Sporadic Ca and Ca+ layers at mid-latitudes: Simultaneous observations andimplications for their formation

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 C a, and 58 nights of Ca+ soundings, at Kuhlungsbom, Germany (54 degreesN, 12 degreesE). The Ca+ soundings have been performed simultaneously and in a c ommon volume with the Ca soundings by two separate lidars. Correlations bet ween sporadic neutral and ionized metal layers are demonstrated through fou r case studies. A systematic study of the variations of occurrence of spora dic Ca and Ca+ layers reveals that neutral and ionized Ca layers are not as closely correlated as expected earlier: (a) The altitude distribution show s 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 fr equent than atom layers. Below 90 km only very few sporadic layers have bee n observed; (b) The seasonal variation of sporadic Ca layers exhibits a min imum of occurrence in summer, while sporadic Caf layers do not show a signi ficant seasonal variation (only the dense Ca+ layers appear to have a maxim um in summer). At midlatitudes sporadic Ca layers are more frequent than sp oradic layers of other atmospheric metals like Na or K. For the explanation of our observations new formation mechanisms are discussed.