External FUV radiation field (in energy density) propagating inside the photoevaporating wind and outer disk surface (reference model of d203-506 with G₀ = 2 × 10⁴ and n_H = 10⁷ cm⁻³). The upper panels (blue spectra) are for a slab position at A_V = 0.1 mag (close to the PDR surface), whereas the lower panels (red spectra) are for a position deeper inside the PDR, at A_V = 0.5 mag. The disk-depth-dependent FUV field results from a FUV radiative transfer calculation that includes gas absorption, H₂ fluorescent emission, and grain absorption and scattering. The resulting FUV continuum and line emission are modulated by strong H I absorption lines (Ly-α and Ly-β lines are indicated in this figure) and by a forest of H₂ absorption and fluorescent emission lines. The black vertical lines show the wavelength position of the main C⁰ pumping transitions at 1277, 1279, and 1280 Å (∼9.6 eV). The vertical dashed gray lines show the wavelength position of the main O⁰ pumping transitions at 1027 and 1040 Å (∼12 eV). These photons are quickly absorbed by small column densities of gas, mainly via C⁰ photoionization below 1101 Å for A_V < 1 mag (Fig. F.1). Lower-energy FUV photons propagate deeper inside the disk and are attenuated by dust grains.