Recently I moved to the north of Norway, so I've seen quite a few auroras. Because the colour is mainly green, I was sure this must be due to electron transitions in $\ce{N2}$ (because the atmosphere is like $75\%$ $\ce{N2}$) Looking into this, however, I found out that the colours of the aurora are mainly due to electron transitions in oxygen gas. Why is it that the electron transitions of oxygen are most prevalent in the colours of the aurora, even though it is far less abundant than nitrogen?
I also read that oxygen gas gives one colour up to $150$ miles altitude(green), and another above $150$ miles (red). Can someone explain the reason for the difference in colour? Does oxygen exist in different oxidation states at different altitudes, leading to different electron configurations => different available electron transitions?
Answer
There is good information at Glowing Gases - Aurorae
There are many factors that need to be considered.
Once an atom or molecule is excited, it can lose the energy by collision or by emission of light.
The longer the lifetime of the excited state, the higher the altitude is required to make radiation vs. collision the way energy is lost.
The atomic oxygen transition with green light has a lifetime of about 1 second, while the transition for red light is 110 seconds, so a higher altitude is required for the red transition to become significant.
For molecular nitrogen, radiative lifetime is only $4 \times 10^{-8}$ seconds according to Electronically excited molecular nitrogen and molecular oxygen in the high-latitude upper atmosphere Ann. Geophys., 26, 1159–1169, 2008
Also, human eyes are more sensitive to green light than red light.
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