Forty years of noctilucent cloud observations near Moscow: Database and simple statistics

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D8, 8443, doi: /2002jd002364, 2003 Forty years of noctilucent cloud observations near Moscow: Database and simple statistics V. A. Romejko and P. A. Dalin 1 Space Research Institute, Russian Academy of Sciences, Moscow, Russia N. N. Pertsev A. M. Oboukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia Received 2 March 2002; revised 4 September 2002; accepted 4 September 2002; published 15 February [1] The observation procedure and relevant database of Moscow (56 N, 37 E) noctilucent cloud (NLC) systematic observations ( ) are described. The longterm series of Moscow NLC data are analyzed and compared to the observations in western Europe. Statistical analysis of seasonally averaged parameters of the NLCs is carried out. Characteristic periods in interannual variability of the integral NLC brightness fluctuation are extracted. Particular attention is paid to decadal (10 years) periodicity in the NLC occurrence and their brightness. A distinct difference between decadal periodicity in NLC parameters and solar activity is found and its possible origin is discussed. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 1650 Global Change: Solar variability; 3332 Meteorology and Atmospheric Dynamics: Mesospheric dynamics; KEYWORDS: noctilucent, clouds Citation: Romejko, V. A., P. A. Dalin, and N. N. Pertsev, Forty years of noctilucent cloud observations near Moscow: Database and simple statistics, J. Geophys. Res., 108(D8), 8443, doi: /2002jd002364, Introduction [2] Noctilucent clouds (NLCs) were observed for the first time in Moscow (Russia) in 1885 by the astronomer W. Ceraski and in Germany by the astronomer T. Backhouse. Some years later, many researchers in Russia, Germany, the UK, Sweden, and the United States were engaged in this affair [Bronshtein and Grishin, 1970; Gadsden and Schroeder, 1989; Schroeder, 1998]. However, these observations were only unsystematic, sporadic registrations of NLCs, and there was no common program of NLC observations till the middle of the 20th century. In 1957, NLC observations began under the programs of the International Geophysical Year and the Quiet Sun Year. A network of sites was created in several regions of the Former Soviet Union and the first catalogues of NLC data were published [Gromova, 1963; Fast, 1972, 1980]. It was the beginning of regular observations of NLCs in several regions of Russia. Now the accumulated data are analyzed. In this paper we describe the procedure of NLC observations near Moscow during the period from 1962 up to now. We give a review of the Moscow NLC database and present some statistical results concerning the NLCs near Moscow. 2. Observations [3] Since the first days of NLC discovery and up to now, the visual observations are the most effective method for 1 Now at Swedish Institute of Space Physics, Kiruna, Sweden. Copyright 2003 by the American Geophysical Union /03/2002JD NLC systematic study. The NLC rather frequent appearance allows observer to be trained quickly and easily during one season. As a rule, a trained observer participates in 3 5 NLC seasonal campaigns. [4] With the adopted technique of visual observations, an observer examines the twilight sky every 15 min during a night, usually from 2300 to 0400 LT. The observations of the NLCs are carried out from the end of May to the middle of July (the most favorable period for NLC occurrence). This technique allows us to create data sets of NLC parameters with a high temporal resolution. [5] The following parameters are visually estimated in the case of the NLC occurrence: 1. Date and time of the observation of a twilight sky (local time) 2. Time of appearance and disappearance of NLCs 3. The brightness of a NLC field on a five-mark scale 4. Morphological forms 5. The intensity of the NLCs (its definition is given below). [6] Besides this, the meteorological conditions for observations are estimated including the area covered by tropospheric clouds (in percentage in the twilight segment of the sky). The estimation of the NLC brightness and classification of morphological forms follow the procedure elaborated by Grishin [1957]. The intensity (dimensionless), which was introduced by V. Romejko in 1979, provides the most generalized characteristic of the NLC field as a whole. This parameter is the integral value of the brightness and general area of the NLC field in a twilight sky. Table 1 presents the value of the intensity dependence on the brightness and the NLC area. PMR 10-1

2 PMR 10-2 ROMEJKO ET AL.: NOCTILUCENT CLOUD OBSERVATIONS NEAR MOSCOW Table 1. The Definition of Intensity Depending on the Brightness (Rows) and Area (Columns) of the NLCs a A S 1/4 1/3 1/2 2/3 3/4 1 M B a B, the NLC brightness by the five-mark scale; A, the area ratio (NLC field)/(twilight part of sky); S, to be applied if only fragments of the NLCs are seen; M, to be applied if multilayered NLCs are seen. [7] After an observation the results are to be filled in a table of standard form containing the following parameters: 1. Date 2. Maximal brightness of the NLCs during the night (marks) 3. Integral brightness (marks), the sum of all the 15 min estimations of NLC brightness; in effect, it is the integral of brightness over time 4. Registered morphological forms 5. Time of the beginning and the end of NLC visibility 6. Duration of NLC visibility 7. Maximal value of intensity 8. Weather conditions. [8] For treatment of the obtained data we have chosen an integral brightness as the single parameter, best describing the level of NLC activity during each night, because it represents both the level of brightness of NLCs and duration of their visibility. 3. Available Database [9] This chapter is devoted to the NLC database, which is the property of Moscow group of NLC observers. Most of the database contains the observations near Moscow (j = 56, l =37 ), although there are also NLC observations in some other regions of Russia [Romejko et al., 2002]. Only the observations near Moscow will be considered below. [10] Although the long-term observations began in 1950 [Grishin, 1952], uniformly organized data of the same value were made since For all of that period, the technique of visual observations including brightness estimation and morphological classification of NLCs has been kept without change. Uniform style of observations and scripts has been ensured by the supervision of N. Grishin (before 1969) and V. Romejko (since 1969) [Romejko, 1989]. This database is unequaled in the world regarding total duration, quantity of the observations and volume of accompanying information. The total number of observations made every 15 min is more than 26,800, i.e., more than 1340 duty nights during 6700 hours. The occurrence number of NLCs for this period ( ) is 426 and the total duration of the NLC visibility is 830 hours. It is necessary to note that the technique of visual observations including the brightness estimation and also the morphological form classification did not change from 1962 until the present. [11] The database contains not only filled tables but also photographs, video materials and theodolite surveys, accompanied some NLC observations. We are planning to transform them to a digital form. Photo and video images are very helpful for more accurate detection of NLC morphological forms and investigations of NLC spatialtemporal structure. [12] At present, the electronic version of the database contains only the information, summarized over every night: the values of maximal brightness of the NLCs, integral brightness, NLC morphological forms, the time of the beginning and the end of twilight sky observations, the duration of NLC visibility and the meteorological conditions. This database essentially differs from west Europe published catalogues [McIntosh and Hallissey, 1974; Schroeder, 1998], which include the diverse notes of the observers instead of precise estimations of the morphological form classification, brightness estimations and meteorological conditions. Particularly, the west Europe databases do not include the cases of the NLC absence at good weather, which is extremely important for correct statistical analysis. 4. Statistical Analysis and Its Results [13] In spite of the fact that many papers [e.g., Vasilyev, 1967; Willmann, 1970; Gadsden, 1998; Sugiyama, 1998] were devoted to NLC statistics and revealed the important statistical properties of NLC occurrences, all of them have one of the following shortcomings: 1. Rather short data sets (less than for two solar cycles) are subjected to the statistical analysis. 2. Diverse data received by different techniques are treated. [14] Besides, many databases do not describe carefully the weather conditions and missing observations which results inevitably in the distortion of NLC statistics due to unexplored statistics of the clear weather nights. The Moscow database, although having a natural territorial restriction, is free from the defects mentioned above. [15] We provide here only the first part of a statistical analysis, examining annual variability of the seasonally averaged parameters of the NLCs. For this we calculated the seasonally averaged NLC parameters. Although the database contains 10 cases of NLC appearance before May or after July, we did not include such unusual observations into the statistics for better uniformity of the NLCs analyzed data restricting our analysis by the season of bimonthly duration between these two dates. In this case we have 35 duty nights per season in average with slight variations from year to year and with the best concentration in late June and early July. For every NLC season the following seasonally averaged parameters were calculated: s 1 the number of nights with NLC occurrence s 2 the integral brightness accumulated for the whole season s 3 the number of clear weather nights s 4 the number of semiclear weather nights. [16] The data sets of s 1, s 2, and s 3 + s 4 are presented in Figures 1 and 2. The curves of solar radioemission index F 10.7 (averaged over June and July) and the number of nights with NLC occurrence in NW Europe [Gadsden, 1998] are given too. We can see from Figure 2 that the

3 ROMEJKO ET AL.: NOCTILUCENT CLOUD OBSERVATIONS NEAR MOSCOW PMR 10-3 Figure 1. Time series: s 2, seasonally accumulated NLC integral brightness, marks (solid line with solid circles), and F 10.7, solar radioflux index, Wm 2 Hz 1, averaged over June and July (dashed line with open circles). numbers of NLC occurrences in NW Europe and in Moscow are correlated well. [17] The values of s 1 and s 2 characterize not only NLC activity as they depend also on the weather conditions for the season; this is well manifested in Figures 1 and 2. That is why two new values (influenced by weather conditions to a much lesser degree) were analyzed: 1. The estimation of probability of the NLC occurrence in clear or semiclear night s 5 ¼ s 1 s 3 þ s 4 2. The normalized integral NLC brightness, i.e., average integral brightness during a clear or semiclear night. autocorrelation functions (being defined for each of two data series) and their spectra have been already used in NLC statistical studies [e.g., Vasilyev, 1967; Sugiyama, 1998]. [20] Figure 3 presents the autocorrelation function for the normalized integral brightness of the NLCs. It is clearly seen that the basic characteristic period is about 10 years. The autocorrelation function for F 10.7 is shown in this figure for the comparison. Since several papers were devoted to the correlation of NLC occurrences with the solar activity [e.g., Gadsden, 1998], in this paper the correlation of seasonally averaged characteristics of the NLCs with the solar activity is also studied based for the much longer data series. The phase shifts between two quasi-periodic processes are often revealed using crosscorrelation functions. Figure 4 presents the cross-correlation functions for three pairs of processes: s 1 and F 10.7, s 3 + s 4 and F 10.7, and s 5 and F The probability of significant correlation for cross-correlation maxima of first and third pairs is not less than 0.98 and about 0.83 for the second pair. It is interesting that all three cross-correlation functions show similar features, namely, the correlation with periodicity of about 10 years. In particular, the phase of all three processes are 2 4 years ahead of F 10.7 (or, equivalently, 6 9 years behind of F 10.7 ). We may also see that the correlation of s 1 and F 10.7 is a result of interference between correlations (s 3 + s 4 and F 10.7 ) and (s 5 and F 10.7 ). However, the curves for s 1 and F 10.7, s 5 and F 10.7 are obviously closer to each other, than to the third curve. Therefore, we may conclude that the weather influence on the correlation of NLC occurrence and F 10.7 is not so strong. It is interesting that Gadsden [1990, 1998] found the same phase shift in the number of nights with NLC occurrence in west Europe with respect to the solar activity. Although the west Europe data do not allow to distinguish the processes s 1 and s 5, the coincidence between the behavior of number of nights with NLCs in s 6 ¼ s 2 s 3 þ s 4 [18] Note that average integral brightness is normalized by the number of clear and semiclear nights, but not by the number of nights with the NLC occurrence. [19] Then the main question arises what physical processes govern s 5 (t) and s 6 (t). To approach the answer we use cross-correlation analysis, which allowed us to estimate the mutual influence of the two processes (x 1 (t), x 2 (t)). This analysis is based on the cross correlation ð CCFðÞ¼ l x 1ðÞ x t 1 Þðx 2 ðt þ lþ x 2 Þ ðsðx 1 Þsðx 2 ÞÞ 1=2 and on the autocorrelation ð ACFðÞ¼ l x 1ðÞ x t 1 Þðx 1 ðt þ lþ x 1 Þ sðx 1 Þ functions where the overline means a time averaging, and s is the variance of time series [Jenkins and Watts, 1969]. The Figure 2. Total number of nights within a season: with NLC in NW Europe [Gadsden, 1998] (diamonds), s 1, with NLC near Moscow (solid circles), s 3 + s 4, with clear or semiclear sky near Moscow (open circles).

4 PMR 10-4 ROMEJKO ET AL.: NOCTILUCENT CLOUD OBSERVATIONS NEAR MOSCOW Figure 3. Autocorrelation functions for s 6 (normed integral brightness, solid line) and F 10.7 (averaged solar radioflux index, dashed line). west Europe and in Moscow with respect to the solar activity makes it clear that the yearly variability of the NLC activity in east Europe and one in west Europe have some common features. [21] Thus the decadal periodicity in the number of the NLC occurrences is determined by the same period of the probability of the NLC occurrence for both clear and semiclear nights (s 5 ) and, less substantially, by the periodicity of the number of these nights (s 3 + s 4 ). How can we estimate statistical reliability of such conclusions? Whether a correlation of solar activity with the specified processes really exists or these processes have random amplitudes at the solar activity frequency, thereby demonstrating a correlation (with random phase shift)? Although there are no good ways to estimate errors of the observers, we believe that those errors are of stochastic character and do not contain periodicities grater than random numbers with the same variance do. In some cases we may distinguish true periodicities from stochastic ones by random permutation technique (RPN). We used it for all time series except for F If the RPN does not reduce essentially the absolute value of the cross-correlation function it means that the correlation is random. In the opposite case, if the value of cross-correlation function is attenuated strongly after the RPN procedure it means that two processes are connected to each other by direct or indirect connection with the probability close to unity. [22] The pattern of a typical RPN test for the correlation of s 5 with F 10.7 is shown in Figure 5. The RPN transform does not change the order of CCF amplitude with decadal periodicity and shifts randomly (from one RPN transform to another one) the CCF phase. The same is true also for s 6.In Figure 6, 10 patterns of RPN are shown. Very small part of RPN transforms may occasionally amplify or attenuate the CCF amplitude. In the study of Romejko et al. [2002], a pattern of essentially decreased CCF amplitude and a loss of the quasi-periodic behavior after an RPN was given. The results based on many RPN transforms show that a contribution of the solar cycle to the decadal variability of s 5 (probability of NLC appearance in clear or semiclear night) and s 6 (normalized integral NLC brightness) is negligible. However, careful examination of Figures 5 and 6 and of many similar results with other RPN patterns gives one more conclusion: the cross-correlation functions between F 10.7 and the randomly permuted values of s 5,orofs 6, almost always have a periodicity with slightly (but distinctly) lower frequency than those before the permutation. We can confirm it by a comparison of main periods in CCFs calculated before RPN and after RPN. The main period, found by the least squares method from the CCF for F 10.7 and s 6, is 9.85 years. The 40 patterns of RPN in s 6 provided Figure 4. Cross-correlation functions for the three pairs of processes: F 10.7 and s 1 (solid thick line), F 10.7 and s 3 + s 4 (solid thin line), and F 10.7 and s 5 (dashed line). Figure 5. Cross-correlation function for solar activity index F 10.7 and probability of NLC appearance in clear or semiclear night s 5 (solid line) and a typical pattern of RPN test for F 10.7 and randomly permuted s 5 (dashed line).

5 ROMEJKO ET AL.: NOCTILUCENT CLOUD OBSERVATIONS NEAR MOSCOW PMR 10-5 Figure 6. Cross-correlation function (thick curve) for solar activity index (F 10.7 ) and normed integral brightness (s 6 ) and patterns of the similar function but with RPN in s 6 (10 thin curves). the main periods in CCFs for F 10.7 and permuted s 6, scattered around years with standard deviation 0.37 year. For this standard deviation a difference between the periods 9.85 and years is significant with the probability more than Thus, the period for the CCF before the RPN procedure is about 1 year less than for the situation after the RPN. The latter period shows simply the main periodicity in F The frequency difference between the decadal oscillations in the solar activity and in NLC parameters may be proved also by Figure 7, depicting autospectra for s 5, s 6 and F 10.7 as well as by the autocorrelation functions. In Figure 3, it may be clearly seen that the periodicity for F 10.7 ACF has a slightly larger period than one for s 6 ACF. The least squares method applied to correlation functions allows us to extract the main sinusoids for s 1 s 6 ACF. Relevant periods are listed in Table 2. From the obtained results we may conclude that the series s 5 and s 6 for seasonally averaged NLC parameters contain a periodicity with the main frequency slightly higher than that for the solar activity. That is why the random numbers are slightly correlated with F 10.7 at the solar activity frequency and random phase; and the small correlation is also observed between s 5 or s 6 with F 10.7 at some intermediate frequency for these two processes, caused by weak mutual resonance at the wings of the relevant spectral lines. The values of period, listed in Table 2, slightly differ from those found by Romejko et al. [2002]. This arises because of a slight change in time series: in this paper the 2001 year values have been added, and the limits for NLC seasons, as described above, have been established. The comparison shows that these changes are not substantial. [23] The s 6 series reveals the most distinct decadal periodicity among all the considered series s 1 s 6. Not only the frequency difference but also a lag of s 6 behind F 10.7 leads to some doubts on the existence of a direct solar activity effect on the NLCs. Figures 4 6 demonstrate that Figure 7. Autospectra for s 5, s 6, and F the minima of s 5 and s 6 delay after the maxima of F 10.7 by about 1 year, and that for s 1 the delay is even greater. [24] Now we compare the temporal dynamics for the normed (s 5, s 6 ) and not normed (s 1 and s 2 ) time series. Since the latter ones may be obtained through multiplying the first ones by the number of clear and semiclear nights (s 3 + s 4 ), it is reasonable to expect that interannual behavior of clear weather would distort the not normed time series in comparison with the normed ones. It does really occur, that follows obviously from autospectra comparison in Figure 7 (fors 5, s 6 ) and in Figure 8 ( for s 1, s 2, s 3 + s 4 ). First of all, such a distortion reveals itself in the decadal periodicity (frequency 0.1 yr 1 ). The number of clear and semiclear nights has a spectral peak at frequency, slightly less than 0.1 yr 1, unlike the normed time series (s 5, s 6 ), having peak frequency slightly more than 0.1 yr 1. For time series (s 1, s 2 ) being distorted by weather, we have a spectral peak at intermediate frequency, more close to 0.1 yr 1. We do not deny a possibility of beats in the system NLC-solar activity, however, much longer time series are required for their extraction. [25] Another interesting feature of interannual variability of NLC parameters is an existence of strong harmonics with periods of 2 5 years, contrasting to the absence of these in solar activity indices. It is noticeable for s 6 and F 10.7 ACF (see Figure 3). The difference between periodicities in NLC parameters and in the solar activity is well seen also in Figure 7, showing autospectra for s 5, s 6 and F The most prominent spectral peak both for s 5 and s 6 Table 2. The Periods Found Through the Extracting of Main Sinusoids From the Autocorrelation Functions for Different Time Series by Least Squares Method Time Series F 10.7 s 1 s 2 s 3 + s 4 s 5 s 6 Period, years 10.4 ± ± ± ± ± ± 0.1

6 PMR 10-6 ROMEJKO ET AL.: NOCTILUCENT CLOUD OBSERVATIONS NEAR MOSCOW Figure 8. Autospectra for s 1 (the number of NLC nights, solid thick line), s 2 (an integral NLC brightness, solid thin line), and s 3 + s 4 (the number of clear and semiclear nights, dashed line). in the range of periods 2 5 years is a peak with a frequency 0.39 yr 1 (period about 2.5 years), resembling the well-known Quasi-Biennial Oscillation. The similar peak exists at the (s 3 + s 4 ) autospectrum (see Figure 8). Multiplying s 5 or s 6 by (s 3 + s 4 ), the nature suppresses this oscillation by a counterphase interference. That is why there is no such spectral peak in s 1 or s 2 autospectra. This analysis shows that statistics of clear nights is necessary and substantial for NLC statistics. Interannual behavior of clear nights number may depend strongly on the location of observing site. Thus the appearance of considered spectral peak for number of nights with NLCs in other databases would not be surprising. For example, a maximum with approximately the same frequency (0.37 yr 1 ) in number of NLC nights was found for another database (containing diverse data on NLCs) by Vasilyev [1967]. He interpreted it as combined oscillation, created by oscillations with periods of 3.7 and 10 years, but for our data such interpretation does not look realistic (see Figure 7). These oscillations seem to be caused either by generation of 4th harmonic in the decadal variation or by other influences, not connected with the decadal variations. [26] As to the oscillations with year period in NLC parameters, they may be caused by some indirect influence of solar activity with periodicity, distorted or masked by other processes. Besides, such periods may arise, from our opinion, on the one hand, in the autooscillations in the atmosphere ocean system [Handorf et al., 1999], and on the other hand, in the periodic variations in the Earth Moon gravitational system [Kropotkina and Shefov, 1976]. In future we will analyze the correlation between various NLC parameters and parameters of the Moon s motion. 5. Conclusions [27] NLCs near Moscow (56 N, 37 E) have been observed by stable techniques almost every summer season for 40 years ( ). Some conclusions, obtained from the analysis of these observations are summarized as follows: 1. The number of nights with NLC occurrence was a basic value used in previous statistical investigations. It has some shortcomings, which do not allow to extract the correct geophysical information from the time series of this parameter. In this paper some more important parameters for NLCs were introduced and their time series were analyzed. 2. The main seasonally accumulated parameters of NLC activity vary quasi-periodically with a period of about 10 years. Besides, NLC time series contain also distinct quasiperiodic variations with periods of 2 5 years, unlike the interannual variations of solar index F The correlation analysis between the seasonally averaged characteristics of NLCs and the solar activity index F 10.7 is carried out. The strongest correlation is observed between the normalized integral NLC brightness for a season and the solar activity index F 10.7 averaged for June and July. The corresponding cross-correlation function has a decadal periodicity and the analyzed processes are almost in antiphase (minimum of the brightness lags behind about 1 year after the maximum of the solar activity). 4. However, tests for statistical reliability do not allow to distinguish unambiguously the found correlation from a random one. Besides, careful analysis shows that periods for NLC characteristics are years less than the period of 10.5 years for F Thus, the decadal periodicity in NLC parameters appears to be of another origin than direct influence of solar activity. The antiphase behavior of normalized integral NLC seasonal brightness and F 10.7 value can change in other epochs. [28] Acknowledgments. The Moscow NLC database was made thanks to the labor of dozens of Moscow enthusiasts NLC observers, who carried out the observations by a strict procedure. The authors are grateful to A.S. Yatsenko for the creation of the electronic version of the NLC database. References Bronshtein, V. A., and N. I. Grishin, Noctilucent Clouds (in Russian), 359 pp., Nauka, Moscow, Fast, N. P., The Catalogue of Observations of Noctilucent Clouds by World Data (in Russian), 194 pp., Tomskiy Univ., Tomsk, Fast, N. P., The Catalogue of Observations of Noctilucent Clouds by World Data (in Russian), 101 pp., Tomskiy Univ., Tomsk, Gadsden, M., A secular change in noctilucent clouds occurrence, J. Atmos. Terr. Phys., 52, , Gadsden, M., The north-west Europe data on noctilucent clouds: A survey, J. Atmos. Sol. Terr. Phys., 60, , Gadsden, M., and W. Schroeder, Noctilucent Clouds, 165 pp., Springer- Verlag, New York, Grishin, N. I., Noctilucent clouds in (in Russian), Meteorol. Gidrol., 6, 22 26, Grishin, N. I., Instructions for Noctilucent Cloud Observations (in Russian), 23 pp., Izdatelstvo Akad. Nauk SSSR, Moscow, Gromova, L. F., Catalogue of observations of noctilucent clouds on the territory of our country for (in Russian), Issledovaniya po klimatologii serebristyh oblakov, Meteorologiya, vol. 6, pp , Izdatelstvo Akad. Nauk SSSR, Moscow, Handorf, D., V. K. Petoukhov, K. Dethloff, A. V. Eliseev, A. Weisheimer, and I. I. Mokhov, Decadal climate variability in a coupled atmosphereocean climate model of moderate complexity, J. Geophys. Res., 104, 27,253 27,275, Jenkins, G. M., and D. G. Watts, Spectral Analysis and Its Applications, 525 pp., Holden-Day, Boca Raton, Fla., Kropotkina, Ye. P., and N. N. Shefov, Influence of lunar tides on the probability of appearance of noctilucent clouds, Izv. Russ. Acad. Sci. Atmos. Oceanic Phys. Engl. Transl., 11, , 1976.

7 ROMEJKO ET AL.: NOCTILUCENT CLOUD OBSERVATIONS NEAR MOSCOW PMR 10-7 McIntosh, D. H., and M. Hallissey, Noctilucent clouds over western Europe during 1973, Meteorol. Mag., 103, , Romejko, V. A., Noctilucent cloud observations in USSR (in Russian), in Katalog dannyh, pp. 9 25, Mezhduved. Geofiz. Komitet, Moscow, Romejko, V. A., N. N. Pertsev, and P. A. Dalin, Long-term observations of Noctilucent clouds in Moscow: The database and statistical analysis (in Russian), Geomagn. Aeron., 42(5), , Schroeder, W., Noctilucent Clouds, 339 pp., Sci. Ed.//IAGA, Bremen, Sugiyama, T., Statistical study of noctilucent cloud occurrence in western Europe, Proc. NIPR Symp. Upper Atmos. Phys., 11, 81 87, Vasilyev, O. B., Astrophysical Research of Noctilucent Clouds (in Russian), 86 pp., Astron. Sov. AN SSSR, Moscow, Willmann, Ch. I., On the spatial-temporal characteristic of the noctilucent cloud occurrence (in Russian), in Fizika mezosfernyh (serebristyh) oblakov. Trudy soveshchaniya po mezosfernym oblakam (Riga, 1968), pp , Zinatne, Riga, P. A. Dalin, Swedish Institute of Space Physics, Box 812, SE Kiruna, Sweden. (pdalin@irf.se; pdalin@iki.rssi.ru) N. N. Pertsev, A. M. Oboukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevsky per. 3, Moscow , Russia. (meso@omega.ifaran.ru) V. A. Romejko, Space Research Institute, Russian Academy of Sciences, ul. Profsojuznaya 84/32, Moscow, GSP-7, , Russia. (vitrom@ museum.ru)