J. Höffner

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.

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.

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.

Temperature and aerosol soundings in the middle atmosphere at different mid and high-latitude lidar stations during day and night

Lidars provide an important tool to measure temperature and minor constituents in the atmosphere up to ~110 km altitude with high accuracy and temporal resolution. The Leibniz-Institute of Atmospheric Physics operates various lidars for the whole range between troposphere and lower thermosphere. The lidars are installed at Kühlungsborn, Germany (54°N, 12°E), at the ALOMAR site, Norway (69°N, 16°E), or in a mobile 20-foot container. Summertime soundings in polar regions as well as coverage of tides and gravity waves require measurements during full daylight. With a standard lidar the daylight background is several magnitudes larger than the signal in the mesosphere. Narrowband spectral filtering by etalons as well as spatial filtering by small fields of view (~50 μrad) are realized instead. At this low FOV turbulence and jitter of the beam pointing affects the signal and have to be compensated. We describe the techniques applied at our lidars. Additionally we will discuss the influence of the etalon filter technique on calculated temperature profiles. The etalon transmission of the Doppler-broadened backscatter signal is temperature dependent and has to be taken into account to avoid systematic errors. Overall, narrow-band lidars provide temperature profiles in the whole range up to the lower thermosphere. We will present observations of temperatures profiles of the lower and middle atmosphere as well as noctilucent clouds (NLC). These quantities provide important insights into the dynamics of the middle atmosphere. Time-resolved and averaged profiles of observations at the different locations will be shown and the results from different latitudes compared.