[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.

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Abstract. Noctilucent clouds (NLC) are the highest clouds in the Earth's atmosphere, observed close to the mesopause at 80-90 km altitudes. Systematic NLC observations conducted in Moscow for the period of 1962-2005 and in Denmark for 1983-2005 are compared and statistical results both for seasonally summarized NLC parameters and for individual NLC appearances are described. Careful attention is paid to the weather conditions during each season of observations. This turns out to be a very important factor both for the NLC case study and for long-term data set analysis. Time series of seasonal values show moderate similarity (taking into account the weather conditions) but, at the same time, the comparison of individual cases of NLC occurrence reveals substantial differences. There are positive trends in the Moscow and Danish normalized NLC brightness as well as nearly zero trend in the Moscow normalized NLC occurrence frequency but these long-term changes are not statistically significant. The quasi-ten-year cycle in NLC parameters is about 1 year shorter than the solar cycle during the same period. The characteristic scale of NLC fields is estimated for the first time and it is found to be less than 800 km.

Abstract.Ground-based spectrographical observations of infrared emissions of the mesopause region have been made at Zvenigorod Observatory (56 N, 37 E), located near Moscow, Russia, for 670 nights of [2000][2001][2002][2003][2004][2005][2006]. The characteristics of the hydroxyl and molecular oxygen (865 nm) airglow, heights of which correspond to 87 and 94 km, are analyzed for finding their response to solar activity. The measured data exhibit a response to the F 10.7 solar radio flux change, which is 30%-40%/100 sfu in intensities of the emissions and about 4.5 K/100 sfu in hydroxyl temperature. Seasonal variations of the airglow response to solar activity are observed. In winter it is more significant than in summer. Mechanisms that may provide an explanation of the solar influence on intensities of the emissions and temperature are considered. Radiative processes not involving atmospheric dynamics appear insufficient to explain the observed effect.

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Long-term systematic observations of noctilucent clouds in the regions of Moscow (Russia), Vilnius (Lithuania), and La Ronge (Canada) are considered. Variables, describing the seasonal activity of noctilucent clouds, are discussed. It is shown that there are no statistically significant trends within time intervals of several recent decades. This result is compared to other known findings on trends in mesospheric clouds. Based on the data of the modern ground-based noctilucent cloud observing network in the northern hemisphere and simultaneous satellite data on mesospheric temperature and humidity, we estimate sensitivity of noctilucent clouds to the relative humidity of the upper mesosphere. Such an approach allows us to discuss possible changes of the upper-mesospheric relative humidity, which are consistent with a zero secular trend in noctilucent cloud activity.