Proceedings with Extended Abstracts (single PDF file) - Radio ...

interactions between tides and planetary waves which are known to take place in the middleatmosphere (for relevant references see Paper B), and the conviction that PW modulateupward propagating tides and through them mediate their signatures in the ionosphere. Also,the role of atmospheric tides on E s formation is well established in the physics of midlatitudesporadic E. Driven by these ideas, we investigated if there is any PW modulation present inthe diurnal and semidiurnal tides measured in Saskatoon and Sheffield and examined ifsimilar effects are concurrently evident in the foEs time series. This, if true, would suggest anew mechanism for the observed relationship between PWs and sporadic E layers.The interaction between PW and atmospheric tidal waves is a nonlinear process whichresembles, for example, amplitude modulation (AM) in communication systems. Thus, onewould expect to find two secondary waves having frequencies respectively equal to the sumand difference of the frequencies of the interacting primary (tidal and planetary) waves. Interms of periods, this means for the 7-day PW that the secondary waves must have periods:(a) for the 12-h tide near 11.1 - 11.2 hours and 12.9 - 13 hours, and (b) for the 24-h tide near21.0 and 28.0 hours. In the following we search for such secondary waves in both theSaskatoon and Sheffield semidiurnal and diurnal tides.The analysis (for details see paper B) showed clear evidence of 7-day modulation on both thediurnal and semidiurnal tides, and on the 24- and 12- hour periodicities in the foEs timeseries. As shown in Figure 2, the 7-day tidal modulation was found to be present in bothamplitude spectra of the zonal (left graphs) and meridional (right graphs) winds measured inSaskatoon (upper panels) and in UK (bottom panels). The two arrow pairs in each plotindicate the position in the spectrum of the secondary peaks discussed previously. Asevidenced in Figure 2, in both, the Saskatoon and UK longitudinal sectors the measured tidesare 7-day amplitude modulated, with the secondary waves being stronger in the zonalcomponent of the semidiurnal tide and the meridional component of the diurnal tide. Toconfirm these results and locate in time the tidal modulations, wavelet transform analysiswas performed on the time series of the tidal amplitudes, after removing the 7-day PW fromthe data in order to avoid any artificial coupling between the tidal amplitudes and the meanwind (again for details see Paper B). The wavelet spectral analysis confirmed the 7-day PWmodulation detected in the correloperiodograms of Figure 2.166The same 7-day PW modulation was also present in the foEs 12- and 24-hour periodicities. Insearch for the 7-day modulation in foEs we applied the same methodology used in the winddata, and looked for spectral peak pairs about the 12-hour periodicity near 11 and 13 hours,and about the 24-hour periodicity near 21.0 and 28.0 hours. Figure 3 shows the foEsamplitude correloperiodograms for Boulder (top panel) and Lannion (bottom panel). As seen,