photoionisation equilibrium models
Employed photoionization equilibrium models to fit observations, taking into account degeneracy between number density $n_e$ and radial location $r$
Technical definition
Models used to describe the balance between photons ionizing gas ions in a region of space, assuming that the energy deposited by these photons is equal to the energy loss through various processes; it's an equilibrium state reached when the inflowing and outflowing energies are balanced.