Red Line (630.0 nm)

Above about 150 miles, oxygen glows red instead of green — but only where the air is thin enough to leave excited atoms undisturbed for minutes. Sky-filling red is the classic mark of an extreme storm.

The 630.0 nm O(¹D → ³P) transition has a ~110 s radiative lifetime, restricting emission to altitudes above ~150–200 km where collisional quenching is negligible. Type-d red auroras characterize severe storms.

Mid-latitude observers often see red first or only — the tall red tops clear the horizon while the green base stays hidden below it.