Higher-order Tides in the Variation of Post-sunset Meridional Winds and, consequently, OI 630.0 nm nightglow emissions over Low-latitudes

Airglow emissions act as a tracer of the altitudinal region of 100 km width centred around 250 km. Various dynamical processes such as neutral, electrodynamics, and atmospheric waves modulate the atmospheric parameters, such as temperature, density, etc., of the ionosphere-thermosphere system (ITS), which vary day-to-day, season, and solar flux as well. We have carried out investigations of ITS by measuring the OI 630.0 nm (redline) nightglow emissions over Gurushikhar, Mt. Abu (24.6°N, 72.7°E, 19°N Mag), a low-latitude location, using the High Throughput Imaging Echelle Spectrograph (HiTIES). On several occasions, a bell-shaped enhancement in these emissions has been noticed around 21 local time, following the typical monotonic decrement after sunset. The cause of this enhancement in redline emissions has been explored by investigating the equatorial electrodynamics and neutral winds. The meridional winds have been obtained by using two digisondes located at equatorial and low-latitude locations. Contrary to the conventional expectation of pre-reversal enhancement bringing additional plasma over the low-latitudes to cause such enhancement, the role of meridional winds has been demonstrated in our study. The poleward winds over the low-latitudes bring additional plasma down to the redline emission altitudes, resulting in the observed enhancement in emissions. The winds at these post-sunset hours are usually equatorward, and thus, this brings the question as to what is the cause for the reversal in wind during those times. We have analysed the winds, electron densities obtained from the global free-run model of Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) data. It has been found that higher-order tides, such as quarter-diurnal tides, play an important role in causing a reversal in meridional winds from their usual equatorward to poleward direction. This clearly explains the root cause for the variation in meridional winds during the post-sunset hours as well as the reason why the post-sunset enhancements in OI 630.0 nm nightglow occur on one night and not the next, even though the electrodynamic forcing was similar on these two occasions. These new findings will be discussed.