Introduction
In the last decades, gravity waves in the mesosphere and
lower thermosphere (MLT) have largely been observed around the world,
primarily due to advances in the development of charge-coupled
device (CCD) cameras. Observations using CCD cameras to monitor
gravity waves in the airglow started with
() during the ALOHA campaign, and nowadays the
imaging of airglow is the principal method used to study high-frequency gravity
waves in the MLT region. Imaging allows for the estimation
of horizontal parameters of gravity waves, like wavelength and
horizontal phase velocity. The gravity wave period can be estimated using
a temporal sequence of images as well (e.g., ,
; ,
). Furthermore, gravity waves are responsible for
the transport of a significant portion of the energy and momentum
between the atmospheric layers. Therefore, gravity waves have
a crucial role in the general circulation of the atmosphere
(, ).
Several airglow emissions come from thermospheric heights, which
coincides with the bottom side of the ionospheric F region. These
emissions have been used to study the morphology and dynamics of
phenomena in the ionosphere, such as equatorial plasma bubbles (e.g.,
, ;
, ;
, ;
, ;
, ), equatorial
ionization anomaly (e.g., , ;
, ), traveling
ionospheric disturbances and gravity waves (e.g.,
Taylor et al., ;
Kubota et al., ;
Garcia et al., ;
Shiokawa et al., ,
; Otsuka et al.,
; Candido et al.,
; Martinis et al.,
; ,
; , ;
, ;
, ), etc. The most
important emission in the thermosphere is the OI 630.0 nm
(hereafter, OI6300) which is a spectral red line and has an intensity
strong enough to be detected by airglow imaging systems.
Using OI6300 airglow images, periodic and quasi-monochromatic waves
were studied around the world revealing their spectral characteristics
(e.g., , ;
, ;
, ;
, ;
, ). The observed
parameters were quite different depending on the location of the observation sites.
Observations of the propagation direction of the
periodic waves showed different anisotropic patterns which could be
related to their sources. Seasonality and solar-cycle
dependencies have peculiarities that depend on the region of
observation. Thus, more observations and studies are necessary to
understand how these waves are generated and how they interact with
the background atmosphere (,
).
used long-term observations of OI6300 images to
study 98 periodic waves over São João do Cariri (7.4∘ S, 36.5∘ W) during almost one solar cycle. The results showed
that most of the observed periods ranged from 10 to 35 min. Periodic
waves with horizontal wavelengths from 100 to 200 km were most
common and the phase speeds were most concentrated from 30 to
180 ms-1. Observations of the propagation direction of the
periodic waves showed anisotropy patterns which could be related to
either the sources or a filtering process by the wind. The largest
occurrence of the waves was during the winter months, and this occurrence had a direct
correlation with the solar activity.
No magnetic influences on occurrence were observed.
In the study of atmospheric waves, the knowledge of the background
wind is crucial to understanding the conditions in which the waves are
propagating . Ground-based observations of periodic
waves using airglow images only allow for estimating the observed
parameters. In this case, the motion of the wind is superposed on to
spectral characteristics of the waves. However, simultaneous
measurements of the background wind combined with the observed
parameters provide sufficient information to calculate the intrinsic
parameters, which consist of determining spectral parameters (period
and phase speed) of the waves excluding the effects of wind.
Besides, the propagation of the high-frequency gravity waves in the
atmosphere depends on the magnitude as well as the direction of the wind
. On one hand for instance, gravity waves propagating parallel to the
wind can easily attain critical levels and be absorbed by the atmosphere. On
the other hand, gravity waves propagating in the opposite direction of the
wind can find a turning level and be reflected due to the wind system.
In the present work, the intrinsic parameters of 24 periodic waves
were calculated and analyzed. Those waves were observed from 2012 to
2014 using OI6300 airglow images of an all-sky imager deployed at
São João do Cariri. A project called the Remote Equatorial Nighttime Observatory of
Ionospheric Regions (RENOIR) was simultaneously
deployed and provided measurements of temperature and wind of the
thermosphere obtained from the OI6300 emission by Fabry–Pérot
interferometers (FPIs; ). Using the FPI measurements,
it was possible to investigate the effects of the thermospheric winds
in the propagation of these periodic waves for the first time over
Brazil.
Observed and intrinsic parameters of medium-scale gravity waves over
São do João do Cariri. Intrinsic parameters are denoted by “i” and
observed by “o”. The propagation direction is represented by ϕ and
increases clockwise from the north. The horizontal components of the wind are
represented by U and V for zonal and meridional components, respectively.
The “Events” column represents the sequential order used in Figs. 4 and 5.
Date
Event
Time interval
λH (km)
τ0 (min)
τi (min)
C0 (ms-1)
Ci (ms-1)
ϕ (∘)
U (ms-1)
V (ms-1)
23 May 2014
1
21:20 → 21:37
131.7
27.4
74
80.2
29.6
121
41.6
-28.7
22 Apr 2012
2
21:57 → 22:24
122.6
11.5
19.2
178
106.1
151.4
66
-45.5
21 Apr 2012
3
21:32 → 22:05
142.6
14.8
31.1
160.7
76.5
158.2
132.1
-37.7
27 May 2014
4
01:12 → 01:39
142
27.5
45.1
86
52.5
56.3
78.56
-57.2
16 Jun 2014
5
23:31 → 23:58
142.6
27.9
41.3
85.3
57.5
68.2
59.92
-75.4
15 Jun 2012
6
21:58 → 22:12
112.9
33.5
44.8
56.2
41.9
17.1
60.2
-3.7
07 May 2013
7
23:40 → 00:02
108.6
14.4
13.9
125.6
130.5
8.1
97.6
-18.7
08 Oct 2013
8
23:18 → 23:36
150.6
17.5
15.4
143.2
162.4
11.3
111.7
-41.7
08 Apr 2013
9
22:43 → 23:02
106.5
63.9
36.5
27.8
48.6
33.7
78
-77.0
15 Aug 2012
10
21:57 → 22:19
114.5
14.6
12.7
130.4
150.5
26.6
57.6
-51.01
15 Aug 2012
11
21:57 → 22:19
114.5
14.6
12.7
130.4
150.5
26.6
57.6
-51.0
14 May 2012
12
21:53 → 22:25
169.6
20.4
16.5
138.6
170.9
6.3
87.6
-42.2
08 Jan 2013
13
00:01 → 00:19
139.6
18.5
14.4
126
161.7
0
92.6
-35.9
09 Jul 2012
14
21:34 → 21:59
150.6
15.7
12.8
159.9
195.7
11.3
70.6
-50.6
25 May 2014
15
21:41 → 22:03
128
18.6
13.9
114.7
153.3
0
75.9
-38.6
06 Dec 2012
16
22:24 → 23:00
152.8
21
14.4
121
176.6
5.7
05.1
-66.1
25 Apr 2012
17
23:25 → 00:01
149.2
20.7
17.2
120.1
144.8
29.1
31
-45.5
16 May 2012
18
21:49 → 22:28
139.1
24.8
16.2
93.5
143.1
5.2
90.4
-58.0
13 Jun 2012
19
23:36 → 23:56
152.8
23
13.9
111
182.9
354.3
128.9
-59.6
07 Sep 2012
20
21:34 → 21:59
190.5
27.5
18.1
115.6
175.7
7.1
87
-71.5
18 May 2012
21
00:50 → 01:18
67.2
38.1
16.5
29.4
67.7
336.8
57.5
-17.0
16 Jul 2012
22
21:48 → 22:27
170.7
32.9
20.7
86.5
137.2
0
50.8
-50.7
14 Apr 2012
23
22:43 → 23:15
171.7
13.4
10.2
214
279.9
116.6
-32.3
83.7
24 Jul 2012
24
02:02 → 02:22
117.8
20.7
13.2
95
149.1
27.5
45.0
-35.6
Instrumentation and observations
Airglow images of two emissions, near-infrared (NIR) OH and OI6300,
have routinely been taken by an all-sky imager installed in São
João do Cariri since 2011. In the present work, data collected from
2012 and 2014 were used to study periodic waves in OI6300 airglow
images. Simultaneous thermospheric wind data were also collected by two
FPIs, one deployed at São João do Cariri
and the other at Cajazeiras (6.9∘ S, 38.5∘ W).
The São João do Cariri's all-sky imager was developed by Keo
Scientific. The fast (f/0.95) imaging system uses a fisheye lens with a
field of view of 180∘. The light is projected through a filter
wheel with 3 inch diameter interference filters. During these
observations only two filters were used, one for the OI6300 emission
and another for the NIR OH emission, which is a notched wide-band
filter. The images are projected onto a CCD of
1024×1024 pixels and each pixel has 13 µm of
width. The sensor has a high quantum efficiency, better than 95 %,
and is cooled down to -70 ∘C to reduce the dark current. Other
technical details about this imager have been published elsewhere (e.g., ).
Images of the OI6300 airglow emission have been
taken using an exposure time of 90 s. Thus, high-frequency
periodic waves with periods greater than 5 min can be detected
by this system. The methodology to determine the spectral parameters
of the periodic waves in the OI6300 emission data was published by
.
The RENOIR project was designed to study the dynamics of the
equatorial thermosphere. An important contribution of this project was
measurements of the thermospheric neutral wind in the equatorial zone
over the South American continent . The
FPIs used in this experiment have an interference filter of 50 mm
diameter combined with an etalon of 42 mm diameter. The
reflectivity of the etalon was adjusted to be 77 % in order to
enhance the transmission of the light at 630 nm without
effective loss of the spectral resolution. A set of lenses produces 11
rings of the interference pattern onto a CCD camera. The CCD has
a resolution of 1024×1024 pixels and each pixel has
13 µm of width. A dual-mirror sky scanner controlled by
two smart motors was used to observe different directions in the
sky. The sky scanning was calibrated using the Sun's position with
an accuracy of 0.2 per degree. An exposure time of 300 s was
used to observe the OI6300 and 30 s of exposure to make
frequency-stabilized laser calibration images. Further details about
the design of the FPI can be found in . Since
the FPIs were separated by less than 250 km, measurements of
thermospheric wind from either the São João do Cariri's FPI or
the Cajazeiras FPI were used to estimate the components of the horizontal
wind to be used in the calculation of the intrinsic frequencies of the
periodic waves. As the observed waves occupied a large portion of the
airglow images, this assumption is quite reasonable.
Histogram for the wavelengths of the periodic waves. <λh>
represents the average and σλh represents the standard deviation of the mean.
![]()
Histograms for the (a) observed periods,
(b) intrinsic periods, (c) observed phase speeds
and (d) intrinsic phase speeds. The number in the brackets
represents the average and the standard deviation of the mean is represented by
σ.
![]()
To calculate the intrinsic frequency of the periodic waves, the
following equation was used:
ωi=ωo-kH⋅U,
where ωo is the observed frequency,
kH is the horizontal wave vector
and U is the horizontal wind interpolated to the time in which the
waves were observed. Winds measured at São João do Cariri were
preferred to be used. However, in some cases, the winds measured at
Cajazeiras were used as well when São João do Cariri's winds were not
available. From the intrinsic frequency, the intrinsic period and intrinsic
horizontal phase speed can be directly estimated.
Table 1 shows all parameters of the studied periodic waves. Notice
that the intrinsic parameters are denoted by the index “i”, the
observed parameters are represented by the index “o”. The
propagation direction is represented by ϕ and increases
clockwise from the north. The horizontal components of the wind
were represented by U and V for zonal and meridional components,
respectively. The “Events” column represents the sequential order
used in Figs. 4 and 5.
Results and discussion
Spectral parameters
From 2012 to 2014, 24 periodic waves were observed in the airglow images with
simultaneous measurement of the horizontal wind by the FPIs. Figure 1 shows
the histogram for the wavelengths of the periodic waves, which ranged mostly
from 100 to 180 km with an average of ∼136 km and standard
deviation of ∼27 km. These results compare favorably with
the results of for long-term observations at the same site.
Thus, the wavelength pattern of the periodic waves used in this study is
representative of the typical waves observed at this site. However,
comparison with other observations in low latitudes reveals, in general, that
the periodic waves observed over São João do Cariri are shorter than
other locations e.g.,.
Figure 2 shows the observed (a, c) and intrinsic (b, d) periods (a, b) and
phase speeds (c, d) for the periodic waves. Most of
them had periods shorter than 40 min with an average of ∼23 min and standard deviation of ∼11 min. Regarding
the phase speed, there were more events between 75 and 175 ms-1
with the average around 113 ms-1 and a standard deviation of ∼43 ms-1. These results are quite similar to the previous
observations at the same site . In order to investigate
the sources of these gravity waves, one can observe that these periodic waves have
faster phase speed as compared to gravity waves observed in the MLT region at the
same location . However, simulations
of the propagation of gravity waves in low latitudes via
ray-tracing showed that medium-scale gravity waves could attain at maximum
200 km of altitude into the thermosphere–ionosphere . In this case, the sources of these two sets of
waves must be different. Since the faster gravity waves are less susceptible
to the wind filtering process in the atmosphere , the periodic waves must have their origin in the thermosphere
and propagate upward.
Figure 2 also shows the distribution of the intrinsic parameters. Most of the
waves had intrinsic periods shorter than 20 min, the average was ∼22 min (almost the same for the observed case) and the standard
deviation was ∼15 min. According to the histogram for the phase
speed, one can see that most waves had intrinsic phase speeds between 125 and
175 ms-1. The average of the intrinsic phase speed was ∼130 ms-1 and the standard deviation was ∼60 ms-1.
One advantage of knowing the intrinsic parameters is that the
propagation characteristics (time and distance prior to dissipation,
for instance) of the gravity waves can be assessed more precisely
. In this study, if most of the waves were launched
from the lower thermosphere (∼120 km height), they could
propagate up to the bottom of the ionospheric F region (∼250 km height) and then be observed by the OI6300 airglow
images. Another important characteristic of these waves is the
inclination of their propagation in the atmosphere. Since they have
horizontal wavelengths of ∼140 km, on average, and the
vertical wavelength must be at least ∼80 km due to
the thickness of the OI6300 layer , these waves must
have an inclination much larger than those waves observed in the MLT,
thus they can propagate quickly in the vertical
.
Vector diagrams for (a) observed and (b)
intrinsic phase velocities. Each circle represents an isoline of
50 ms-1.
![]()
Propagation direction
Figure 3 shows the propagation direction of the periodic waves. The
chart on the left is for the observed phase velocity and the chart on
the right is for the intrinsic phase velocity. Most of the waves
propagated to the northeast, north and northwest and four of them propagated
to the southeast, which is the same anisotropy presented by
. Comparing the intrinsic phase velocity with the
observed one, it is possible to see that the wind is decelerating
most of the waves which propagate to the north. Regarding the waves
propagating southeastward, three of them were decelerated.
The explanation of the increase and decrease in the observed period
due to the wind can be interpreted from Eq. (1). When the periodic
waves are propagating in the same direction as the wind, the second
term of the right-hand side is maximized, the observed frequencies increase
and the observed periods are reduced compared to the intrinsic
periods. Otherwise, if the wave propagates anti-parallel to the wind direction,
the observed frequencies are reduced, resulting in an increase in the
observed periods.
Figure 4 shows this effect on the periodic waves observed in this
study. The horizontal dashed line represents 90∘ between the
wave and wind vector. For all periodic waves observed below this line,
the intrinsic periods are longer than the observed periods. When the
wind is close to 90∘, there is not much difference and for
angles greater than 90∘, the observed periods are longer than the intrinsic periods.
For event 9, the discrepancy was the largest and it happens due to
the strength of the observed wind during the occurrence of this wave.
On one hand, gravity waves propagating parallel to the background wind can reach critical
levels and be absorbed by the medium. On the other hand, if the propagation
is anti-parallel to the wind, the gravity waves can be reflected due to
turning levels. These are the reasons why fast gravity waves propagate easily
into the atmosphere .
The measurements of the horizontal wind are really useful to discuss the propagation of the periodic waves.
Further, in the lower part of the thermosphere which corresponds to the range of the OI6300 airglow layer according to the
horizontal wind model (HWM-14; ∼ 200–300 km), the wind does not change quickly in time and
altitude .
Diagram for the angle between the propagation of periodic
wave and wind vector for all 24 waves. The observed and
intrinsic periods are shown on the right vertical axis.
![]()
Vector diagrams for the winds (dashed arrows) and phase
velocity (solid arrows) observed during the summer (a),
autumn (b), winter (c) and spring
(d).
![]()
Blocking diagrams due to the wind for the summer
(a), autumn (b), winter (c) and spring
(d). Theoretically, propagation of periodic waves were
prohibited in the shaded areas due to the action of critical
levels.
![]()
One topic that was poorly discussed in is the
anisotropy of the propagation direction of the periodic waves. This
kind of anisotropy can have two origins: either associated with the
anisotropy of the sources themselves or the filtering process of the
gravity due to the wind system. Figure 5 shows a vector diagram for
the phase velocity of the periodic waves (solid arrows) and horizontal
wind vector (dashed arrows). The numbers in front of the arrows
indicate the event numbers shown in Table 1 and Fig. 4.
The first important result from this analysis is that the wind direction derived
from the FPI (∼250 km) during the evening over São
João do Cariri is typically southeastward and it corresponds to the
time interval of maximum occurrence of the periodic waves. As
a consequence of this wind pattern, most of the observed periodic
waves were propagating almost orthogonal to the wind and it is
certainly the most important reason why many observed periodic waves
had a propagation direction to the north at the observation site.
Furthermore, according to the Thermosphere–Ionosphere–Electrodynamics General Circulation Model
TIE-GCM;, the horizontal wind in the lower
thermosphere (below 180 km height) during the evening is
southwestward. Thus, periodic waves generated in the lower levels of
the thermosphere are not allowed to propagate almost parallel to the
southwest–northeast direction and, in the levels near the OI6300
layer, the blocking area is moved to be almost parallel to
the southeast–northwest direction. As a result, only faster periodic waves
are able to skip the filtering effects and can be observed in this
direction. The results of sustain this
hypotheses indicating that the wind filtering is the main reason for
the observed anisotropy in the propagation direction of periodic
waves, even if the sources of them are isotropic.
Even so, few events were observed in the direction parallel or
anti-parallel to the wind. In all cases, the phase speeds of the
periodic waves were greater than the magnitude of the wind and it
agrees with the theory. Figure 6 shows the blocking diagrams for the
seasons calculated using the HWM-14 model for the heights below the
OI6300 layer and from 18:00 to 24:00 LT. This time interval coincides
with the time of maximum occurrence of periodic waves as mentioned
above. According to the theory of the filtering process of the waves
due to the wind system, propagation of periodic waves into the shaded
area is not allowed; since in these cases, the waves would attain
critical levels. Further details on the calculations of the
blocking diagrams were published by and
. It is important to observe that none of the
studied periodic wave were into the prohibited areas, i.e., indicating
that all periodic waves have enough phase speed to skip the
critical levels.
Furthermore, during the summer, autumn and winter, the main blocking
areas are almost orthogonal to the propagation direction of the
periodic waves, which help to explain the observed results. During the
spring, the blocking diagram had a large area almost parallel to the
propagation of the waves; however, the waves had phase velocity
greater than the prohibited area, and were able to overcome the absorption
level.
Thus, the usage of the blocking diagram reinforced with the FPI
measurements of the wind in the same altitude of the periodic wave
satisfactorily explain the propagation direction of the periodic waves
over São João do Cariri.