Atmospheric Ca and Ca+ layers:: Midlatitude observations and modeling

We report on a comprehensive set of observations of the upper atmospheric Ca and Ca+ layers. The observations were obtained by ground-based lidars at Kuhlungsborn, Germany (54 degreesN, 12 degreesE), between December 1996 and December 1998. During this period, 112 nights of Ca soundings and 58 nights of Ca+ soundings were realized. The Ca layer has an average column abundance of 2.1(.)10(7) cm(-2) cm, centered around 90.3 km with a mean peak density of 22 cm(-3) at 89.9 km altitude. The Ca+ dominates the total Ca amount above 90 km and has an average column abundance of 4.9(.)10(7) cm(-2). Because the vaporization of cosmic dust is the most probable source of atmospheric metals, the column densities of the metals within the atmosphere are often compared with the abundance in chondritic CI meteorites. We show that the atmospheric Ca is severely depleted with respect to other metals such as Na and Fe, compared with their relative abundances in CI chondrites. We present a one-dimensional steady state chemistry model of the nighttime Ca and Ca+ layers, based on new laboratory studies of CaO reaction kinetics. This model is able to reproduce satisfactorily the characteristic features of the annual mean layers and to provide a possible explanation for the unusual seasonal variation of the Ca layer which exhibits a pronounced summertime enhancement around 87 km.