Atmospheric Ca and Ca+ layers: Midlatitude observations and modeling

We report on a comprehensive set of observations of the upper atmospheric C a 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 night s of Ca+ soundings were realized. The Ca layer has an average column abunda nce of 2.1(.)10(7) cm(-2) cm, centered around 90.3 km with a mean peak dens ity 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). Bec ause the vaporization of cosmic dust is the most probable source of atmosph eric metals, the column densities of the metals within the atmosphere are o ften compared with the abundance in chondritic CI meteorites. We show that the atmospheric Ca is severely depleted with respect to other metals such a s Na and Fe, compared with their relative abundances in CI chondrites. We p resent a one-dimensional steady state chemistry model of the nighttime Ca a nd Ca+ layers, based on new laboratory studies of CaO reaction kinetics. Th is model is able to reproduce satisfactorily the characteristic features of the annual mean layers and to provide a possible explanation for the unusu al seasonal variation of the Ca layer which exhibits a pronounced summertim e enhancement around 87 km.