Long-period variations of scattered twilight.

Photoelectric photometry and spectral observations of twilight are used to investigate long-period variations in scattered twilight. The observations were made during two solar activity cycles and at certain times when dust of terrestrial and space origin was present in the upper atmosphere. An anticorrelation between the intensity of scattered twilight and solar activity is discovered, along with a direct correlation between atmospheric turbidity near a height of 75 km and solar activity. It is found that the atmospheric density high in the upper thermosphere varies in phase with solar activity cycle, while the density in the mesosphere and lower thermosphere varies in antiphase with solar activity cycle. The long-term variations in scattered twilight are shown to have a period of 5 to 6 years and to be associated with solar activity.