Semi-annual Variation in Density at a Height of 90 km

Abstract

A SEMI-ANNUAL variation in upper-atmosphere density, with maxima in April and October and minima in January and July, was first detected in 1961 by Paetzold and Zschorner1 from the analysis of satellite orbits at heights of 210–650 km. By 1965 it seemed that the main features of the semi-annual variation had been established. Using observational values of density obtained at heights between 350 and 750 km, Jacchia2 found that the variation with height of the semi-annual effect was similar to that shown by other types of upper-atmosphere variation. He therefore suggested that the effect could be represented by assuming temperature variations in the thermosphere, the magnitude of the temperature variations being proportional to the average 10.7 cm solar radiation flux, which is often used as the best available index for solar extreme ultraviolet radiation.