This gorgeous Astronomy Picture of the Day shows the Milky Way, the Tre Cime di Lavaredo peaks of the Italian Alps, and…airglow.
Unlike aurorae powered by collisions with energetic charged particles and seen at high latitudes, airglow is due to chemiluminescence, the production of light in a chemical reaction, and found around the globe. The chemical energy is provided by the Sun’s extreme ultraviolet radiation. Like aurorae, the greenish hue of this airglow does originate at altitudes of 100 kilometers or so dominated by emission from excited oxygen atoms. More easily seen near the horizon, airglow keeps the night sky from ever being completely dark.
What a composition.
(Click the image to illuminate.)
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