Airglow
Encyclopedia
Airglow is the very weak 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 are removed).
during the day, luminescence caused by cosmic ray
s striking the upper atmosphere, and chemiluminescence caused mainly by oxygen
and nitrogen
reacting with hydroxyl
ions at heights of a few hundred kilometers. 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
can observe much fainter objects than current ground-based telescopes at visible wavelengths.
The airglow at night may be bright enough to be noticed by an observer, and is generally bluish in color. 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)
, molecular 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.
W cm−2 µm−1. If we take the example of a V=28 star observed through a normal V band filter (
µm bandpass, frequency 
Hz), the number of photons we receive per square cm of telescope aperture per second from the source is 
:
(where
is Planck's constant; 
is the energy of a single photon of frequency 
).
At V band, the emission from airglow is V = 22 per square arcsecond at a high-altitude observatory on a moonless night; in excellent seeing
conditions, the image of a star will be about 0.7 arc-seconds across with an area of 0.4 square arc-seconds, 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,
:
The signal-to-noise for an ideal groundbased observation with a telescope of area
(ignoring losses and detector noise), arising from Poisson
statistics, is only:
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
photons arrive from the air-glow, and approximately 
arrive from the source; so the S/N ratio is about 35.
We can compare this with "real" answers from exposure time calculators. For an 8 m VLT 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. Space telescopes don't have to worry about the view field restrictions, since they are not impacted by airglow.
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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 conditions.
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 nightglow within the Earth's atmosphere, and to develop technology for future spacecraft. Though SwissCube-1 is rather small (10x10x10 cm) and weights less than 1 Kg, it carries a small telescope which will allow to obtain images of the nightglow, a luminescence phenomena occurring at 100 km of height above the Earth surface. 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 phenomena in the near-infrared. The range measured is between 500 – 61400 photons, with a resolution of 500 photons.
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.
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
by a planetary atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
. In the case of Earth's atmosphere, this optical phenomenon
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
causes the night sky
Night sky
The term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
to never be completely dark (even after the effects of starlight
Starlight
Starlight refers to the visible radiation emitted by stars other than the Sun.Starlight may also refer to:-Comics:* Starlight , Dr...
and diffused
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
from the far side are removed).
Development
The airglow phenomenon was first identified in 1868 by Swedish scientist 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 which would be present in Earth's atmosphere, and astronomers have verified that such emissions are present.Description
Airglow is caused by various processes in the upper atmosphere, such as the recombination of ions which were photoionized by the sunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
during the day, luminescence caused by cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s striking the upper atmosphere, and chemiluminescence caused mainly by oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
reacting with hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
ions at heights of a few hundred kilometers. It is not noticeable during the daytime because of the scattered light from the Sun
Diffuse sky radiation
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky...
.
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
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
can observe much fainter objects than current ground-based telescopes at visible wavelengths.
The airglow at night may be bright enough to be noticed by an observer, and is generally bluish in color. 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
Extinction (astronomy)
Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...
reduces the apparent brightness of the airglow.
One airglow mechanism is when an atom of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
combines with an atom of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to form a molecule of nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
(NO). In the process a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
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)
, molecular oxygen (O), sodium (Na) and lithium (Li). See Sodium layer
Sodium layer
Refers to a layer within the Earth's mesosphere of unbound, non-ionized atoms of sodium. The altitude of this layer is usually located between 80–105 km and has a depth of about . The sodium comes from the ablation of meteors...
.
The sky brightness is typically quoted in units of astronomical magnitudes per square arcsecond of sky.
How to calculate the effects of airglow
We need first 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 -- W cm−2 µm−1. If we take the example of a V=28 star observed through a normal V band filter ( µm bandpass, frequency Hz), the number of photons we receive per square cm of telescope aperture per second from the source is :(where
is Planck's constant; is the energy of a single photon of frequency ).At V band, the emission from airglow is V = 22 per square arcsecond at a high-altitude observatory on a moonless night; in excellent seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
conditions, the image of a star will be about 0.7 arc-seconds across with an area of 0.4 square arc-seconds, 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,
:The signal-to-noise for an ideal groundbased observation with a telescope of area
(ignoring losses and detector noise), arising from PoissonPoisson distribution
In probability theory and statistics, the Poisson distribution is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time and/or space if these events occur with a known average rate and independently of the time since...
statistics, is only:
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
photons arrive from the air-glow, and approximately arrive from the source; so the S/N ratio is about 35.We can compare this with "real" answers from exposure time calculators. For an 8 m VLT 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
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
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. Space telescopes don't have to worry about the view field restrictions, since they are not impacted by airglow.
Induced airglow
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. These radiowaves interact with the ionosphere to induce faint but visible optical light at specific wavelengths under certain conditions.
Experimental observation
SwissCube-1SwissCube-1
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 nightglow within the Earth's atmosphere, and to develop technology for future spacecraft. It has also been used for amateur radio...
is a Swiss satellite operated by Ecole Polytechnique Fédérale de Lausanne
École polytechnique fédérale de Lausanne
The École polytechnique fédérale de Lausanne is one of the two Swiss Federal Institutes of Technology and is located in Lausanne, Switzerland.The school was founded by the Swiss Federal Government with the stated mission to:...
. The spacecraft is a single unit CubeSat
CubeSat
A CubeSat is a type of miniaturized satellite for space research that usually has a volume of exactly one liter , has a mass of no more than 1.33 kilograms, and typically uses commercial off-the-shelf electronics components...
, which was designed to conduct research into nightglow within the Earth's atmosphere, and to develop technology for future spacecraft. Though SwissCube-1 is rather small (10x10x10 cm) and weights less than 1 Kg, it carries a small telescope which will allow to obtain images of the nightglow, a luminescence phenomena occurring at 100 km of height above the Earth surface. The first SwissCube-1
SwissCube-1
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 nightglow within the Earth's atmosphere, and to develop technology for future spacecraft. It has also been used for amateur radio...
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 phenomena in the near-infrared. The range measured is between 500 – 61400 photons, with a resolution of 500 photons.
Observation of airglow on other Solar System planets
The Venus ExpressVenus Express
Venus Express is the first Venus exploration mission of the European Space Agency. Launched in November 2005, it arrived at Venus in April 2006 and has been continuously sending back science data from its polar orbit around Venus. Equipped with seven science instruments, the main objective of the...
spacecraft contains an infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
sensor which has detected near-IR emissions from the upper atmosphere of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
. The emissions come from nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
(NO) and from molecular oxygen. Scientists had previously determined in laboratory testing that during NO production, ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
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.
External links
- Description and Images
- Sky Brightness Information for Roque de los Muchachos ObservatoryRoque de los Muchachos ObservatoryRoque de los Muchachos Observatory is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands...
- 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 Swiss Satellite