Apostolov, E. M.

The aim of this work within the WP 3.1 of the COST 271 Action is the characterization of the variability introduced in the F-region ionosphere by -Planetary Wave Signatures- (PWS) and -Gravity Wave Signatures- (GWS). Typical patterns of percentage of time occurrence and time duration of PWS, their climatology and main drivers, as well as their vertical and longitudinal structure have been obtained. Despite the above characterization, the spectral distribution of event duration is too broad to allow for a reasonable prediction of PWS from ionospheric measurements themselves. GWS with a regular morning/evening wave bursts and specific GWS events whose arising can be predicted have been evaluated. As above, their typical pattern of occurrence and time duration, and their vertical structure have been obtained. The latter events remain in the ionospheric variability during disturbed days while additional wave enhancements of auroral origin occur. However, both types of disturbances can be distinguished.

Seasonal and solar cycle variations of the quasi-two- and five-day oscillation amplitudes off f0F2 are evaluated by moving periodogram analysis. The 23 year time series (1964-1986) of fOF2 hourly values of Kaliningrad (54.7°N, 20.62°E), covering the solar cycles 20 and 21, is used for the analysis. Long term variations of these amplitudes are modulated by the 11-year solar cycle and are simultaneously influenced by the geomagnetic activity. The annual variation of the quasi-two- and five-day oscillation amplitudes has very clear maxima near the equinoxes. The mechanism of the influence of the travelling planetary waves in the meteor wind region by vertical plasma drift to the F2-layer electron density maximum is discussed.