6-HOUR ZONALLY SYMMETRICAL TIDAL OSCILLATIONS OF THE WINTER MESOPAUSEOVER THE SOUTH-POLE STATION

Four complete cycles of a 6-h quasi-monochromatic variation of OH Mein el (3,1) and (4,2) band airglow brigtness (I) and rotational distribut ion temperature (T) were observed with a near infrared (1.0-1.7 mu) Mi chelson Interferometer operating at the South Pole Station in Antarcti ca. The simultaneous occurrence of a quasi-periodic variation in I and T suggests the presence of a coherent wave phenomenon in the mesopaus e region. A power spectral density analysis of a continuous 24-h recor ding of I and T on 24 May 1992, indicates a 6+/-1-h period oscillation (Sivjee et al., EOS 74 (Suppl.), 48, 5A 11A-10 1993a). The propagatio n of 6-h wave-like disturbances in the high latitude winter mesopause is substantiated by more recent extended-period airglow measurements a t approximately 81.6-degrees-N latitude around Eureka, Canada (Sivjee et al, EOS 74 (Suppl.), 43, 5A 21A-4, 1993b; Sivjee et al, Planet. Spa ce Sci. 42, in press, 1994). The period and coherency of the oscillati on point to a tidal origin, while its occurrence at a geographic pole suggests a zonally symmetric tide. Unlike the globally dominant migrat ing tides, and other zonally propagating tides, for which the quantiti es that generate airglow fluctuations vanish at the pole, the 6-h zona lly symmetric standing tide has its maximum amplitude in these quantit ies at the poles. To examine whether the observed airglow variations w ere consistent with a zonally symmetric non-migrating tide, comparison s with airglow theory were made in terms of Krassovsky's ratio (Ann. G eophys. 28, 739, 1972) (the ratio of the fractional change in brightne ss to the fractional change in temperature). The observed value was 9/-3 for amplitude and 30-degrees +/- 15-degrees for phase, with I lead ing T. The agreement with the predicted amplitudes is good for the mod es which are sharply peaked near the pole. The predicted phase of I re lative to T is not as large as the observed value, but is in the right sense and a mixture of modes could readily account for the larger obs erved phase. The latter is consistent with the predicted phase at the 95% confidence level. The magnitude of the predicted Krassovsky's rati o for a 6-h period gravity wave is reasonably close to the observed va lue. However, we rule out a gravity wave source because the predicted phase indicates that the temperature fluctuation leads the brightness fluctuation, opposite to what is observed, and the absolute difference between the observed and predicted phase is quite large. We conclude that considerations based on tidal theory and a model of wave driven f luctuations in OH airglow strongly favor an interpretation in terms of a zonally symmetric tide over zonally propagating tides and gravity w aves.