Seasonal and diurnal variations in foF2 trends

Median values of the F-2 layer critical frequencies observed at ionospheric stations Slough, Juliusruh, and Rome are analyzed in detail. Long-term trends in f(o)F(2), k(f(o)), for the period 1985-2009 are found using the method described earlier by authors. The dependence of the trend value k(f(o)) on local time and month of the year is studied. For all three stations, similar seasonal and diurnal behaviors of k(f(o)) are obtained. The maximum values of the negative trends in f(o)F(2) fall on the noon period of the day (1000-1600 LT). In the seasonal behavior, two maxima of the absolute value of k(f(o)) (at the end of winter and beginning of spring and middle of fall) are observed. The absolute values of the trends are small in the summer months. Possible relation to trends in thermospheric parameters which follow from the obtained conclusions on variations in the f(o)F(2) trends are discussed. Arguments are presented in favor of a decrease in the atomic oxygen concentration due to an increase in the eddy diffusion as the main cause of negative f(o)F(2) trends. It is shown that a negative trend in the atomic oxygen concentration on the order of 10% per decade is able to explain the negative trend in f(o)F(2) in winter. Three possible causes of the difference in f(o)F(2) and h(m)F(2) trends obtained by different authors are formulated and discussed. They are the following: the use of the entire series of data and the absence of the allowance for diurnal and seasonal variations in the trends.