A quarter moon is visible in this oblique view of Earth's horizon and airglow.

The airglow is the very weak emission of light by the Earth's atmosphere; as a result, the night sky is never completely dark. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five It was first noticed in 1868 by Anders Ångström. Anders Jonas Ångström ( August 13, 1814 &ndash June 21, 1874) was a Physicist in Sweden, one It is caused by various processes in the upper atmosphere, such as the recombination of ions which were photoionised 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. Photoionisation is the physical process in which an incident Photon ejects one or more Electrons from an Atom, Ion or Molecule The Sun (Sol is the Star at the center of the Solar System. For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on Chemiluminescence (sometimes " chemoluminescence " is the emission of Light ( Luminescence) with limited emission of heat as the result of a chemical Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Hydroxyl in Chemistry stands for a molecule consisting of an Oxygen atom and a Hydrogen atom connected by a Covalent bond. It is not noticeable during the daytime because of the scattered light from the Sun. Diffuse sky radiation is Solar radiation reaching the Earth 's surface after having been scattered from the direct solar beam by Molecules

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 Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into

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. Extinction is a term used in Astronomy to describe the absorption and Scattering of Electromagnetic radiation emitted by Astronomical objects

One mechanism that produces airglow occurs when an atom of nitrogen combines with an atom of oxygen to form a molecule of nitric oxide (NO). Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Nitric oxide or nitrogen monoxide is a Chemical compound with Chemical formula N[[Oxygen O]] In the process a photon is emitted. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena 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 OH, OI and NaI.

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. ^[1] If we take the example of a V=28 star observed through a normal V band filter (B = 0. 2 µm bandpass, frequency Hz), the number of photons we receive per square meter of telescope aperture per second from the source is N_s:

(where h is Planck's constant; hν is the energy of a single photon of frequency ν). The Planck constant (denoted h\ is a Physical constant used to describe the sizes of quanta.

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. Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by Turbulence in the Earth's atmosphere 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, N_a:

The signal-to-noise for an ideal groundbased observation with a telescope of area A (ignoring losses and detector noise), arising from Poisson statistics, is just:

Now we can do a quick calculation for 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. In Probability theory and Statistics, the Poisson distribution is a Discrete probability distribution that expresses the probability of a number of events 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 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 would take nearer 30 minutes.

It should be clear from this calculation that reducing the size of the seeing disc can make much fainter objects detectable against the air-glow; unfortunately, adaptive optics techniques that reduce the diameter of the seeing disc of an Earth-based telescope by an order of magnitude only as yet work in the infra-red, where the sky is in any case much brighter. Adaptive optics (AO is a Technology used to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion Space telescopes don't have to worry about seeing discs.

See also

• Polar aurora
• Optical phenomena

References

1. ^ High Energy Astrophysics: Particles, Photons and Their Detection Vol 1, Malcolm S. An optical phenomenon is any observable event which results from the interaction of Light and Matter. Longair, ISBN 0-521-38773-6

External links

• 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