We observed a gravity wave (GW) signature in the OH
emission layer in the upper mesosphere, and 4 h later, a medium-scale
travelling ionospheric disturbance (MSTID) in the OI 630 nm emission layer.
Spectral analysis of the two waves showed that both have almost the same
wave characteristics: wavelength, period, phase speed and propagation
direction, respectively, 200 km, 60 min, 50 m s-1, toward the southeast. From
the gravity wave ray-tracing simulation for the mesospheric gravity wave, we
found that the wave came from a tropospheric deep convection spot and
propagated up to the 140 km altitude. Regarding the same wave
characteristics between mesospheric GW and ionospheric MSTID, the two
possible cases are investigated: a direct influence of the GW oscillation in
the OI 630 nm emission height and the generation of a secondary wave during
the GW breaking process. This is the first time to report an observational
event of gravity wave propagation from the troposphere, mesosphere to
thermosphere–ionosphere in the South American region.
Introduction
A deep cloud convection in the troposphere generates vertical (up and down)
air-mass movement launching a variety of gravity waves into the
stratosphere. Atmospheric gravity waves (GWs) have important roles in
transporting the energy and momentum from the lower to upper atmosphere and
ionosphere. A part of energy and momentum is deposited in the mesosphere
lower thermosphere (MLT) region through wave breaking and altering the
background wind field. Some of the GWs produce secondary waves and propagate
further upwards into the thermosphere where it modulates ionospheric plasma
(Hocke and Schlegel, 1996; Nicolls et al., 2014). A part of medium-scale
travelling ionospheric disturbances (MSTIDs) has its origin in the passage
of gravity waves in the ionosphere (Otsuka, 2018). Observations of GW
propagation in the thermosphere have been carried out by many researchers
since Hines (1960) presented theoretical background for the GW
propagation in the ionosphere. Rottger (1973) suggested the role of GWs in
the ionospheric irregularities.
GW observations in the mesosphere have been carried out by measuring short
period temporal variation of the mesospheric airglow (hydroxyl and atomic
oxygen OI 557.7 nm emissions) by airglow photometers in 1970–1990 (e.g.
Takahashi et al., 1999). After 1990, airglow digital imagers were used to
monitor GWs in two-dimensional forms (e.g. Taylor et al., 2009; Dare-Idowu
et al., 2020; Nyassor et al., 2021). Dynamical processes in the mesosphere to
thermosphere were studied by OH and oxygen 630.0 nm airglow imaging by
Kubota et al. (2000), Taori et al. (2013), and most recently by Ramkumar et
al. (2021). In case of GWs in the stratosphere, satellite-onboard GPS radio
occultation measurements have made it possible to observe GWs by vertical
profile of the temperature variability on a global scale (Tsuda, 2014; Xu et
al., 2017).
There are many previous works on the GW propagations in the stratosphere,
mesosphere, and ionosphere individually. However, it has been difficult to
monitor an event of GW propagating through the troposphere up to the
ionosphere. Smith et al. (2013) observed GW waves in the OH and OI 557.7 nm
emission layers in the mesosphere to lower thermosphere (MLT) region (85–100 km) and OI 630.0 nm emission layer (around 240 km altitude) in the
thermosphere and discussed on the mountain waves from the mesosphere to
ionosphere. They attributed the wave structure in the ionosphere as due to
secondary waves. Azeem et al. (2015), for the first time, reported the
occurrence of circular GW structures in the stratosphere, mesosphere, and
ionosphere during a tropospheric convective storm. They observed concentric
wave structures in the stratosphere by the Atmospheric Infrared
Sounder (AIRS) onboard Aqua satellite (https://airs.jpl.nasa.gov/, last access: 30 September 2022), and by an
optical imaging radiometer (VIIRS) onboard Suomi satellite
(https://www.nasa.gov/mission_pages/NPP/main/index.html, last access: 30 September 2022), and
in the ionosphere by ground-based GPS receivers. Prior to this work,
Nishioka et al. (2013) has reported concentric gravity waves in the
ionosphere which were induced by a severe convective system (supercell) in
the troposphere. The concentric waves lasted for more than 7 h. Nyassor
et al. (2021) reported the first mesospheric concentric gravity waves
excited by thunderstorm. Takahashi et al. (2020) presented the generation
and propagation of MLT GWs and concentric MSTIDs in the ionosphere during a
deep convection activity in the troposphere over the South American
continent.
Regarding propagation of GWs from the lower to upper atmosphere, Vadas (2007), for the first time, studied propagation property of GWs from the
troposphere to the thermosphere for the horizontal wavelength of 10 to 1000 km and the period of 10 to 100 min. The author presented the GW dissipation
altitudes depending on their horizontal wavelength and period. In case of
the horizontal wavelength of 200 km and its period of 60 min, for example,
the model predicts dissipation above 120 km altitude. It means that the
dissipation produces a body force and generates secondary waves.
There is a difficulty to observe a gravity wave from its origin (source) in
the troposphere following up to the thermosphere. During the upward
propagation, it could change its wave characteristics under the background
atmospheric condition, dissipating and producing secondary waves changing
the horizontal wavelength, phase speed, and propagation direction (Vadas and
Crowley, 2010). It would take several hours to reach from the troposphere to
the mesosphere lower thermosphere (Vadas and Liu, 2013), which makes it
difficult to follow the wave step by step. Recent observation of the
concentric wavefronts in the stratosphere, mesosphere, and thermosphere by
Azeem et al. (2015) would be rather a rare case. Further observational
evidence would be necessary to clarify the propagation processes. The
purpose of the present work is to report a case of gravity wave propagation
directly from a tropospheric convection to the mesosphere and
thermosphere/ionosphere. For investigating the propagation of gravity waves,
data from the airglow OH imager in the mesosphere, OI 630 nm imager from the
thermosphere/ionosphere, and ionosondes are used.
Observations
Airglow observation has been carried out at Bom Jesus da Lapa (hereafter
BJL), 13.3∘ S, 43.5∘ W, geomag.14.1∘ S, since
2019. The observation site is located under the equatorial ionospheric
anomaly (EIA) belt. Equatorial plasma bubbles can also be frequently
observed. An all-sky airglow imager equipped with 3 in. optical
interference filters (for 630.0, 557.7 and OH-NIR (710–930 nm)) takes
180∘ wide images with a time sequence of ∼ 5 min.
Exposure time for each filter is 15 s for the OH-NIR and 90 s for the OI
630.0 and 557.7 nm images. The imager characteristics have been presented
by Wrasse et al. (2021). In the present study we used the image data from
December 2019 to September 2020. During this period, we selected 13 d of
observation to analyse wave structures in the OI 630 nm images.
In the present work, the data from the ionosondes were used to observe the
vertical drift of the F-layer and to calculate the electron density profile.
Three digital ionosondes (DPS-4) (http://www.digisonde.com/instrument-description.html, last access: 30 September 2022) have been operated,
one at São Luís (2.6∘ S, 44.2∘ W, geomag.
3.9∘ S), Fortaleza (3.9∘ S, 38.4∘ W, geomag.
6∘ S), and Cachoeira Paulista (22.7∘ S, 45.0∘ W, geomag. 18.1∘ S). The DPS-4 sounder has a 500 W peak power,
covering a frequency range from 0.5 to 30 MHz. Ionograms are taken with a
time interval of 10 min.
ResultsOH images
On the night of 18–19 January 2020, around 22:30 to 23:30 UT (19:30–20:30 LT), the airglow OH images showed two wave structures passed over
BJL. Figure 1 shows the OH images at the moment of one of the wavefronts
passing over the zenith. The images are projected on the geographic
coordinates. The horizontal extension of the image is approximately 500 km
and the blue dot indicates the location of the BJL observation site. In
Fig. 1a, there are two wave structures, one is shorter wavelength in the
northwest side of the image (top left side) (GW-1) and another is a longer
wavelength, one wavefront over the zenith (blue dot) and the other at the
southeast (SE) portion, indicated by the blue arrows (GW-2). Three
sequential images with a time interval of 10 min indicate that the
wavefront is moving toward SE (indicated by a red arrow in Fig. 1b). The
propagation mode can be clearly seen if keograms are made by using the
meridional and longitudinal cuts as a function of time. Figure 2 shows the
keograms (zonal and meridional cuts) between 22:30 and 28:30 (04:30) UT.
Looking at the GW-2, two bright bands propagating from north (N) to south
(S) and west (W) to east (E) can be seen. The broad bright band passing over
the zenith from northeast (NE) to southwest (SW) through the night is the
galactic Milky Way.
Geographically coordinated OH images observed at Bom Jesus da Lapa (BJL), 13.3∘ S, 43.5∘ W, geomag. 14.1∘ S, on the night of 18–19 January 2020 at 23:30 UT (a), 23:41 UT (b), and 23:51 UT (c). Blue dots indicate the zenith of BJL. The blue arrows indicate the wave fronts of the longer wavelength one (GW-2). The red arrow indicates the direction of propagation.
Keograms of OH images observed at BJL in the night of 18–19 January 2020. The zenith crossing sliced image along the W–E direction (a) and the S–N direction (b) are shown as a function of time from 22:00 to 30:00 (06:00) UT. The red colour rectangular boxes are where the FFT (fast Fourier transform) spectral analysis was taken.
For calculating the wave characteristics (horizontal wavelength, period,
phase velocity), we used fast Fourier transform (FFT) spectral analysis
(Wrasse et al., 2007; Figueiredo et al., 2018; Essien et al., 2018). Image
samples used in the calculation are indicated by red boxes in Fig. 2. The
wave characteristics of the longer wave (GW-2) are the horizontal
wavelength of 217.9 ± 12 km, the period of 60.6 ± 03 min, the
phase speed of 59.9 ± 5 m s-1, and the propagation direction of
148.5 ± 10∘ (clockwise from north in degrees). For the short
wave (GW-1), the wave characteristics were also obtained: the horizontal
wavelength of 36.2 ± 1 km, the period of 15.8 ± 0.8 min, the phase
speed of 38.3 ± 2 m s-1, and the propagation direction of 135.0 ± 10∘.
OI 630 nm image
Figure 3 presents three sequential images of the OI 630 nm emission between
03:20 and 03:48 UT. The top three panels are original images and the bottom
three panels are the residual images which are subtracted from the 1 h
averaged image. From the residual images, one can see two dark bands in the
southwest of BJL propagating toward east, which seem to be the medium-scale
travelling ionospheric disturbance, named as MSTID-1. We checked any
contamination of the OH emission in the OI 630 nm image. No such wavelike
structure could be seen in the OH images during the same period. The bright
OI 630 emission intensity over the northwest part of the sky should be the
midnight downward drift of the F-layer accompanied by the midnight
temperature maximum (MTM) in the thermosphere (Colerico et al., 1996;
Figueiredo et al., 2017). One can also notice the presence of the equatorial
plasma bubbles (EPBs) (at least two depletions) in the northwest of BJL,
which are also drifting toward the east. The difference between the EPBs and
MSTID-1 is clear to see. The EPBs are extending from the Equator side and
the MSTID is elongated from the south. During the 28 min of the time
interval, from Fig. 3a to c, a dark band moved toward the east by
∼ 90 km. In order to get the wave characteristics of MSTID-1
from the OI 630 images, we used the FFT spectral analysis for the OI 630
keogram (not shown here), which is similar to the OH image analysis
mentioned above (Wrasse et al., 2007). The results are the horizontal
wavelength of 201.7 ± 13 km, the period of 64.2 ± 33 min, the
phase speed of 52.4 ± 27 m s-1, and the propagation direction of 113.2 ± 10∘. The characteristics of the wave propagation in the
OH emission layer and OI 630 nm emission layer are summarized in Table 1.
The movement of wave fronts of MSTID-1 is presented in the supporting file (OI 6300_movie.mp4).
Geographically coordinated OI 630 nm images observed at Bom Jesus da Lapa (BJL) at 03:20 (a, d), 03:34 (b, e), and 03:48 UT (c, f) on the night of 18–19 January 2020. The top three images are original and the bottom three images are residual subtracted from the 1 h averaged image. Blue dots indicate the location of BJL. MSTID-1, EPBs (upper left corner), and MTM can be seen (see text).
Wave Characteristics obtained by the OH images in the MLT region (GW-1 and GW-2), and OI 630 nm images in the thermosphere (MSTID-1). The values in parentheses are error ranges. “Az” indicates the propagation direction (clockwise from north).
Through the evening to midnight over the Bom Jesus da Lapa (BJL) airglow
observation site (17∘ S, 38∘ W) on 18–19 January 2020,
we observed a relatively long wavelength and slow speed GW (GW-2) in the OH
emission layer (∼ 87 km altitude) at around 23:00 UT. In 4 h later (03:00 UT), we observed a wave structure in the OI 630 nm
emission layer (∼ 240 km altitude) (MSTID-1). The two
different waves, one from MLT and the other from the thermosphere, had
almost the same wave characteristics, i.e. a same horizontal wavelength
(210 ± 10 km), same period (62 ± 5 min), and the same phase speed
(55 ± 5 m s-1). The propagation directions of the two emissions,
however, are slightly different, OH showing 149∘ N against OI 630
being 113∘ N, the difference of 36∘. The OH wavefronts
are extended longer than 500 km. On the other hand, the OI 630 wave was
limited in the southern sky with a relatively short duration
(∼ 60 min). Such coincident occurrence of the wave structure
called our attention to further investigate whether these waves have the
same origin from the lower atmosphere, i.e. both are primary waves, or one of the
waves in the thermosphere was due to a secondary wave generated in the lower
atmosphere.
GW ray tracing and tropospheric convection origin
For studying the wave propagation, we used a wave ray-tracing method
(Paulino et al., 2013; Vadas et al., 2019; Nyassor et al., 2021) to find out
the source of the waves in the lower atmosphere. The wind model used in this
work was according to the NRLMSISE-00 (Picone et al., 2002) and horizontal
wind model (HWM14) (Drob et al., 2015). Figure 4 presents the ray-tracing
trajectories of the GW-2 for the case of no-wind and with-wind model.
Ray tracing (backward and forward) of the observed gravity wave (λh=217.9 km) starting at 87 km altitude at BJL on the night of 18 January 2020 at 23:30 UT. The vertical trajectory versus time (a) and horizontal distance (b) are shown. The red line is of the case of no-wind and the blue line is of the with-wind model. The blue triangle is the starting point of the ray tracing. The background map is the cloud top temperature from NOAA GOES-16 meteorological satellite data (10.35 μ) on 18 January 2020 at 19:00 UT.
The simulation started from 87 km altitude at 23:30 UT and went down
(backward tracing) to the ground level at around 19:00 UT, crossing the
tropopause (∼ 15 km) at around (7.0∘ S,
46.5∘ W) at 19:30 UT. There is a difference of around 200 km of
the tropopause crossing positions between the no-wind and with-wind
trajectories, which can be assumed to be an error range in the present
study. Then, we search for any convective system in this region. Figure 4
also shows the cloud top temperature map produced by GOES-16 (10.35 μ
radiation map) at 19:00 UT (https://www.cptec.inpe.br/, last access: 30 September 2022). The
convection system spread over the tropical zone 0–10 S can be seen. It is
the Intertropical Convergence Zone (ITCZ). One can notice that there is the
lowest temperature spot (-80 ∘C) at (8.5∘ S,
46.5∘ W), where a deep convection was in progress. One can notice
that this convection spot is located very close (in an error range of 200 km) to the GW-2 trajectory at the tropopause height. According to the
GOES-16 maps, this convection spot started at around 18:00 UT, developing
into a much larger area from 19:00 to 23:00 UT and decreasing the intensity
after 00:00 UT. During the 5 h of activity, the convection spot should
generate up and down streams inside of the convection cell producing a
variety of GWs. The present ray tracing suggests that the observed gravity
wave in the OH emission layer started from this convection spot propagating
up to the lower thermosphere.
GW breaking in the thermosphere and generation of a secondary wave
The forward ray tracing shown in Fig. 4, on the other hand, went up to 130 km with the momentum flux in the maximum and then it lost the amplitude of
oscillation at around 140 km, indicating dissipation of the wave energy. The
wave dissipation occurred at the location of (18∘ S, 40∘ W) where we observed the wave structure in the OI 630 nm image starting at
around 03:00 UT. According to Vadas and Crowly (2010), GW dissipation
produces a body force and generates secondary waves. The secondary waves
have a variety of wave characteristics. In our present case, we understand
that the primary wave observed at the MLT region dissipates in the lower
thermosphere, then a secondary wave reaches at the OI 630 emission height,
which is located at around 240 km altitude. If this is the case of what
happened, the secondary wave had the same characteristic as the primary
wave. Vadas and Becker (2018) have discussed the small- and large-scale
secondary waves. According to them, there will be two kinds of secondary
waves, one is small-scale waves (short horizontal wavelengths) that will be
produced during the primary wave breaking process, and the other one is the
much longer wavelength (thousands of km) which is produced by a body force
generated after the primary GW dissipation. The latter is dependent on the
spatial scale of the body force. Bossert et al. (2017), for example, observed
secondary waves near the primary (mountain) wave breaking area and found
that the horizontal wavelengths are shorter than the primary waves. Smith et
al. (2013) reported nearly simultaneous observations of mesospheric GWs by OH
airglow and thermospheric GWs by OI 630 nm images. According to their
observation, the horizontal wavelength of the OH wave against the OI 630 nm
wave is 106 km vs. 255 km, the phase speed is 49.5 m s-1 vs. 104 m s-1, and the
period is 36.5 min vs. 42.7 min. Our present case (relatively short
wavelength and low phase speed) could be the first case, i.e. a secondary
wave generated during the primary wave breaking process.
Possible direct influence of primary wave in the ionosphere
The other possibility of the presence of GWs is a direct influence of the
primary wave in the OI 630 nm emission layer. The ray-tracing simulation for
the MLT GWs did show its dissipation at around 140 km (Fig. 4). According
to Vadas (2007), the signature of GWs in the thermosphere could be
observable even at one or two local density-scale heights (15–20 km at
around 150 km altitude) above the dissipation altitude. If this is the case,
the influence of the primary wave could reach at least at the altitude of
170–180 km where the F-layer bottom side is located. It is worth checking,
therefore, the OI 630 nm emission height over BJL during the GW occurrence
(03:00–04:00 UT).
The airglow OI 630 nm emission is produced by the dissociative recombination
process in the ionosphere:
O2++e→O1D+O3P,1S,
where O(1D) is an excited state of atomic oxygen responsible to emit a
photon of 630.0 nm. The emission rate, therefore, depends on the
concentration of the electron density [e] and its height profile (Chiang et
al., 2018). The electron density profile, especially its bottom side
profile, could be estimated by ionograms. Unfortunately, there is no
ionosonde at BJL. During the passage of the waves at around 03:00–03:30 UT
(00:00–00:30 LT), two DPS ionosondes, one at Fortaleza (7∘ S,
38∘ W), located at the north of BJL, and the other at Cachoeira
Paulista (22.7∘ S, 45.0∘ W), located at the south of
BJL, were in the routine observation mode. The Fortaleza ionogram showed the
F-layer peak height (hmF2) at 220 ± 10 km. It is a mean altitude during
the period of 03:00 and 03:20 UT when the ionogram was free from the spread
F condition. It is very low altitude, due to the midnight collapse of the
ionosphere (Gong et al., 2012). On the other hand, the ionosonde at
Cachoeira Paulista observed the peak altitude at 260 ± 10 km. From the
estimated electron density profiles, we calculated the OI 630 nm volume
emission rates based on the equation presented by Chiang et al. (2018). The
peak emission altitude at Fortaleza was at 200 km. On the other hand, at
Cachoeira Paulista it was at 240 km. The peak altitude at Fortaleza is 40 km
lower than Cachoeira Paulista. The BJL site is located between the two
ionosonde sites. Therefore, we assume that the OI 630 nm emission peak
altitude at BJL might be between 200 and 240 km. In this case, a possibility that the bottom side of the OI 630 nm emission layer would be disturbed
by the primary wave cannot be ruled out. The difference of the propagation
direction of 36∘ between the OH and OI 630 nm wave fronts could be
due to the different wind fields between the two emission altitudes.
Conclusions
We observed two gravity waves, one at the OH emission height
(∼ 87 km) and the other at OI 630 nm emission height
(<240 km), which showed the same wave characteristic. Although the
two waves look to be similar, the wave observed in the ionosphere might be a
secondary wave. However, a direct influence of the primary wave in the OI
630 nm emission layer at around 200 km altitude cannot be ruled out. Both
the waves have their origin from a convective spot in the ITCZ region. This
is the first time reporting the direct evidence of GW propagation from the
troposphere to the ionosphere by optical imaging measurements in the South
American region.
Data availability
Airglow image data and ionosonde data used in the present study are
available at the EMBRACE data centre website
(http://www2.inpe.br/climaespacial/portal/en/#, EMBRACE, 2022). The satellite infrared
thermal images (Fig. 7) are obtained from the Geostationary Operational
Environmental Satellite System 16 (GOES 16) data
(http://satelite.cptec.inpe.br/home/index.jsp, CPTEC, 2022), provided by the Center for
Weather Forecasting and Climate Studies (CPTEC) in Brazil. Two atmospheric
models were used in computing the gravity wave ray tracing (Fig. 6); one
is the MSIS-E-00 Atmospheric Model: https://ccmc.gsfc.nasa.gov/modelweb/models/msis_vitmo.php (CCMC, 2022)
and the other is the empirical Horizontal Wind Model (HWM14) (Drob et al.,
2015).
Author contributions
HT was responsible for data analysis, interpretation, and text editing. CAOBF was responsible for data analysis and interpretation. PE worked to carry out data processing and interpretation. CMW was responsible for data interpretation and discussion. DB was responsible for data analysis and interpretation. PKN worked to carry out data analysis and interpretation. IP was responsible for ray-tracing data analysis. FE contributed to data handling and scientific discussion. GMR and AHRS were responsible for imager operation and airglow observation.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “From the Sun to the Earth's magnetosphere–ionosphere–thermosphere”. It is not associated with a conference.
Acknowledgements
We thank the Brazilian Ministry of Science, Technology and Innovation (MCTI) and the Brazilian Space Agency (AEB), who supported the present work under the grants PO 20VB.0009. The present work was supported by CNPq (Conselho Nacional de Pesquisa e
desenvolvimento) under the grants 310927/2020-0, 150569/2017-3,
161894/2015-1, 303511/2016, 300322/2022-4, and 306063/2020-4;
Fundação de Amparo à Pesquisa do Estado de São Paulo
(FAPESP) under the grants 2018/09066-8 and 2019/22548-4; and
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
(CAPES) under the process BEX4488/14-8. Igo Paulino thanks Fundação
de Amparo à Pesquisa do Estado da Paraíba for the grants Demanda
Universal Edital 09/20221 and Edital PRONEX termo de concessão 002/2019.
Review statement
This paper was edited by Dalia Buresova and reviewed by Jan Laštovička and one anonymous referee.
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