One should note that modeling an absorption spectrum is generally easier than an emission one. It requires only a correct computation of the thermal and ionization equilibrium, which give the fractional ion abundances, i.e. the populations of the ground levels, and thus the equivalent widths of the resonance lines, which can be compared to those deduced from the observations through a curve of growth analysis implying no line transfer. However it can happen (and this is indeed the case for the Warm Absorber), that emission lines are also produced by the absorbing medium, and one should thus worry about the line transfer.

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Since Titan is made for Thomson-thick media, the transfer is treated in the semi-isotropic two-stream Eddington approximation. It means that the optical thickness is multiplied by a factor $\sqrt{3}$ with respect to the normal direction.

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... lines

In He-like ions, the first resonant line ( $1{\rm s}^2\ ^1{\rm S}{-} 2{\rm p} ^1{\rm P}^{\rm o}$ ) is called w, the forbidden line $1{\rm s}^2\ ^1{\rm S} {-} 2{\rm s} ^3\rm S$ is called z, and the intercombination lines $1{\rm s}^2\ ^1{\rm S}{-}2{\rm p} ^3{\rm P}^{\rm o}_{1,2}$ are called x and y.

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