Abstract
Our Na LIDAR observations indicate that at high latitudes in summer the neutral sodium layer frequently attains a double-peaked structure. The main layer with a maximum near 90 km altitude is supplemented by a secondary, narrow layer near 95 km altitude. Under almost identical geophysical conditions a similar structure has also been observed in the ionized sodium profile by a rocket-borne mass spectrometer experiment. We report the following results concerning secondary sodium layers: (a) the neutral Na layers form only during a period of 6 h centered on 23:00 local time; (b) the ionized Na + layers are accompanied by layers of Fe + and Mg + at the same altitudes, with Fe + being about 10 times as abundant as Na +. It appears likely that the formation of secondary Na layers observed frequently above the LIDAR site is not solely a “sodium phenomenon”, but part of a more comprehensive layering process for metal atoms and ions. For the first time we derive experimentally from our in situ measurments Na + Na density ratios which are close to 0.5 near the peaks of both the main and secondary layers. This value is larger than predicted by current models of the sodium layer topside.
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