Solar activity effect and diurnal variation in the upper atmosphere

It has been shown that the relation between density variations in the upper atmosphere derived from satellite drag measurements and the solar flux in the decimeter range can be given by ρ∼Sm where ρ is the density, S the solar flux, and the exponent m is a function of altitude and local time. This function has been derived for the altitudes 350, 650, and 1300 km (Figs. 1, 2, and Table 1). Further, the temperature variation at 650 km as function of solar activity, measured by the 20- or 10.7-cm flux, has been calculated for the diurnal maximum (14 h LT) and minimum (5 h LT). To explain the calculated temperature-variation, it is suggested that in addition to the heating of the F layer by solar extreme ultraviolet radiation, another important energy source must be present that also increases toward the maximum of the solar cycle.