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Airglow (also called nightglow) is a faint emission of light by a planetary
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
. In the case of
Earth's atmosphere The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
, this
optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the sun or moon wit ...
causes the
night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include ...
never to be completely dark, even after the effects of starlight and diffused sunlight from the far side are removed. This phenomenon originates with self-illuminated gases and has no relationship with
Earth's magnetism Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
or sunspot activity.


History

The airglow phenomenon was first identified in 1868 by Swedish physicist
Anders Ångström Anders is a male name in Scandinavian languages and Fering North Frisian, an equivalent of the Greek Andreas ("manly") and the English Andrew. It originated from Andres via metathesis. In Sweden, Anders has been one of the most common names fo ...
. Since then, it has been studied in the laboratory, and various chemical reactions have been observed to emit electromagnetic energy as part of the process. Scientists have identified some of those processes that would be present in Earth's atmosphere, and astronomers have verified that such emissions are present.
Simon Newcomb Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian–American astronomer, applied mathematician, and autodidactic polymath. He served as Professor of Mathematics in the United States Navy and at Johns Hopkins University. Born in N ...
was the first person to scientifically study and describe airglow, in 1901 Airglow existed in pre-industrial society and was known to the ancient Greeks. "Aristotle and Pliny described the phenomena of ''Chasmata'', which can be identified in part as auroras, and in part as bright airglow nights."Google Books: SCIENCES OF THE EARTH, An Encyclopedia of Events, People, and Phenomena, 1998, Garland Publishing, PAGE 3
Google Books
access date 25 June 2022.


Description

Airglow is caused by various processes in the upper
atmosphere of Earth The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
, such as the recombination of atoms which were photoionized by the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
during the day, luminescence caused by
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s striking the upper atmosphere, and chemiluminescence caused mainly by
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
reacting with
hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...
free radicals at heights of a few hundred kilometres. It is not noticeable during the daytime due to the glare and scattering of
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
. Even at the best ground-based observatories, airglow limits the
photosensitivity Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicit ...
of
optical telescope An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through elect ...
s. Partly for this reason,
space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launch ...
s like Hubble can observe much fainter objects than current ground-based telescopes at visible wavelengths. Airglow at night may be bright enough for a ground observer to notice and appears generally bluish. Although airglow emission is fairly uniform across the atmosphere, it appears brightest at about 10° above the observer's
horizon The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
, since the lower one looks, the greater the mass of atmosphere one is looking through. Very low down, however, atmospheric
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
reduces the apparent brightness of the airglow. One airglow mechanism is when an atom of
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
combines with an atom of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
to form a molecule of
nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
(NO). In the process, a
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
is emitted. This photon may have any of several different wavelengths characteristic of nitric oxide molecules. The free atoms are available for this process, because molecules of nitrogen (N2) and oxygen (O2) are dissociated by solar energy in the upper reaches of the atmosphere and may encounter each other to form NO. Other chemicals that can create air glow in the atmosphere are hydroxyl (OH), atomic oxygen (O), sodium (Na), and lithium (Li). The sky brightness is typically measured in units of
apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
per square arcsecond of sky.


Calculation

In order to calculate the relative intensity of airglow, we need to convert apparent magnitudes into fluxes of photons; this clearly depends on the spectrum of the source, but we will ignore that initially. At visible wavelengths, we need the parameter S0(V), the power per square centimetre of aperture and per micrometre of wavelength produced by a zeroth-magnitude star, to convert apparent magnitudes into fluxes – . If we take the example of a ''V''=28 star observed through a normal ''V'' band filter ( bandpass, frequency ), the number of photons we receive per square centimeter of telescope aperture per second from the source is ''N''''s'': :N_s = 10^\times\frac (where ''h'' is Planck's constant; ''hν'' is the energy of a single photon of frequency ''ν''). At ''V'' band, the emission from airglow is per square arc-second at a high-altitude observatory on a moonless night; in excellent seeing conditions, the image of a star will be about 0.7 arc-second across with an area of 0.4 square arc-second, and so the emission from airglow over the area of the image corresponds to about . This gives the number of photons from airglow, ''N''''a'': :N_a = 10^\times\frac The signal-to-noise for an ideal ground-based observation with a telescope of area ''A'' (ignoring losses and detector noise), arising from Poisson statistics, is only: :S/N = \sqrt\times\frac If we assume a 10 m diameter ideal ground-based telescope and an unresolved star: every second, over a patch the size of the seeing-enlarged image of the star, 35 photons arrive from the star and 3500 from air-glow. So, over an hour, roughly arrive from the air-glow, and approximately arrive from the source; so the S/N ratio is about: :\frac \approx 36. We can compare this with "real" answers from exposure time calculators. For an 8 m unit
Very Large Telescope The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m acro ...
telescope, according to th
FORS
exposure time calculator, 40 hours of observing time are needed to reach ''V'' = 28, while the 2.4 m Hubble only takes 4 hours according to th
ACS
exposure time calculator. A hypothetical 8 m Hubble telescope would take about 30 minutes. It should be clear from this calculation that reducing the view field size can make fainter objects more detectable against the airglow; unfortunately,
adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
techniques that reduce the diameter of the view field of an Earth-based telescope by an order of magnitude only as yet work in the infrared, where the sky is much brighter. A space telescope isn't restricted by the view field, since it is not affected by airglow.


Induced airglow

Scientific experiments have been conducted to induce airglow by directing high-power radio emissions at the Earth's
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
.
Vol. 32, L23106, , 2005 These radiowaves interact with the ionosphere to induce faint but visible optical light at specific wavelengths under certain conditions.SwissCube-1 image came down on 18 February 2011 and was quite black with some thermal noise on it. The first airglow image came down on 3 March 2011. This image has been converted to the human optical range (green) from its near-infrared measurement. This image provides a measurement of the intensity of the airglow phenomenon in the
near-infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
. The range measured is from 500 to 61400
photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
, with a resolution of 500 photons. spacecraft contains an
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
sensor which has detected near-IR emissions from the upper atmosphere of
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
. The emissions come from
nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
(NO) and from molecular oxygen. Scientists had previously determined in laboratory testing that during NO production,
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
emissions and near-IR emissions were produced. The UV radiation had been detected in the atmosphere, but until this mission, the atmosphere-produced near-IR emissions were only theoretical.


Gallery

La_Silla’s_Great_Dane.jpg, Hues of red and green lighting up the sky are produced by airglow. Anything But Black.jpg, Airglow over
Paranal Observatory Paranal Observatory is an astronomical observatory operated by the European Southern Observatory (ESO). It is located in the Atacama Desert of northern Chile on Cerro Paranal at altitude, south of Antofagasta. By total light-collecting area, it ...
. Airglow in Allier (FRANCE) during the night at the 13th August of 2015.jpg, Airglow in Auvergne (France) on 13 August 2015.


See also

*
Earthlight ''Earthlight'' is a science fiction novel by British writer Arthur C. Clarke, published in 1955. It is an expansion to novel length of a novella of the same name that he had published four years earlier. Overview ''Earthlight'' is a scie ...
*
Ionized air glow Ionized-air glow is the luminescent emission of characteristic blue–purple–violet light, often of a color called Electric blue (color), electric blue, by air subjected to an energy flux either directly or indirectly from solar radiation. ...
*
Optical phenomena Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the sun or moon with ...
*
Polar aurora An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
*
Zodiacal light The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction ...


References


External links


Description and Images
for
Roque de los Muchachos Observatory Roque de los Muchachos Observatory ( es, Observatorio del Roque de los Muchachos, ORM) is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands, Spain. The observatory site is operated ...

''Night-side Glow Detected at Mars'' Space.com interview
*[http://www.iop.org/EJ/abstract/0957-0233/8/4/016 ''An improved signal-to-noise ratio of a cool imaging photon detector for Fabry - Perot interferometer measurements of low-intensity air glow'' by T P Davies and P L Dyson]
Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 13SwissCube, The first Swiss Satellite
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