Noctilucent clouds getting brighter

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D14, 4195, /2001JD001345, 2002 Noctilucent clouds getting brighter J. Klostermeyer Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany Received 1 October 2001; revised 14 January 2002; accepted 22 January 2002; published 20 July [1] Noctilucent cloud (NLC) observations in northwest Europe show a significant secular increase in the occurrence frequency between 1964 and Although model computations predicted a brightness increase due to increasing stratospheric methane concentrations and a consequent increase in the number of observed NLCs, earlier investigations of the histograms of the solar depression angles at the first and last NLC sightings did not reveal any noticeable brightness change. However, a reinvestigation of the histograms based on statistical model predictions, c 2 tests, and least squares fitting clearly indicates that the NLC brightness increased by a factor of the order of 5 between the periods and The brightness increase goes along with a significant lengthening of the NLC season. Considering recently published trends in the upper mesospheric water vapor mixing ratio and temperature strongly suggests that the brightness increase is caused by an increase of the water vapor concentration rather than a decrease in the temperature. For the analysis also predicts at least a doubling of the NLC brightness between dusk and dawn, in good agreement with lidar observations. INDEX TERMS: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere composition and chemistry; 1610 Global Change: Atmosphere (0315, 0325); KEYWORDS: noctilucent clouds, NLC brightness, NLC season 1. Introduction [2] Analyzing visual observations of noctilucent clouds (NLCs) over northwest Europe, Gadsden [1985, 1990, 1998] found that the number of nights in which NLCs were detected shows a steady secular increase that is modulated by a 10.4-year cycle. Although this modulation is not the subject of the present study, we note in passing that it seems to be related to the solar cycle by a hitherto unknown mechanism. The huge secular increase from 20 nights with observed NLCs in the mid-1960s to more than 40 nights in the mid-1990s is in all probability not caused by changes of observers habits or observational conditions such as tropospheric cloud coverage and therefore points to substantial long-term changes in the high-latitude upper mesosphere near 83 km. [3] Considering the observed increase of the methane concentration and the resulting increase of the water vapor mixing ratio by methane oxidation in the stratosphere and mesosphere, Thomas et al. [1989] predicted a long-term increase in NLC brightness. This process should in turn raise the chance to detect the clouds during twilight conditions and thereby lead to higher and higher NLC occurrence frequencies. Similarly, Gadsden [1990] made the proposition that a small systematic cooling of the summertime high-latitude mesopause region might also enhance the brightness and consequently the occurence frequency of NLCs. There is, however, a problem with these interpretations: Gadsden [1997, 1998] investigated the data of earliest Copyright 2002 by the American Geophysical Union /02/2001JD NLC appearence at dusk and latest disappearence at dawn and could not detect any noticeable secular increase in cloud brightness. [4] Besides the visual observations of NLCs, satellite data of so-called polar mesospheric clouds (PMCs) at visible and ultraviolet wavelengths became available since 1969 [Donahue et al., 1972; Thomas, 1984]. Almost certainly, NLCs and PMCs consist of small ice particles with sizes of several tens of nanometers, and there seems to be no principal difference between the two phenomena. During the past 10 years, ground-based lidars were also successfully used for NLC studies [Thayer et al., 1995; von Zahn et al., 1998]. Both the satellite and lidar methods yield quantitative descriptions of the NLC brightness, mostly in terms of the optical depth or the backscatter ratio. Unlike visual observations, the data thus are not influenced by physiological peculiarities of individual observers. Probably even more importantly, the satellite data are not affected by lower atmospheric phenomena such as haze and clouds. However, the existing satellite and lidar time series are too short to reveal secular trends. Therefore the visual NLC observations are the only ones that can be used at present to reconcile the contradiction between the positions of Thomas et al. [1989] and Gadsden [1997, 1998]. 2. Simulated and Observed Histograms [5] Lidar observations and model simulations indicate that the diurnal variation of the NLC brightness is strongly controlled by semidiurnal and diurnal tides [von Zahn et al., 1998; Klostermeyer, 2001]. In an otherwise undisturbed polar mesopause region the clouds should therefore travel AAC 1-1

2 AAC 1-2 KLOSTERMEYER: NLC GETTING BRIGHTER around the Earth with the horizontal phase trace speed of the tides which, at the latitudes of the visual NLC observations, is 250 m/s and thus 1 order of magnitude larger than typical wind speeds. Consequently, the appearance and disappearance of a cloud is primarily caused by its growth and decay within the field of view of an observer rather than by the advection of an existing cloud through the boundaries of the field of view. This simple picture may of course become more complicated by the occurrence of nonmigrating tides or planetary and gravity waves, for example, but it is confirmed by the fact that the advection of relatively bright clouds through the boundaries of the field of view is generally not seen (M. Gadsden, personal communication, 2001). Excluding the advection of bright clouds will, in particular, allow us to describe the brightness of clouds at the times of their first and last sightings as a function of the solar depression angle. [6] In a clear night the observed zenith brightness of the twilight sky decreases by 5 orders of magnitude if the solar depression angle b changes from 0 to 12 and, at b > 12, approaches the almost constant brightness of the night sky light resulting essentially from airglow and starlight. Observations at directions other than zenith and during overcast skies show essentially the same variation with the exception of a brightness shift which, for the present purposes, can be considered as independent of b [Koomen et al., 1952; Möller, 1957]. In the following we normalize the sky brightness B s by its value B s0 at b =0 and use logðb s =B s0 Þ ¼ bðþ; b where for convenience, log denotes the decadal logarithm and bðþ¼ 3:46 b 10 1 b 2: b 2 þ 1: b 3 for 0 b 15.5, bðbþ ¼ 5:60 ð1þ ð2aþ ð2bþ for b > Equation (2a) is a least squares fit to the data given by Möller [1957] for cloudless skies and is shown in Figure 1. It should be stressed that B s0 is a variable depending on the position at the sky and the visibility conditions in the troposphere and stratosphere. [7] NLCs can be detected against the twilight sky only if the contrast c exceeds a threshold value. In the literature the term contrast has been defined in many different ways [e.g., Dietze, 1973; Avaste, 1993]. In the following, c ¼ B c =B s will be used where B c denotes the brightness caused by the cloud particles alone so that the total NLC brightness is equal to B c + B s. With the exception of a minus sign, equation (3) is the definition used by Dietze [1973]. The threshold value c 0 depends, among other things, on the turbidity of the troposphere and stratosphere; on the stratospheric ozone content; and, very sensitively, on the degree of adaptation of the eye [Dietze, 1973]. For the first and last sightings in the ð3þ Figure 1. Normalized sky brightness as a function of the solar depression angle. The dots indicate data obtained from Möller [1957], and the continuous curve represents the least squares fit b(b). evening and morning twilights, c = c 0. Then equations (1) and (3) yield logðb c =B s0 Þ¼bðbÞþlogc 0 ; describing the relationship between the cloud brightness and the solar depression angle for any values of B s0 and c 0. For observations at a given latitude l, b is subject to the additional constraint b b max ¼ 90 l d; where d is the solar declination. [8] According to Gadsden [1997], the large majority of all NLCs have been observed at solar depression angles between 5 and 15, i.e., when b(b) lies in the range from 2 to 5.6. Estimating the range of log c 0 is more difficult because only a little information is available from the literature. From Avaste [1993], who studied NLC observations from space and used another definition of the contrast, we obtain values of c between 0.4 and 4, indicating that c 0 may be of the order of 0.5. G. Thomas (personal communication, 1997), on the other hand, mentioned c 0 = 5 for visual observations from the ground. If we assume that experienced observers allow for more than 15 min for the adaptation of their eyes, changes of atmospheric and physiological conditions should not cause changes in c 0 by a factor >10 [Dietze, 1973]. In a rough approximation we will therefore assume c 0 = 3 (log c 0 = 0.5) and neglect any variance of log c 0 in regard to the stronger variations of b(b). [9] Since B c /B s0 can vary over several orders of magnitude, it is reasonable to approximate its probability density by a lognormal one, ð4þ ð5þ " # fðþ¼ x pffiffiffiffiffi 1 ðx mþ2 exp 2p s 2s 2 ; ð6þ

3 KLOSTERMEYER: NLC GETTING BRIGHTER AAC 1-3 owing to the fact that the gradient of b(b) approaches zero with increasing b and that b cannot exceed 14. The two samples contain 266 and 360 visible clouds, respectively, with the difference originating predominantly in the small b range. Because of the limited size of the samples, these numbers vary slightly for different realizations. [11] Figure 3 shows the b histograms obtained by Gadsden [1997] from visual observations of the beginning and of the end of NLC displays in the periods and Both diagrams exhibit the same characteristic feature as the simulated histograms in Figure 2: The difference between the total numbers of observed NLCs in the two periods originates mainly in the range of small b values. We note that the observed samples contain a few clouds being visible at b > 14 because there are observers at Figure 2. Simulated histograms of the solar depression angles at the first or last sighting for two cloud samples with m = 4.5 (dashed lines) and m = 3.5 (solid lines). where m and s denote the mean value and the standard deviation of x =log(b c /B s0 ), respectively. To demonstrate the effects of a secular increase in B c on the histograms of the solar depression angle, computations with the ad hoc values m = 4.5, m = 3.5, and s = 1.0 have been performed. We note that the mean value of a difference is equal to the difference of the mean values [Cramér, 1974], m ¼ logb c logb s0 ; ð7þ and assume that any change in logb s0 can be excluded. Then the choice of the m and s values implies that the corresponding mean values of the cloud brightness differ by a factor of 10. Taking into account that the sky is likely to be clear on 3 nights each week, that the NLC season lasts 3 months [Gadsden and Schröder, 1989], and that the observational period extends over 10 years, there are 394 nights to look for NLCs. Therefore two samples, each with 394 values of x, have been realized by means of normally distributed random numbers, and the corresponding b values for the first and last sightings at dusk and dawn have been obtained by solving equation (4) numerically. In equation (5), l =56 and d =20 have been used because most NLC reports came from observers at latitudes between 54 N and 58 N and because 20 is the mean of the solar declination during the NLC season extending approximately from 19 May to 19 August [Gadsden, 1998]. Consequently, all solutions of equation (4) with b >14 or with complex values represent invisible clouds. The resulting b histograms are shown in Figure 2. [10] The main difference between both histograms occurs at b <10, where the histogram for m = 3.5 can obviously be obtained from the histogram for m = 4.5 by shifting the latter one by b 2. According to equation (4), this shift is produced by the almost constant gradient of b(b) for log(b c /B s0 ) = 1. At b >10, however, the difference in the two histograms, if it is significant at all, is much smaller Figure 3. (top) Histograms of the solar depression angles observed at the first noctilucent cloud (NLC) sighting in the periods (dashed lines) and (solid lines); the total numbers are 228 and 367, respectively. (bottom) Same as top but for the last NLC sighting; the total numbers are 221 and 356, respectively. (Data from Gadsden [1997]).

4 AAC 1-4 KLOSTERMEYER: NLC GETTING BRIGHTER l <56 and because d can be smaller than 20, in particular at the end of the NLC season. [12] In conclusion, the qualitative agreement between the simulated and the observed histograms indicates clearly that the observed increase in the total number of NLCs is compatible with the hypothesis that on average, the cloud brightness increased by roughly 1 order of magnitude between the two observational periods. 3. The C 2 Tests [13] The hypothesis of a long-term brightness increase seems to be at odds with the statement of Gadsden [1997, p. 2099] that the data do not contain evidence of an overall increase in brightness of noctilucent clouds over the last three decades. This conclusion has been drawn from c 2 tests of the null hypothesis that the shapes of the small b tails of the histograms for do not differ from those for Only observed frequencies at b 7, but none at b >7, were taken into account. [14] Figure 3 indicates, however, that in particular for , the frequencies at b 7 may be too small for detecting any significant difference in the shapes of the small b tails. Therefore we assume now the more common null hypothesis that the samples for both periods have been drawn from the same population and simply partition the clouds into two classes with b 7 ( bright NLCs ) and b >7 ( dim NLCs ). Then the null hypothesis is equivalent to the assumption that the probability for detecting bright NLCs did not change between the two observational periods. The frequencies are given in Table 1 and yield c 2 = 16.4 and c 2 = 45.1 for the first and last NLC sightings, respectively, with a degree of freedom equal to 1. (For details of c 2 tests see, e.g., Cramér [1974].) The NLC populations in the two periods thus are significantly different, in agreement with the impression gained from Figure 3. In particular, the probability for observing bright clouds in the period was larger than that in the period The error probability is <0.01%. [15] These conclusions do not depend sensitively on the choice of 7 as a limit between the classes of bright and dim NLCs. It should be mentioned that the observed frequencies in Table 1 can also be combined for studying any differences in the NLC populations at the first and last sightings within the same period. For no significant difference has been found, but for the probability for observing bright clouds at dawn is larger than that at dusk (error probability = 6%). 4. Estimating the Brightness Increase [16] In the following we will call the set of visible and invisible clouds polar mesospheric clouds (PMCs) and use the term NLC for visible clouds only. A PMC is invisible if it is kept out of sight by tropospheric and stratospheric turbidity and clouds or if the contrast does not exceed the threshold value c 0 during the night (dark PMCs). [17] According to Gadsden [1998], the histograms in Figure 3 contain observations from longitudes between 20 E and 11 W and from latitudes south of 61 N (mostly from Denmark and Scotland). Therefore it seems to be Table 1. Observed Frequencies for the First (Last) Noctilucent Cloud (NLC) Sighting Period Bright NLCs 30 (20) 100 (120) Dim NLCs 198 (201) 267 (236) extremely difficult to assess the exact fraction of nights with favorable visibility conditions in the troposphere and stratosphere for at least one of the observers. However, it is reasonable to assume that the PMC brightness does not depend on the lower atmospheric visibility conditions. Then an inaccuracy in the fraction of clear nights affects only the total number of PMCs rather than the mean and the variance of their brightness. [18] Since the observed histograms contain no information about dark PMCs, the mean and the variance of log(b c / B s0 ) cannot be estimated by calculating moments. Therefore we assume again the lognormal probability density (equation (6)) and transform it into a frequency function of the solar depression angle, g(b). Then b histograms can be computed from ( ) gðþdb b ¼ rn yn pffiffiffiffiffi exp ½xðÞ m b Š2 dxðþ b 2p s 2s 2 db; ð8þ db where x (b) =log(b c /B s0 ) is given by equation (4), r is the fraction of clear nights, n y is the number of years of any observational period, and n is the annual rate of PMCs. Because of the quasi-10-year cycle in the NLC observations, n must be considered as a mean value over the corresponding period. We recall that m and s represent the mean and the standard deviation of log (B c /B s0 ), respectively. It should also be mentioned that certain fractions of the NLC reports could not be included in the observed histograms because b values are missing (18% for and 6% for ). With the assumption that there is no bias in favor of any b range the effect of the missing b values may be corrected by factors of 0.82 and 0.94, respectively, on the right-hand side of equation (8). Inclusion of these factors, however, would only make sense if the fraction of clear nights, r, was known with sufficient accuracy, which is not the case. [19] In principle, the parameters n, m, and s can be estimated by approximating the observed b histograms (Figure 3) by either least squares fits or least c 2 fits of g(b) [Cramér, 1974]. As compared with the first method, the second one gives more weight to the tails of the histograms and automatically yields a test of whether the assumed lognormal brightness distribution is compatible with the observed histograms. The observed low b tails, however, are probably influenced by the fact that occasionally high cirrus clouds are taken for NLCs [Fogle and Haurwitz, 1966]. Furthermore, according to equation (2b), dx/db = 0 and consequently g =0atb > 15.5 so that no NLCs are predicted at very large depression angles. Since there are tiny fractions of observed NLCs at b > 15.5, least c 2 fits would require additional data manipulations or more sophisticated theoretical considerations. Therefore we have used least squares fits which are less sensitive to the observed histogram tails. An example of a fitted frequency

5 KLOSTERMEYER: NLC GETTING BRIGHTER AAC 1-5 Table 3. Median and Quartiles of y = B c /B s0 for the First (Last) Noctilucent Cloud (NLC) Sighting Period y ( ) ( ) y ( ) ( ) y ( ) ( ) curve is shown in Figure 4, and the parameter estimates for r = 3/7 (3 clear nights per week) are given in Table 2. [20] Between the two periods the annual PMC rates grew by a factor of 2. It should be mentioned that the mean lengths of the NLC season also increased from 62 days during to 87 days during These lengths have been obtained from the dates of the first and last NLC sightings in each year. The difference in the season lengths is significant with an error probability of only 0.2%. The estimated annual PMC rates in Table 2 thus indicate that roughly speaking, one PMC occurred each night during the NLC seasons. [21] Table 2 shows an increase in m between the two periods which can be attributed to a growth of log B c because in all probability, significant secular changes of log B s0 can be excluded (see equation (7)). Similarly, if we assume that B c and B s0 are independent variables and that the variance of B s0 did not change between both periods, the estimated increase in s is due to a growth of the variance of B c. For a lognormal distribution the parameters m and s 2 are not identical to the mean and the dispersion of B c /B s0. Some ideas about the location and dispersion of the distributions of B c /B s0 can be gained from the median together with the lower and upper quartiles (Table 3). [22] A comparison of the results for the first and last sightings within each period indicates that during , there is almost no change in the brightness distribution in the course of the night whereas during , the distribution is shifted to much larger values. The brightness increase between the periods and may be best characterized by an increase of the median values by a factor of 3 at dusk and a factor of 6 at dawn. Figure 4. Histogram of the solar depression angle at the last NLC sighting for and fitted frequency curve. Table 2. Parameters of the Frequency Function of log(b c /B s0 ) for the First (Last) Noctilucent Cloud (NLC) Sighting Period n 47 (51) 97 (87) m 4.0 ( 3.9) 3.5 ( 3.1) s 0.8 (1.0) 1.3 (1.2) 5. Discussion and Conclusions [23] The estimates of the annual PMC rate vary with the assumed fraction of clear nights; an increase of r yields a decrease of n and vice versa. However, the ratio of the n values of two different samples as well as the estimates of m and s do not depend on the choice of r. The secular brightness increase of PMCs obviously implies a corresponding brightness increase of NLCs because even the lower quartiles of B c /B s0 exceed the threshold value of caused by the night sky light by at least 1 order of magnitude. The conclusion that there is a secular brightness increase relies essentially on equation (7) and the assumption that log B s0 remained constant between the two periods under consideration. We note that equation (7) is valid without any restriction concerning the nature of the dependence between the two terms on the right-hand side [e.g., Cramér, 1974]. Therefore equation (7) even holds if, for example, a correlation between B c and B s0 were caused by the fact that both quantities vary with the sky position of a cloud at its first or last sighting. Estimating the distribution of B c in units of cd/m 2 (candela per square meter) is beyond the scope of this study because adequate knowledge about the distribution of B s0 is missing. Of course, under clear sky conditions this distribution may be estimated from the data of Koomen et al. [1952], but haze and clouds in the lower atmosphere cause additional unknown variations, in particular near the horizon [Dietze, 1973]. [24] From the existence of deep depletions of the electron concentration in the summer polar mesopause region, Reid [1990] has concluded that there is a layer of ice clouds with particles which in general are smaller than 10 nm but may occasionally grow to larger sizes and then become visible as NLCs. Moreover, the particles capture free electrons and thereby give rise to strong VHF radar echoes (so-called polar mesosphere summer echoes, PMSEs) [Cho et al., 1992; Klostermeyer, 1994]. The occurrence frequency of PMSEs in the Northern Hemisphere during June and July is close to 100% [Kirkwood et al., 1998], indicating that the layer of ice clouds is a quasi-permanent phenomenon. The result that a PMC occurred virtually each night during the NLC seasons thus is in good agreement with the observed PMSE frequency. [25] According to Table 3, there is a brightness increase by a factor of 2 or 3 between dusk and dawn in the period

6 AAC 1-6 KLOSTERMEYER: NLC GETTING BRIGHTER von Zahn et al. [1998] found a similar increase in the maximum backscatter ratio of NLCs measured by a lidar over Andøya (69 N, 16 E) within the years The increase is part of a diurnal variation of the backscatter ratio, which is dominated by a semidiurnal component and has minima near 30 shortly before local midnight and maxima between 60 and 110 in the early morning hours. [26] The agreement of the estimated nocturnal brightness increase with the result of von Zahn et al. [1998] yields further evidence that at least the order of magnitude of the estimated secular brightness increase is correct. Both an increase of the middle-atmospheric water vapor [Thomas et al., 1989] and a decrease of the mean mesopause temperature [Gadsden, 1990] have been proposed as possible origins of the secular increase in the number of observed NLCs. At a height of 70 km, Thomas et al. estimated an increase in the water vapor mixing ratio from 5.2 ppmv in 1940 to 6.0 ppmv in 1990 as a result of an increase in methane, yielding an average trend near 0.02 ppmv/y. Studying the saturation pressure of water vapor, Gadsden postulated a temperature decrease of 7 K over 20 or 30 years at the mesopause, i.e., a trend of roughly 0.3 K/y. Recently published trends of the water vapor mixing ratio and of the temperature in the upper mesosphere at latitudes between 65 N and 71 N are equal to 0.2 ppmv/y in the period [Nedoluha et al., 1998] and to 0.02 K/y in the period [Lübken, 2000] so that, in all probability, the increase of the PMC brightness is caused by a tremendous trend in the water vapor. The trend reported by Nedoluha et al. [1998], however, is much larger than the water vapor trend associated with the increasing methane concentration and is probably not valid for the whole period from 1964 to We therefore assume an average trend of 0.1 ppmv/y, an initial mixing ratio of 4 ppmv in 1970, and a mean mesopause temperature of 132 K over the NLC period. Over 20 years the model predictions of Thomas et al. [1989] then yield a brightness increase by a factor of 4.5, which is in good agreement with the values found in section 4. [27] According to Evans et al. [1998] and Nedoluha et al. [1998], presently available mesurements of stratospheric methane do not show a systematic secular trend and can only partly account for the observed trend in the water vapor so that in all probability there is an increase in the upward transport of water. Any significant change of the vertical velocity of the air in the upper polar mesosphere, however, can be excluded because it would produce corresponding changes of the adiabatic cooling, in contradiction to the measurements of Lübken [2000]. In agreement with Evans et al. [1998] and Nedoluha et al. [1998] it is therefore suggested that the increased upward water transport is a consequence of an increasing amount of water entering the middle atmosphere from the troposphere. [28] The observed increase in the water vapor concentration together with an almost unchanged mesospheric temperature profile should lead to an earlier beginning and a later end of the NLC season. This conclusion is in agreement with a statement of Gadsden [1985] that during , clouds have been observed both earlier and later in the summer than during Comparing histograms of the day number of cloud observations for and , Gadsden [1998] found an increase of 12 days in the difference between the ninth and the first deciles, in qualitative agreement with the present results that the length of the NLC season obtained from the first and last cloud sightings in each year has increased from 62 to 87 days. However, despite the 12- day difference between the ninth and first deciles, Gadsden [1998] concluded that there has been no change in the length of the season concomitant with the secular change of the NLC occurrence frequency. For a final discussion about the lengthening of the NLC season it would be necessary to assess the relationship between the two definitions of the length of the season as well as the significance of the 12-day difference found by Gadsden. This is beyond the scope of the present study, but whichever definition of the length of the NLC season is adopted, the lengthening of the season alone can in no case fully account for the secular increase in the NLC occurrence frequency. [29] Acknowledgments. I am grateful to Michael Gadsden for answering many questions and generously providing his NLC database. The computations were performed on the workstation cluster of the Gesellschaft für wissenschaftliche Datenverarbeitung in Göttingen. References Avaste, O., Noctilucent clouds, J. Atmos. Sol. Terr. Phys., 55, , Cho, J. Y. N., T. M. Hall, and M. C. Kelley, On the role of charged aerosols in polar mesospheric summer echoes, J. Geophys. Res., 97, , Cramér, H., Mathematical Methods of Statistics, Princeton Univ. Press, Princeton, N. J., Dietze, G., Der Einfluss atmosphärischer und physiologischer Bedingungen auf die Homogenität von Beobachtungen leuchtender Nachtwolken, Gerl. Beitr. Geophysik, 82, , Donahue, T. M., B. 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7 KLOSTERMEYER: NLC GETTING BRIGHTER AAC 1-7 spheric water vapor as observed by the Halogen Occultation Experiment and the ground-based Water Vapor Millimeter-wave Spectrometer from , J. Geophys. Res., 103, , Reid, G. C., Ice particles and electron bite-outs at the summer polar mesopause, J. Geophys. Res., 95, 13,891 13,896, Thayer, J. P., N. Nielsen, and J. Jacobsen, Noctilucent cloud observations over Greenland by a Rayleigh lidar, Geophys. Res. Lett., 22, , Thomas, G. E., Solar Mesosphere Explorer measurements of polar mesospheric clouds (noctilucent clouds), J. Atmos. Terr. Phys., 46, , Thomas, G. E., J. J. Olivero, E. J. Jensen, W. Schroeder, and O. B. Toon, Relation between increasing methane and the presence of ice clouds at the mesopause, Nature, 338, , von Zahn, U., G. von Cossart, J. Fiedler, and D. Rees, Tidal variations of noctilucent clouds measured at 69 N latitudebyground-basedlidar, Geophys. Res. Lett., 25, , J. Klostermeyer, Max-Planck-Institut für Aeronomie, D-37191, Katlenburg-Lindau, Germany. ( jkloste@gwdg.de)