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This question describes how some energy transitions become more prominent as you higher in the atmosphere. Short summary: Some electron transitions are "forbidden" by quantum mechanical selection rules, and so have a larger time delay from time of excitation until time of photon emission. Thus, low in the atmosphere (high pressure), it's more likely that the species loses the excitation energy by collision than emission.

However, as you go higher in the atmosphere, you see red (due to energy transition in atomic oxygen). The characteristic green line (also due to atomic oxygen, but different transition) disappears.

Why don't you see a mix of the two colours? After all, from the collision rate argument you would expect fast transitions (like the green of atomic oxygen) to become even more prominent.

Why is it that some transitions are less likely as you increase in height?

(I have a feeling that the answer to this question will also explain why nitrogen does not play a more central role in the colour of the aurora.)

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  • $\begingroup$ I'm not an atmospheric scientist, but I'm guessing it has something to do with the relative abundance of nitrogen/oxygen at different atmospheric heights. Correct me if I'm wrong, but the ratio of oxygen to nitrogen is not the same at all heights, right? $\endgroup$ – getafix Dec 16 '16 at 2:11
  • $\begingroup$ There is a variation. @DavePhD posted the following link in the referenced question: malagabay.files.wordpress.com/2014/07/… $\endgroup$ – Adroit Dec 16 '16 at 12:02
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Why don't you see a mix of the two colours? After all, from the collision rate argument you would expect fast transitions (like the green of atomic oxygen) to become even more prominent.

There can be a mix of colors. However, according to Auroral electron energy derived from ratio of spectroscopic emissions Journal of Geophysical Research Volume 79, Pages 5181–5186, the ratio of the atomic oxygen versus molecular nitrogen bands are a function of "a characteristic energy of the precipitating electron flux".

Why is it that some transitions are less likely as you increase in height?

Well, consider Atmospheric Composition and Vertical Structure. Figure 3 shows how dramatically pressure drops as altitude increases. This means a change in altitude of 15 kilometers could reduce the concentration of the species of interest by a factor of 10.

In addition, dissociation of the $\ce{O2}$ and $\ce{N2}$ to atomic O and N increases with altitude. See especially Fig. 6 which plots the points where monoatomic oxygen starts to exceed diatomic oxygen, and where monoatomic oxygen starts to exceed diatomic nitrogen in concentration.

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