Wave Effects in Variations of the Partial Reflection Radar Signals in Twilight Conditions

For various heliogeophysical conditions during solar terminator passages results of observations of amplitude variations of the partial reflection radar signals (the observatory Tumanny of the Polar Geophysical Institute, 69.0N, 35.7E) at the heights of the D-region of the ionosphere are given. Temporary variations of signal amplitudes which were reflected in the interval of the heights of 75-90 km and their spectra were analyzed. It was found that the components of the time spectrum of the variations which corresponding to resonant frequencies of the atmosphere (the acoustic cut-off and the Brunt-Väisälä ones) in certain cases were intensified. On the basis of the theory of acoustic-gravity waves and the empirical model of neutral structure and temperature of the atmosphere (NRLMSISE-00) identification of the experimental periods corresponding to the atmospheric resonances was executed and calculation of temperature of the neutral atmosphere and sound speed at mesospheric heights was carried out. The received results showed satisfactory consent with data of other independent observations.