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 to never be completely dark, even
after the effects of starlight and diffused sunlight from the far side
3 Calculation of the effects of airglow
4 Induced airglow
5 Experimental observation
6 Observation of airglow on other Solar System planets
8 See also
10 External links
Airglow in Allier, France, on the night of 13 August 2015
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
Comet Lovejoy passing behind Earth's airglow on December 22, 2011.
Airglow is caused by various processes in the upper atmosphere, such
as the recombination of atoms which were photoionized by the sun
during the day, luminescence caused by cosmic rays striking the upper
atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen
reacting with hydroxyl ions at heights of a few hundred kilometres. It
is not noticeable during the daytime because of the scattered light
from the sun.
Even at the best ground-based observatories, airglow limits the
sensitivity of telescopes at visible wavelengths. Partly for this
reason, space-based telescopes such as the
Hubble Space Telescope
Hubble Space Telescope can
observe much fainter objects than current ground-based telescopes at
The airglow at night may be bright enough to be noticed by an observer
and is generally bluish in colour. Although airglow emission is fairly
uniform across the atmosphere, to an observer on the ground it appears
brightest at about 10 degrees above the horizon, because the lower one
looks, the greater the depth of atmosphere one is looking through.
Very low down, however, atmospheric 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 (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 species that can create air glow in the atmosphere are
hydroxyl (OH), atomic oxygen (O), sodium (Na) and lithium
(Li). See Sodium layer.
The sky brightness is typically quoted in units of astronomical
magnitudes per square arcsecond of sky.
Calculation of the effects of airglow
The airglow above the horizon, captured from the ISS.
Two images of the sky over the
Gakona facility using the
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.
See also: Apparent magnitude
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 — S0(V) =
6988400000000000000♠4.0×10−12 W cm−2 µm−1. If we
take the example of a V=28 star observed through a normal V band
filter (B = 6993200000000000000♠0.2 μm bandpass, frequency ν
≈ 7014600000000000000♠6×1014 Hz), the number of photons we
receive per square centimeter of telescope aperture per second from
the source is Ns:
displaystyle N_ s =10^ -28/2.5 times frac S_ 0 (V)times B hnu
(where h is Planck's constant; hν is the energy of a single photon of
At V band, the emission from airglow is V = 22 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 V = 23. This
gives the number of photons from airglow, Na:
displaystyle N_ a =10^ -23/2.5 times frac S_ 0 (V)times B hnu
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:
displaystyle S/N= sqrt A times frac N_ s sqrt N_ s +N_ a
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
7007130000000000000♠1.3×107 arrive from the air-glow, and
approximately 7005130000000000000♠1.3×105 arrive from the source;
so the S/N ratio is about:
displaystyle frac 1.3times 10^ 5 sqrt 1.3times 10^ 7
We can compare this with "real" answers from exposure time
calculators. For an 8 m unit
Very Large Telescope
Very Large Telescope telescope, according
to the FORS exposure time calculator you need 40 hours of observing
time to reach V = 28, while the 2.4 m Hubble only takes 4 hours
according to the 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 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 they are not
affected by airglow.
SwissCube-1's first airglow image of the Earth (shifted to green from
near IR) captured on March 3, 2011.
Scientific experiments have been conducted to induce airglow by
directing high-power radio emissions at the Earth's ionosphere.
These radiowaves interact with the ionosphere to induce faint but
visible optical light at specific wavelengths under certain
SwissCube-1 is a 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 x 10 x 10 cm) and weighs less
than 1 kg, it carries a small telescope for obtaining images of
the airglow. The first
SwissCube-1 image came down on February 18,
2011 and was quite black with some thermal noise on it. The first
airglow image came down on March 3, 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.
Observation of airglow on other Solar System planets
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. Scientists had previously determined in laboratory
testing that during NO production, ultraviolet emissions and near-IR
emissions were produced. The UV radiation has been detected in the
atmosphere, but until this mission, the atmosphere-produced near-IR
emissions were only theoretical.
Hues of red and green light up the sky are produced by airglow.
Airglow over Paranal Observatory.
Wikimedia Commons has media related to Airglow.
Ionized air glow
^ "Austrian Software Tools Developed for ESO". www.eso.org. European
Southern Observatory. Retrieved 6 June 2014.
^ A. B. Meinel (1950). "OH Emission Bands in the Spectrum of the Night
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^ A. B. Meinel (1950). "OH Emission Bands in the Spectrum of the Night
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^ F. W. High; et al. (2010). "Sky Variability in the y Band at the
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^ Origin of Sodium and Lithium in the Upper Atmosphere
^ High Energy Astrophysics: Particles,
Photons and Their Detection Vol
1, Malcolm S. Longair, ISBN 0-521-38773-6
^ HF-induced airglow at magnetic zenith: Thermal and parametric
instabilities near electron gyroharmonics. E.V. Mishin et al.,
Geophysical Research Letters
Geophysical Research Letters Vol. 32, L23106,
HAARP Overview Archived 5 March 2009 at the Wayback Machine..
Naval Research Laboratory.
^ SwissCube official website
^ Garcia Munoz, A.; Mills, F. P.; Piccioni, G.; Drossart, P. (2009).
"The near-infrared nitric oxide nightglow in the upper atmosphere of
Venus". Proceedings of the National Academy of Sciences. 106 (4):
985–988. Bibcode:2009PNAS..106..985G. doi:10.1073/pnas.0808091106.
ISSN 0027-8424. PMC 2633570 . PMID 19164595.
^ Piccioni, G.; Zasova, L.; Migliorini, A.; Drossart, P.; Shakun, A.;
García Muñoz, A.; Mills, F. P.; Cardesin-Moinelo, A. (2009-05-01).
"Near-IR oxygen nightglow observed by VIRTIS in the
atmosphere". Journal of Geophysical Research: Planets. 114 (E5):
E00B38. Bibcode:2009JGRE..114.0B38P. doi:10.1029/2008je003133.
^ Wilson, Elizabeth (2009). "PLANETARY SCIENCE Spectral band in Venus'
'nightglow' allows study of NO, O". Chemical & Engineering News.
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^ "La Silla's Great Dane". www.eso.org. Retrieved 26 March 2018.
^ "Anything But Black". www.eso.org. Retrieved 20 September
Description and Images
Sky Brightness Information for Roque de los Muchachos Observatory
Night-side Glow Detected at Mars Space.com interview
Stereoscopic Observations of
HAARP Glows from HIPAS, Poker Flat, and
Nenana, Alaska by R.F. Wuerker et al.
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 13
SwissCube The first Swi