a) Ionization structure of the DM Tau disk model. The region bounded by blue lines is what we have defined as the HCO⁺ molecular layer. b) and c) relative contributions of charged species to the ionization level over the whole disk and the HCO⁺ molecular layer. As a disk average, panel b), it is clear the charge is dominated by C⁺ and H⁺, the grey region representing all other charged species. In the HCO⁺ molecular layer, panel c), defined as the region that contributes 90% of the total HCO⁺ column density, HCO⁺ supplies a majority of the charge with large contributions from H⁺, C⁺ and S⁺.