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Abstract
Abstract. We report on the observations of 188 sporadic layers of either Ca atoms and/or Ca ions that we have observed during 112 nights of lidar soundings of Ca, and 58 nights of Ca+ soundings, at Kühlungsborn, Germany (54° N, 12° E). The Ca+ soundings have been performed simultaneously and in a common volume with the Ca soundings by two separate lidars. Correlations between sporadic neutral and ionized metal layers are demonstrated through four case studies. A systematic study of the variations of occurrence of sporadic Ca and Ca+ layers reveals that neutral and ionized Ca layers are not as closely correlated as expected earlier: (a) The altitude distribution shows the simultaneous occurrence of both sporadic Ca and Ca+ layers to be most likely only in the narrow altitude range between 90 and 95 km. Above that region, in the lower thermosphere, the sporadic ion layers are much more frequent than atom layers. Below 90 km only very few sporadic layers have been observed; (b) The seasonal variation of sporadic Ca layers exhibits a minimum of occurrence in summer, while sporadic Ca+ layers do not show a significant seasonal variation (only the dense Ca+ layers appear to have a maximum in summer). At mid-latitudes sporadic Ca layers are more frequent than sporadic layers of other atmospheric metals like Na or K. For the explanation of our observations new formation mechanisms are discussed.Key words. Ionosphere (ion chemistry and composition; ionosphere-atmosphere interactions; mid-latitude ionosphere)
Highlights
Metal atoms reside in the upper atmosphere in two types of layers: (1) In permanent, broad “regular” layers covering the altitude range 80 to 110 km and (2) in transient, narrow “irregular” layers occuring almost exclusively above 90 km altitude
The irregular layers remained of minor interest for almost two decades until the observations of von Zahn et al (1987, 1988) and von Zahn and Hansen (1988) concerning irregular layers observed at high latitudes which those authors called “sudden” sodium layers
This is similar to the behavior of sodium: Hansen and von Zahn (1990) reported the mean altitude of 75 Nas observed above Andenes, Norway (69◦N, 16◦E) to be 95 km, while the density peak of the regular Na layer was found to be at 90 km altitude
Summary
Clemesha (1995) concluded in his review that a detailed explanation for the formation of sporadic metal layers, their occurrence, their geographical distribution, and their high variability cannot be given yet. Strong sporadic Ca+ and Ca layers existed already when we started the record of density profiles at 19:30 and about 20:00 UT, respectively Both the Cas and Ca+s layers were observed near 95 km altitude and both increased in density. During the night March 4/5, 1997, a number of Ca+s and Cas layers were observed over a wide range of altitudes (Fig. 5). The Ca+ ion profiles showed strong sporadic layer activity above 105 km altitude from the beginning of the observations until the end (Fig. 6). High layers have been observed with number densities of more than 5 cm−3 above 105 km These intense high layers show a distinct maximum in summer. The higher Ca+s (above 100 km) are dominated by annual variation, with maximum in summer
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