We consider a unique case of a propagating internal gravity wave that has generated in situ a compact and thin layer of noctilucent clouds (NLC) at 82.7-85.2 km with a characteristic horizontal scale of 65-70 km, as observed in the Moscow region on the night of 18-19 July 2013. This particular transient isolated gravity wave together with the whole NLC layer suddenly appeared in the clear twilight sky and lasted about 1 h traveling eastward, which differs significantly from previously observed cases of gravity waves propagating through preexisting NLC layers. Our model studies demonstrate that the wave had a tropospheric source connected to the passage of an occluded front. The wave was likely generated due to strong horizontal wind shears at about 5 km altitude.

Electron diffraction studies of the structural characteristics of Ar–CO2 solid solutions are carried out over the entire range of their mutual concentrations. The regions of low concentrations of both components are analyzed in detail and the relative excess volumes Δv/v are determined for the CO2 impurity in solid Ar (Δv/v≃0.8), and for the Ar impurity in solid CO2 (Δv/v≃0.28). The experimental results and relationships are compared with the results obtained from the semi-quantitative theory on the basis of known atom–atom potentials. An expression relating the intensity of superstructural reflections I(super) (including the reflection 210) with the long-range orientational order parameter η is obtained. It is found that I210∝η2 to a high degree of precision. The experimental dependence of the orientational order parameter η on the argon concentration in solutions rich in CO2 is obtained.

Using the transmission high-energy electron diffraction technique, we studied the temperature dependence of the diffraction reflection intensities from the solid phase of CO 2. To deduce absolute values of the orientational order parameter, we modified the existing reconstruction method to account for the triatomic shape of the molecule. It is shown that for triatomics solids a higher-rank order parameter, h 4 , is important and cannot be neglected. Application of the modified approach yielded h and h 4 values as a function of temperature over the range from 6 to 70 K. Both orientational order parameters proved to be a nonmonotone function of temperature.

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