Ionization parameter ($U=Q_H/(4\pi r^2{n}_{H}c)$)

The ionization parameter compares the local density of hydrogen-ionizing photons with the hydrogen number density. For an unattenuated isotropic source the displayed form uses its ionizing photon production rate $Q_H$. Ionic fractions depend on $U$ together with the radiation spectrum, temperature, recombination rates and shielding, so $U$ alone does not determine every emission-line ratio.