During the night of August 5/6, 1989 for the first time a noctilucent cloud (NLC) was detected and measured by a lidar instrument. The observations were made with our ground‐based narrow‐band Na lidar located at Andenes, Norway (69°N, 16°E geographic coordinates). In wavelength the lidar was operated both at the Na D2 resonance line of 589 nm as well as 5 Doppler widths shifted away. The altitude resolution was 200 m. The NLC developed at about 22:20 UT, reached its maximum backscatter cross section at 23:05 UT and became unobservable at around 00:10 UT. During this period the NLC exhibited the following properties: (a) its altitude ranged between 83.4 and 82.2 km; (b) its full width at half maximum ranged between 1.4 and 0.3 km; (c) the ratio of measured backscatter intensity from the NLC to the calculated Rayleigh signal from 82.6 km reached 450; (d) its volume backscatter cross section maximized at 6.5·10−9 m−1 sr−1.
The almost explosive growth of narrow sodium layers at altitudes near 95 km was observed by ground‐based LIDAR located at 69°N, 16°E. Starting from typical conditions at these altitudes (sodium density approximately 1 to 2× 10³ cm−3) the density is observed to increase by large factors within a few minutes in a narrow layer, the half height of which is typically 1 km only. One of the more spectacular cases involved an increase by a factor 25 within 5 min and a layer width of 0.8 km (full width at half maximum). We propose that the sodium forming these sudden layers is released from upper atmospheric dust by energetic auroral particles. Prior to the auroral bombardment the dust particles were concentrated into a narrow layer by appropriate meteorological processes. Our observations thus imply that considerable amounts of sodium are stored on the surface of upper atmospheric dust.
During August 1989, extended twilight and nighttime measurements with the European incoherent scatter (EISCAT) UHF radar were performed under PCA conditions. This provided an excellent data quality in the altitude region of 70 to 90 km throughout the three nights of August 12 to 15, 1989. A sophisticated control program allowed the measurement of the spectral width in the altitude region mentioned. In general, the measured spectral width deviates significantly from model values based on temperatures measured simultaneously by Na lidar combined with CIRA 88 temperature and density values. The observed spectra are up to 2 or 3 times narrower. In our observations the deviation tends to increase with increasing altitude. We also find that earlier spectral width measurements published by other workers are often narrower than current D region theory predicts. The possible reasons for this phenomenon are discussed.[1978]. Measurements by means of incoherent scatterhave been made since the late sixties, first covering only scattered power. In 1977 the first spectral width measurements were performed [Harper, •:,, ol The European: ß lnvuIICl tilt •i:tttCl 1 auar (EIS-CAT) in northern Scandinavia joined the latter activities in 1982 [Kofman et al., 1984] using the pulse-to-pulse correlation technique. However, all spectral width measurements made in recent years have been somewhat limited by the rather low electron densities usually found below about 90 km under normal geomagnetic conditions. The occurrence of a polar cap absorption (PCA) event in August 1989 provided sufficiently high electron densities, allowing extended spectral width measurements throughout the range of 70 to 92 km. In addition, during several EISCAT observations the temperature profile in the mesopause region was measured by means of a Na lidar experiment [Fricke and yon Zahn, 1985] only 129 km west of the EISCAT site. These data allowed the calculation of much more reliable theoretical spectral widths.After a short review of the experimental setup and of the commonly accepted theoretical expressions for the spectral width we will present the results of a comparison of our spectral width measurements with the values to be expected from theory. In the discussion we will compare the results obtained during the PCA event with data 1153
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