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
Movement toward a predominantly electronic journal collection
Purpose – To address the UNLV Libraries movement toward a predominantly electronic journal collection including evaluation of library collection and reevaluation of organizational structure, staff resources and workflow to find the best ways to provide library users with timely and reliable access to electronic resources.Design/methodology/approach – A range of UNLV Libraries initiatives in building and managing electronic resources is described to demonstrate how the library moved from a predominantly print environment to a predominantly electronic environment. The paper consists of sections: development of the electronic resources collection, reengineering acquisitions/periodicals, new responsibilities and new skills, next steps, and conclusion.Findings – Since 1999, the composition of the Libraries journal collection has been dramatically changed. The percentage of print‐only subscriptions decreased from 59 percent in 1990 to 20 percent in 2004, while electronic journals jumped from 35 percent to 75 pe...