# Airglow

(Redirected from Night glow)

Airglow over the VLT platform[1]

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 are removed.

## History

Airglow in Auvergne (France) on 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 present.

## Description

Comet Lovejoy passing behind Earth's airglow on 22 December 2011, captured from the ISS

Airglow is caused by various processes in the upper atmosphere of Earth, 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 free radicals at heights of a few hundred kilometres. It is not noticeable during the daytime due to the glare and scattering of sunlight.

Even at the best ground-based observatories, airglow limits the photosensitivity of optical telescopes. Partly for this reason, space telescopes 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, since 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),[2][3][4] atomic oxygen (O), sodium (Na), and lithium (Li).[5]

The sky brightness is typically measured in units of apparent magnitude per square arcsecond of sky.

## Calculation of the effects of airglow

The airglow above the horizon, captured from the ISS.
Airglow.
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.

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) = 4.0×10−12 W cm−2 µm−1.[6] If we take the example of a V=28 star observed through a normal V band filter (B = 0.2 μm bandpass, frequency ν ≈ 6×1014 Hz), the number of photons we receive per square centimeter of telescope aperture per second from the source is Ns:

${\displaystyle N_{s}=10^{-28/2.5}\times {\frac {S_{0}(V)\times B}{h\nu }}}$

(where h is Planck's constant; is the energy of a single photon of frequency ν).

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:

${\displaystyle N_{a}=10^{-23/2.5}\times {\frac {S_{0}(V)\times B}{h\nu }}}$

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 1.3×107 arrive from the air-glow, and approximately 1.3×105 arrive from the source; so the S/N ratio is about:

${\displaystyle {\frac {1.3\times 10^{5}}{\sqrt {1.3\times 10^{7}}}}\approx 36.}$

We can compare this with "real" answers from exposure time calculators. For an 8 m unit 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.

## Induced airglow

SwissCube-1's first airglow image of the Earth (shifted to green from near IR) captured on 3 March 2011.

Scientific experiments have been conducted to induce airglow by directing high-power radio emissions at the Earth's ionosphere.[7] These radiowaves interact with the ionosphere to induce faint but visible optical light at specific wavelengths under certain conditions.[8]

## Experimental observation

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 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.[9]

## 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.[10][11] 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.[12]

## References

1. ^ "Austrian Software Tools Developed for ESO". www.eso.org. European Southern Observatory. Retrieved 6 June 2014.
2. ^ Meinel, A. B. (1950). "OH Emission Bands in the Spectrum of the Night Sky I". Astrophysical Journal. 111: 555. Bibcode:1950ApJ...111..555M. doi:10.1086/145296.
3. ^ A. B. Meinel (1950). "OH Emission Bands in the Spectrum of the Night Sky II". Astrophysical Journal. 112: 120. Bibcode:1950ApJ...112..120M. doi:10.1086/145321.
4. ^ High, F. W.; et al. (2010). "Sky Variability in the y Band at the LSST Site". The Publications of the Astronomical Society of the Pacific. 122 (892): 722–730. arXiv:1002.3637. Bibcode:2010PASP..122..722H. doi:10.1086/653715.
5. ^ Donahue, T. M. (1959). "Origin of Sodium and Lithium in the Upper Atmosphere". Nature. 183 (4673): 1480–1481. Bibcode:1959Natur.183.1480D. doi:10.1038/1831480a0.
6. ^ High Energy Astrophysics: Particles, Photons and Their Detection Vol 1, Malcolm S. Longair, ISBN 0-521-38773-6
7. ^ 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
8. ^ NRL HAARP Overview Archived 5 March 2009 at the Wayback Machine. Naval Research Laboratory.
9. ^ SwissCube official website
10. ^ 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.
11. ^ Piccioni, G.; Zasova, L.; Migliorini, A.; Drossart, P.; Shakun, A.; García Muñoz, A.; Mills, F. P.; Cardesin-Moinelo, A. (1 May 2009). "Near-IR oxygen nightglow observed by VIRTIS in the Venus upper atmosphere". Journal of Geophysical Research: Planets. 114 (E5): E00B38. Bibcode:2009JGRE..114.0B38P. doi:10.1029/2008je003133. ISSN 2156-2202.
12. ^ Wilson, Elizabeth (2009). "PLANETARY SCIENCE Spectral band in Venus' 'nightglow' allows study of NO, O". Chemical & Engineering News. 87 (4): 11. doi:10.1021/cen-v087n004.p011a. ISSN 0009-2347.
13. ^ "La Silla's Great Dane". www.eso.org. Retrieved 26 March 2018.
14. ^ "Anything But Black". www.eso.org. Retrieved 20 September 2016.