Aurora colors come from atoms that are trying to shed some of their energy.

In very thin air, oxygen survives in a delicate, high-energy state that emits green light. In denser air, atomic collisions knock oxygen to a lower-energy state that emits red. Nitrogen is a robust emitter that glows bright in even lower, denser layers of the atmosphere.

Beautiful science.

https://www.swpc.noaa.gov/content/aurora-tutorial #aurora #science #space #nature