Features of Variations in the F 2-Layer Critical Frequency During the Sudden Stratospheric Warnings of 1966–2009 According to Data from Kaliningrad and Irkutsk Stations

A study of the wave changeability in the F 2-layer critical frequency over Kaliningrad (54.6° N, 20° E) and Irkutsk (53° N, 103° E) stations during strong sudden stratospheric warmings is presented. Low solar activity was a common factor for all selected cases. The results of wavelet analysis of the time variations in foF 2 showed that an amplification of the oscillations with periods of 4–10 days occurred in the majority of the warmings within a period of 1965–2009. The amplitude of long-period (25–30 days) oscillations, vice versa, decreased. A similar analysis performed for the 1975–1976 winter, during which there were no warmings, showed that the amplitudes of the foF 2 oscillations with periods of 25–30 days, conversely, were very stable and changed insignificantly. The amplification of the variability in the foF 2 oscillation spectrum in other frequency ranges also is another manifestation of stratospheric warmings over Kaliningrad and Irkutsk.