Emission spectrum of Atomic Oxygen Note that the half-life of N2 in the excited state is very low, hence it can emit at lower altitudes and higher densities. Atomic O (as opposed to molecular O2) exists above 80 km alt, formed by photodissociation.

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   AkaSci 🛰️ (akasci@fosstodon.org)'s status on Wednesday, 14-Aug-2024 11:14:48 JST AkaSci 🛰️

   in reply to

   The ISS aurora images show red colors at high altitudes. Why?

   Atomic O is unusual in terms of its return to ground state after excitation: it takes ~0.7 sec to emit green light and up to 2 mins for red.
   - At low altitudes, collisions with other atoms or molecules absorb the excitation energy and prevent emissions.
   - At 100-200 km, O density is sufficiently low to allow green emission.
   - Above 200 km, O density is very low which allows time to emit red light.

   https://en.wikipedia.org/wiki/Aurora
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