Seasonal behavior of middle-latitude short-wave mesospheric radio echoes

Using the radar of the "Sura" facility, we studied the diurnal and seasonal behavior of mesospheric middle-latitude short-wave radio echoes. It was, found that the diurnal dependence of the intensity of reflected signals is mainly determined by the solar illumination of the mesosphere, i.e., by photoionization intensity. We revealed an almost permanent and season-independent presence of a thin turbulent layer at altitudes of 82 to 84 km whose radio echoes are maximal in summer and winter and much weaker in spring and fall. Sporadic summer radio echoes with increased intensity, which are observed at somewhat greater (by 2 to 4 km) altitudes, apparently correspond to the phenomenon of mesospheric summer echo (MSE) observed in the VHF range.[PUBLICATION ABSTRACT]