The diurnal variation in tides is clearly seen in the broad picture of meridional wind variation. Typically, winds are poleward and equatorward during the day and nighttime, respectively (Figure 5). The small-scale variations are seen due to other tidal components, such as, semi-diurnal, terdiurnal, and quarter-diurnal. Higher-order tides, such as terdiurnal and quarter-diurnal, have been investigated at lower thermosphere altitudes using the temperature and wind data obtained from both measurements and models (Moudden and Forbes, 2013; Guharay et al., 2018; Jacobi et al., 2017; Pancheva et al., 2021). In a recent work, the climatological mean amplitude of QDTs has been shown to be comparable with semi-diurnal and terdiurnal in the thermospheric altitude of around 250 km as measured by incoherent scatter radar over Arecibo (Gong et al., 2023). The quarter-diurnal and terdiurnal tides in the upper atmospheric altitudes can be generated in different ways. Both the migrating and non-migrating tides can play a significant role here. However, thermal excitation by the solar heating and non-linear wave-wave interaction can generate the higher-order tides of different wavenumbers in the middle and upper atmosphere (Xu et al., 2012, 2014; Moudden and Forbes, 2013; Geißler et al., 2020). Although the amplitude of these higher-order tides is significantly small compared to diurnal tides, they nonetheless seem to play an important role in the dynamics of the thermosphere, especially in the post-sunset hours. Here, we show that the QDTs are more effective in causing the reversal in meridional winds after sunset, which has not been reported so far, to the best of our knowledge.

5. Summary:

The OI 630.0 nm nightglow emission brightness typically shows a monotonic decrease after sunset. On many occasions, an enhancement in emissions has been observed during post-sunset hours as measured by HiTIES over Mt. Abu, a low-latitude location in India. In a comprehensive and detailed investigation, the presence of poleward wind has been shown to be responsible for such enhancement in emissions at low latitudes as the poleward winds bring down the plasma to lower altitudes (Saha et al., 2021). The cause of such reversal in the usually equatorward winds at post-sunset hours has been examined in this study. In order to address the optical observations, free-running WACCM-X simulations have been carried out for a three-month duration, which also showed an increase in electron density during post-sunset hours on many occasions in the altitudinal region of 250 km, as also shown in the digisonde measurements in our earlier study (Saha et al., 2021). The WACCM-X simulations show a reversal of equatorward wind coincident with the time of enhancement in electron density, which serves as an independent confirmation of our observations reported earlier (Saha et al., 2021). In the present work, the variations of meridional winds obtained from WACCM-X have been analysed to understand the cause of such reversals during the post-sunset time. Different tidal periodicities, such as diurnal, semidiurnal, terdiurnal, and quarter-diurnal, are fitted using least square method, which reveals very interesting information on the amplitudes and phases of each of the components and their association with the direction of the meridional wind. The phase and amplitude of higher-order tides play a crucial role in the nighttime thermospheric dynamics. Whenever the amplitudes of higher-order tides fall in the same phase, the magnitudes of wind reversal get enhanced. Thereby, a strong tidal contribution has been found to be the cause behind the poleward reversal of meridional winds after sunset, which causes an increase in electron density as well as enhancement in OI 630.0 nm emission. Especially, QDTs play the dominant role in the variation of meridional wind. This also explains why such reversals in winds do not occur on all the nights in a given season.  Thus, the redline OI 630.0 nm emission enhancements in the post-sunset time can also serve as an indicator of the reversal of winds and the existence of the strength of the higher-order tides at that time.

6. Acknowledgement:

The nighttime optical data, used in this study, has been obtained by the Physical Research Laboratory, Ahmedabad, India. This work is supported by the Department of Space, Government of India. FL was supported by NASA Contract 80GSFC18C0061 to the University of Colorado, Boulder, USA.

7. Open Research:

The data used to represent the figures in this work can be accessed from https://osf.io/gteq9/. A netCDF file containing all the relevant WACCM-X simulated data used in this study is available at the following link: https://doi.org/10.5281/zenodo.8400600.

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