Observations of the sodium layer at high latitudes in summer

Abstract

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.