We have also verified the amplitude of higher-order tides by analysing the WACCM-X simulated winds using a 7h band-pass filter (Moudden and Forbes, 2013; Gong et al., 2023). The least square fitting method was applied after processing the winds of a given location by a 7h band-pass filter. The 7h band-pass filter diminishes the contribution arising from semi-diurnal and diurnal tides, which predominantly contribute to the meridional wind variation. The effect of aliasing, if any, due to these tides will also be reduced using this technique (Moudden and Forbes, 2013). The amplitude of QDTs obtained using this calculation aligns with the result obtained by the earlier-mentioned method, wherein the wave number and the corresponding tidal periodicities were used. This demonstrates that the higher-order tides do not contain the aliasing effect in the analysis as carried out in this study.
 

4. Discussion:

In the previous section, we have discussed the post-sunset enhancement in electron density as obtained from WACCM-X over low-latitudes which agrees well with enhancements in the OI 630.0 nm nightglow emissions after sunset (Figure 1). The spectral analysis and least square fit indicate the contribution of tidal components in the meridional wind variation obtained from WACCM-X. The least square fit provides the amplitude and phase information of different tidal components such as diurnal, semi-diurnal, terdiurnal, and quarter-diurnal, which depicts a picture that helps in understanding the tidal contribution to the meridional wind variation. The top panel (panel a) of Figure 7 shows the amplitude variation of four tidal components during the period from January to March. The amplitudes of diurnal tides can be seen to be larger as compared to the other three components, which show reasonably similar amplitudes, as also mentioned above for a few days. The quarter-diurnal tides have been shown to be responsible for the reversal of post-sunset hour meridional winds, as demonstrated for a few days, as shown in Figure 5. The variations in phase and amplitude of the higher-order tides, such as terdiurnal and quarter-diurnal are depicted in the panels b and c of Figure 7, respectively. The phases of QDTs coincide with the time of wind reversal, and the amplitudes of QDTs are also larger when the model shows an increase in the electron density. This is in simultaneity with the time when a maximum in poleward wind magnitudes occurs in the post-sunset hours. The bottom panel (panel d) of Figure 7 shows the changes in wind magnitudes from those at the time of the abatement of equatorward wind (Vmin, as shown in Figure 5d) and its peak value after that reversal (Vmax, as shown in Figure 5d). Depending on the amplitude and phase of QDT, the change in wind magnitude shows day-to-day variations (Figure 7). For example, the reduction/absence in wind reversal can be seen, as shaded with light blue colour, with the simultaneous reduction in amplitude of QDT. Besides, an increase in the amplitude of QDT and significant changes in wind magnitudes are seen, as shaded in salmon colour. Reference lines are drawn in Figure 7 (panels c and d) for the amplitude of QDT and magnitude of wind reversal at the values 16 and 25 ms-1, respectively. It can be seen that whenever the magnitudes of wind reversal exceed 25 ms-1, the amplitudes of QDTs are greater than 16 ms-1, and the times of peak phase values are seen to be around or before midnight. As discussed earlier, the time of the phases of different tides is also important. When the peaks in the phases of different periodicities, such as QDTs, terdiunal, and semidiurnal tides align in time, there can be further amplification in the wind magnitudes. For instance, we have seen a larger magnitude in wind reversal on DOY 7 due to the synchronization of phases of these three tidal components (Figure 5d). Such phase matching of these three tidal components is also seen for days 14 and 17. As the phases of QDTs are seen to be shifted beyond midnight towards the end of March, no reversal in winds in the post-sunset hours is observed. In this way, the amplitudes and phases of different tidal components, such as quarter-diurnal, terdiurnal, and semidiurnal tides, come out as important factors for the occurrence of reversal in the post-sunset wind, with the QDTs as the major contributor.