Six-hour zonally symmetric tidal oscillations of the winter mesopause over the South Pole Station

Abstract

Four complete cycles of a 6-h quasi-mono-chromatic variation in OH Meinel (3,1) and (4,2) band airglow brightness ( I) and rotational distribution temperature ( T) were observed with a near infrared (1.0–1.7 μ) Michelson Interferometer operating at the South Pole Station in Antarctica. The simultaneous occurrence of a quasi-periodic variation in I and T suggests the presence of a coherent wave phenomenon in the mesopause region. A power spectral density analysis of a continuous 24-h recording 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 propagation of 6-h wave-like disturbances in the high latitude winter mesopause is substantiated by more recent extended-period airglow measurements at ∼81.6°N latitude around Eureka, Canada (Sivjee et al, EOS 74 (Suppl.), 43, 5A 21A-4, 1993b; Sivjee et al, Planet. Space Sci. 42, in press, 1994). The period and coherency of the oscillation point to a tidal origin, while its occurrence at a geographic pole suggests a zonally symmetric tide. Unlike the globally dominant migrating tides, and other zonally propagating tides, for which the quantities that generate airglow fluctuations vanish at the pole, the 6-h zonally symmetric standing tide has its maximum amplitude in these quantities at the poles. To examine whether the observed airglow variations were consistent with a zonally symmetric non-migrating tide, comparisons with airglow theory were made in terms of Krassovsky's ratio ( Ann. Geophys. 28, 739, 1972) (the ratio of the fractional change in brightness to the fractional change in temperature). The observed value was 9 ± 3 for amplitude and 30° ± 15° for phase, with I leading T. The agreement with the predicted amplitudes is good for the modes which are sharply peaked near the pole. The predicted phase of I relative 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 observed phase. The latter is consistent with the predicted phase at the 95% confidence level. The magnitude of the predicted Krassovsky's ratio for a 6-h period gravity wave is reasonably close to the observed value. 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 fluctuations in OH airglow strongly favor an interpretation in terms of a zonally symmetric tide over zonally propagating tides and gravity waves.