Explanation:
In this serene night skyscape, the Milky Way's graceful arc stretches over
prominent peaks in the Italian Alps known as Tre Cime di Lavaredo. A 180 degree
wide-angle panorama made in four exposures on August 24, the scene does look to
the north and the sky is suffused with an eerie greenish light. Still, the
subtle glowing bands are not aurorae, but 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.
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