Structure of the emission region: structure in abundances (a) and structure in line emissivity (b) as a function of the visual extinction A_V and of the thickness for Z = 0.014, n_H = 100 cm⁻² and U = 10⁻³. The HII region is located at the left of the ionization front (vertical dashed red line), and the PDR and molecular region lie to its right. We stopped our simulation at A_V = 10 (vertical dashed green line).