Ionization rate ζ _CR by cosmic rays coming from one side of the disk as a function of the depth χ from the disk surface. The abscissa is normalized by the attenuation length of cosmic rays λ _CR ≈ 96 g cm ⁻², and the ordinate is normalized by the ionization rate at the disk surface ζ ⁽⁰⁾ _CR/2 and the exponential attenuation factor exp(−χ/λ _CR). The solid line shows the ionization rate obtained by numerical integration of Equation (A6). The ionization rates given by the empirical formula (A9) with γ = 1, 3/4, and 2/3 are also shown for some values of χ/λ _CR by different marks.