"The incredible night photos and time-lapse movies NASA has been sharing with us provoke questions about our planet. That thin-yellow atmospheric line separating earth from space, for example, that we see in all of the night shots provokes two questions: (1) how thick is this line? and (2) why is this line colored the way it is?
The visible yellow and green/blue capped line represents atmosphere reaching ~100km above the surface of the earth. The colors are not reflected light, and not pollution, but rather are light generated from the components in the atmosphere itself. Yes, the atmosphere gives off its own light, in a chemiluminescent process called "airglow" or "night glow.""