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.
