Rainbow Bands of Airglow in Gravity Waves above Pico Island Were seen from NOAA/NASA Satellite in Space

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Featured as NASA´s Astronomy Picture of the Day (APOD).

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.


Dr. Steve Smith, Senior Research Scientist at Center for Space Physics of Boston University, is a specialist in airglow and has explained the rare phenomenon captured on these images: “The photo is very other-worldly but it is indeed a photo of two, maybe three, airglow layers with gravity waves propagating through them in the upper mesosphere. The green is most likely due to atomic oxygen near 96 km. The red is probably due to hydroxyl (OH) near 87 km. Possibly sodium from near 90 k m also but the orange may also just be the response of the system to the combination of the red and green airglow. The awesome rainbow appearance of the patterns is easily explained. A GW wave propagates upwards at an angle and because airglow layers reside at slightly different altitudes, the waves reach each layer at a slightly different times. The vertical scale size of the gravity waves is also much larger than the height differences between the layers: OH = 87 km | Na = 90 km | O2 = 94 km | OI = 96 km.

I exploit the fact of the altitude separation of the four brightest airglow layers in my work to determine the scale sizes and other things associated with the GWs. Sometimes atomic oxygen can also produce red emission. This occurs due times of auroral activity particularly at high latitudes but occasionally in the mid-latitudes during strong aurora. This red-line OI emission, as it is called, originates from 250 km (much higher) in the thermosphere and would not match wave patterns seen in the green-line OI emission.

Solar activity has little effect on gravity wave activity in the mesosphere, at least at the latitude of the Azores. There may be effects at high latitudes though. Also solar activity seen on the sun’s disc doesn’t necessarily impact the Earth. Flares and other solar phenomena that are rich in x-rays and UV radiation can cause immediate effects in the earth’s upper atmosphere. Also, even though a CME my be observed occurring on the sun, it may not affect the Earth because they usually miss us because of the sun’s rotation. The Kp index – a measure of the solar activity prior to the time of this photo was at 2-3, which means a relatively quiet time. At the location of the Azores, one would need a Kp index of 6-7 to see auroral emission.”

Dr. Steven D. Miller, Senior Research Scientist – Deputy Director Cooperative Institute for Research in the Atmosphere – Colorado State University, have commented the fact that the photos were taken at high altitude and this would probably be the reason why the greens are seen so well, since oxygen in the lower atmosphere will absorb much of that downwelling emission.

Dr. Les Cowley, explain in Atmospheric Optics page that 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. 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.

After receiving the first results of the Suomi-NPP VIIRS satellite, I decided to start processing another 12 images taken in the same place and time, to create a wide-angle – panoramic view – of more than 200º where it is clearly visible the airglow activity combined with strong ripples from gravity waves occurring in a huge part of the sky from West to Northeast (from left to right on the panoramic image above). At the same time, Dr. Martin Stevák, starts the final stage of processing the images taken by Suomi-NPP VIIRS satellite but in different bands and the result is shown below. These are in Transverse Mercator projection, centered at 38.5N 28W, remapped to 1km pixel size (for Azores).

20151117_0305-UTC_NPP-VIIRS_TM-1KM_DNB-netDNB image
20151117_0305-UTC_NPP-VIIRS_TM-1KM_M15-netM15 IR band image
20151117_0305-UTC_NPP-VIIRS_TM-1KM_Night-Microphysical-RGB-netNight-Microphysical RGB image
Above – NOAA/NASA Suomi-NPP satellite images taken approx. at 03:05 UTC. In DNB band, the waves in airglow (nightglow) appear as bands transverse to the jet-stream, passing above the Azores region. To discriminate the airglow waves from cloud formations, it is essential to compare the DNB image with other satellite bands, such as M15 IR band, or even better with more advanced image products, such as Night-Microphysical RGB image. Data: NOAA CLASS archive, processed by Martin Setvák from Satellite Dept. of the Czech Hydrometeorological Institute. Data processing: ENVI, VCTK plugin and Adobe Photoshop CS5.

Below, you can see a comparison of images taken by me from Earth, as seen from the island of Pico, Azores (yellow donut on the graphic), versus the images captured by the equipment on board of NOAA/NASA Suomi-NPP satellite, as seen from Space, at approximately the same time. The waves are not matching in the same exact position due to a parallax shift of the airglow waves in the DNB image – in reality  waves at this part of the DNB image are located some 100 – 150 km to the east than they appear to be.


More information about Suomi-NPP satellite and its bands can be found on the following links: http://www.nasa.gov/mission_pages/NPP/main/index.html | http://npp.gsfc.nasa.gov/suomi.html | http://rammb.cira.colostate.edu/projects/npp/http://www.pnas.org/content/109/39/15706.full.pdf | http://www.mdpi.com/2072-4292/5/12/6717 

And basic information about RGB image products can be found here:

Below, on (A) we can see the DNB image, that can show the Gravity Waves appearing high in the atmosphere, but on (B) Night-Microphysical RGB image can show a different waves from low atmosphere, they are very likely part of the same spectrum of waves launched by the small island of Flores. The (C) is an output from EUMETRAIN’s ePort (http://www.eumetrain.org/eport.html), for 2015-11-17 00 UTC, based on Meteosat WV6.2 band and showing the geopotential (“height”) of the 300 hPa pressure level (in cyan) and isotachs (yellow) at the same level (both from the ECMWF model). The red arrow indicates the jet-stream above the Azores. The (D) is the WV band 27 from Terra MODIS satellite taken at 00:20 UTC. The images are in the same projection and pixel size as the Soumi-NPP images (A & B). The (E) is a Meteosat-10 satellite in the Airmass-RGB product, documenting the dynamics of the atmosphere in the area, and namely showing the jet stream above Azores, taken at 03:00 UTC, the arrow points to the jet-stream band, and the polygon indicates roughly the area covered by NPP images. To watch a 24h loop from Meteosat-10 satellite, click here.


Below is a illustration from a combination of Google Earth Maps from Azores region and Stellarium planetarium software, that can show through the yellow dot, where I was located in the island and which direction my camera was pointing to the sky – as we can see in the illustration at right.

Gravity waves (disturbances to the density structure of the atmosphere whose restoring forces are gravity and buoyancy) comprise the principal form of energy exchange between the lower and upper atmosphere. Wave breaking drives the mean upper atmospheric circulation, determining boundary conditions to stratospheric processes, which in turn influence tropospheric weather and climate patterns on various spatial and temporal scales. Despite their recognized importance, very little is known about upper-level gravity wave characteristics. The knowledge gap is mainly due to lack of global, high-resolution observations from currently available satellite observing systems. The capability of the Day/Night Band (DNB) on the NOAA/NASA Suomi National Polar-orbiting Partnership environmental satellite to resolve gravity structures near the mesopause via nightglow emissions at unprecedented subkilometric detail, are impressive. On moonless nights, the Day/Night Band observations provide all-weather viewing of waves as they modulate the nightglow layer located near the mesopause (∼90 km above mean sea level). These waves are launched by a variety of physical mechanisms, ranging from orography to convection, intensifying fronts, and even seismic and volcanic events. Cross-referencing the Day/Night Band imagery with conventional thermal infrared imagery also available helps to discern nightglow structures and in some cases to attribute their sources. The capability stands to advance our basic understanding of a critical yet poorly constrained driver of the atmospheric circulation. More about this study and scientific paper which includes the picture of my TWAN colleague Jeff Dai , led by Dr. Steven D. Miller, Dr. Steve Smith and others…

Below in gray scale are DNB images captured by Suomi-NPP VIIRS satellite above Pico island, Azores, as a result of a pre-processing enhanced by a High-Pass filter. Taken on 17th November of 2015 at 02h05AM (03h05 GMT. Local time for the Azores is GMT-1,) approximately at same time I took the pictures from Earth between 1h43 and 2h00AM, pointing to the same direction of the sky. Those waves seen at ~2AM local were right overhead at that time. The waves can change rapidly over the course of an hour, so must have been a very expansive wave train, which seems to be consistent with a large-scale forcing like a frontal system. The result shows the ripples of atmospheric Gravity Waves (left and right image).

Suomi NPP is in orbit around Earth at 834 kilometers (about 518 miles), well above the nightglow layer. The day-night band imagery contains signals from the upward emission of the nightglow layer and the reflection of the nightglow emissions from clouds and Earth’s surface. While nightglow is a well-known phenomenon, it is not typically considered by meteorological sensors. In fact, scientists were surprised at Suomi NPP’s ability to detect it. During the satellite’s check-out procedures, scientists thought this light source was a problem with the sensor until they realized that they were seeing the faintest light in the darkness of night. Learn more about the VIIRS day-night band and nighttime imaging of Earth in this story: Out of the Blue and Into the Black.

In conclusion, Dr. Steven D. Miller,  has shared his thoughts related to this picture and beyond…by saying: “is that it would be very nice to have such observations on a geostationary satellite platform so as to be able to observe the wave motion and from that information infer important properties of the energy transfer.  The community has a growing interest for such observations now that we are seeing them in “snapshot mode” from the Day/Night Band.  The hope is that the publicity gained by such nice photos as yours will help to raise awareness on our need to better characterize the full atmospheric circulation if we hope to represent climate processes accurately in the models we appeal to for insight on climate change.”

The TWAN The World at Night ) have now one of the best collection of hight quality images in the world with many different views of Airglow and Gravity Waves phenomenon captured by TWAN photographers spread around the globe, that could be a great scientific tool to help scientists understanding better this atmospheric phenomenon, as seen from different places on Earth and comparing them to the satellite images from Space.

Professional astronomer and TWAN photographer Yuri Beletsky, in an interesting answer to a question made by our colleague Anthony Ayiomamitis about how does airglow could impact the professional work done by astronomers in places with professional telescopes in sites like ESO – European Southern Observatory, saying that: “It really depends on many factors (imaging vs spectroscopy, field of view, overlapping with interesting spectroscopic lines, etc..). Therefore the answer is – it does affect the observations, but it does not necessarily affect particular scientific case you are trying to solve using those observations.”

* “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.”

Airglow photographs and compiled text with story by © Miguel Claro – The article was written based in the collected information and collaboration of each specialist and intervenient in different fields of working that have analyzed this images from Pico, and that have been specifically mentioned along the article, now published at (www.miguelclaro.com) with his knowledge and review.


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