Auroral Colours, Iceland

Images by Parag Mahajani of Pune, India. Canon DSLR 550D, 50 mm lens, 15-60s. HDR images using Photomatix Pro 5.0.
The red light of oxygen (singlet D to ground state triplet P) is evidently at greater altitude than its green and shorter lifetime singlet S to singlet D state transition.

Both transitions are spin forbidden hence they have very long lifetimes and we only see this light at very low pressures where collisions cannot quench the excitation first.

A hint of ionised molecular nitrogen emission tops both.
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The culprits responsible for auroral colours are oxygen atoms, nitrogen atoms and molecular nitrogen (N2).   They are electronically excited by collisions with energetic particles spilling from Earth’s magnetotail overfilled during solar storms.

Atomic oxygen at 100-150 km produces intense green.   At greater altitudes of 150 to 500 km collisions are so infrequent that another oxygen excited state can survive enough to radiate red light.

Molecular nitrogen is a smaller contributor.   Its excited ion gives purple-blue at very high altitudes and sometimes, in violent displays, a red-purple fringe below the green curtains of glowing atomic oxygen.