The standard PN model. a)–c) Ionizing continuum spectra in three different positions of the nebula, as indicated in the cartoon representing the ionization structure of the standard PN in panel d) (the PDR is not to scale). e) Radial density profile of H⁰, H⁺, H⁻, H₂, H, and H in the ionized region. Densities are relative to the total density of H nuclei. f) Radial gas temperature profile in the ionized region. The blue band in both panels indicates the TZ. A color version of this figure is available online.