Aurorae have a red color at high altitudes caused by the excitation of atomic oxygen and the subsequent emission at about $630 \,\text{nm}$. This happens at high altitudes because at that height there aren't many particles, so collisions and loss of excitation aren't a problem (the red emission is caused by the D1 excitation state, which takes $150 \,\text{s}$ to emit light, quite a lot), while lower down they are.
But why is there green emission (also caused by oxygen, S1 excitation state precisely) only at lower altitudes? Can't they also happen higher up since they take only about $1 \,\text{s}$ to emit light? Is the reason the scarcity of the S1 excitation state in respect to the D1 (for whatever reason)? And what determines at what wavelength an atom will emit?
Previously this question has been called duplicate, because, in fact, there is an identical question on this site. However, the approved answer just talks about nitrogen (which, apparently, also contributes to the aurorae's green) and not oxygen, which is what I (actually we; both me and the author of the similar question) really asked.
Also, according to the approved answer of the same post I linked before, nitrogen is the one who is most responsible of green emissions, in contrary to what the vast majority of internet sites say (i.e. oxygen is the one who emits both green and red light).
