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.
Enhanced seasonal variations for chemical rates with inverse temperature dependencies: Application t
On a observe que les pertes associees a la reaction Na+O 2 +M→NaO 2 +M, (qui est reliee negativement a la temperature) varient annuellement d'un facteur quatre a 80-85 km et 60°N alors qu'elles varient d'un facteur inferieur a deux a 30°N. En utilisant un modele recent de la concentration en sodium on montre que ces pertes sont d'un facteur deux ou trois a 60°N et respectivement a 80 et 85 km. Les autres facteurs que l'altitude semblent moins importants
Indian Fig Cactus (Opuntia Ficus-Indica (L.) Miller) in the Americas: an Uncertain History
Abstract: 
 Opuntia ficus-indica (L.) Miller is a globally important cactus species with a long history of human use. In addition to the importance of O. ficus-indica as a food for humans and livestock, it also has long served a role in the production of cochineal, a once important textile dye. This cactus also is intimately interwoven into the story of Cactoblastis cactorum (Berg) (South American cactus moth), one of the best-known examples of biological weed control. In this regard, O. ficus-indica has been a target weed as well as an unintended host in C. cactorum invasion. Although the distribution and human-assisted movement of this cactus throughout much of the world is well documented, surprisingly little has been written about its history and movement within the Americas. Literature associated with Opuntia and their phytophagous pests in North and South America was examined in an attempt to determine the history of O. ficus-indica in the Americas. Available information suggests with some certainty that O. ficus-indica originated in present-day Mexico, but the timing of its introduction to South America is much less clear. Taxonomic difficulties associated with distinguishing among Opuntia species and the long pre-Columbian history of exchange between South American and Mesoamerican cultures are two primary complicating factors associated with dating the dispersal of this species to South America. Additionally, previous evidence in support of a pre-Columbian introduction to South America relied on questionable identification of insect-based textile dyes. These uncertainties are significant because clearer knowledge of the history of O. ficus-indica introduction to South America could provide a more complete biological and ecological context to efforts at managing this species in its native and introduced ranges, particularly with respect to the threat posed by C. cactorum in North American and Argentinean plantations.