ON KRASSOVSKYS RATIO FOR TER-DIURNAL HYDROXYL OSCILLATIONS IN THE WINTER POLAR MESOPAUSE

Past observations of ter-diurnal oscillations in hydroxyl brightness a nd rotational temperature of the winter polar mesopause have revealed two apparently conflicting facets. The 8 h wave is a commonly observed feature, possibly the most recurring tide of that region. On the othe r hand, temperature changes were observed to lead changes in brightnes s, implying a negative phase of Krassovsky's ratio. (Krassovsky's rati o is the ratio of normalized airglow brightness variation to normalize d temperature variation.) This qualitatively supports a gravity wave e xplanation according to predictions of current dynamical-chemical mode ls of fluctuations in airglow emissions. But the ter-diurnal oscillati ons are not likely due to gravity waves because of the persistence of the oscillations. Here a statistically-significant sample (23 individu al days) of clear ter-diurnal waves of the winter polar mesopause over Eureka, NWT (80 degrees N), is considered in order to establish the r ange of observed amplitude and phase of Krassovsky's ratio and compare with the predicted values. The observed amplitude of Krassovsky's rat io was 2.6-8.1 with the majority of cases (74%) at 4 +/- 1. The phase of Krassovsky's ratio was negative for all 23 cases, i.e. changes in t emperature led changes in brightness. In the majority of cases the neg ative phase was significantly different from zero. Theoretical calcula tions for two different model atmospheres (both isothermal and non-the rmal) were performed using a current model of tidally driven hydroxyl airglow fluctuations. The predicted phase of Krassovsky's ratio was sl ightly positive (less than or equal to +30 degrees) for an 8 h gravita tional zonally symmetric tide-of meridional index 2-5 and a migrating tide of meridional index 3-7. It is concluded that the theoretical pre dictions of Krassovsky's ratio of oscillations in hydroxyl emission in duced by zonally symmetric tides and migrating tides as applied to the two model atmospheres do not agree with the above observations the wi nter polar mesopause. Neither do the theoretical predictions of Krasso vsky's ratio for gravity wave-driven fluctuations in OH nightglow from an extended, dissipative emission region match the observed value for the vast majority (87%) of days with clear ter-diurnal oscillations, The consistently negative phases over Eureka probably reflect unique c onditions in the very high latitude winter mesopause region characteri zed by strong lapse rates in the emission layer or background minor co nstituent concentrations that differ significantly from model predicti ons. (C) 1997 Published by Elsevier Science Ltd.