Ionization rate weighted column densities N_ζ (Eq. (5)) as a function of radius for HCO⁺ (left column) and N₂H⁺ (right column). N_ζ is normalized to the total column density of the respective molecule. The individual colored solid lines show the fraction of the total column density dominated by a certain ionization source. Red is for X-rays (N_ζX), blue for SPs (N_ζSP) and black for CRs (N_ζCR). Each row corresponds to one model. On the right hand side the model descriptions are provided (see Table 5). The grey shaded area marks the region where more than 50% of the column density arise from regions above the CO ice line (i.e. the warm molecular layer).