Sporadic sodium layers and the average vertical distribution of atmospheric sodium: Comparison of different Nas layer strengths

In an earlier study of sporadic sodium layers (Nas) [Simonich, D.M., Clemesha B.R., Batista, P.P. Sporadic layers and the vertical distribution of atmospheric sodium. Adv. Space Res. 35, 1976–1980, 2005], observed by lidar at Sao Jose do Campos (23°S, 46°W) we found that, although individual profiles give the impression that Nas layers involve sodium additional to the normal background layer, there is very little difference between the long-term averages of profiles with and without the presence of Nas. This led us to conclude that Nas layers result from the redistribution of an omnipresent source, rather than an additional source mechanism. We have now extended this study to investigate whether or not the relative magnitude of Nas layers influences this conclusion. To this end we manually characterized all the profiles obtained in the time interval from 1900 to 2200 LT for the years 1993–2004. This involved registering the upper and lower limits to each Nas layer observed, the height of the peak and the sodium concentration at each of these three points. We then computed average profiles for Nas layers of differing strengths, with strength defined as the concentration at the peak divided by the mean of the concentrations at the upper and lower boundaries. For strengths up to 4 the results confirmed our earlier conclusion but for Nas layer strengths greater than 4 we found a significant difference between the average profiles with and without Nas. For the strong Nas layers both the average abundance and the average peak sodium concentration were about 10% greater than for layers without Nas. This leads to the possibility that a different mechanism might be responsible for very stronger sporadic layers although we do not think this very likely.