Total ion (black), H₂O⁺ (blue), and H₃O⁺ (red) ion number density profile at a solar zenith angle of χ = π∕2 and for six different outgassing rates Q: 10²⁵ s⁻¹ (left panel, circles), 10²⁶ s⁻¹ (left panel, dashed line), 10²⁷ s⁻¹ (left panel, pluses), 10²⁵ s⁻¹ (right panel, circles), 10²⁹ s⁻¹ (right panel, dashed line), 10³⁰ s⁻¹ (right panel, pluses). The number densities were scaled accordingly to be plotted as a dimensionless parameter. The vertical line corresponds to the lowest upper bound of the plasma density for χ = π∕2 in dimensionless units, i.e., 0.1657 × 2∕π ≈ 0.1055 (see Eq. (24)). The expanding velocity is set to U_n = 600 m⋅s⁻¹.