Observations of the sodium layer at high latitudes in summer
At altitudes between 80 and 100 km the upper atmosphere contains a layer of free sodium atoms presumably maintained by the almost continuous inflow and vaporization of cosmic dust particles. This sodium layer has been studied by ground-based photometers and lidar (laser radar), rocket-borne instrumentation, and satellite-borne photometers. Sodium measurements have been carried out at low, middle and high geographic latitudes. Lidar observations show that the peak of the layer commonly occurs close to the altitude of 90 km, with maximum densities of ˜5,000 atoms cm−3. In the 1960s photometric measurements established that the sodium layer shows seasonal variations which become more pronounced towards high latitudes (for a review of these early measurements, see ref. 1). Later lidar measurements yielded the following ratios of midsummer over midwinter column densities: 4.5 to 1 at 40° N (ref. 2), 3 to 1 at 44° N (ref. 3), and 6 to 1 at 51° N (ref. 4). Polar latitudes, however, are constantly sunlit in summer and the measurement of Na density profiles under these particular conditions have not yet been possible by ground-based methods. The state of our theoretical understanding of the Na layer has recently been reviewed by Kirchhoff5. Here we report on results of the first lidar measurements of the Na layer performed at polar latitudes in summer. Our sodium lidar instrument located at the And0ya Rocket Range, Norway (69° N, 16° E) gave measurements over ten days during July 1986 and July 1987. We find the sodium layer confined entirely to the lower thermosphere. The mean values for the layer parameters are as follows: the sodium column density is 6×l08cm−2; the number density at the layer peak is 1.2×103 cm−3; the altitude of the layer peak is 93 km, and the halfwidth of the layer is 6 km. Comparing these values with previous lidar measurements made at the same site in winter we obtain the following ratio of winter to summer densities: 10 to 1 for the sodium column density and 5 to 1 for the sodium number density at the layer peak.
Sporadic Ca and Ca + layers at mid-latitudes: Simultaneous observations and implications for their formation
Abstract. 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 Ca, and 58 nights of Ca+ soundings, at Kuhlungsborn, Germany (54° N, 12° E). The Ca+ soundings have been performed simultaneously and in a common volume with the Ca soundings by two separate lidars. Correlations between sporadic neutral and ionized metal layers are demonstrated through four case studies. A systematic study of the variations of occurrence of sporadic Ca and Ca+ layers reveals that neutral and ionized Ca layers are not as closely correlated as expected earlier: (a) The altitude distribution shows 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 frequent than atom layers. Below 90 km only very few sporadic layers have been observed; (b) The seasonal variation of sporadic Ca layers exhibits a minimum of occurrence in summer, while sporadic Ca+ layers do not show a significant seasonal variation (only the dense Ca+ layers appear to have a maximum in summer). At mid-latitudes sporadic Ca layers are more frequent than sporadic layers of other atmospheric metals like Na or K. For the explanation of our observations new formation mechanisms are discussed. Key words. Ionosphere (ion chemistry and composition; ionosphere-atmosphere interactions; mid-latitude ionosphere)
Ceramic microwave antennas for mobile applications
Due to their medium to high permittivity values, ceramic microwave materials are attractive as building blocks for miniaturised antenna solutions. Further advantages are low dielectric losses and an excellent temperature stability of standard ceramic materials. Yet, the permittivity of the ceramic material also has a strong influence on the radiation efficiency and bandwidth of the antenna. This implies that in the design process a suitable compromise has to be found between the degree of miniaturisation on the one hand and the antenna efficiency and bandwidth on the other. We describe two approaches towards efficient miniaturised ceramic microwave antennas: the dielectric resonator antenna (DRA) with partial metallisation and the shorting post patch antenna. We focus on design steps taken to ensure both a high degree of miniaturisation and high values of the radiation efficiency, the choice of the ceramic material and the experimental performance of the antennas.