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\begin{document}

\title{OI 630.0 nm Post-Sunset Emission Enhancement as an Effect of Tidal
Activity over Low-Latitudes}



\author[1]{Sovan Saha}%
\author[1]{Duggirala Pallamraju}%
\author[1]{Sunil Kumar}%
\author[2]{Fazlul I Laskar}%
\author[3]{Nicholas Michael Pedatella}%
\affil[1]{Physical Research Laboratory}%
\affil[2]{Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA}%
\affil[3]{National Center for Atmospheric Research (UCAR)}%


\vspace{-1em}



  
  \date{May 25, 2024}


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OI 630.0 nm post-sunset emission enhancement as an effect of tidal
activity over low-latitudes

Sovan Saha\textsuperscript{1}, Duggirala Pallamraju\textsuperscript{1},
Sunil Kumar\textsuperscript{1}, Fazlul I. Laskar\textsuperscript{2}, and
Nicholas M. Pedatella\textsuperscript{3}

\section*{\texorpdfstring{\textsuperscript{1} Space and Atmospheric
Sciences Division, Physical Research Laboratory, Ahmedabad,
India}{1 Space and Atmospheric Sciences Division, Physical Research Laboratory, Ahmedabad, India}}\label{space-and-atmospheric-sciences-division-physical-research-laboratory-ahmedabad-india}

\section*{\texorpdfstring{\textsuperscript{2}Laboratory for Atmospheric
and Space Physics, University of Colorado, Boulder, CO,
USA}{2Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA}}\label{laboratory-for-atmospheric-and-space-physics-university-of-colorado-boulder-co-usa}

\section*{\texorpdfstring{\textsuperscript{3}High Altitude Observatory,
National Center for Atmospheric Research, Boulder, CO,
USA}{3High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO, USA}}\label{high-altitude-observatory-national-center-for-atmospheric-research-boulder-co-usa}

Corresponding author: Sovan Saha {(sovansaha93@gmail.com)}.

\textbf{~}

\section*{Key Points:}\label{key-points}

\selectlanguage{ngerman}·~~~~~~~~ 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.

~

\section*{Abstract:~~}\label{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 (24.6\textsuperscript{o}N, 72.7\textsuperscript{o}E,
19\textsuperscript{o}N 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.

\section*{1. Introduction:}\label{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 (24.6\textsuperscript{o}N,
72.7\textsuperscript{o}E, 19\textsuperscript{o}N 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; Yi\selectlanguage{polish}ğ\selectlanguage{english}it 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?

\section*{2. Data used:}\label{data-used}

\subsection*{2.1. Optical data (OI 630.0 nm nightglow
emissions):}\label{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\selectlanguage{ngerman}×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.

\subsection*{2.2. WACCM-X simulation:}\label{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.

\section*{3. Data analysis and results:}\label{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\textless{}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.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure01/Figure01}
\caption{{\emph{This is OI 630.0 nm nightglow variations with post-sunset
enhancement in emissions for 72 nights from January to March for the
years 2013-2016. Typical variation of OI 630.0 nm nightglow emissions
has been shown for a given night taken in 2016. ~}
{\label{621972}}%
}}
\end{center}
\end{figure}

Figure 2 shows the electron density (top row), meridional wind (middle
row), and temperature (bottom row) at 250 km altitude for the location
of Mt. Abu as simulated by WACCM-X for three sample days. On these three
days, an increase in electron densities is seen after sunset which
coincides with the reversal in meridional winds from equatorward to
poleward direction. Enhancements in electron densities occur at 21, 23,
and 21 LT, respectively, for the three given nights, and the meridional
winds show a reversal in the poleward direction at that time. The
variation in electron densities at 250 km shows a one-to-one relation
with the OI 630.0 nm nightglow emissions for geomagnetic quiet time
period, which has been established by the estimated OI 630.0 nm
nightglow emissions, wherein the measured electron densities were used
as inputs (Saha et al., 2021). Typically, the meridional winds turn
equatorward after sunset, but interestingly, a reversal in equatorward
wind is seen in the estimated relative variation in meridional winds for
a period of 2-3 hours (Saha et al., 2021). The WACCM-X model also shows
a similar reversal in meridional winds after sunset, as shown in Figure
2.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure02/Figure02}
\caption{{\emph{The electron densities, meridional winds, and temperatures at 250
km altitude are shown for three representative nights obtained from the
WACCM-X simulation. An increase in electron densities is seen at 21, 23,
and 21 LT, respectively (top panel). Simultaneously, the meridional wind
is poleward directed at that time (middle panel). Moreover, the neutral
temperature decreases due to the reversal in meridional wind at that
time (bottom panel).}
{\label{554813}}%
}}
\end{center}
\end{figure}

A cumulative picture of the nature of the meridional winds during
nighttime is shown in Figure 3 for the months of January, February, and
March. During this period, on several occasions, the model shows
occurrences of reversal in winds during post-sunset hours. The top
panels of Figure 3 show the variation in the meridional winds on those
nights when they remain equatorward without any reversal towards
poleward direction. In contrast, the bottom panels correspond to those
nights when there is a reversal in the meridional winds (or a cessation
in their equatorward movement). It can be seen in all the plots that the
winds are usually poleward in the daytime and their magnitudes start
decreasing during sunset time (typically after 18 LT), and turn
equatorward after around 21 LT. It can be noted in the bottom panels
that there is a reversal of meridional winds wherein a poleward turn of
the meridional winds or weakening of equatorward winds is noted during
21 LT - 01 LT, beyond which it turns again in the equatorward direction.
As we move from January to March, the magnitudes of equatorward wind
increase during post-sunset hours due to climatological effects, so the
number of occurrences of the post-sunset wind reversal decreases. In
addition, the model output is derived from a free-run simulation, and it
is noteworthy that geophysical events, such as sudden stratospheric
warming (SSW), did not occur during the winter analysed in the present
study. The simulated variations in the meridional winds are thus not
driven by the occurrence of SSW. In the following, we will analyse and
discuss the meridional winds obtained from WACCM-X to understand the
nature of variation of post-sunset electron density/ redline nightglow
emissions for several nights to obtain a systematic picture.

~\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure03/Figure03}
\caption{{\emph{The nighttime variations in meridional winds obtained from WACCM-X
for three different months with two categories, when the meridional wind
reversal can be seen during post-sunset hours (bottom panels) and when
there is no reversal in meridional winds (top panels).}
{\label{922786}}%
}}
\end{center}
\end{figure}

Spectral analysis of the WACCM-X meridional winds has been carried out
for different days using fast Fourier transform (FFT) for a 24-hour
window. Figure 4 shows the meridional winds obtained from the WACCM-X
simulation (top panels), and the periodogram analysis (bottom panels)
for the three representative nights shown in Figure 2. A residue of
14-hour running average has been considered to investigate the presence
of periods smaller than 14 hours. The horizontal red solid lines in the
bottom panels indicate a significance level of 80\%. The nights used in
Figure 4 show a reversal in meridional winds. The significant presence
of higher-order tidal periodicities (6h, 8h, and 12h) can be noted on
these nights, which motivated further investigations on assessing the
contribution of these higher-order tides in the WACCM-X meridional
winds.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure04/Figure04}
\caption{{\emph{The meridional wind and power spectral analysis for the three
nights shown in Figure 2 in the top and bottom panels, respectively.
80\% significance is shown in the FFT power spectrum using red solid
lines.}
{\label{546398}}%
}}
\end{center}
\end{figure}

Based on the above discussion, the hourly meridional winds from WACCM-X
are used to estimate the amplitudes of different tidal components for
the periodicities at which the FFT analysis showed significant power
using the least square fitting method (Venkateswara Rao et al., 2011;
Pancheva et al., 2021). We have used the variation of meridional winds
simulated by WACCM-X for all the longitudes with a cadence of
2.5\textsuperscript{o} and the latitude has been taken
23.68\textsuperscript{o}, very close to the observation location. Then,
the least square fitting has been performed using the following
equation: {}

{}\(A=A_0+\sum_{s=-4}^5\sum_{i=0}^3A_iSin(\frac{2\pi}{T_i}t+s\selectlanguage{greek}λ\selectlanguage{english}+\selectlanguage{greek}ϕ\selectlanguage{english}_i),\ T_i=6,8,12,24\dots\dots\dots..(1)\)

where ~denotes the mean value, and , , s, and ~denote the amplitudes,
phases of different tidal components, wave numbers, and longitudes,
respectively. A window of 24 hours is used in this least square fitting.
The amplitudes and phases of these components are obtained from the
least square fitting which is further used to derive the temporal
behaviour of the winds for a particular tidal component over a given
location. The reconstructed winds for all the tidal components are shown
in Figure 5 along with the model winds (black solid line) for six
representative nights. For each of the nights, different tidal
components are identified by different colours, and the sum of these
amplitudes and A\textsubscript{0} is shown in the black dash-dot-dot-dot
line. A good comparison between the magnitudes of WACCM-X wind and the
resultant amplitudes as obtained from equation 1, serves as a validation
of the analysis of the least square fit. It can be seen that the diurnal
component (magenta dotted line) shows larger magnitude, whereas the
amplitudes of other three components, quarter-diurnal (QDT, red
long-dashed line), terdiurnal (blue dashed line), and semidiurnal (teal
dash-dot line) show comparable magnitudes with each other. The effect of
aliasing can arise in the higher-order tidal amplitudes when two tidal
components possess the same zonal wave numbers. In our analysis, the
model simulation produces continuous data with an hour cadence.
Therefore, there is no issue on sampling of the data. Further, we
separated the wave numbers of different tidal periodicities using the
least square fitting. Therefore, the effect of aliasing will not reflect
in the obtained amplitudes of different tides. As the migrating
component of diurnal and semi-diurnal tides have larger magnitude, they
can be aliased into higher-order tides with same wave numbers with
greater magnitudes (Moudden and Forbes, 2013). However, the amplitudes
of wave numbers 1 and 2 in QDTs and terdiurnal tides appear very small.
We have also used a 7h filter to remove the contribution of higher-order
tides which we have also discussed in later. The amplitudes of QDTs
obtained after 7h filtering remain the same as calculated from equation
1. The result further verifies that the obtained amplitudes do not
contain the effect of aliasing in the higher-order tides.

Broadly, the winds are poleward and equatorward in the daytime and
nighttime, respectively, as seen in the strong diurnal component. The
amplitudes and phases of different tidal components contribute
differently to the resultant variations of meridional winds. The subtle
variations in the winds can be due to the influence of higher-order
tidal contributions. For example, the winds are poleward in the daytime,
but due to the opposite phase of other tidal components, a double-humped
structure can be seen in the model wind during daytime. After sunset,
the winds turn equatorward. On occasions, the meridional winds reverse
their direction from equatorward to poleward after sunset (Figure 5).
The shaded regions of Figure 5 indicate the duration when the reversal
in meridional winds can be seen. On such occasions, we can see that the
magnitudes of the QDT vary concurrently with the winds thereby, it is
likely responsible for causing the reversal in the meridional winds. The
magnitude of the winds after post-sunset reversal gets even stronger
when the phases of the other three tidal components match one another.
For example, on the day of year 7 (Figure 5d), we see a larger increase
in the magnitude of winds during post-sunset time, and interestingly,
quarter-diurnal, terdiurnal, and semidiurnal tides seem to be occurring
in phase. Figure 6 shows the reconstructed winds while QDT has not been
considered in the reconstruction. A clear difference can be seen,
especially in the wind variation during post-sunset hours. When the QDT
is absent, the reversal in winds is not being generated and the winds
show the usual variation of poleward to equatorward.

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure05/Figure05}
\caption{{\emph{Four tidal components obtained by the least square fitting in the
WACCM-X simulated meridional wind output for 6 sample days. The shaded
regions indicate the post-sunset hours when the reversal in meridional
winds can be seen. The changes in wind magnitudes are calculated from
minimum (V}\textsubscript{\emph{min}}\emph{) and maximum
(V}\textsubscript{\emph{max}}\emph{) wind values during such reversals.}
{\label{617627}}%
}}
\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure06/Figure06}
\caption{{\emph{Only difference with Figure 5 is that QDT has not been taken while
reconstructing the winds. The post-sunset winds do not show reversal
clearly without the inclusion of QDT.}
{\label{270085}}%
}}
\end{center}
\end{figure}

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.

~

\section*{4. Discussion:}\label{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 (V\textsubscript{min}, as shown in
Figure 5d) and its peak value after that reversal (V\textsubscript{max},
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\textsuperscript{-1}, respectively. It can be seen that whenever the
magnitudes of wind reversal exceed 25 ms\textsuperscript{-1}, the
amplitudes of QDTs are greater than 16 ms\textsuperscript{-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.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure07/Figure07}
\caption{{\emph{The amplitudes of different tides are shown in panel (a) for
different days from January to March as obtained from the WACCM-X
simulated meridional winds. Panels (b) and (c) show the phases and
amplitudes of the quarter-diurnal and terdiurnal tidal components for
the same duration, respectively. The bottom panel (d) depicts the change
in wind magnitude after the post-sunset reversal in the wind. The shaded
regions with salmon colour indicate the presence of wind reversal, and
the magnitude of wind reversal can be seen to correspond with the
amplitude of QDT. The blue shaded regions indicate the days when
reversals were not seen, and the amplitudes of QDT were small. Reference
lines have been marked for magnitude of wind reversal and maximum
amplitude of QDT at the values of 25 and 16
ms}\textsuperscript{\emph{-1}}\emph{, respectively. The phase of QDT, a
reference line marked at 24 LT, also plays an important role. Whenever
the magnitudes of wind reversal are greater than 25
ms}\textsuperscript{\emph{-1}}\emph{, on most occasions, the amplitudes
of QDT are greater than 16 ms}\textsuperscript{\emph{-1}} \emph{and
phases are present at pre-midnight time.}
{\label{652630}}%
}}
\end{center}
\end{figure}

~We have investigated the post-sunset enhancement in electron density,
which follows the reversal in meridional winds after sunset for January
to March as obtained from WACCM-X simulations. Out of a total of 90 days
from January to March, we have found such reversal in winds on 39 days
during post-sunset times in the WACCM-X simulation. The amplitude of
different tidal components, as well as the superposition of those
components, were calculated and compared with the change in wind
magnitudes after the reversal. As can be seen, the broad meridional wind
pattern follows the diurnal tidal component. Therefore, we removed the
contribution of the large background diurnal component. The changes in
wind magnitudes have been calculated considering the contribution of
quarter-diurnal, terdiurnal, and semi-diurnal components only, which are
shown to be responsible for the reversal in wind. The changes in
amplitude of different tidal components have been calculated between the
time of V\textsubscript{min} and V\textsubscript{max} (as shown in
Figure 5d). Figure 8 shows the relation between the changes in amplitude
of the (a) quarter-diurnal, (b) terdiurnal, and (c) semidiurnal tides
with the changes in wind magnitudes wherein the diurnal contribution has
been subtracted. The amplitudes of QDT show a clear positive relation
with the change in wind magnitude, whereas the other tidal components
contribute at small magnitudes. Collectively, when the phases of these
tides match, they also give rise to larger changes in the wind
magnitudes. Therefore, based on the detailed analysis and results as
depicted in Figures 5, 6, 7, and 8a, it can be concluded that the
magnitudes of quarter-diurnal tide play a major role in the reversal of
meridional winds as seen during post-sunset hours.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure08/Figure08}
\caption{{\emph{\textbf{}} \emph{The x-axis depicts the change in wind magnitude
during the post-sunset wind reversal obtained by subtracting the
contribution of diurnal tide amplitudes. The change in tidal components
during the wind reversal period is depicted on the y-axis. Different
panels show variations in the changes in amplitudes of different tides
with the change in wind magnitudes. The quarter-diurnal tide shows the
best correlation with the wind reversal.}
{\label{130061}}%
}}
\end{center}
\end{figure}

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; Gei\selectlanguage{ngerman}ßler 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.

\section*{5. Summary:}\label{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.

\section*{6. Acknowledgement:}\label{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.

\section*{7. Open Research:}\label{open-research}

The data used to represent the figures in this work can be accessed from
\url{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: \url{https://doi.org/10.5281/zenodo.8400600}.

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