Sodium layer

For a typical thickness of 5 km this corresponds to volume density of roughly 8000 sodium atoms/cm3.

[1] Atoms of sodium in this layer can become excited due to sunlight, solar wind, or other causes.

Once excited, these atoms radiate very efficiently around 589 nm, which is in the yellow portion of the spectrum.

Astronomers have found the sodium layer to be useful for creating an artificial laser guide star in the upper atmosphere.

In 1939 the British-American geophysicist Sydney Chapman proposed a reaction-cycle theory to explain the night-glow phenomenon.

A thin bright yellow light beam goes straight up into the sky from an optical instrument
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 .)