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the 12-h and 24-h periodicities in foEs are present at both stations, with the diurnalperiodicity being dominant. Figure 3 shows clearly that the 24- and 12-hour periodicities infoEs are both also 7-day amplitude modulated. The presence of the 7-day modulation wasalso confirmed by wavelet transform analysis (see again for details Paper B).4. SummaryA large amplitude, 7-day period westward propagating S = 1 planetary wave (PW) has beenreported from ground radar and satellite wind measurements in the mesosphere-lowerthermosphere (MLT) during the second half of August and well into September, 1993.Following recent suggestions that planetary waves might play a role in the formation ofmidlatitude sporadic E layers , we have analyzed, for the period from August 1 to September30, 1993, the sporadic E critical frequency (foEs ) time series from 8 midlatitude ionosondestations covering a longitudinal zone from about 58 0 E to 157 0 W. The analysis revealed thatall 8 station foEs data showed a strong 7-day periodicity, occurring concurrently with the 7-day planetary wave reported elsewhere. This provided the first direct proof in favor of a PWrole on E s formation. The PW-E s relationship was investigated further by considering thevariations in the mesospheric neutral wind measured with radars in Canada and UK. Ouranalysis showed clearly that E s is affected indirectly by the PW through the action of thediurnal and semidiurnal tides which are strongly modulated by the same PW, apparentlythrough a nonlinear interaction process at altitudes below 100 km. This 7-day PWmodulation was found to be clearly present simultaneously in the amplitude of the zonal 12-hour tidal wind, the meridional 24-hour tidal wind, and in both, the 12-hour and 24-hourperiodicities which existed in the foEs time series. These results suggested an explanation forthe observed relation between sporadic E layers and planetary waves.5. AcknowledgementsThis work was supported by the Greek Secretariat for Research and Technology and theBritish Council in Athens through a Greek-British Collaboration Research grant for 2001-2003. Support was also provided by the European Office of Aerospace Research andDevelopment (EOARD), Air Force office of Scientific Research, Air Force ResearchLaboratory, under contracts F61775-01-WE004 and FA8655-03-1-3028 to C. Haldoupis.We thank H. G. Muller, for providing the Sheffield, and to C. Meek and A. Manson for theSaskatoon, wind data.6. ReferencesClark, R. R., M. D. Burrage, S. J. Franke, A. H. Manson, C. E. Meek, N. J. Mitchell, and H.G. Muller, Observations of 7-day planetary waves with MLT radars and UARS/HRDIinstrument, J. Atmos. Sol.-Terr. Phys., 64, 1217, 2002.Haldoupis, C., and D. Pancheva, Planetary waves and midlatitude sporadic E layers: Strongexperimental evidence for a close relationship, J. Geophys. Res., 107, ido10.1029/2001JA000212, 2002.Mathews, J.D. Sporadic E: current views and recent progress, J. Atmos. Sol.-Terr. Phys., 60,413, 1998.Pancheva, D., C. Haldoupis, C. E. Meek, A. H. Manson, and N. Mitchell, Evidence of a rolefor modulated atmospheric tides in the dependence of sporadic E layers on planetary waves,J. Geophys. Res., 108, ido10.1029/ 2002JA009788, 2003.167