Sodium layer

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A FASOR used at the Starfire Optical Range for LIDAR and laser guide star experiments is tuned to the sodium D2a line and used to excite sodium atoms in the upper atmosphere. FASOR stands for Frequency Addition Source of Optical Radiation, and for this system it is two single mode and single frequency solid state IR lasers, 1.064 and 1.319 micrometres, that are frequency summed in a LBO crystal within a doubly resonant cavity.

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 (50–65 miles) and has a depth of about 5 km (3 miles). 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.

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.

Astronomers 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 telescopes can perform much closer to their theoretical limit of resolution.

The sodium layer was first discovered in 1929. In 1939 the British-American geophysicist Sydney Chapman proposed a reaction-cycle theory to explain the night-glow phenomenon.