Changes with Time in the Relation between the Critical Frequency and Height of the F 2 Layer

The change with time in parameter S, which characterizes the relation of the critical frequency foF2 to the height hmF2 of the ionospheric F 2 layer is considered. The results of measurements by the vertical sounding method at two stations (Moscow and Juliusruh) are analyzed. The dependence of foF2 on hmF2 is plotted for three intervals: 1957–1980, 1996–2023, and 2011–2023. Five near-noon LT moments and two seasons (winter (January and February) and summer (June and July)) are considered. It is found for both stations and both seasons that the S value has been increasing systematically from the earlier to later periods. At the same time, the winter S values are approximately by a factor of 3 higher than the summer ones for all periods. It is suggested that the found changes in parameter S could provide valuable information on long-term variations (trends) in thermospheric parameters with the help of current theoretical models of the TIEGCM or WACCM-X type.