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This video, which shows time-lapse views of Earth from the ISS, consistently shows some sort of hard boundary in the atmosphere, below which there seems to be significant reflection from sunlight and/or Earth's cities and above which there seems to be nothing impeding the view of black space. What is this boundary? I don't think its the edge of the atmosphere, partly because the atmosphere has no hard edge and partly because the aurorae (an atmospheric phenomenon) seen in the video exist, at least in part, above this boundary.

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Yes this is neither aurora, nor reflection of any light. This is airglow, that comes from upper layers of atmosphere. The brightest emission is green 558nm light from excited atomic oxygen in a layer 90-100 km high. At that height most of the oxygen is in atomic state, not molecular, and is constantly being exposed to solar UV radiation.

Here you can find more details. airglow formation

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