Relation of long-period variations in the critical frequencies of the F 2 layer to geomagnetic activity

The relation of the long-period variations in the midnight and noon values of the critical frequency of the ionospheric F 2 layer at three midlatitude stations (Irkutsk, Moscow, and Boulder) to the daily mean index of geomagnetic activity in years of different solar activity has been studied. It has been found that the correlation coefficients between the above parameters depend on time of day, season, and solar activity level. The correlation coefficients are higher at night than in the daytime, especially at low solar activity. The highest absolute values of the correlation coefficient most often appear during equinoxes: April–May and September–October. It has been shown that the variability of the critical frequencies of the midlatitude ionospheric F 2 layer depends not only on geomagnetic activity but also (to a considerable degree) on the effect of the lower atmosphere.