Timchenko, A. V., Bessarab, F. S., Koren'kov, Yu. N., Koren'kova, N. A., Borchevkina, O. P., and Ratovsky, K. G.
A study of the wave changeability in the F2-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 foF2 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 foF2 oscillations with periods of 25–30 days, conversely, were very stable and changed insignificantly. The amplification of the variability in the foF2 oscillation spectrum in other frequency ranges also is another manifestation of stratospheric warmings over Kaliningrad and Irkutsk. [ABSTRACT FROM AUTHOR]