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What's up with Atomic Oxygen emissions in red and green?

O is unusual in terms of its return to ground state after excitation: it takes ~0.7 sec to emit the 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 high altitudes, O density is v low which allows time for oxygen to emit red light.
- At 100-200 km, O density is sufficiently low to allow green emission.

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