Density cuts of four simulations of an ionization front propagation with a modulated interface. The UV flux at the top of the box is 10⁹ photons per second per square centimetre, the box is 4 × 2 × 2 parsecs (half of the cut is shown). The dense gas is initially at 500 H/cm³ at a temperature around 25 K (thermal equilibrium from Fig. 1). The (base width)/height ratios of the modulations are respectively 2 for case (a), 1.4 for case (b), 0.9 for case (c) and 0.5 for case (d).