View source for Airglow

{{short description|Faint emission of light by a planetary atmosphere}}
{{redirect|Night glow|the hot air balloon event|Balloon glow|luminance of the night sky caused by artificial light sources|skyglow}}
{{Use dmy dates|date=November 2019}}
[[File:Airglow in Auvergne (France) on 13th of August 2015.jpg|thumb|Airglow over [[Auvergne, France]]]]
[[File:ISS-42 Starry Night (2).jpg|thumb|Yellow, green and red bands of airglow along Earth's limb as seen from space.]]

'''Airglow''' (also called '''nightglow''') is a faint emission of light by a planetary [[atmosphere]]. In the case of [[Earth's atmosphere]], this [[optical phenomenon]] causes the [[night sky]] never to be completely dark, even after the effects of [[starlight]] and [[diffuse sky radiation|diffused sunlight]] from the far side are removed. This phenomenon originates with self-illuminated gases and has no relationship with [[Earth's magnetic field|Earth's magnetism]] or [[sunspot]] activity.

== History ==
The airglow phenomenon was first identified in 1868 by Swedish physicist [[Anders Ångström]]. 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]] was the first person to scientifically study and describe airglow, in 1901.<ref>M. G. J. Minnaert, ''De natuurkunde van 't vrije veld'', Deel 2: ''Geluid, warmte, elektriciteit''. § 248: ''Het ionosfeerlicht''</ref>

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."<ref name='SCIENCES OF THE EARTH'>''Sciences of the Earth, An Encyclopedia of Events, People, and Phenomena'', 1998, Garland Publishing,  p. 35, [https://books.google.com/books?id=vdqXVddh0hUC&dq=Simon+Newcomb+airglow&pg=PA35 via Google Books], access date 25 June 2022.</ref>

== Description ==
{{further|Sodium layer}}
[[File:Ionosphere and its constituents.jpg|thumb|upright=1.5|Types and layering of airglow above Earth]]

Airglow is caused by various processes in the upper [[atmosphere of Earth]], such as the recombination of atoms which were [[photoionization|photoionized]] by the [[Sun]] during the day, luminescence caused by [[cosmic ray]]s striking the upper atmosphere, and [[chemiluminescence]] caused mainly by [[oxygen]] and [[nitrogen]] reacting with [[hydroxyl]] free radicals at heights of a few hundred kilometres. It is not noticeable during the daytime due to the [[glare (vision)|glare]] and [[diffuse sky radiation|scattering]] of [[sunlight]].

Even at the best ground-based observatories, airglow limits the [[photosensitivity]] of [[optical telescope]]s. Partly for this reason, [[space telescope]]s like [[Hubble Space Telescope|Hubble]] can observe much fainter objects than current ground-based telescopes at [[visible spectrum|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]], since the lower one looks, the greater the [[air mass (astronomy)|mass of atmosphere]] one is looking through. Very low down, however, atmospheric [[extinction (astronomy)|extinction]] reduces the apparent brightness of the airglow.

One airglow mechanism is when an atom of [[nitrogen]] combines with an atom of [[oxygen]] to form a molecule of [[nitric oxide]] (NO). In the process, a [[photon]] 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 (N<sub>2</sub>) and oxygen (O<sub>2</sub>) 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),<ref name=meinel50a>
{{cite journal
 |first= A. B.
 |last= Meinel
 |title= OH Emission Bands in the Spectrum of the Night Sky I
 |journal= Astrophysical Journal
 |date= 1950
 |doi= 10.1086/145296
 |volume= 111
 |page= 555
 |bibcode= 1950ApJ...111..555M
}}</ref><ref name=meinel50b>
{{cite journal
 |author= A. B. Meinel
 |title= OH Emission Bands in the Spectrum of the Night Sky II
 |journal= Astrophysical Journal
 |date= 1950
 |doi= 10.1086/145321
 |volume= 112
 |page= 120
 |bibcode= 1950ApJ...112..120M
 |doi-access= free
}}</ref><ref name=high10>
{{cite journal
 |first= F. W.
 |last= High
 |title= Sky Variability in the y Band at the LSST Site
 |journal= The Publications of the Astronomical Society of the Pacific
 |date= 2010
 |doi= 10.1086/653715
 |volume= 122
 |issue= 892
 |pages= 722–730
 |bibcode= 2010PASP..122..722H
 |arxiv= 1002.3637
 |s2cid= 53638322
 |display-authors= etal
}}</ref> atomic oxygen (O), sodium (Na), and lithium (Li).<ref>{{cite journal |title=Origin of Sodium and Lithium in the Upper Atmosphere |journal=[[Nature (journal)|Nature]]|volume=183 |issue=4673 |pages=1480–1481 |doi=10.1038/1831480a0 |year=1959 |last1=Donahue |first1=T. M. |bibcode=1959Natur.183.1480D |s2cid=4276462}}</ref>

The [[sky brightness]] is typically measured in units of [[apparent magnitude]] per square [[minute and second of arc|arcsecond]] of sky.

== Calculation ==
{{see also|Apparent magnitude}}
[[File:Antarctic aurora ESA313457.jpg|thumb|upright=1.5|Airglow as pinkish orange sodium line at just below one hundred kilometers and a faint green line, at the [[Outer space#Boundary|edge of space]] and the lower edge of the [[thermosphere]] (invisible),  sandwiched between green and red bands of [[aurora]]e stretching over several hundred kilometers upward and the pink [[mesosphere]], white and blue [[stratosphere]], as well as orange [[troposphere]] [[afterglow]] and silhouettes of clouds at the bottom.]]
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 ''S''<sub>0</sub>(''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 – {{nowrap|''S''<sub>0</sub>(''V'') {{=}} {{val|4.0|e=-12|u=W⋅cm<sup>−2</sup>⋅µm<sup>−1</sup>}}}}.<ref>''High Energy Astrophysics: Particles, Photons and Their Detection'' Vol 1, Malcolm S. Longair, {{ISBN|0-521-38773-6}}</ref> If we take the example of a {{nowrap|1=''V'' = 28}} star observed through a normal ''V'' band filter ({{nowrap|''B'' {{=}} {{val|0.2|u=µm}}}} bandpass, frequency {{nowrap|ν ≈ {{val|6|e=14|u=Hz}}}}), the number of photons we receive per square centimeter of telescope aperture per second from the source is ''N''<sub>s</sub>:
: <math>N_\text{s} = 10^{-28/2.5}\times\frac{S_0(V) \times B}{h\nu}</math>
(where ''h'' is the [[Planck constant]]; ''hν'' is the energy of a single photon of frequency ''ν'').

At ''V'' band, the emission from airglow is {{nowrap|''V'' {{=}} 22}} per square arc-second at a high-altitude observatory on a moonless night; in excellent [[Astronomical seeing|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 {{nowrap|''V'' {{=}} 23}}. This gives the number of photons from airglow, ''N''<sub>a</sub>:
: <math>N_\text{a} = 10^{-23/2.5}\times\frac{S_0(V) \times B}{h\nu}</math>

The signal-to-noise for an ideal ground-based observation with a telescope of area ''A'' (ignoring losses and detector noise), arising from [[Poisson distribution|Poisson]] statistics, is only:
: <math>S/N = \sqrt{A}\times\frac{N_\text{s}}{\sqrt{N_\text{s}+N_\text{a}}}</math>

If we assume a 10&nbsp;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 {{val|1.3|e=7}}<!-- this value was 1.3e7±3500 photons, but the ±3500 makes no sense as 1.3e7 implies an uncertainty of ±5e5, which is far greater--> arrive from the air-glow, and approximately {{val|1.3|e=5}} arrive from the source; so the ''S''/''N'' ratio is about:
: <math>\frac{1.3 \times 10^5}{\sqrt{1.3 \times 10^7}} \approx 36.</math>

We can compare this with "real" answers from exposure time calculators. For an 8 m unit [[Very Large Telescope]] telescope, according to the [http://www.eso.org/observing/etc/bin/gen/form?INS.NAME=FORS1++INS.MODE=imaging FORS] exposure time calculator, 40 hours of observing time are needed to reach {{nowrap|1=''V'' = 28}}, while the 2.4&nbsp;m Hubble only takes 4 hours according to the [https://archive.today/20050209215954/http://apt.stsci.edu/webetc/acs/acs_img_etc.jsp ACS] exposure time calculator. A hypothetical 8 m Hubble telescope would take about 30 minutes.

This calculation shows that reducing the view field size can make fainter objects more detectable against the airglow; unfortunately, [[adaptive optics]] 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]] is not restricted by the view field, since it is not affected by airglow.

== Induced airglow ==
[[File:Induced Airglow (HAARP).gif|thumb|upright=1.5|Two images of the sky over the [[HAARP]] [[Gakona]] facility using the NRL-cooled [[CCD imager]] at 557.7 nm. The field of view is approximately 38°. The left-hand image shows the background star field with the HF transmitter off. The right-hand image was taken 63 seconds later with the HF transmitter on. Structure is evident in the emission region.]]

Scientific experiments have been conducted to induce airglow by directing high-power radio emissions at the Earth's [[ionosphere]].<ref name='agu2005'>[https://web.archive.org/web/20090726023615/http://www.agu.org/pubs/crossref/2005/2005GL023864.shtml HF-induced airglow at magnetic zenith: Thermal and parametric instabilities near electron gyroharmonics].  E.V. Mishin et al., [[Geophysical Research Letters]] Vol. 32, L23106, {{doi|10.1029/2005GL023864}}, 2005</ref> These radiowaves interact with the ionosphere to induce faint but visible optical light at specific wavelengths under certain conditions.<ref name='nrlHAARP'>[http://www.nrl.navy.mil/content.php?P=04REVIEW106 NRL HAARP Overview] {{webarchive|url=https://web.archive.org/web/20090305223823/http://www.nrl.navy.mil/content.php?P=04REVIEW106 |date=5 March 2009}}.  [[Naval Research Laboratory]].</ref> The effect is also observable in the radio frequency band, using [[ionosonde]]s.

== Experimental observation ==
[[SwissCube-1]] is a [[Switzerland|Swiss]] satellite operated by [[Ecole Polytechnique Fédérale de Lausanne]]. The spacecraft is a single unit [[CubeSat]], which was designed to conduct research into airglow within the Earth's atmosphere and to develop technology for future spacecraft. Though SwissCube-1 is rather small (10&nbsp;cm × 10&nbsp;cm × 10&nbsp;cm) and weighs less than 1&nbsp;kg, it carries a small telescope for obtaining images of the airglow. The first [[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]]. The range measured is from 500 to 61400 [[photons]], with a resolution of 500 photons.<ref name='EPFL_Sopace_Center_airglow'>[http://swisscube.epfl.ch/ SwissCube official website]</ref>

== Observation of airglow on other planets ==
The ''[[Venus Express]]'' spacecraft contains an [[infrared]] sensor which has detected near-IR emissions from the upper atmosphere of [[Venus]]. The emissions come from [[nitric oxide]] (NO) and from molecular oxygen.<ref name="Garcia MunozMills2009">{{cite journal|last1=Garcia Munoz|first1=A.|last2=Mills|first2=F. P.|last3=Piccioni|first3=G.|last4=Drossart|first4=P.|title=The near-infrared nitric oxide nightglow in the upper atmosphere of Venus|journal=Proceedings of the National Academy of Sciences|volume=106|issue=4|date=2009|pages=985–988|issn=0027-8424|doi=10.1073/pnas.0808091106|bibcode=2009PNAS..106..985G|pmid=19164595|pmc=2633570|doi-access=free}}</ref><ref>{{cite journal|first1=G.|last1=Piccioni|first2=L.|last2=Zasova|first3=A.|last3=Migliorini|first4=P.|last4=Drossart|title=Near-IR oxygen nightglow observed by VIRTIS in the Venus upper atmosphere|journal=Journal of Geophysical Research: Planets|date=1 May 2009|issn=2156-2202|pages=E00B38|volume=114|issue=E5|doi=10.1029/2008je003133|first5=A.|last5=Shakun|first6=A.|last6=García Muñoz|first7=F. P.|last7=Mills|first8=A.|last8=Cardesin-Moinelo|bibcode=2009JGRE..114.0B38P|url=https://zenodo.org/record/1064109|doi-access=free}}</ref> Scientists had previously determined in laboratory testing that during NO production, [[ultraviolet]] 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.<ref name="Wilson2009">{{cite journal|last1=Wilson|first1=Elizabeth|title=Planetary Science – Spectral band in Venus' 'nightglow' allows study of NO, O|journal=Chemical & Engineering News|volume=87|issue=4|date=2009|page=11|issn=0009-2347|doi=10.1021/cen-v087n004.p011a}}</ref>

== Gallery ==
<gallery widths="200px" heights="150px">
Airglow over La Silla’s Great Dane.jpg|Hues of red and green lighting up the sky are produced by airglow.<ref>{{cite web|title=La Silla's Great Dane|url=http://www.eso.org/public/images/potw1813a/|website=www.eso.org|access-date=26 March 2018}}</ref>
Airglow over Paranal Observatory, Chile.jpg|Airglow over [[Paranal Observatory]].<ref>{{cite web|title=Anything But Black|url=http://www.eso.org/public/images/potw1638a/|website=www.eso.org|access-date=20 September 2016}}</ref>
Panoramic shot of the VLT platform.jpg|Airglow over the [[Very Large Telescope|VLT]] platform<ref>{{cite web|title=Austrian Software Tools Developed for ESO|url=http://www.eso.org/public/announcements/ann14041/|website=www.eso.org|publisher=European Southern Observatory|access-date=6 June 2014}}</ref>
Airglow in France (01-21-2023).jpg|Airglow over Dordogne, France.
Tumblr inline ph0ungGXF31tzhl5u 500-1.gif|Airglow timelapse from space, with a broad red band of airglow.
</gallery>

== See also ==
* [[Earthlight (astronomy)|Earthlight]]
* [[Ionized-air glow]]
* [[Optical phenomena]]
* [[Polar aurora]]
* [[Zodiacal light]]

== References ==
{{reflist}}

== External links ==
{{Commons category|Airglow}}
* [http://www.atoptics.co.uk/highsky/airglow1.htm Description and Images]
* [http://www.not.iac.es/weather/skybrightness.html Sky Brightness Information] for [[Roque de los Muchachos Observatory]]
* [http://www.space.com/scienceastronomy/mars_glow_050131.html ''Night-side Glow Detected at Mars'' Space.com interview]
* [https://web.archive.org/web/20060901225612/http://www.hipas.alaska.edu/hipasweb/air_glow.htm ''Stereoscopic Observations of HAARP Glows from HIPAS, Poker Flat, and Nenana, Alaska'' by R.F. Wuerker ''et al.'']
* [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]
* [http://www.stsci.edu/hst/stis/performance/background/documents/handbooks/currentIHB/c06_exptime6.html Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 13]
* [http://swisscube.epfl.ch/ SwissCube| The first Swiss Satellite]

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