Abstract
Abstract. Airglow observations with a Spectral Airglow Temperature Imager (SATI), installed at the Sierra Nevada Observatory (37.06° N, 3.38° W) at 2900-m height, have been used to investigate the presence of tidal variations at mid-latitudes in the mesosphere/lower thermosphere region. Diurnal variations of the column emission rate and vertically averaged temperature of the O2 Atmospheric (0-1) band and of the OH Meinel (6-2) band from 5 years (1998-2003) of observations have been analysed. From these observations a clear tidal variation of both emission rates and rotational temperatures is inferred. It is found that the amplitude of the daily variation for both emission rates and temperatures is greater from late autumn to spring than during summer. The amplitude decreases by more than a factor of two during summer and early autumn with respect to the amplitude in the winter-spring months. Although the tidal modulations are preferentially semidiurnal in both rotational temperatures and emission rates during the whole year, during early spring the tidal modulations seem to be more consistent with a diurnal modulation in both rotational temperatures and emission rates. Moreover, the OH emission rate from late autumn to early winter has a pattern suggesting both diurnal and semidiurnal tidal modulations.
Highlights
Airglow emissions have been used to study the chemical and dynamical behaviour of the atmosphere in those regions where the emission takes place
The diurnal tide for the OH emission rate is predominant in spring, as for the O2 emission rate and both temperatures, and seems to be present from late autumn to early winter. This tidal behaviour of the OH emission rate during winter suggests that the diurnal type of modulation is present, the conclusion has to be considered with caution, because, as can be seen in Table 4, the standard deviation after the fitting is significantly smaller for December, while for January the standard deviation after the fitting is very close to that obtained by considering a semidiurnal tidal modulation in fitting of the data
The observed amplitude temperature growth factors are greater during May and August but, due to the undetectability or small amplitude variations detected in the OH rotational temperatures from May to August, the growth temperature factor during these months is affected by large uncertainties, and there is no confidence in the growth factor obtained
Summary
There is an enormous body of ground-based observations of the mesosphere and lower thermosphere by the global radar network (e.g. Jacobi et al, 1999; Pancheva et al, 2000, 2002; Riggin et al, 2003; Forbes et al, 2004; Manson et al, 2004; Portnyagin et al, 2004) that have been used to study the tidal behaviour of the atmospheric winds. Studies of the influences of atmospheric tides on airglow emission and temperature using long-term, ground-based observations (e.g. Wiens and Weill, 1973; Petitdidier and Teitelbaum, 1977; Scheer and Reisin, 1990; Reisin and Scheer, 1996; Takahashi et al, 1998; Choi et al, 1998) and satellite airglow observations (Abreu and Yee, 1989; Burrage et al, 1994; Shepherd et al, 1995, 1998) have shown a large variation in diurnal behaviour of the airglow emission rates as a function of the year and of the latitude. We analysed the tidal variation found from longterm, ground-based airglow observations at 37.06◦ N latitude These observations have been made by a Spectral Airglow Temperature Imager (SATI) instrument placed at the Sierra Nevada Observatory. The amplitudes of the variations observed during late autumn to spring are greater by more than a factor of two than those observed during the rest of the year
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