Rainbow Bands Of Airglow In Gravity Waves Above Pico Island
Taken by Miguel Claro on November 16, 2015 @ Pico Island, Azores
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During a climb to the highest mountain of Portugal (2351m), Pico mountain, in Pico island – Azores, I stopped at about 1200 meters to appreciate the views and photograph the lights coming from the island of Faial in the middle of the Atlantic Ocean, during the night in a rare occasion with only a few clouds with part of the “winter” Milky Way visible. Above the low clouds, I have captured strange “rainbow bands” of airglow and between them, the Andromeda Galaxy M31. The bands are actually huge parallel structures in the thermosphere 90 km upwards. Perspective makes them appear to converge. This Gravity Waves* (not confuse with gravitational waves) propagating upwards from disturbances lower down in the atmosphere, are likely the source of the bands. The wave amplitude increases with height (reducing density) and wavelengths can be thousands of kilometers.

Airglow is the light of electronically and/or vibration-rotationally excited atoms and molecules high in Earth’s atmosphere, by solar ultra-violet radiation. In this image, we can see almost each possible airglow color appearing on a single band. The green airglow is from oxygen atoms (1S ->1D) 90-100 km high. The red/orange could be yet more oxygen airglow, this time from atoms 150-300km high where the atmosphere is so sparse and collisions so infrequent that the atoms have time to radiate ‘forbidden’ light (1D ->3P) before losing their electronic excitation in impacts with other atoms and molecules. Deep red banded airglow is likely emission from vibrationally excited OH radicals in a layer ~86km high. The bands are caused by gravity waves propagating upwards from the lower atmosphere. They modulate the local pressure, temperature and specie concentrations. Blue airglow is much much fainter and not very obvious on the image. Excited molecular oxygen at ~95 km high can produce it. The excitation is indirect. Possible routes are via daylight dissociation of N2 and NO or twilight recombination of NO+ whose reaction products generate excited O2. The oxygen then decays by emitting blue multi-wavelength banded radiation (Herzberg bands) if it is not first collisionally de-excited.

* “Drop a stone into a pool of water. The spreading ripples are gravity waves. The waves occur between any stable layers of fluids of different density. When the fluid boundary is disturbed, buoyancy forces try to restore the equilibrium. The fluid returns to its original shape, overshoots and oscillations then set in which propagate as waves. Gravity or buoyancy is the restoring force hence the term – gravity waves.”

Info source: All the technical information used in the description above and related to airglow, was consulted in the specialized website of Atmospheric Optics, by Les Cowley. To know more about this phenomenon and others, please click here: http://www.atoptics.co.uk/

Comments
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wish there was eclipsed sun also on this image
Posted by tsap 2016-03-09 11:50:47
Stunning! It will be picture on my desktop =) Thanks
Posted by Lilium13 2016-03-13 00:48:31
Muito Bom!
Posted by jonn 2016-04-01 11:59:10
51 thumbs up
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