Spectral energy contributions of quasi-periodic oscillations (2±35 days) to the variability of the foF2

The relative contributions of quasi-periodic oscillations from 2 to 35 days to the variability of foF2 at middle northern latitudes between 42°N and 60°N are investigated. The foF2 hourly data for the whole solar cycle 21 (1976±1986) for four European ionospheric stations Rome (41.9°N, 12.5°E), Poitiers (46.5°N, 0.3°E), Kaliningrad (54.7°N, 20.6°E) and Uppsala (59.8°N, 17.6°E) are used for analysis. The relative contributions of di€erent periodic bands due to planetary wave activity and solar ̄ux variations are evaluated by integrated percent contributions of spectral energy for these bands. The observations suggest that a clearly expressed seasonal variation of percent contributions exists with maximum at summer solstice and minimum at winter solstice for all periodic bands. The contributions for summer increase when the latitude increases. The contributions are modulated by the solar cycle and simultaneously in ̄uenced by the long-term geomagnetic activity variations. The greater percentage of spectral energy between 2 to 35 days is contributed by the periodic bands related to the middle atmosphere planetary wave activity.