Ratio between the recombination rate Γ_rec and the ionization rate Γ_{UV − ion}, as a function of the altitude counted from the planet center and the electron density at 3 R_p. Solid and dashed lines show isovalues of the ratio. In the striped red zone photoionization is dominant (Γ_{UV − ion} ≥ 100Γ_rec) and the probability that an ionized particle recombines is close to zero. With altitudes below the equilibrium altitude (solid black line), the lifetime of a neutral magnesium atom is significantly extended, and in the blue striped zone below the recombination altitude (solid blue line) the probability that it is ionized is close to 0 (Γ_rec ≥ 100Γ_{UV − ion}).