La foto che proponiamo oggi arriva dal Portogallo ed è stata realizzata da noto astrofotografo Miguel Claro (del gruppo #TWAN) e riguarda una manifestazione rarissima e molto difficile da registrare, stiamo parlando delle bande color arcobaleno dell’Airglow. Questa immagine è stata rilanciata anche dalla #NASA tramite la sua rubrica #APOD.

L’airglow è una debole luminescenza emessa dall’atmosfera terrestre visibile di notte. Ecco la spiegazione dell’autore: “During a climb to the highest mountain of Portugal (2351m) – Pico mountain, located in Pico island, Azores – with a very hard weather conditions due to a strong winds and rain during almost the entire photo expedition with my team colleagues, 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, in a rare occasion with only a few clouds and part of the “winter” Milky Way visible as a background of a temporarily clear sky. Above the low clouds, I have captured strange “rainbow bands” of airglow and between them, the Andromeda Galaxy M31 on the top left of the picture. The air glows all of the time, but it is usually hard to see. A disturbance however – like an approaching storm – may cause noticeable rippling in the Earth’s atmosphere. The bands are actually huge parallel structures in the thermosphere 90 km upwards. Perspective makes them appear to converge. These atmospheric Gravity Waves* (not confound with gravitational waves related to Einstein) 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.

Dr. Martin Setvák from Czech Hydrometeorological Institute Satellite Department, Praha, Czech Republic, has processed the Suomi-NPP VIIRS data to check if these gravity waves in airglow can be found in NOAA/NASA Soumi-NPP satellite Day/Night Band (DNB) image. The “day-night band,” of the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite has indeed captured these glowing ripples in the night sky. The day-night band detects lights over a range of wavelengths from green to near-infrared and uses highly sensitive electronics to observe low light signals. The structures of the bands can be seen below, above the Pico and Faial islands in Azores, Portugal.

Airglow is a layer of nighttime light emissions caused by chemical reactions, light of electronically and/or vibration-rotationally excited atoms and molecules high in Earth’s atmosphere, by solar ultra-violet radiation. A variety of reactions involving oxygen, sodium, ozone, and nitrogen result in the production of a very faint amount of light. In fact, it’s approximately one billion times fainter than sunlight. Technically speaking, airglow occurs at all times. During the day it is called “dayglow,” at twilight “twilightglow,” and at night “nightglow”. There are slightly different processes taking place in each case, but in the image above the source of light is nightglow or airglow.

In the first colorful image captured above Pico, we can see a rare event where is distinguished almost each possible airglow color that is appearing on a single band (Gravity Wave) showed like a rainbow. Below is the Airglow Spectrum designed and explained by the expert Dr. Les Cowley from Atmospheric Optics. Green light from excited oxygen atoms dominates the glow. The atoms are 90-100 km (56-62 mile) high in the thermosphere. The weaker red light is from oxygen atoms further up. Sodium atoms, hydroxyl radicals (OH) and molecular oxygen add to the light”.
Ringraziamo l’autore per averci messo a disposizione il suo lavoro.