Forbidden Light, Banded Airglow, Hungary
In the far sky the Milky Way spans the heavens. In the near sky, at left, a Perseid dust speck finally burns away after wandering the solar system for eons. Higher up in the atmosphere, excited oxygen atoms paint the sky with green banded airglow.

Images ©Ádám Szabó, shown with permission.

Ádám Szabó took these images at Baja Observatory, Hungary - possibly the first images of airglow from Hungary. "I was there to watch the maximum of Perseids during a five night camp with my three friends. In the evening of 14th Aug. No significant meteor activity was seen, so I decided to take a few Milky way pictures. These pictures were taken with a Samyang 8mm fisheye borrowed from the leader of the observatory, Tibor Hegedüs. I noticed that pictures taken with the same white balance, showed differences in the sky background colors.. ..It came in my mind, that this must be airglow. Before that moment, I had taken pictures to freeze the stars with acceptable noise, but with the Milky way bright enough. Then I took pictures with lower ISO, because I saw that the airglow bands were moving relatively slowly - so this was my chance to record them with finer details." The airglow in the top image was seen by Ádám with his unaided eyes.
Green airglow is so-called ‘forbidden’ radiation. The sun’s far ultraviolet light excites atoms and molecules high in Earth’s thermosphere. Excited oxygen atoms (direct or indirect products of the UV) can retain their energy for seconds. For an atom that is an unimaginable time span, something like the age of the Earth to us. Quantum theory predicts naturally and unasked out of its mathematics that atoms are allowed only discrete and fixed energies. More so, the maths produces ‘selection rules’ that specify how an excited state can decay to a lower energy and emit light. Airglow transitions ‘disobey’ these rules but the disobedience has a price, the excited states take an age to decay. The transitions are not unphysical, higher order radiation theories permit them.

Airglow transitions can only occur in the near vacuum of the thermosphere because, at lower altitudes and higher pressures, collisions with other atoms and molecules deactivate the excited oxygen first. There is a balance between sufficient concentration to produce significant light and collisional de-excitation. The result is that green airglow occurs in a narrow zone between 90 and 100 km high well seen from orbit.

The potential airglow is further influenced by density (gravity) waves from the lower air layers. These give the banding.

Airglow is usually very weak and needs a very dark sky free of moonlight and light pollution. It is hard to see visually but try some photographs, it will probably show up more often than is thought.

Atmospheric
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