OI 630.0 nm post-sunset emission enhancement as an effect of tidal activity over low-latitudesSovan Saha1, Duggirala Pallamraju1, Sunil Kumar1, Fazlul I. Laskar2, and Nicholas M. Pedatella31 Space and Atmospheric Sciences Division, Physical Research Laboratory, Ahmedabad, India2Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA3High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO, USACorresponding author: Sovan Saha (sovansaha93@gmail.com). Key Points:·         OI 630.0 nm post-sunset emission enhancement over low-latitudes is consistent with the presence of poleward meridional winds.·         WACCM-X simulated meridional winds show a poleward wind reversal during post-sunset hours, on occasion.·         Quarter-diurnal tides seem to play significant role in reversing the meridional winds after sunset. Abstract:   The OI 630.0 nm airglow emission variability provides salient information on the dynamical changes taking place in the upper atmosphere at around 250 km. The emission rates vary with the changes in the ambient electron densities and the neutral constituents that are associated with these emissions. On several occasions, enhancements in these emissions are observed during post-sunset hours as measured from Mt. Abu (24.6oN, 72.7oE, 19oN Mag), a low-latitude location at Indian longitudes. These enhancements occur following the typical monotonic decrease in emission intensity after sunset. The presence of poleward meridional wind was shown to be the cause for such observed emission enhancements. However, climatologically, meridional winds are equatorward during these times. In this study, the cause of such reversal in winds in the post-sunset hours has been investigated using the variation in electron densities and winds simulated by the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X), which also shows enhancements in electron densities similar to those observed in the post-sunset OI 630.0 nm nightglow emissions, and simultaneous reversal in winds as well. The amplitudes and phases of different components of tides obtained from WACCM-X meridional winds reveal a significant contribution of quarter-diurnal tides to the observed reversal in the meridional winds during post-sunset hours. The change in the tidal amplitudes is proportional to the changes in wind magnitude during that wind reversal.1. Introduction:The OI 630.0 nm (redline) nightglow emissions typically decrease monotonically after sunset along with the decrease in the incidence of solar radiation. The nightglow emission intensities continue to be small throughout the night and again start increasing towards morning twilight time. In one of our recent studies, an enhancement in OI 630.0 nm nightglow emissions after sunset was reported (Saha et al., 2021) in measurements over Mt. Abu (24.6oN, 72.7oE, 19oN Mag), a low-latitude location in the Indian longitude sector. The enhancements in nightglow emissions were observed between 20-22 hrs local time. Firstly, the strength of pre-reversal enhancement in the zonal electric field was investigated as a possible cause as it has the potential to bring in equatorial and off-equatorial plasma to the tropical latitudes, such as Mt. Abu. However, it could not satisfactorily explain the observations of enhancement in redline emissions during post-sunset time. The variation in meridional winds, however, showed that they were either poleward or that there was a cessation in the equatorward direction during those times. This observation was explained in terms of the altitudinal movement of the ionosphere to be responsible for the observed enhancements in the redline airglow. As the poleward wind contributes to a decrease in the ionospheric height (Saha et al., 2021), thereby, it provides greater reactants for the dissociative recombination mechanism to cause an enhancement in the emissions. Such a decrease in the height of the ionosphere in the nighttime has been observed during midnight hours, and it is called as the midnight temperature maximum (MTM) (e.g., Mayr et al., 1979; Herrero and Spencer, 1982; Herrero et al., 1983; Sastri and Rao, 1993; Colerico et al., 1996; Fesen, 1996; Mesquita et al., 2018). Consequently, brightness in redline emission was also enhanced during those events (Colerico and Mendillo, 2002).The upper atmosphere shows different kinds of variability associated with neutral winds (e.g., Meriwether et al., 1985; Gurubaran et al., 1995; Jyoti et al., 2004; Abdu et al., 2006; Kumar et al., 2022) and electrodynamics (e.g., Pallamraju et al., 1996; Karan et al., 2016; Karan and Pallamraju, 2017; Saha and Pallamraju, 2022). The atmospheric waves have a significant impact on the upper atmosphere at different altitudes (Vadas, 2007; Yiğit and Medevdev, 2009; Miyoshi et al., 2014; Singh and Pallamraju, 2017; Mandal et al., 2020; 2022). Optical and radio techniques have been used to characterize atmospheric waves (e.g., Oliver et al., 1994; Pallamraju et al., 2014, 2016; Mandal et al., 2019; Kumar et al., 2023a). Several model studies have also been carried out, which include the lower atmospheric forcing and describe the thermospheric variation quite satisfactorily (Roble and Ridley, 1994; Fesen, 1996; Laskar et al., 2013; Liu et al., 2018). It is known that atmospheric tides play an essential role in the variation of temperature and winds in the mesosphere and the thermosphere. Several dynamical processes that occur in the upper atmosphere have been explained using the variation in atmospheric tides (e.g., Thayaparan, 1997; Akmaev, 2001; Chau et al., 2009; Guharay and Franke, 2011; Laskar et al., 2014; Guharay et al., 2018; Pedatella and Harvey, 2022; Kumar et al., 2023b). The atmospheric tidal waves are generally generated in the lower atmosphere and they can propagate to the thermosphere where they eventually dissipate due to the molecular viscosity and thermal diffusivity. In the thermosphere, the tides can be generated in-situ as well. The influence of semidiurnal and other higher-order tides was seen during MTM (e.g., Mayr et al., 1979; Herrero et al., 1983; Fesen, 1996; Colerico and Mendillo, 2002), which caused an increase in the nightglow emission intensity during midnight and post-midnight hours. A significant magnitude of MTM was seen in simulations by the Whole Atmosphere Model, and lower atmospheric forcing was found to contribute to the MTM (Akmaev et al., 2009; Fang et al., 2016).In this work, we have used a free-running version of the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) to understand the dynamics prevalent during the post-sunset hours over low-latitudes. Electron densities and meridional winds have been obtained from the WACCM-X at 250 km altitude (which is the altitude of the peak emission of OI 630.0 nm). Interestingly, WACCM-X simulated electron density also occasionally shows an increase after sunset, and the increase is consistent with the presence of poleward winds in the simulation. Therefore, these simulations provide an independent confirmation of the interpretation made in our earlier work that the poleward turning of the usually equatorward meridional wind, or cessation of equatorward wind during post-sunset hours is the cause for the observed enhancements in the OI 630.0 nm emissions from Mt. Abu (Saha et al., 2021). The question we ask in this study is what is the cause for the reversal of meridional wind from its usual equatorward direction at post-sunset hours over low-latitudes?2. Data used:2.1. Optical data (OI 630.0 nm nightglow emissions):An optical spectrograph, High Throughput Imaging Echelle Spectrograph (HiTIES) (Chakrabarti et al., 2001), is used to measure the nocturnal OI 630.0 nm airglow emission variability over a low-latitude location, Mt. Abu. The OI 630.0 nm nightglow emissions originate from an altitude region peaking at around 250 km, with a half-value width of around 70 km. HiTIES is a slit spectrograph, and the spectra around OI 630.0 nm nightglow emissions are imaged onto a 1K×1K CCD chip. Image processing has been carried out to remove the dark counts, scattered lights that arise due to starlight, zodiacal light, and the vignetting and Van-Rhijn effects.2.2. WACCM-X simulation:The WACCM-X is a community developed whole atmosphere model that provides simulations of the variabilities in the Earth’s atmosphere-ionosphere-thermosphere regions (Liu et al., 2018). The model captures chemical, thermodynamical, electrodynamical, ionization, and physical processes from the surface of the Earth to 500 to 700 km (depending on solar activity) altitudes. The simulation used WACCM4/CAM4 physics, as described in Marsh et al. (2013), Neale et al. (2013), and Liu et al. (2018). The chemistry used in the middle atmosphere chemistry as described in Davis et al. (2023) as well as the ionosphere-thermosphere modifications described in Liu et al. (2018). The model is capable of providing upper atmospheric neutral and ionospheric parameters and dynamics, which are coupled to the lower atmosphere. In this study, the analysis has been carried out using hourly data obtained from a free-run simulation of WACCM-X, where the lower atmospheric dynamics and day-to-day variability are internally generated by the model, for a constant solar flux value of 70 solar flux unit and Ap value of 1 to represent moderate solar activity and geomagnetic quiet conditions, to which the measured data pertains. The global variations in meridional winds, temperatures, and electron densities obtained from WACCM-X are used in this study. The variations in these parameters are analysed to understand the post-sunset emission enhancement observed in the nightglow data. The analyses and results are discussed below.3. Data analysis and results:Typically, OI 630.0 nm nightglow emissions show a monotonic decrease after sunset as the ionization stops, but the recombination process of ions and electrons continues uninterruptedly. On several occasions, an enhancement in emissions is observed around 20 to 22 LT following a decrease after sunset (Saha et al., 2021). Figure 1 shows the variations in OI 630.0 nm nightglow enhancements during post-sunset hours for the period of Jan-Mar for the years 2013 to 2016 over Mt. Abu. The thick blue line of Figure 1 shows the typical OI 630.0 nm nightglow variation for a given night in 2016. A total of 72 nights (out of 185 nights) during that period showed an enhancement in post-sunset emissions, as depicted in this figure. Note that all these days were geomagnetically quiet with Ap<20 nT. The left-side y-axis represents the emission rates (in Rayleighs) for the years 2013, 2014, and 2015, whereas, the values shown on the right-side y-axis are for the year 2016. Due to the decrease in the values of solar flux in 2016, the nightglow emission rates are relatively lower as expected, owing to reduced electron number densities. The amount of increase in emissions during post-sunset hours also varies depending on the solar flux (Saha et al., 2021) ranging from a few tens of Rayleigh to around 200 Rayleigh. The occurrence time of nightglow enhancement does not show any dependence on solar flux and seasons.