 Solution

Population can move between the levels by electron-impact excitation and de-excitation, spontaneous emission, radiative absorption and stimulated emission. Other collision partners and radiative cascades can matter in a fuller atom. In the elementary coronal approximation, the plasma is dilute, optically thin, steady, and weakly illuminated at the transition frequency. Electron impacts predominantly excite the lower level, while spontaneous emission predominantly empties the upper level; collisional de-excitation and photon trapping are negligible. The population balance isA key condition is $n_e{C}_{21}\ll A_{21}$, together with insignificant external pumping and competing channels. A Maxwellian electron Maxwell-Boltzmann distribution lets the tabulated rate coefficient be expressed using one temperature.