Evidence of a role for modulated atmospheric tides in the dependence of sporadic <i>E</i> layers on planetary waves

<jats:p>A large amplitude, 7‐day period westward propagating S = 1 planetary wave (PW) of global response has been reported from ground radar and satellite wind measurements in the mesosphere‐lower thermosphere (MLT) during the second half of August and well into September 1993. Following recent suggestions that PW might play a role in the formation of midlatitude sporadic <jats:italic>E</jats:italic> layers (<jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub>), <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#jgra16912-bib-0007"><jats:italic>Haldoupis and Pancheva</jats:italic> [2002]</jats:ext-link> found a strong 7‐day periodicity present in all stations concurrently with the 7‐day planetary wave reported elsewhere, by analyzing sporadic <jats:italic>E</jats:italic> critical frequency (<jats:italic>foEs</jats:italic>) time series from eight midlatitude ionosonde stations covering a longitudinal zone from about 58°E to 157°W. This study provided the first direct proof in favor of a PW role on <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> formation. In the present paper we further investigate this role by considering the same PW event and correlating the 7‐day periodicity in <jats:italic>foEs</jats:italic> directly with concurrent variations in the mesospheric neutral wind measured with atmospheric radars in Saskatoon, Canada, and in Sheffield, United Kingdom. Although our analysis cannot exclude a direct PW role on <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> formation, it shows clearly that <jats:italic>E</jats:italic><jats:sub><jats:italic>s</jats:italic></jats:sub> is affected indirectly by the PW through the action of the diurnal and semidiurnal tides which are strongly modulated by the same PW, apparently through a nonlinear interaction process at altitudes below 100 km. This 7‐day PW modulation 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‐hour periodicities which existed in the <jats:italic>foEs</jats:italic> time series. The results here provide a new physical explanation for the observed relation between sporadic <jats:italic>E</jats:italic> layers and planetary waves.</jats:p>