The Sodium Layer

A natural global layer (usually about 3 miles thick) of sodium atoms exists between about 50 and 65 miles (80 and 105 km) altitude. The sodium originates from the ablation of meteors. The atoms are naturally excited and emit a weak glow near a wavelength of 589 nm (yellow) known as "the sodium D lines". Above the layer, sodium exists in its ionized form (which does not emit yellow light) and below the layer, sodium exists as chemical compounds such as sodium oxide (which also do not emit yellow light).


Although originally a curiosity, the sodium layer has been found to have useful applications:

Other Metallic Vapor Layers

Other metals such as iron (Fe), potassium (K) and calcium (Ca) have also been observed to form metal layers. The iron layer is typically about 55 miles (88 km) altitude and about 2 miles thick.

Sudden Sodium Layers

Occasionally certain areas may experience a sudden appearance of an enhanced layer of sodium (Sudden Sodium Layer) or other metallic atoms such as iron or potassium (more generally called a Sudden Atom Layer). The cause of Sudden Atom Layers is not known; the leading explanation is that these layers result from the neutralization of descending sporadic E layers.

Historical Outline

1929 Sodium (Na) layer discovered by Vesto Melvin Slipher
1939 Chapman proposes theory to explain layer
1960's Routine observations begin
1969 Lidar observations begin
1976 Iron (Fe) layer detected
1985 First extensive laboratory studies of Na atom chemistry relevant to atmosphere
1989 Lidar observations of iron layer begin