Yuri Beletsky took this image and those below at Paranal, Chili. Paranal, part of the European Southern Observatory, is the world’s most advanced optical observatory with four 8.2m and four auxiliary 1.8m telescopes that can be optically linked into a giant interferometer.

The site, high in the Atacama Desert was chosen for its ultra-dark and steady skies but Earth’s skies are rarely completely dark.  Take away the last vestiges of twilight, take away moonlight, try to evade the pollution of thoughtlessly bright and unshielded artificial lights and we find that sometimes the upper atmosphere itself glows – airglow.

Fortunately for astronomy the brightness of the red airglow in these pictures was exceptional.     

Airglow is light emitted by excited atoms and molecules 85-300 km high in the atmosphere.   The excitation derives from extreme ultra violet radiation from the sun (aurorae are collisionally excited).    Green is the most common airglow colour, see Yuri's third image below.   The less common red airglow is probably from vibrationally and rotationally excited OH radicals in a band 87 km high. Oxygen atoms at 150 – 300 km might also have formed it.

The airglow at left is banded.    Waves propagating up from the lower atmosphere influence collisional de-excitation that competes with the airglow-forming radiative de-excitation.

A yellow laser beam shines up from the Yepun telescope to create an artificial star in the upper atmosphere that controls the telescope's adaptive optics.