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.
Theory of the atmospheric sodium layer: a review
The advent of the laser-radar technique allowed sodium densities and sodium emissions to be monitored simultaneously, and great scientific progress has been achieved since. We describe these results in a broad sense, keeping a balance between theory and experiment, addressing the following subjects: evolution of the photochemical theory, the model for diurnal variations, the sodium source, tidal effects, and problems at the top of the layer.
Equilibrium temperatures of small particles in the Earth's upper atmosphere (50–110 km)
Abstract Aerosols absorb radiation from the solar and planetary radiation fields, exchange sensible heat by collisions with the ambient gas, and emit thermal radiation. If changes in phase are possible, they also lose or acquire latent heat. As a result of equilibrium between source and loss terms, particles acquire a temperature Tp that can be substantially different from the ambient gas temperature T g∗ Differences Tp − Tg, in excess of 100 K are obtained above 60 km in daytime, depending on size, refractive index, and planetary albedo; negative differences can be obtained at night. The relative contribution of the energy exchange terms also varies; in particular, the fraction of absorbed radiation transferred into the ambient gas by collisions changes significantly with particle size and altitude. The attainment of high Tp's may have important consequences in several upper atmospheric phenomena, such as the sublimation of alkali metals ( Fiocco and Visconti , 1973), the formation of noctilucent clouds, and other surface-related effects. The dependence of the heating rate, characterizing the exchange of energy between particles and gas, on various factors such as the planetary albedo may have a bearing on upper atmospheric behavior when sufficient particle concentrations exist.