Long-term variations in the critical frequency of the midlatitude F2 layerat noon

Based on an analysis of the midday data of the midlatitude ionospheric stat ions in the Northern Hemisphere, it is indicated that the quiet monthly med ian f(0)F2m depends also on the relative sunspot number R* averaged for 11 years: an increase in R* results in decreasing f(0)F2m two years later. Thi s dependence is shown to be the main cause of long-term f(0)F2 variations a t all longitudes. On this basis, it is assumed that longterm variations in the solar UV radiation flux and (or) in the thermospheric parameters should depend on R*. It is indicated that a linear time variation (trend) of the relative f(0)F2 values is a less important cause of the longterm variations in f(0)F2. This ionospheric trend is found to be longitude-dependent: it i s significant and maximal at longitudes of 100 degrees -290 degrees E and v irtually absent over Europe. This effect is apparently associated with the fact that the ionospheric trend is the sum of trends caused by the greenhou se effect P and the secular variations in the magnetic field M. The trends in P and M are shown to be insignificant, but their sum results in a signif icant trend at certain longitudes.