Dye-laser observations of the nighttime atomic sodium layer

Observations of the free atomic sodium layer near 90 km have been made as part of performance tests on a tunable dye-laser radar. Altitude profiles of the layer obtained during parts of four nights in the fall of 1971 are consistent with those obtained by other groups but show two interesting additional features. The first is a sharp decrease in density that terminates the layer on the bottom side at a variable altitude near the mesopause. This decrease appears to become sharp only some time after twilight, suggesting that the sodium consumption mechanism undergoes a change as the nighttime chemistry is established. The second feature is a fourfold increase in sodium-layer content during a 4-hour period surrounding the transit of the radiant of the Geminids meteor shower on the night of December 13–14, 1971, when the shower was at its peak.