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A 5 km (3.1 mi; 16,000 ft) deep sodium layer is located between 80–105 km (50–65 mi; 262,000–344,000 ft). Made of unbound, non-ionized atoms of sodium, the sodium layer radiates weakly to contribute to the airglow. The sodium has an average concentration of 400,000 atoms per cubic centimeter.
This band is regularly replenished by sodium sublimating from incoming meteors.
originally posted by: Dan00
I don't know enough about this stuff to know what to think.