Seasonal and diurnal variations in f_oF_2 trends

摘要

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_oF_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 maximumvalues of the negative trends in f_oF_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_oF_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_oF_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_oF_2 in winter. Three possible causes of the difference in f_oF_2 and h_mF_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.