M. Alpers

The mesopause altitude: Only two distinctive levels worldwide?

A large number of temperature profiles of the altitude range 80 to 105 km were obtained between 71°S and 54°N latitude from late April to early July 1996. The measurements were performed by a ship-borne lidar, resolving the Doppler broadening of the K(D1) resonance line. The most notable result of this field campaign is the finding that throughout our observation series the mesopause altitude was located at altitudes of either 100±3 km or 86±3 km. The high “winter” level extended from 71°S to 23°N, the low “summer” level from 24°N until the end of the field observations at 54°N. Our latitudinally distributed observations indicate strongly a worldwide bimodal character of the mesopause altitude. Furthermore, our mesopause temperatures are generally lower than previonsly measured in the northern hemisphere.

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°N, 12°E), 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 , 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.

Daylight rejection with a new receiver for potassium resonance temperature lidars.

For daytime operation a new receiver for potassium resonance temperature lidars was developed that includes a twin Faraday anomalous dispersion optical filter and a high-performance photon-counting avalanche photodiode instead of a photomultiplier. The design of a flat spectral filter shape over the lidar scan range avoids instrumental error on the temperature results. Initial temperature measurements under daylight conditions were realized.

First simultaneous measurements of neutral and ionized iron densities in the upper mesosphere

We report on the results of a multi-instrument study of a sporadic Fe layer and a sporadic E layer which took place in the night of September 20, 1991, at the Andoya Rocket Range (69°N, 16°E). A ground-based lidar was used to measure mesospheric Fe densities, while simultaneously rocket-borne probes and an ion mass spectrometer measured the profile of total ion density and ion composition, respectively. All instruments observed the sporadic layers near 97 km altitude. Radar tracking of chaff clouds established the wind field in the 90- to 100-km region with high spatial resolution, while the EISCAT incoherent scatter radar measured the E fields in the F region. The major ion in the sporadic E layer turned out to be Mg+. The narrow Fe+ layer was 1 km lower than the sporadic Fe+ layer. The ratio of densities [Fe+]/[Fe] in the peaks of the sporadic layers was found to be 1.75.

NLC particle properties from a five‐color lidar observation at 54°N

On June 13/14, 1998, a first simultaneous five-color lidar measurement of a noctilucent cloud was realized. The experiment was made possible by the fact that at the site of our Institute (54°N), dark summer nights allow lidar observations of overhead noctilucent clouds (NLCs) without expensive spectral filters. We determine the size distribution of the NLC particles assuming spherical ice particles with a monomodal, lognormal size distribution. To this end we introduce a new method for calculation of the NLC particle properties. For the latter the following ranges are found: Particle number density N = 260–610 cm−3, median radius rm = 20.2–27.5 nm, and distribution width σ= 1.5–1.6. Cross checks using different wavelength combinations confirm the robustness of the NLC particle property results. The results are similar to those from recent lidar work on noctilucent clouds, observed at arctic latitudes.

Upper atmosphere Ca and Ca+ at mid‐latitudes: First simultaneous and common‐volume lidar observations

We report on simultaneous, common-volume observations of Ca and Ca+ layers in the 80 to 125 km altitude range by groundbased resonance lidar. The measurements were carried out during 9 nights in summer 1995 on the German island Rugen (55°N, 13°). We observed a permanent Ca layer with an average peak density of 31 atoms/cm³ at 87 km altitude. The average Ca column abundance was 2.2×107 cm;−2. No permanent Ca+ layer was found, but sporadic Ca+ layers occured frequently in the altitude range 90–120 km. Peak densities of these thin layers varied between 65 and 3900 atoms/cm³. We demonstrate the interesting possibilities of groundbased simultaneous common-volume observations of neutral and ionized calcium in the lower thermosphere.

Sporadic Fe and E layers at polar, middle, and low latitudes

First observations of sporadic iron layers (FeS) at low and middle southern latitudes are reported. These and new measurements from northern polar latitudes are combined to study the latitudinal dependence of FeS layer properties and their correlation with simultaneously observed sporadic E layers (ES). We also compare the individual properties of FeS and sporadic sodium layers (NaS): (1) The overall appearance rate for FeS is much higher than for NaS. (2) FeS layers do not show the marked minimum in appearance. (3) Both the normal and the sporadic Fe layers are considerably more dynamic than in the case of Na layers. (4) Concerning the shape, FeS layers are typically broader, slower in growing and longer lasting than NaS layers. (5) The dependence of appearance rate of sporadic metal layers versus, local time and latitude shows a complex pattern. (6) The correlation with ES appears to be weaker for FeS than for NaS.

Simultaneous K and Ca lidar observations during a meteor shower on March 6–7, 1997, at Kühlungsborn, Germany

We report about observations of meteor trails by ground-based lidars which were obtained with two metal resonance lidars monitoring simultaneously the same air column at meteor trail heights. The lidars are located at the site of the Leibniz-Institute of Atmospheric Physics (54°N, 12°E). More specifically, we have used K and Ca lidars to study meteor trails with respect to (1) their absolute K or Ca abundances, (2) their duration in the laser beams, (3) the altitude distributions of the K and Ca trails, and (4) the reaction of the regular K and Ca layers to the occurence of a (yet unknown) meteor shower. Lidar observations during the night of March 6–7, 1997, began around 1820 UT. They indicated the start of an unknown meteor shower at ∼2200 UT, which we could observe until 0430 UT of March 7. Within 4 h after 2200 UT, the column densities of the regular K and Ca layers increased by factors of 2 and 4, respectively. During the period of the shower, our lidars registered 26 Ca trail events, but only 2 K trail events. Hence we observe for the two metals a noticeable difference between their column density enhancements and rates of trails. The rate of Ca trails was quite similar to that observed by our lidars near the peak of the Lyrids on April 22–23, 1997. The Ca trails were observed in the altitude range from 81 to 98 km with a mean altitude of 89.6 km. In all of the lidar-observed meteor trails, it was either K or Ca which could be detected. Metal densities in the trails ranged from ∼90 to 360 cm−3 in the case of K and from 50 to 2700 cm−3 for Ca.

Multiwavelength lidar observation of a strange noctilucent cloud at Kühlungsborn, Germany (54°N)

On July 6/7, 1997, we observed a noctilucent cloud (NLC) by lidar at Kuhlungsborn, Germany (54°N, 12°E) using four laser wavelengths (393, 423, 532, and 770 nm). While at the near-ultraviolet and visible wavelengths clear backscatter signals were detected, the NLC did not produce significant backscattering at the near-infrared wavelength 770 nm. The latter signature can not be explained by backscattering on any size distribution of homogeneous water ice spheres. In this work we discuss the spectral backscatter signatures of particles with various shapes and compositions. We found three different particle types matching the experimental results, but there are physical arguments excluding these candidates as well. Our lidar observations of this strange NLC event appear to contradict the common understanding of NLC particle formation and sublimation. We emphasize the importance of multiwavelength observations for an in-depth interpretation of noctilucent cloud lidar backscatter data, including also the infrared part of the spectrum.