Airglow and wind measurements from the Brazilian equatorial region were used
to investigate the presence and the effects of the 3–4-day ultrafast Kelvin
waves in the MLT. The airglow integrated intensities of the OI557.7 nm,
O
The planetary-scale waves along with the atmospheric tides strongly influence
the large-scale dynamics in the mesosphere and lower thermosphere (MLT)
region. In the equatorial MLT, many studies aimed to investigate the
contribution of such waves to the total dynamics budget of that region. In
this context, the 3–4-day ultrafast Kelvin wave (UFKW) has been
investigated due to its effects on the MLT neutral dynamics and ionosphere.
The UFKW is the third Kelvin wave mode, in addition to the slow and fast
Kelvin wave modes. According to the linear theory, the Kelvin waves are
equatorially trapped waves that propagate eastward and upward and that
present only perturbation in the zonal direction (Andrews, 1987). Salby et
al. (1984) observed the UFKW for the first time in satellite measurements. The
first reports of the UFKW activity in the MLT region go back to the 1990s
(Vincent and Lesicar, 1991; Vincent et al., 1993). The UFKW presents zonal
phase speed ranging approximately from 100 to 130 m s
The effects of the Kelvin waves on the atmospheric composition occur through the transport of constituents and temperature-induced variations, which affects the production and loss in photochemical reactions. Investigations of the effects of the Kelvin waves on the stratospheric ozone revealed their influence on the fluctuations of the ozone concentrations and temperature (Timmermans et al., 2004, 2005). Forbes (2000) predicted in his dynamical model that the effects of the UFKW on the composition and temperature should affect the airglow emissions coming from 90 and 110 km altitudes. From ground-based airglow measurements, Takahashi et al. (2002) associated, possibly for the first time, the 3–4-day airglow variability with an UFKW. Afterwards, Lichstein et al. (2002), using the wave field from the Forbes (2000) model in an unidimensional chemistry–dynamical model, demonstrated that 3–4-day oscillations in the airglow reported by Takahashi et al. (2002) were consistent with the signature of an UFKW. Moreover, they showed that vertical winds are the dominant contributor to the UFKW-induced airglow variations. Buriti et al. (2005) extended the study of Takahashi et al. (2002) and found a year-to-year variation in the frequency of occurrence of the 3–4-day oscillations in the airglow.
Experimental measurements and numerical modeling have shown the influence of the UFKW in the day-to-day variability of the equatorial ionosphere (Takahashi et al., 2007; England et al., 2012; Onohara et al., 2013). One possible way for the planetary-scale waves to communicate their signatures to the ionosphere is through modulations of the atmospheric tides, which are able to penetrate into the ionosphere. Such modulations can occur due to the nonlinear interaction between the tides and planetary-scale waves. Theoretical predictions and observational evidence have shown that the nonlinear interaction between tides and planetary-scale waves can dramatically affect both neutral and ionized atmosphere dynamics (Teitelbaum and Vial, 1991; Pancheva, 2001; Pedatella and Liu, 2013). Recent advances have pointed out the potential of the secondary waves to induce variability in the ionosphere. For example, from a modeling perspective, Gan et al. (2017) showed that secondary waves generated by the nonlinear interaction of the diurnal and semidiurnal solar migrating tides with the 6-day planetary wave contribute to the shorter-term variability in the ionosphere. They observed that the relatively large magnitude and vertical wavelengths of two secondary waves (21 h period westward zonal wavenumber 2 and 13 h period westward zonal wavenumber 1) in the E-region zonal wind are favorable for an efficient dynamo modulation and responsible for the prominent signals of the respective waves in the vertical ion drift and peak electron densities. Yue et al. (2016), also from a modeling perspective, indicated that secondary waves resulting from the nonlinear interaction between the migrating diurnal tide and the quasi-2-day planetary wave induce corresponding oscillations in the vertical ion drift and ionospheric electron densities. Most of these studies address the nonlinear interaction between tides and the 2-day, 5-day and 16-day planetary waves. Concerning the UFKW, England et al. (2012) investigated the nonlinear interaction between them and the tides in the MLT and its effects in the ionosphere. Their findings show that the nonlinear interaction of the UFKW with the diurnal tide is seen between 82 and 88 km, and a resultant 3-day periodicity in the diurnal tide is seen to propagate up to altitudes of approximately 150 km. This could have a significant impact on the ionosphere via modulation of the E-region dynamo and it would carry the 3-day periodicity to higher altitudes. Effects of such interactions in the composition, however, have been little studied.
Airglow measurements can provide information about the composition and temperature of the MLT and, along with wind measurements, have the potential to expand the view of the MLT dynamics. In this study, we investigate the presence of 3–4-day oscillations associated with the UFKW in the MLT airglow and wind. Additionally, we examine the interaction between the UFKW and diurnal tide and its effects on the airglow variability.
The airglow intensities have been measured in the Brazilian equatorial region at
São João do Cariri (7.4
Number of nights per month with airglow observations in 2005.
Example of the nightly OH airglow intensity between 26 October and
8 November 2005
Monthly normalized Lomb–Scargle periodogram of OI5577
To search for the 3–4-day periodic variations in the airglow we applied the Lomb–Scargle periodogram (Lomb, 1976; Scargle, 1982) (hereafter LS periodogram), which is suitable to search for periodic variations in nonregularly sampled data as the airglow. The airglow time series, corresponding to an entire period of observation, are built up by putting in sequence all the nights with measurements. Afterwards, the mean intensity for the corresponding time interval is subtracted from the original time series and the LS periodogram is calculated for the residuals. This procedure is performed for each emission and for each period of observation. Figure 1 shows an example of the OH intensity measured between 26 October and 8 November in 2005 and its corresponding LS periodogram, which shows the periodicities present in the data. Each group of data corresponds to one night of observation.
The neutral-wind components are calculated by using measurements performed by the meteor radar also installed at Cariri observatory. The radar operates at 35.24 MHz frequency and uses one transmitter antenna and an array of five receiving ones. The radar transmits pulses of 12 kW peak power, which is partially reflected back by the ionized trail produced during meteor ablation. From reflected echoes, zonal and meridional wind components are retrieved. For this study, we calculated the zonal and meridional winds in layers centered at 82, 85, 88, 91, 94 and 98 km altitudes at a time resolution of 2 h. The planetary wave dominant periodicities in zonal and meridional wind were identified by means of the Morlet wavelet transform (Torrence and Compo, 1998), which also works as a band-pass filter. Additionally, harmonic analysis based on the least mean squared fitting was used to extract the amplitude and phases of the planetary waves periodicities in the wind and airglow.
Wavelet spectra of the zonal
Band-passed zonal and meridional wind at 88, 94 and 98 km. Cutoff periods are from 2.8 to 4.5 days.
Vertical amplitude
Lomb–Scargle (LS) spectra
The airglow and OH rotational temperature time series have been analyzed by
applying the LS periodogram as described in the previous section. Figure 2
shows the normalized LS spectra of OI5577, O
The 3–4-day periodic variations in the airglow and temperature are observed
in all seasons. They appear in January, March, July, August and November. We
call attention to the 3–4-day periodic variation in March, which is known
for its effects on the ionosphere (see Takahashi et al., 2007). Despite being
well characterized, this oscillation does not appear in the OI5577 emission.
As will be discussed later, this may be related to effects of the atmospheric
tides. The amplitude of variation, relative to the mean intensity, induced by
the 3–4-day oscillations are 18–45 % in OI5577, 17–43 % in O
The presence of 3–4-day oscillations in the airglow in all seasons agrees with previous reports based on MLT wind and satellite-borne temperature measurements. This way, investigations concerning the variability of the 3–4-day induced oscillations in the MLT wind indicate the presence of semiannual variations in their occurrence, with intensification around equinoxes (Yoshida et al., 1999; Tsuda et al., 2002; Davis et al., 2012). Additionally, based on the temperature measurements of the SABER/TIMED satellite, Forbes et al. (2009) showed that the UFKW exists intermittently at amplitudes of the order of 3–10 K between 80 and 120 km during all months of the year, with variability at periods typically in the 20–60-day range.
To complement the information about the 3–4-day oscillations observed in the airglow, we searched for common periodic oscillations in the MLT neutral wind. Figure 3 shows the periodic oscillations in the MLT zonal and meridional wind at 90 km revealed by means of the wavelet transform. The zonal wind spectrum is marked by periodic oscillations at periods between 3 and 4 days and periods from 5 to 8 days. However, the meridional wind spectrum is primarily dominated by quasi-2-day oscillations, which are particularly strong during the austral summer in January, although they are also observed around spring equinox. The white hatched areas delimited by vertical black lines indicate the intervals with coincident airglow measurements. From the airglow LS periodogram and the wind wavelet spectra, Figs. 2 and 3, respectively, one can identify common oscillations at periods near 2 days, 3–4 and 5–7 days on several occasions throughout the year. In the 3–4-day period range, the wind power spectrum indicates the presence of common oscillations in the airglow and in the zonal wind in March (days 60 to 80) and August (days 210 to 230). Although there are other signal enhancements in the 3–4-day band present in the wind spectra, none of them entirely matches with those observed in the airglow. That is the case of the 3–4-day oscillations identified in the airglow in January (days 2 to 9), July (days 210 to 225) and October–November (days 300 to 320).
To investigate additional features of the 3–4-day oscillations in the winds,
Fig. 4 shows the filtered zonal and meridional winds at 88, 94 and 98 km
altitudes, which correspond approximately to the nominal altitude peaks of
the OH, O
The zonal and meridional band-passed winds indicate that the other two cases
(January and July) with 3–4 oscillations observed in the airglow do not
exhibit the characteristics of an UFKW. In January, significant amplitudes
are observed in the meridional wind at 94 and 98 km, while in the zonal wind
component the amplitudes are much lower. Considering the case of July, during
almost the entire period of airglow measurements the amplitudes in the
meridional wind are higher than in the zonal wind. Just at the end of the
airglow observations, the 3–4-day amplitudes increase in the zonal wind.
Younger and Mitchell (2006) investigated the wind-field variability in the
equatorial MLT at Ascension Island (8
Diurnal tide amplitude in the zonal
To provide additional evidence for an UFKW interpretation of the common
3–4-day oscillation in the airglow and wind, we investigate their vertical
structures. From harmonic analysis, we extracted the amplitudes and phases of
the 3–4-day oscillation in the zonal wind observed in March, August and
October–November. Figure 5 shows the results. The numbers on the top
indicate the time interval (days of year) analyzed. Amplitudes are found to
increase from near 10 m s
Characteristics of the 3–4-day oscillations observed simultaneously in the airglow and wind.
The 3–4-day oscillation observed in March in the airglow presents some
interesting features that are worthy of investigation. From the spectra in
Fig. 2, one can see that the peaks in the LS periodogram correspond to
periodic oscillations of about 3.5 days in the OH and O
Fourier amplitude spectrum of the zonal
The LS periodograms of Fig. 2 were obtained using the airglow data acquired
during the whole night. We performed additional LS periodogram analysis
without considering the entire airglow nightly measurements. Instead, we take
the airglow data only within specific time intervals during the night. This
is because it is well established from ground-based and satellite-borne
measurements and modeling studies that the atmospheric tides strongly affect
the MLT equatorial airglow and exhibit pronounced nocturnal variation (e.g.,
Shepherd et al., 1995; Yee et al., 1997; Takahashi et al., 1998). In
addition, the tides can interact with other waves (e.g., Teitelbaum and Vial,
1991; Pancheva, 2001; England et al., 2012; Alves et al., 2013), leading to
changes in their amplitudes and phases, which in turn could affect the
airglow emissions. As there was a 1-day gap on 4 March in the airglow data,
we take for this particular analysis the data from 5 to 14 March, which
present excellent quality and continuity, and are still long enough to allow
the study of the 3–4-day oscillations. First we analyzed the
airglow time series built only with data obtained between 18:00 and
00:00 LT. In this case, the periodicities in the 3–4-day band exhibit
essentially the same features as those observed in the LS periodogram built
with the whole-night data presented in Fig. 2; i.e., the OH and O
The time of acquisition of the data can affect the observed airglow and inferred temperature variability, which is related to effects of the atmospheric tides (e.g., Reisin and Scheer, 2017). The influence of the solar tides in the MLT airglow is well known. This way, in the equatorial region, the airglow variability induced by the tides is strong before local midnight, particularly in the OI5577 emission, and after this time, their effects are weaker (Shepherd et al., 2005). Then, the variability of the tides on the scale of days could affect the airglow emissions. The source of variability of the atmospheric tides include, as well as other factors, the nonlinear interaction with planetary-scale waves. As proposed by Teitelbaum and Vial (1991), when a nonlinear interaction between a tide and planetary wave takes place, the result is the generation of secondary waves whose frequencies are the sum and difference of the primary waves (tide and planetary wave) frequencies. As well as this, the amplitude of the tides are modulated in the periods of the planetary waves. Previous investigations of England et al. (2012) using meteor radar, incoherent scatter radar and satellite wind data, along with satellite-borne temperature data, reported the evidence of the nonlinear interaction between the UFKW and the diurnal tide in the MLT and its effects in the E-region of the ionosphere. To investigate a possible interaction between the UFKW and atmospheric tides, we analyzed the amplitudes of the diurnal tide in the zonal and meridional wind during the presence of the 3–4-day UFKW in March. The amplitudes of the diurnal and semidiurnal tides were extracted by applying the harmonic analyses to a 2-day long moving window and stepped forward by 1 day. As the diurnal tide is dominant at Cariri latitude (e.g., Lima et al., 2007), only the amplitudes of the diurnal tide in the zonal (Fig. 7a) and meridional (Fig. 7b) winds together with the wavelet spectra of the zonal (Fig. 7c) and meridional (Fig. 7d) diurnal tide amplitudes at 91 km are plotted. The amplitudes of the diurnal tide in the zonal wind are weak between days 60–65 and 70–75. Additionally, they present a quasiperiodic enhancement of about 4 days between days 65 and 70 around 92 km. The amplitude of the diurnal tidal in the meridional wind also exhibits a similar decrease during days 60–65 and quasiperiodic enhancement between days 65 and 75, but in this case, it is longer than 5 days. The wavelet spectra of both zonal and meridional diurnal tide amplitudes at 91 km confirm the presence of the aforementioned periodic variations.
Figure 8 shows the Fourier amplitude spectra of the zonal (a)
and meridional (b) wind at 91 km using data from days 60 to
75. The zonal and meridional wind spectra present clear signatures of the
diurnal tide. As expected, the diurnal tide is stronger in the meridional
wind. The UFKW signature appears as a peak at the frequency of
0.25 cycles day
Secondary waves generated from the nonlinear interaction between tides and
planetary waves may propagate upward and induce ionospheric variability (e.g.,
England et al., 2012; Yue et al., 2016; Gan et al., 2017). This way, we
investigated the amplitude and phase vertical structures of the
0.75 cycles day
The periodic variations in the tidal amplitudes and the presence of the
predicted 0.75 cycles day
In this study, we investigated the presence and the effects of the 3–4-day oscillations in the equatorial MLT airglow and wind. The 3–4-day oscillations in the OI557.7 nm, O
The data used in this study are available upon request to authors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Space weather
connections to near-Earth space and the atmosphere”. It is a result of the
6
We are thankful to CAPES for partially supporting this research. The present work was also supported by CNPq under the grant 30.5461/2015-0. The topical editor, Christoph Jacobi, thanks Quan Gan and one anonymous referee for help in evaluating this paper.