponds to 0.5 to 1.5 (contours mark 0.8, 0.9, 1.1, 1.4), while in the right the range 0.75 to 1.25 is shown (contours 0.8, 0.95, 1.1, 1.2). The white curve near the bottom of the frames is a radial cut of the autocorrelation at ∆y = 0. The last two rows display the modelled I/F and τ_phot versus ring longitude, for B = B′ = 12° and ∆λ_⊙ = 0°. Symbols mark the minimum of modelled I/F and τ_phot. Both the autocorrelation plots and the photometric curves are averages over 40 snapshots; for each snapshot 10⁵ photons were followed for 36 ring longitudes separated by 10°.