Bubble diagram of the model and the included processes. Collision processes are drawn in blue, and the radiative processes (involving a photon) are drawn in red, where the other particles involved in the process are shown along the relevant line. The corresponding rate coefficients for processes that appear in the model are shown in brackets: R for photodetachment (see Eq. (6) and Sect. 2.1.1), k_H for associative detachment (see Sect. 2.1.2), k_3H for three-body recombination of hydrogen (see Sect. 2.1.3), and k_e for electron collisional detachment (see Sect. 2.1.4).