Several semi-empirical models of the terrestrial upper atmosphere are presently available. These models take into account solar activity effects by using the solar decimetric flux as an index. Such a procedure is a consequence of the lack of continuous determinations of the solar spectrum directly responsible for the physical structure of the upper atmosphere. Variations of the thermopause temperature are discussed. Using five sets of solar irradiances measured in the ultraviolet and in the extreme ultraviolet, the penetration of solar radiation is analyzed as a function of solar activity. Several examples of absorption profiles and ion production rates are discussed for variable conditions. Various energetic effects are also described. All computations are made for physical conditions above Scheveningen (52.08° N) where the 14th ESLAB symposium was held.