Plasma density measurements from a polar‐orbiting Defense Meteorological Satellite Program satellite at 840 km have shown the existence of significant plasma depletions near the magnetic equator in the postsunrise hours. The phenomenon is observed regularly during magnetically quiet times near the equinoxes. The magnitude of the depletion depends upon longitude and the time elapsed since sunrise in the lower F region. It is significantly less pronounced or absent near the solstices. During magnetically active periods the low‐latitude topside ionosphere is modified due to vertical E × B plasma drifts in the predawn sector. The quiet time equinox observations are interpreted in terms of a diurnal flux tube filling model. It is suggested that the observed equatorial depletions result from a pair of plasma fronts that are moving upward from conjugate ionospheres and that are associated with the early stages of the postsunrise refilling process.

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Monthly averages of the zonal and meridional neutral wind data recorded over Obninsk (55°N), Heiss Island (80.5°N) and Molodezhnaya (67°S), using meteor radar technique, during the period 1967-71 are analyzed in detail in terms of prevailing and periodic components. Results of the geographical variation of the data reveal the presence of comparatively higher amplitudes of the wind components at Molodezhnaya station. It is observed that higher amplitudes of the semi-diurnal component are mostly associated with lower phases at all the three stations. The momentum flux due to the diurnal and semi-diurnal wind components are calculated for all the seasons at each site. The results are compared with the measurements made at other latitudinal stations. Zusammenfassung iJber atmosphiidsche Gezeitenwindiinderungen in 95 km H6heEs werden Monatsmittel der mit Radarmessungen yon Meteoritenbahnen tiber Obninsk (55°N), Heiss Island (80,5°N) und Molodeshnaya (67°S) in den Jahren 1967 bis 1971 gemessenen zonalen und meridionalen Windbeobachtungsdaten in bezug auf vorherrschende und periodische Komponenten analysiert. Aus den geographischen Unterschieden der Beobachtungsdaten ergeben sich verhiiltnism~iNg gr6gere Amplituden an der Station Molodeshnaya. Es wurde beobachtet, dag an allen drei Stationen gr6fSere Amplituden der halbtiigigen Komponente meist mit kleineren Phasen verbunden sind. Ftir alle Jahreszeiten wird for jede Station der Impulsflug zufolge der t~iglichen und halbt~igigen Windkomponente berechnet. Die Ergebnisse werden mit Messungen an Stationen in anderen Breiten verglichen.