Journal of Geophysical Research: Space Physics
RESEARCH ARTICLE
10.1002/2017JA023919
Key Points:• Long-term analysis of the horizontal
parameter of wave structures
observed by airglow imager• Clear seasonal variation of
propagation direction of gravity
waves in mesopause region• Relationship between medium-scale
traveling ionospheric disturbancesand solar activity

Correspondence to:K. Shiokawa,
shiokawa@nagoya-u.jp

Citation:
Takeo, D., K. Shiokawa, H. Fujinami,
Y. Otsuka, T. S. Matsuda, M. K. Ejiri,
T. Nakamura, and M. Yamamoto(2017), Sixteen year variation ofhorizontal phase velocity andpropagation direction of mesosphericand thermospheric waves in
airglow images at Shigaraki, Japan,
J. Geophys. Res. Space Physics,
122, doi:10.1002/2017JA023919.

Received 18 JAN 2017
Accepted 17 JUL 2017
Accepted article online 20 JUL 2017

Sixteen year variation of horizontal phase velocity
and propagation direction of mesosphericand thermospheric waves in airglow
images at Shigaraki, Japan
D. Takeo1 , K. Shiokawa1 , H. Fujinami1
T. Nakamura2 , and M. Yamamoto3, Y. Otsuka1 , T. S. Matsuda2, M. K. Ejiri2,

1 Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan, 2 National Institute of Polar

Research, Tachikawa, Japan, 3 Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan

Abstract We analyzed the horizontal phase velocity of gravity waves and medium-scale travelingionospheric disturbances (MSTIDs) by using the three-dimensional fast Fourier transform method
developed by Matsuda et al. (2014) for 557.7 nm (altitude: 90–100 km) and 630.0 nm (altitude: 200–300 km)
airglow images obtained at Shigaraki MU Observatory (34.8∘ N, 136.1∘ E, dip angle: 49∘ ) over ∼16 years
from 16 March 1999 to 20 February 2015. The analysis of 557.7 nm airglow images shows clear seasonalvariation of the propagation direction of gravity waves in the mesopause region. In spring, summer, fall,
and winter, the peak directions are northeastward, northeastward, northwestward, and southwestward,
respectively. The diﬀerence in east-west propagation direction between summer and winter is probably
caused by the wind ﬁltering eﬀect due to the zonal mesospheric jet. Comparison with troposphericreanalysis data shows that the diﬀerence in north-south propagation direction between summer andwinter is caused by diﬀerences in the latitudinal location of wave sources due to convective activity inthe troposphere relative to Shigaraki. The analysis of 630.0 nm airglow images shows that the propagation
direction of MSTIDs is mainly southwestward with a minor northeastward component throughout the
16 years. A clear negative correlation is seen between the yearly power spectral density of MSTIDs and F10.7
solar ﬂux. This negative correlation with solar activity may be explained by the linear growth rate of the
Perkins instability and secondary wave generation of gravity waves in the thermosphere.
1. Introduction
Many studies have investigated wave-like structures in the upper atmosphere, such as gravity waves andmedium-scale traveling ionospheric disturbances (MSTIDs) by using measurement techniques such as radar,lidar, and optical airglow imaging. Radar and lidar observations provide the vertical structure of these waves
with high spatial resolution. However, it is diﬃcult to observe the horizontal structure of gravity waves by using
these measurement techniques. In contrast, airglow imaging is useful for investigating the two-dimensional
horizontal structure of waves in the upper atmosphere, providing the horizontal phase velocity with highspatial and temporal resolutions. There are two major airglow line emissions from the atomic oxygen at wavelengths of 557.7 nm at altitudes of 90–100 km and 630.0 nm at altitudes of 200–300 km. The former emission
can be used as a manifestation of atmospheric density variations in the mesopause region, while the latteremission represents variations of both atmospheric and plasma density variations in the bottomside of theionospheric F layer.
Gravity waves aﬀect the wind ﬁeld and thermal balance in the mesosphere and lower thermosphere region
and cause large-scale pole-to-pole circulation, because the waves propagate from the lower to the upper
atmosphere with momentum transport. The vertical propagation of gravity waves and its momentum transport depend on the horizontal phase velocity of gravity waves according to the dispersion relation of waves.
Therefore, the study of the horizontal phase velocity of gravity waves is important.

©2017. American Geophysical Union.
All Rights Reserved.

TAKEO ET AL.

The horizontal parameters of waves have been examined in many studies by using airglow images [e.g., Taylor
et al., 1993; Hecht et al., 1994; Swenson et al., 1999; Yue et al., 2010]. Nakamura et al. [1999] made 18 month longterm imaging observations of gravity waves at Shigaraki, Japan, and discussed the seasonal variation of the
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propagation direction of gravity waves. Ejiri et al. [2003] also investigated the seasonal variation of the propagation direction of gravity waves at Shigaraki and Rikubetsu, Japan, for 1 year. However, long-term variations
in small-scale gravity waves observed by airglow imagers for more than 10 years have not been studied yet,because previous studies were performed by visual inspection, which requires a long time to analyze a large
number of airglow images. Recently, Matsuda et al. [2014] developed a three-dimensional fast Fourier transform (3-D FFT) method to obtain the power spectra of airglow intensity variation in the horizontal phasevelocity domain. This method is suitable for rapidly deriving the horizontal phase velocity characteristics ofatmospheric gravity waves from a large amount of imaging data.
MSTIDs also have been studied by using airglow images [e.g., Garcia et al., 2000; Makela and Kelley., 2003;
Martinis et al., 2010; Narayanan et al., 2014]. Taylor et al. [1998] reported MSTID observations during theSporadic E Experiment over Kyushu (SEEK) campaign. These were the ﬁrst 630.0 nm airglow measurementsof nighttime MSTIDs in Japan. Kubota et al. [2000] and Saito et al. [2001] observed sequential MSTIDs, which
propagated southwestward over Japan, by using ﬁve airglow imager sites in Japan. Shiokawa et al. [2003a]
performed statistical analysis of MSTIDs obtained at two airglow imager sites in Japan by visual inspection.
However, long-term variations of MSTIDs observed by airglow imaging for more than 10 years have not yet
been studied.
In this study, we used the 3-D FFT method developed by Matsuda et al. [2014] to analyze the horizontal phasevelocity spectrum of gravity waves and MSTIDs by using 557.7 nm (altitude of 90–100 km) and 630.0 nm(altitude of 200–300 km) airglow images obtained at Shigaraki MU Observatory (34.8∘ N, 136.1∘ E) over
∼16 years. This study enables us to investigate the long-term variation of horizontal phase velocity and the
propagation direction of gravity waves and MSTIDs and their relationship to other atmospheric and solar
activity parameters.

2. Observation
We used airglow images at Shigaraki MU Observatory that were collected over ∼16 years from 16 March 1999
to 20 February 2015 by the all-sky airglow imager No. 1 of the Optical Mesosphere Thermosphere Imagers
owned by the Institute for Space-Earth Environmental Research, Nagoya University [Shiokawa et al., 1999,
2009]. This system has a ﬁsh-eye lens with a ﬁeld of view of 180∘ , multiple interference ﬁlters with a wheel
to change ﬁlters, and a cooled charge-coupled device (CCD) camera with a resolution of 512 × 512 pixels.
The imager 1 was calibrated on 8 January 1998, 12 December 2000, 26 October 2006, 21 February 2007, and
19 April 2012, by using calibration facilities in the National Institute of Polar Research, Japan. The wavelengthsof airglow images we used were 557.7 nm in the mesopause region (altitudes from 90 to 100 km) and 630.0 nm
in the thermosphere (altitudes from 200 to 300 km). The sampling periods of these airglow images were 5 min(16 March 1999 to 9 July 2000) or 5.5 min (10 July 2000 to 20 February 2015) for each wavelength. We usedairglow images only during clear-sky nights because we could not observe gravity waves when there wascontamination by clouds.

3. Analysis Method
We used the 3-D FFT method developed by Matsuda et al. [2014] to obtain the three-dimensional wave
number spectrum of the periodic structures in airglow images. There have been many studies of gravity waves
that have analyzed airglow images by visual inspection. However, visual inspection is time consuming andcauses artiﬁcial bias depending on the person analyzing the images. In contrast, the horizontal phase velocity
spectrum can be obtained easily by the 3-D FFT analysis method.
First, we removed dark counts and the oﬀset of the CCD camera by subtracting the average counts at thefour corners of the images where the sky image was not projected. We assume that the dark count is uniform
over the CCD array. We also removed localized bright spots caused by stars by using an 11 pixel median ﬁlter.
Next, we calculated the absolute intensity of the airglow at each image pixel by subtracting the contaminationof the background continuum emission, which was passed through the 557.7 and 630.0 nm band-pass ﬁlters.
The intensity of the background continuum emission was measured by a 572.5 nm band-pass ﬁlter every
30 min, and we assumed that the intensity was the same at 572.5, 557.7, and 630.0 nm. We calculated thenormalized perturbation of the airglow intensity, ΔIi = (Ii − ̄I)∕̄I, where Ii and ̄I are the airglow intensities at theconsidered time and averaged over ±30 min of the time, respectively. Next, we converted the all-sky images
obtained by the ﬁsh-eye lens to equidistant coordinates with horizontal sizes of 400×400 km for the 557.7 nm
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images and 800 × 800 km for the 630.0 nm images. For this conversion, we assumed airglow heights of 95 km
for the 557.7 nm images and 250 km for the 630.0 nm images. Shefov et al. [2007] has shown that the heightvariation of 630 nm emission layer is less than ±20 km during solar maximum and minimum periods. Next,
we used the 3-D FFT method of Matsuda et al. [2014] to obtain wave number spectra of gravity waves, and weconverted the wave number spectra to horizontal phase velocity spectra by using equation (1).
vx =

𝜔k
𝜔l
, vy = 2 2
k 2 + l2
k +l(1)

Here vx and vy are the zonal and meridional components of the horizontal phase velocity, respectively, 𝜔 is thefrequency, k is the zonal wave number, and l is the meridional wave number. For the conversion, we changed
volume element by using the Jacobian determinant as shown by equations (2) and (3).
dvx dvy d𝜔 = J ⋅ dkdld𝜔
| 𝜕vx
| 𝜕k
| 𝜕v
J = || y
| 𝜕k
| 0
|

𝜕vx
𝜕l
𝜕vy
𝜕l

0

𝜕vx |
|
𝜕𝜔
𝜕vy ||
𝜕𝜔 ||

(2)

(3)

1 ||

The spectra were obtained for horizontal wavelengths from 10 to 100 km for 557.7 nm and from 40 to 400 km
for 630.0 nm. Finally, we integrated the horizontal phase velocity spectrum in all frequency ranges from 1/10.9
to 1/60.0 min (for the 5.0 min cadence) and from 1/11.0 to 1/60.5 min (for the 5.5 min cadence) for both 557.7
and 630.0 nm images.

4. Results and Discussion
In this section, we present the long-term variation of the horizontal phase velocity spectra and propagation
direction for the mesospheric and thermospheric wave structures observed in airglow images, and we discuss
the results. In the analysis, we deﬁned the four seasons as follows: (1) spring: from 21 February to 19 April for
2 months, centering on the spring equinox; (2) summer: from 20 April to 20 August for 4 months, centeringon the summer solstice; (3) fall: from 21 August to 19 October for 2 months, centering on the fall equinox; and(4) winter: from 20 October to 20 February for 4 months, centering on the winter solstice.
Summer and winter are longer than spring and fall because the dynamical variation of the middle atmosphereis divided into summer and winter types.
4.1. Results From 557.7 nm Airglow Images
Figure 1 shows the horizontal phase velocity spectra for the four seasons averaged over 16 years from 1999
to 2015 at Shigaraki, Japan. The power spectral density (PSD) indicates the power of the observed waves,corresponding to the square of the wave amplitude. These spectra are averages of all the 16 year spectra
obtained in the analysis for each season. The power spectral density is calculated by summing all the power
spectral density for 2–4 h time segments and dividing the summation by the number of time segments.
The low-velocity region (0 to 20 m/s) shows high power spectral density because the white noise of airglow
images contribute to enhance the power spectral density at the region of small wavelengths of images where
the power spectral density from natural waves becomes relatively minor. The seasonal variations in the propagation direction of the gravity waves can be clearly identiﬁed in the higher-velocity region. In spring, thepeak is northeastward and southwestward with a boundary of the power spectral intensity between yellow
and green at ∼70 m/s and ∼40 m/s, respectively. In summer, the signiﬁcant peak is north and northeastward
with a boundary at ∼70 m/s. In fall, the peak is northwestward with a boundary at ∼60 m/s. In winter the peakis southwestward with a boundary at ∼50 m/s. These seasonal variations of propagation direction are fairly
consistent with those obtained by visual inspection in previous research (Nakamura et al. [1999] for Shigaraki
and Ejiri et al. [2003] for Shigaraki and Rikubetsu).
Figure 2 shows the 16 year variation of the horizontal phase velocity spectrum and the propagation directionof gravity waves in the mesopause region obtained from the 3-D FFT analysis of 557.7 nm airglow images.
These 16 year variations are obtained by integrating the horizontal velocity spectra for all phase velocities
from 20 to 150 m/s. The spectra at 0–20 m/s were not used because of the white noise contamination of
airglow images. Figure 2a shows the variations for all seasons, whereas Figures 2b–2e are for spring, summer,
fall, and winter, respectively.
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Figure 1. Sixteen year averages of horizontal phase velocity spectra in the mesopause region for (a) spring, (b) summer,(c) fall, and (d) winter. The tops of the spectra are pointing north, and the right sides are pointing east. Color bar
indicates power spectral density (PSD) of the observed waves. The radius from the center indicates the phase velocity
of gravity waves from 0 to 150 m/s.

In Figure 2a, the seasonal variation from the northeast, northwest, and southwest, which is repeated every
year, is clearly visible. In Figures 2b–2e, more detailed variations in the propagation directions for the four
seasons are seen. There is a clear diﬀerence in propagation direction between summer and winter. In summer,
the propagation direction of gravity waves is continuously northeastward throughout the 16 years. However,in winter, the propagation direction of gravity waves is roughly southwestward but shows a much wider
spread than in summer. In spring and fall, the propagation direction of gravity waves is either northeastward(summer type) or southwestward (winter type), and the predominant direction depends on the year. This isprobably because the dynamics of the middle atmosphere are divided into summer and winter types, andspring and fall are transitional seasons.
In Figures 2b–2e, we also ﬁnd several distinctive long-term variations in spectral intensity. In particular, thewinter spectra shows clear variation of 5–7 years. Shorter-scale variations of 3–4 years can be also seenin summer.
4.2. Discussion of 557.7 nm Results
Next, we discuss the east-west anisotropy of the propagation direction during summer and winter, thenorth-south anisotropy of the propagation direction during summer and winter, and the characteristic yearlyvariations of spectral intensity in winter.
East-west anisotropy between summer and winter is probably caused by ﬁltering of gravity waves by themesospheric jet, which has a maximum wind velocity of about 70 m/s in the mesosphere [e.g., Taylor et al., 1993].
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Figure 2. Sixteen year variation of horizontal phase velocity spectrum amplitude and propagation direction of gravity
waves in the mesopause region obtained by 3-D FFT analysis of 557.7 nm airglow images. The horizontal axis shows
the year from 1999 to 2015, and the vertical axis shows the azimuth angles. (a) All seasons, (b) spring, (c) summer,(d) fall, (e) winter, and (f ) F10.7 solar ﬂux. Deep blue regions indicate years with no data.

The direction of the mesospheric jet is westward in summer and eastward in winter in both hemispheres. The
dispersion relation of gravity waves is given as follows [Hines, 1960].
m2 =

N2
1
− k2 −(c − u)24H2(4)Here m and k are vertical and horizontal wave numbers, respectively; c is horizontal phase velocity of gravity
waves; u is background wind velocity; N is the Brant-Väisälä frequency; and H is scale height. According toequation (4), the waves that have a horizontal phase velocity (c) equal to the background wind velocity (u)cannot propagate further, because the vertical wave number (m) becomes inﬁnity (critical level). Thus, if the
propagation direction of the gravity waves is the same as that of the mesospheric jet and the horizontal phasevelocity of the gravity wave is smaller than the mesospheric jet speed, the wave cannot propagate. This windﬁltering eﬀect causes the observed east-west anisotropy of the propagation direction in summer and winter[e.g., Nakamura et al., 1999; Ejiri et al., 2003].
This north-south anisotropy may be caused by the diﬀerence of the location of tropospheric gravity wave
sources relative to Shigaraki. In this study, we investigated this possibility by using the tropospheric verticalﬂow velocity given by the ERA-Interim, which is one of the most reliable global atmospheric reanalysis data
for the troposphere. We considered the source of the observed gravity waves as the convective activity inthe troposphere, because the horizontal wavelengths of the observed waves are rather small (10–100 km).
The gravity waves from the lower stratospheric jet stream would be in larger scale, considering the scale sizeof the jet stream. The orographic waves would be a stationary wave which does not have horizontal phasevelocity. For ERA-Interim, the time resolution is 6 h and both longitudinal and latitudinal spatial resolution are
0.75∘ . We used the pressure vertical velocities at 400 hPa on the pressure coordinates, which corresponds toan altitude of about 7 km in the middle latitudes. This height was chosen because it is the middle part of thetroposphere. Thus, the upward ﬂow in this height indicates existence of rather strong convection activity.
Figures 3a and 3b show the pressure vertical velocity (negative: upward) averaged over summer and winterat 400 hPa on the pressure coordinates (∼7 km in altitudes) from the European Centre for Medium-Range
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Figure 3. (a) Summer and (b) winter averages of the pressure vertical velocity (negative: upward) at 400 hPa at
the pressure coordinates calculated by ERA-Interim for the dates when the 557.7 nm airglow images are available.
(c) Summer and (d) winter correlation coeﬃcients between the yearly power spectrum amplitude of the horizontal
phase velocity and yearly vertical velocity in Figures 3a and 3b, respectively. Red regions (negative correlationcoeﬃcients) indicate a positive correlation between the upward ﬂow and high power spectral density because
negative vertical velocity indicates upward ﬂow. The small black circle indicates the Shigaraki airglow imager station.

Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) [Dee et al., 2011]. The small black circle indicatesthe airglow imager station at Shigaraki. We averaged the data for the dates when the 557.7 nm airglow images
are available. This data set corresponds to particular weather conditions when the night sky was clear over
Shigaraki. In summer, there is a region of strong upward velocities from the south to the east of Japan due
to the Baiu rain band on a line approximately from (20∘ N, 120∘ E) to (40∘ N, 170∘ E) and at latitudes from 0∘ N
to 10∘ N near the equator. If gravity waves were generated in these regions, the propagation direction at
Shigaraki would be northward and westward, while the westward propagating waves will be ﬁltered out by
the mesospheric jet in summer. In winter, there is a region of strong vertical velocities in the northeast of Japandue to wintry extratropical cyclones. If gravity waves were generated in this region, the propagation direction
would be southwestward. These results indicate that the observed north-south anisotropy of gravity waves
in the 557.7 nm airglow images could be caused by the diﬀerence in the location of gravity wave sources inthe troposphere.
The distance between these possible source regions of gravity waves and Shigaraki could be more than
1000 km. This distance may be too far for the horizontal propagation of small-scale gravity waves fromthe troposphere to the mesopause region [e.g., Alexander, 1996; Horinouchi et al., 2002]. According to
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Suzuki et al. [2013] and Parihar and Taori [2015], atmospheric gravity waves ducting in the mesopause region
propagate over long distances (1200–2000 km). Walterscheid et al. [2001] reported that ducting waves may
propagate over more than 2000 km distances. A mesospheric duct is necessary for propagation of small-scale
gravity waves over such long distances from possible source regions in the troposphere to the mesopouseregion over Shigaraki.
Next, we discuss the 5–7 year variation of the spectral intensity of the horizontal phase velocity in winter.
We investigated the correlation between the yearly variations of the spectral intensity and the following
four parameters: tropospheric vertical ﬂow velocity obtained by the ERA-Interim reanalysis data; the NINO3
index [Torrence and Compo, 1998], which is a proxy for the El Niño–Southern Oscillation and has a 2–7 year
periodicity; the AO index [Thompson and Wallace, 1998], which is a proxy for the Arctic Oscillation and has
a 6–15 year periodicity; and the F10.7 solar ﬂux [Tapping, 1987], which is a proxy for solar activity and has
an 11 year periodicity. El Niño–Southern Oscillation, Arctic Oscillation, and solar activity aﬀect climate at
middle latitudes through causing long-term variations of temperature, atmospheric pressure, and wind ﬁeld
[Sakai and Kawamura, 2009; Park et al., 2011]. These long-term variations may cause the variation of thespectral intensity of the horizontal phase velocity in winter.
Figures 3c and 3d show the correlation coeﬃcient between the yearly averages of the spectral intensities of
the horizontal phase velocity and the yearly averages of the tropospheric vertical ﬂow velocities. This analysis
is made to identify possible connection between the yearly variations of gravity wave power observed in the
airglow images and the yearly variations of the possible source convection activities in the troposphere. In
summer in Figure 3a, there are strong upward ﬂow regions in the Baiu front and at latitudes from 0∘ N to 10∘ N
near the equator, as noted before. In Figure 3c, several high correlation regions (red) are also seen in thesestrong upward ﬂow regions. In winter, the region of strong vertical velocities in the northeast of Japan also corresponds to the high correlation region in Figure 3d. These positive correlations between the spectral intensityof the gravity waves in the 557.7 nm airglow images and the tropospheric vertical ﬂow velocity suggestthat the yearly variation of gravity wave power in the airglow images can be caused by the variations of thetropospheric source activities, though there may be some other causes for the yearly variation of gravity wave
power in airglow images, such as yearly variations of mesospheric wind velocities.
On the other hand, the correlation coeﬃcients of the spectral intensity of gravity waves in winter with the
NINO3 index and with the AO index are 0.034 and −0.151, respectively, indicating no correlation with theseparameters. The correlation coeﬃcients with the F10.7 solar ﬂux is 0.032, indicating no correlation between
the solar activity and the gravity wave intensity in the mesopause region. We used airglow images for only
clear-sky nights, which may aﬀect these low correlation coeﬃcients.
4.3. Results From 630.0 nm Airglow Images
Figure 4 shows the horizontal phase velocity spectra obtained from 630.0 nm airglow images in four seasons
averaged over 16 years from 1999 to 2015 at Shigaraki, Japan. In these spectra, the major propagation direction of the waves is always southwestward, consistent with many previous studies by visual inspection, andthe waves probably correspond to nighttime MSTIDs [e.g., Saito et al., 1998; Garcia et al., 2000; Shiokawa et al.,2003b; Otsuka et al., 2004]. A minor northeastward propagating component is also seen in these four panels.
This northeastward propagating component is newly identiﬁed by the 3-D FFT method. In spring and fall,
the peak is southwestward with a power spectral intensity boundary between yellow and green at ∼110 m/s.
In summer, the boundary is at ∼180 m/s, and in winter, the boundary is at ∼140 m/s. In summer, the highest
power spectral density and horizontal phase speed of the wave are observed. The minor northeastward
propagating component is also the largest power spectral density during summer.
Figure 5 shows the 16 year variation of the horizontal phase velocity spectra and propagation direction ofwaves in the thermosphere obtained from the 3-D FFT analysis of 630.0 nm airglow images. These 16 year variations are obtained by integrating the horizontal velocity spectra for all phase velocities from 40 to 300 m/s.
The spectra at 0–40 m/s were not used because of the white noise contamination of airglow images. Figure 5ashows the variations for all seasons, whereas Figures 5b–5e show those for spring, summer, fall, and winter,
respectively.
In Figure 5a, the propagation direction of waves is always southwestward with a minor northeastward propagating component throughout the 16 years. The power spectral intensities show long-term variations,increasing between 2001 and 2007 and then decreasing after ∼2009. Figures 5b–5e show the variations
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Figure 4. Sixteen year averages of horizontal phase velocity spectra in the thermosphere obtained from the 630.0 nm
airglow images for (a) spring, (b) summer, (c) fall, and (d) winter. The tops of the spectra are pointing north, and theright sides are pointing east. The radius from the center indicates the phase velocity of MSTIDs from 0 to 300 m/s.

during the four seasons. All seasons have the major southwestward component and the minor northeastward
component. The variations in the power spectral intensities also show a similar trend, in which the intensity
increases between ∼2001 and ∼2007 and then decreases after ∼2009. Figure 5f shows the yearly average of
the F10.7 solar ﬂux. A clear negative correlation is seen between the variation in the power spectral intensities
and the F10.7 solar ﬂux, with correlation coeﬃcients of −0.894 and −0.663 for summer and winter, respectively.
4.4. Discussion of 630.0 nm Results
According to Hunsucker [1982], the wavelength of large-scale TIDs (LSTIDs) is over 1000 km, while the wavelength of MSTIDs is several hundred kilometers. The term small-scale TIDs has not been used after Hunsucker[1982]. The scale sizes of waves we studied using the 630.0 nm airglow images are 40–400 km. Thus, weconsider the waves we analyzed in this paper using the 630.0 nm airglow images as MSTIDs.
Figure 5 shows a rather clear negative correlation between the variation in the power spectral intensities andthe F10.7 solar ﬂux. Some papers reported the negative relationship in occurrence of nighttime MSTIDs withsolar activity [Duly et al., 2013; Narayanan et al., 2014]. To explain this negative correlation, we consider thegrowth rate of the Perkins instability, which plays an important role in generating the nighttime MSTIDs at
middle latitudes [Perkins, 1973; Yokoyama et al., 2009]. Equation (5) shows the linear growth rate of the Perkins
instability.
g sin2 I
𝜈in Hn(5)

VARIATION OF GRAVITY WAVES AND MSTIDS

8𝛾=

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Figure 5. Sixteen year variation of horizontal phase velocity spectra and propagation direction of waves in thethermosphere obtained from 3-D FFT analysis of 630.0 nm airglow images. The horizontal axis shows the year from1999 to 2015, and the vertical axis shows the azimuth angles. (a) All seasons, (b) spring, (c) summer, (d) fall, (e) winter,and (f ) F10.7 solar ﬂux. Deep blue areas indicate years with no data.

Here I is the magnetic inclination, g is the acceleration due to gravity, 𝜈in is the ion-neutral collision frequency,and Hn is the scale height. 𝜈in and Hn are aﬀected by solar activity. The typical values of 𝜈in and Hn in the lower
thermosphere are 6 s−1 and 45 km, respectively. When solar activity is small, 𝜈in and Hn are small. Then, thelinear growth rate of the Perkins instability, 𝛾 , will be large. This could cause the clear negative correlation
between the power spectral intensities and F10.7 solar ﬂux in Figure 5.
Otsuka et al. [2007] and Yokoyama et al. [2009] pointed out that the sporadic E layer instability plays a major rolein generating the nighttime MSTIDs, and the Perkins instability helps to amplify its perturbation. In this study,
the clear negative correlation implies that the Perkins instability plays also an important role in generating
the nighttime MSTIDs at middle latitudes, in addition to the E -F coupling process in the nighttime ionosphereat middle latitudes.
Gravity waves propagating from the lower atmosphere can initiate the ionospheric instabilities that may
result in the MSTIDs. Vadas et al. [2014] suggested that gravity wave penetration is less important andsecondary wave generation is more important during the solar quiet time. This is because during solar
minimum, the background density decreases more rapidly with increasing altitudes, causing the kinematic
viscosity to increase more rapidly with altitudes. Kinematic viscosity makes gravity waves fragile, allowing
secondary waves to be generated. The secondary waves propagate to the thermosphere and may trigger the
MSTIDs. This scenario may also explain the negative correlation between MSTIDs activity in the thermosphereand the solar activity, which is consistent with our long-term analysis.
Many previous studies of 630 nm airglow images by visual inspection reported that nighttime MSTIDs propagate mainly southwestward. Yokoyama et al. [2009] suggested that the southward neutral wind in the
sporadic E layer arising from atmospheric tides produces MSTIDs propagating southward. However, Figure 5
shows a minor northeastward component of MSTIDs, which is newly identiﬁed by this 3-D FFT method.
If the southward neutral wind in the sporadic E layer always makes MSTIDs propagate southward, we could
not explain the northeastward propagating component. Shiokawa et al. [2008] reported an event observed
at Paratunka, Kamchatka (north of Japan), during which the MSTIDs propagated ﬁrst southwestward, and
TAKEO ET AL.

VARIATION OF GRAVITY WAVES AND MSTIDS

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Journal of Geophysical Research: Space Physics

10.1002/2017JA023919

then northeastward. If the variation of the neutral wind in the sporadic E layer is caused by atmospheric tides,
it may exhibit latitudinal dependence on their characteristics. Thus, more studies of this sort of 3-D FFT analysis for stations at diﬀerent latitudes may contribute to understanding the mechanisms that determine thepropagation direction of MSTIDs.

5. Summary and Concluding Remarks
In this study, we used the 3-D FFT method developed by Matsuda et al. [2014] to analyze the horizontal phasevelocity spectra of gravity waves and MSTIDs observed in 557.7 nm (altitude of 90–100 km in the mesopause
region) and 630.0 nm (altitude of 200–300 km in the thermosphere and ionosphere) airglow images obtainedat Shigaraki MU Observatory (34.8∘ N, 136.1∘ E) over ∼16 years, from 16 March 1999 to 20 February 2015. We
found several interesting features of the long-term variation of these horizontal phase velocity spectra. Our
results can be summarized as follows.

Acknowledgments
We thank Y. Katoh, M. Satoh, T. Katoh,
Y. Hamaguchi, Y. Yamamoto, and T.
Adachi of the Solar-Terrestrial Environment Laboratory/Institute forSpace-Earth Environmental Research(ISEE), Nagoya University, for theirhelpful support with the airglow
imaging observations. The MU
radar at Shigaraki belongs to and isoperated by the Research Institutefor Sustainable Humanosphere,
Kyoto University. The all-sky airglow
images used in the present study
are available at ISEE, Nagoya
University. ERA-Interim data used in
Figure 3 are available at the European
Centre for Medium-Range Weather
Forecasts (ECMWF). The NINO3
index, AO index, and F10.7 index
used in this paper are available at
http://www.data.jma.go.jp/gmd/cpd/
db/elnino/index/soi.html, http://www.
cpc.ncep.noaa.gov/products/precip/
CWlink/daily_ao_index/ao.shtml,
and http://omniweb.gsfc.nasa.gov/
form/dx1.html, respectively.
This research is supported by
the JSPS Core-to-Core Program,
B. Asia-Africa Science Platforms, and
JSPS KAKENHI grants (JP 15H05815
and JP 16H06286).

TAKEO ET AL.

1. In the mesopause region in the 557.7 nm airglow images, there are clear seasonal variations in the propagation direction of gravity waves. In spring, the peak is northeastward and southwestward with a boundaryof the power spectral intensity between yellow and green at ∼70 m/s and ∼40 m/s respectively. In summer,
the signiﬁcant peak is north and northeastward with a boundary at ∼70 m/s. In fall, the peak is northwestward with a boundary at ∼60 m/s. In winter the peak is southwestward with a boundary at ∼50 m/s. Thisseasonal variation of the propagation direction is repeated every year.
2. The propagation directions between summer (northeastward) and winter (southwestward) are anisotropic
in the mesopause region. The east-west anisotropy is probably caused by the ﬁltering of gravity waves by
the mesospheric jet. The north-south anisotropy is possibly caused by the diﬀerence in the location of gravity wave sources. By comparing the tropospheric reanalysis data (vertical velocity at 400 hPa) of ERA-Interim,
we identiﬁed the gravity wave source in the troposphere as the Baiu front and the tropical convection
activities in the south of Japan in summer and the wintry low pressure in the northeast of Japan in winter.
3. There are several distinctive long-term variations in the spectral intensities of the gravity waves in themesopause region. In particular, the winter spectra shows clear variation of 5–7 years. There are positive
correlations between the yearly power spectral intensity of gravity waves and the yearly upward velocity at
400 hPa at several regions in the south of Japan in summer and in northeastern Japan in winter. However,there is no clear correlation between the spectral variation and the NINO3 and AO indices and F10.7 solar
ﬂux in winter.
4. In the thermosphere and ionosphere in 630.0 nm airglow images, the propagation direction of waves isalways southwestward throughout the 16 years. These waves probably correspond to the nighttime MSTIDs
reported in previous work. In spring, summer, fall, and winter, the peak velocities of the spectra are lessthan ∼110, ∼180, ∼110, and ∼140 m/s, respectively. In summer, the highest power spectral density andhorizontal phase velocity of the waves are observed. We also found a clear minor peak for the northeastward
propagating component of MSTIDs.
5. The power spectral intensities of the MSTIDs for four seasons show long-term variation, increasing between
2001 and 2007, and then decreasing until 2011. There is a clear negative correlation between these variations of the power spectral intensities and F10.7 solar ﬂux. This anticorrelation of MSTIDs with solar activity
is consistent with the linear growth rate of the Perkins instability and the secondary wave generation ofgravity waves in the thermosphere.

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