Sodium layer
Encyclopedia
Refers to a layer within the Earth
's mesosphere
of unbound, non-ion
ized atom
s of sodium
. The altitude of this layer is usually located between 80–105 km (50–65 miles) and has a depth of about 5 km (3.1 mi). The sodium comes from the ablation
of meteors. Atmospheric sodium below this layer is normally chemically-bound in compounds such as sodium oxide
, while above the layer the atoms tend to be ionized.
The density varies with season; the average column density is roughly 4 billion sodium atoms/cm2.
Atoms of sodium in this layer are typically in an excited state, and radiate weakly at a wavelengths around 589 nm, which is in the yellow portion of the spectrum. These radiation bands are known as the sodium D lines
. The resulting radiation has been termed night glow.
Astronomer
s have found the sodium layer to be useful for creating an artificial laser guide star
in the upper atmosphere. The star is used by adaptive optics
to compensate for movements in the atmosphere. As a result, optical telescope
s can perform much closer to their theoretical limit of resolution.
The sodium layer was first discovered in 1929 by American astronomer Vesto Slipher
. In 1939 the British
-American
geophysicist Sydney Chapman
proposed a reaction-cycle theory to explain the night-glow phenomenon.
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
of unbound, non-ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ized atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
. The altitude of this layer is usually located between 80–105 km (50–65 miles) and has a depth of about 5 km (3.1 mi). The sodium comes from the ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
of meteors. Atmospheric sodium below this layer is normally chemically-bound in compounds such as sodium oxide
Sodium oxide
Sodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....
, while above the layer the atoms tend to be ionized.
The density varies with season; the average column density is roughly 4 billion sodium atoms/cm2.
Atoms of sodium in this layer are typically in an excited state, and radiate weakly at a wavelengths around 589 nm, which is in the yellow portion of the spectrum. These radiation bands are known as the sodium D lines
Fraunhofer lines
In physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer . The lines were originally observed as dark features in the optical spectrum of the Sun....
. The resulting radiation has been termed night glow.
Astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s have found the sodium layer to be useful for creating an artificial laser guide star
Laser guide star
Laser guide stars are a form of artificial star created for use in astronomical adaptive optics imaging.Adaptive optics systems require a wavefront reference source in order to correct atmospheric distortion of light...
in the upper atmosphere. The star is used by 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...
to compensate for movements in the atmosphere. As a result, optical telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s can perform much closer to their theoretical limit of resolution.
The sodium layer was first discovered in 1929 by American astronomer Vesto Slipher
Vesto Slipher
Vesto Melvin Slipher was an American astronomer. His brother Earl C. Slipher was also an astronomer and a director at the Lowell Observatory....
. In 1939 the British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
-American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
geophysicist Sydney Chapman
Sydney Chapman (astronomer)
Sydney Chapman FRS was a British mathematician and geophysicist. His work on the kinetic theory of gases, solar-terrestrial physics, and the Earth's ozone layer has inspired a broad range of research over many decades....
proposed a reaction-cycle theory to explain the night-glow phenomenon.