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Wave-4 structure of the neutral density in the thermosphere and its relation to atmospheric tides
Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 45–51
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Wave-4 structure of the neutral density in the thermosphere and its relation to atmospheric tides Yasunobu Miyoshi a,n, Hidekatsu Jin b, Hitoshi Fujiwara c, Hiroyuki Shinagawa b, Huixin Liu a a
Department of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan National Institute of Information and Communication Technology, Koganei, Japan c Faculty of Science and Technology, Seikei University, Musashino, Japan b
a r t i c l e i n f o
abstract
Article history: Received 1 August 2011 Received in revised form 25 November 2011 Accepted 1 December 2011 Available online 20 January 2012
The generation mechanism for the 4-peak longitudinal structure of the neutral density in the upper thermosphere is examined using an atmosphere–ionosphere coupled model. Our result indicates that the wave-4 structure of the neutral density in the upper thermosphere is caused by the upward propagation of the eastward diurnal tide with zonal wavenumber 3 (DE3) and the eastward semidiurnal tide with zonal wavenumber 2 (SE2) from the troposphere. The wave-4 structure of the neutral density in the equatorial region is mainly generated by the DE3, while the SE2 is important for the generation of the wave-4 structure in middle latitudes. Our simulation demonstrates that the wave4 structure is evident at the height range from 150 km to 500 km. Furthermore, we examine the day-today variation of the wave-4 amplitude of the neutral density at 400 km height and its relation with the SE2 and DE3 amplitudes at various heights. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Thermosphere Atmospheric tide Modeling
1. Introduction Behaviors of the equatorial thermosphere/ionosphere are unique and interesting. These structures in the ionosphere and thermosphere are known as the Equatorial Ionization Anomaly (EIA) and the Equatorial Mass density Anomaly (EMA), respectively. The EIA is explained by a fountain effect, which is driven by a large-scale eastward electric field during daytime. Many observational and theoretical studies have revealed various behaviors of the EIA (e.g., Rishbeth, 2000; Lin et al., 2007). As for the EMA, detailed behaviors are revealed by satellite observations (Hedin and Mayr, 1973; Liu et al., 2007). Using an atmosphere– ionosphere coupled model, Miyoshi et al. (2011) studied the generation mechanism of the EMA. They showed that the density trough along the dip equator is basically generated by the ion drag force, which is prominent at higher solar activity levels. The 4-peak (wave-4) longitudinal structure of the ionosphere/ thermosphere has recently been revealed by quasi-Sun-synchronous satellites. The wave-4 structure of the electron density near the EIA is a well-known phenomenon (e.g., Immel et al., 2006; Lin et al., 2007). This wave-4 in the ionosphere is explained by modulation of the day-time E-region electric field generated by the eastward diurnal tide with zonal wavenumber 3 (DE3; Immel n
Corresponding author. Tel.: þ81 92 642 2683; fax: þ 81 92 642 2684. E-mail addresses: [email protected] (Y. Miyoshi), [email protected] (H. Jin), [email protected] (H. Fujiwara), [email protected] (H. Shinagawa), [email protected] (H. Liu). 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.12.002
et al., 2006; Hagan et al., 2007). The wave-4 structure is also found in neutral parameters in the thermosphere, such as density ¨ (Liu et al., 2009; Lei et al., 2010), zonal wind (Luhr et al., 2007) and temperature (Forbes et al., 2009). Liu et al. (2009) investigated the generation mechanism of the wave-4 structure of the EMA. They suggested that the upward propagation of the DE3 to the upper thermosphere is one of plausible candidates for the generation of the wave-4 structure of neutral density. However, the wave-4 structure of the EMA has an antisymmetric component about the equator (e.g., Figure 1 of Liu et al., 2009). This antisymmetric component cannot be explained by the DE3, because the DE3 in the upper thermosphere is almost the symmetric structure about the equator. Bruinsma and Forbes (2010), England et al. (2010) and Oberheide et al. (2011) demonstrated the importance of the eastward semidiurnal tide with zonal wavenumber 2 (SE2) for generation of the wave-4 structure. They predicted the vertical and latitudinal structures of the SE2 using the Hough function extension method. Using the CHAMP and GRACE satellites, Forbes et al. (2009) showed the wave-4 structure of the temperature component due to the semidiurnal tide in the upper thermosphere. However, effects of the SE2 on the wave-4 structure of neutral density in the upper thermosphere are not well known. Thus the question of the generation mechanism for the wave-4 structure of neutral density is still open. The purpose of this study is to investigate the generation mechanism of the wave-4 structure of neutral density using an atmosphere–ionosphere coupled model. Jin et al. (2011) have recently developed a new whole
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atmosphere–ionosphere coupled model, which is called Groundto-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). This model solves the ionosphere–thermosphere interaction self-consistently, including the electrodynamics. Using this model, we study the mechanism for the wave-4 longitudinal modulation of neutral density. The descriptions of the GAIA model used in this study and numerical simulation are presented in Section 2. Results and discussion are presented in Section 3. Concluding remark follows in Section 4.
2. Descriptions of the GCM and numerical simulation A new whole atmosphere–ionosphere coupled model (GAIA) has been recently developed by coupling three models: a whole atmosphere general circulation model (GCM; Miyoshi and Fujiwara, 2003), an ionosphere model (Shinagawa, 2009) and an electrodynamics model (Jin et al., 2008). This model solves the ionosphere–thermosphere interaction self-consistently, including the electrodynamics. The electrodynamics model treats the closure of global ionospheric currents induced both by the neutral wind and by the setup of a polarization electric field under the assumption of equipotential geomagnetic field lines. The present model assumes a tilted dipole for the geomagnetic field configuration. A detailed description of the GAIA model is found in Jin et al. (2011). For neutral atmospheric part the GAIA model covers the whole range from the ground surface to the exobase and contains a full set of physical processes appropriate for investigating vertical coupling processes in the troposphere, stratosphere, mesosphere and thermosphere as well as the interaction with the ionosphere (Miyoshi and Fujiwara, 2003, 2006, 2008; Fujiwara and Miyoshi, 2006, 2010). Effects of the surface topography and land–sea contrast are also taken into account. A more detailed description of these physical processes is given by Miyoshi and Fujiwara (2003) and Miyoshi (1999). Using this model, we can simulate the excitation of atmospheric waves (such as tides, gravity waves and planetary waves) in the whole atmosphere, including their upward propagation into the upper atmosphere self-consistently without posing any boundaries between the lower and upper regime. The wave-4 structure and the amplitude of the DE3 peak during the period from August to October at low solar activity (e.g., Forbes et al., 2008). Therefore, in this study, the solar F10.7 cm flux is fixed at 70 10 22 W/m2/Hz, and geomagnetically quiet condition is assumed. The data are sampled at 1-h intervals from 1 September to 31 October.
3. Result and discussion 3.1. Wave-4 structure of the neutral density The longitude–geographic latitude distribution of neutral density at 400 km height in a fixed local time frame is shown in Fig. 1a. This is averaged over September and October at 12–13 LT, when the EMA is most evident. The density trough is located near the dip equator, while the density maximum appears around 25–351 geomagnetic latitudes. However, the equatorial trough of the neutral density along the dip equator obtained in this study is weaker than the CHAMP observation (Liu et al., 2009). The solar flux level during the CHAMP observation is high, while that in this study is low. Thus, the difference of the EMA structure is due to the difference of the solar activity level. The wave-4 longitudinal structure of neutral density is prominent in the northern hemisphere, but less obvious in the southern hemisphere. The density maximum in the southern hemisphere is found around 110–1701E, and other peaks appear at 201E and 2701E. In order to compare the longitudinal variation of neutral density with that of electron density, the global distribution of electron density at 12–13 LT is presented in Fig. 1b. The wellknown EIA structure and the density trough aligned with the dip equator are visible. Fig. 1b also shows a wave-4 longitudinal variation of electron density at the south crest of EIA peaking at 201E, 1101E, 1901E and 2601E. On the other hand, the 4-peak structure at the north crest of EIA is less obvious. However, the distinct peaks are located at 2001E and 2701E, and other weak peaks are found around 01E and 1101E. It is noteworthy that the location of the four peaks of EMA is somewhat displaced from that of EIA. Furthermore, the wave-4 structure of the electron density disappears at 25–401 geomagnetic latitudes, where the prominent wave-4 structure of neutral density has its maximum. These features of the wave-4 structures of EIA and EMA are in good agreement with those obtained by the CHAMP satellite (Liu et al., 2009; Huang et al., 2011). The amplitude of the wave-4 component of neutral density along 301 geomagnetic latitude is 3.80 10 14 kg m 3, while that along 301 geographic latitude is 3.78 10 14 kg m 3. The difference of the amplitude between the geomagnetic and geographic coordinates is small. The horizontal structures of the tides are generally described in geographic coordinate. Therefore, geographical latitudes are used to extract the wave-4 feature in neutral density. In order to examine the wave-4 structure of neutral density, the zonal wavenumber 4 component of neutral density is extracted. Fig. 2a shows the global distribution of the zonal wavenumber 4 component of neutral density at 12–13 LT. The amplitude of the wave-4 component at 12–13 LT is larger in the northern hemisphere than in the southern hemisphere. The phase difference between the
Fig. 1. (a) Longitude–geographic latitude distribution of the neutral mass density during 12–13 LT at a height of 400 km averaged over September and October. Units are in 10 12 kg m 3. (b) Same as in (a) except for electron density. Units are in 1012 m 3.
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Fig. 2. (a) Longitude–latitude distribution of the zonal wavenumber 4 component of neutral density at 12–13 LT. Units are in 10 14 kg m 3. (b) Longitude–latitude distribution of the zonal wavenumber 4 component due to the diurnal and semidiurnal variations. (c) The distribution of the zonal wavenumber 4 component due to the diurnal variation. (d) The distribution of the zonal wavenumber 4 component due to the semidiurnal variation.
northern and southern hemispheres increases with increasing latitude and the phase at 401N is almost out of phase with that at 401S. However, the phase difference in this study is larger than that obtained by the CHAMP observations (e.g., Figure 1 of Liu et al., 2009). The longitudinal distribution of neutral density in Liu et al. (2009) was average during the period from 14 LT to 18 LT (4 h average), while that in this study was average during the period from 12 LT to 13 LT (1 h average). The 4 h average is not appropriate, because the semidiurnal tide plays an important role on the longitudinal structure of the neutral density.
3.2. Local time variation of the wave-4 structure The local time variation of the wave-4 structure of the neutral density at 400 km height is examined here. The upper row of Fig. 3 shows the longitude–local time distributions of the wave-4 components at the equator, 301N and 301S. The wave-4 component at the equator moves eastward with a period of 24 h, indicating the importance of the DE3. The wave-4 components at 7301 latitudes move eastward with time and have semidiurnal variations. The middle and lower rows of Fig. 3 display the distributions of the wave-4 components due to the diurnal and semidiurnal tides, respectively. The wave-4 structure at the equator is determined by the diurnal tide (Fig. 3b), and the wave-4 component due to the semidiurnal tide at the equator (Fig. 3c) is negligibly small. The diurnal variation of the wave-4 component at 301N (Fig. 3e) is nearly in phase with that at 301S (Fig 3h), while the semidiurnal variation of the wave-4 component at 301N (Fig. 3f) is out of phase with that at 301S (Fig. 3i). At 7301 latitudes, the amplitude of the wave-4 component due to the semidiurnal tides is 3 10 14 kg/m3, which is larger than that due to the diurnal tides by a factor of 2. However, the diurnal tides modulate the local time variation of the wave-4 structure. For example, the amplitude of the wave-4 component at 301N (301S) is relatively small during the period from 23 LT to 03 LT (from 12 LT to 16 LT).
The global distribution of the wave-4 component due to the diurnal tide (the semidiurnal tide) at 12–13 LT is shown in Fig. 2c (Fig. 2d). The wave-4 component due to the diurnal tide is almost symmetric about the equator and has a maximum at the equator. The wave-4 component due to the semidiurnal tide is antisymmetric about the equator and has maxima at 730–351 latitudes. These features are characteristics of the SE2 (Oberheide et al., 2011). At 710–151 latitudes, the amplitude of the wave-4 component due to the diurnal tide is comparable to that due to the semidiurnal tide. Fig. 2b shows the distribution of the wave-4 component due to superposition of the diurnal and semidiurnal tides. In the northern hemisphere, the longitudinal variation of the wave-4 component due to the semidiurnal tide at 12–13 LT is nearly in phase with that due to the diurnal tide, so that the longitudinal variation due to the diurnal plus semidiurnal tides is enhanced. On the other hand, at 12–13 LT in the southern hemisphere, the longitudinal variation of the wave-4 component due to the semidiurnal tide is almost out of phase with that due to the diurnal tide. This is the reason for the larger amplitude of the wave-4 component in the northern hemisphere. The wave-4 structure due to superposition of the diurnal and semidiurnal tides (Fig. 2b) is almost the same as the wave-4 structure due to all the tidal components (Fig. 2a) except for the poleward of 601 latitude. This result indicates that the wave-4 structure of the neutral density is generated by both the diurnal and semidiurnal tides. 3.3. Vertical structures of DE3 and SE2 We showed that the SE2 and DE3 play an important role on the generation of the wave-4 structure at 400 km. In this subsection, we will show the vertical structures of SE2 and DE3 and their influence on the wave-4 structure of neutral density at various heights in the thermosphere. In order to compare the present result with the results by previous studies, Fig. 4 shows the vertical structures of amplitude and phase of the temperature component due to DE3. In the thermosphere, the amplitude has a peak at the equator, and the phase structure is symmetric about the equator. This means
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Fig. 3. (a) Longitude–local time distribution of the wave-4 component of neutral density at the equator at 400 km height. Units are in 10 14 kg m 3. (b) Same as in (a) except for the component due to the diurnal variation. (c) Same as in (a) except for the component due to the semidiurnal variation. (d) Longitude–local time distribution of the wave-4 component of neutral density at 301N. (e) Same as in (d) except for the component due to the diurnal variation. (f) Same as in (d) except for the component due to the semidiurnal variation. (g) Longitude–local time distribution of the wave-4 component of neutral density at 301S. (h) Same as in (g) except for the component due to the diurnal variation. (i) Same as in (g) except for the component due to the semidiurnal variation.
Fig. 4. (a) Latitude–height distribution of the DE3 amplitude in the temperature component. Units are in K. (b) Same as in (a) except for the phase distribution. Units are in h. Phase is defined as the universal time of maximum at 01E.
that the symmetric mode is dominant above 80 km height. The vertical structure of DE3 in the thermosphere indicates the upward propagation of the symmetric mode of DE3 from the troposphere to the thermosphere. The amplitude in the mesosphere has a peak at 151N, and the phase below 80 km height is not symmetric about the equator. In the mesosphere and stratosphere, both the symmetric and antisymmetric modes exist, and superposition of the symmetric and antisymmetric modes makes the latitudinal phase structure complicated (Forbes et al., 2008). Fig. 5 shows the distributions of amplitude and phase of temperature due to SE2. The amplitude in the thermosphere maximizes around 725–351 latitudes, with the largest amplitude of 7 K. The equatorial minimum of the amplitude and the
latitudinal phase structure indicate the dominance of the antisymmetric mode in the thermosphere. Obviously, the vertical structure of SE2 in the thermosphere shows the upward propagation of the antisymmetric mode of SE2 from the troposphere to the thermosphere. The latitudinal phase structure below 80 km height is very complicated, and it is due to superposition of the symmetric and antisymmetric modes. The discussion concerning the behaviors of the symmetric and antisymmetric modes in the mesosphere and thermosphere was found in Oberheide et al. (2011). In the lower thermosphere, second symmetric (HME3) and antisymmetric (HME4) modes, which cannot propagate into the upper thermosphere, have considerable amplitudes. Furthermore, the first symmetric (HME1) mode is not negligibly small below the lower
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Fig. 5. (a) Same as in Fig. 4a except for SE2. (b) Same as in Fig. 4b except for SE2.
Fig. 6. (a) Longitude–geographic latitude distribution of the neutral mass density during 12–13 LT at a height of 200 km. Units are in 10 10 kg m 3. (b) As in (a) except for 150 km height. Units are 10 9 kg m 3.
thermosphere. The features of DE3 and SE2 obtained in this study are quite similar to those using the Hough mode extensions of the TIMED observation (Oberheide et al., 2011). The propagation of DE3 and SE2 from the lower atmosphere to the thermosphere implies that the longitudinal wave-4 structure in a fixed local time frame is considered to be evident not only in the upper thermosphere but also in the lower thermosphere. The longitudinal variation of neutral density at various heights is examined here. Fig. 6 shows the global distributions of neutral density at 12–13 LT at altitudes of 150 and 200 km. A prominent 4-peak structure is evident not only at low latitudes but also at middle latitudes of the northern hemisphere. The weak wave-3 and wave-4 longitudinal variations are visible in the southern hemisphere. Thus, the longitudinal wave-4 structure of neutral density is prominent above 150 km height although the wave-4 longitudinal structure of the electron density is weak in the 150–200 km height region (Lin et al., 2007). Using the WINDII data, Shepherd (2011) showed the wave-4 structure of neutral density at an altitude of 250 km, which is consistent with the present result. 3.4. Day-to-day variation of the wave-4 amplitude Forbes et al. (1997) indicated that the seasonal variation of the convective activity in the tropics affects seasonal variation of the tidal amplitude in the mesosphere and lower thermosphere using a Global Scale Wave Model (GSWM). Using a whole atmosphere GCM, Miyoshi and Fujiwara (2003) showed that day-to-day variations in tropical convective activity produced day-to-day variations in the intensity of diurnal tides that are propagating into the upper atmosphere. The DE3 and SE2 in the upper thermosphere, which produce the wave-4 structure of neutral
density, are considered to be excited by latent heating in the troposphere. Therefore in this subsection, we examine day-to-day variation of the wave-4 amplitude of neutral density at 400 km height and its relation to the day-to-day variations of DE3 and SE2 amplitudes at various heights. The day-to-day variation of amplitude of the zonal wavenumber 4 component of neutral density at 12–13 LT in September and October is estimated. The upper row of Fig. 7 shows the time variations of wave-4 amplitudes at 301N and the equator. Fluctuations of the wave-4 amplitude with periods of 10–20 days are evident both at 301N and at the equator. However, the variations at 301N and at the equator do not correlate with each other. Namely, the wave-4 amplitude at 301N has peaks on 10–15 September, 4–5 October and 18–19 October, while the wave-4 amplitude at the equator attenuates on 11–12 September, 21–23 September, 3–4 October and 21–22 October. The lower row of Fig. 7 shows the time variations of amplitudes of SE2 and DE3 temperature components. Significant fluctuations of the SE2 and DE3 amplitudes with periods of 10–20 days are found. It is clearly seen that the day-to-day variation of wave-4 amplitude of neutral density at 301N (equator) is correlated with the variation of SE2 (DE3) amplitude. Thus, the temporal variation of SE2 amplitude produces the day-to-day variation of wave-4 amplitude of neutral density at 301N, while the day-to-day variation of wave-4 amplitude at the equator is determined by the temporal variation of DE3 amplitude. Next, we examine the day-to-day variations of SE2 and DE3 amplitudes at various heights. Fig. 8a and b shows the variations of DE3 amplitudes at 120 and 400 km heights, respectively. There is a good correlation between the amplitude at 400 km height and the amplitude at 120 km height. The day-to-day variations of the
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Fig. 7. (a) Temporal variation of the amplitude of wave-4 component of neutral density at 301N during 12–13 LT. 5-day running mean is performed. Units are in 10 14 kg m 3. (b) As in (a) except for the variation at the equator. (c) The temporal variation of amplitude of temperature due to SE2 at 301N. Units are in K. (d) As in (c) except for the DE3 at the equator.
shows the variations of the symmetric component due to the DE3 at 40 and 70 km heights, respectively. The variations of DE3 amplitudes at 40 and 70 km heights are similar to those at 120 and 400 km heights. Namely, the amplitude minima around 10–12 September, 20–23 September, 3–4 October and 20–22 October are evident at all heights. This result indicates that the day-to-day variation of DE3 amplitude at 400 km height is caused by the DE3 variation of the lower atmospheric origin. The day-to-day variations of SE2 amplitudes at 120 and 400 km heights are shown in Fig. 9a and b, respectively. The temporal variation of SE2 amplitude at 120 km height is different from that at 400 km height. The variation of the amplitude due to the antisymmetric mode at 120 km height is examined (Fig. 9c). The antisymmetric component has minima on 25 September and on 12 October, and these minima are also evident at 400 km height. The variation of the antisymmetric component at 120 km height is similar to the variation of the SE2 amplitude at 400 km height. Thus, the difference of the variations at 400 km and at 120 km is explained by the fact that the symmetric modes have considerable amplitudes in the lower thermosphere. Fig. 9d and e shows the day-to-day variations of amplitudes of the antisymmetric mode at 40 and 70 km heights, respectively. Again, the amplitude minima at 40 and 70 km heights are evident around 25 September and 12 October. Thus, the temporal variations of antisymmetric amplitude of SE2 at 40 and
Fig. 8. (a) Temporal variations of the DE3 amplitude in the temperature at the equator at a height of 400 km. Five-day running mean is performed. Units are in K. (b) As in (a) except for 120 km height. (c) Temporal variations of the DE3 amplitude due to the symmetric component at the equator at 70 km height. (d) As in (c) except for 40 km height.
symmetric component due to DE3 in the stratosphere and mesosphere are examined, because the antisymmetric modes have considerable amplitudes below the mesopause region. Fig. 8c and d
Fig. 9. (a) Temporal variations of the SE2 amplitude in the temperature at 351N at an altitude of 400 km. Units are in K. (b) Same as in (a) except for 120 km height. (c) Temporal variations of the SE2 amplitude due to the antisymmetric component at 351N at 120 km height. (d) As in (c) except for 70 km height. (e) As in (d) except for 40 km height.
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70 km heights are somewhat similar to those at 120 and 400 km heights. This result shows the day-to-day variation of SE2 amplitude in the upper thermosphere is closely related to the variations of antisymmetric amplitude of SE2 in the lower atmosphere. The previous studies (e.g., Forbes et al., 1997) showed that latent heat release in the troposphere is the main source of the SE2 and DE3 in the thermosphere. In the next step, the day-to-day variation of convective activity in the tropics and its impact on the day-to-day variations of DE3 and SE2 amplitudes should be evaluated. This will be the subject of a future study.
4. Concluding remarks Using the GAIA model, the generation mechanism for the wave-4 structure of neutral density and its day-to-day variation are studied. Liu et al. (2009) discussed the generation mechanism of the wave-4 structure of neutral density at 400 km height. They concluded that the ion drag force associated with the EIA’s wave4 structure is not a major cause for the wave-4 structure of neutral density. They suggested that the upward propagation of DE3 to the upper thermosphere is one of the plausible candidates for the generation of the wave-4 formation of neutral density, because the phase relation between the neutral density and zonal ¨ ¨ wind at 400 km height (Hausler and Luhr, 2009) is in good agreement with the theoretical predictions by Oberheide et al. (2009). Our result confirms that the wave-4 structure of neutral density at 400 km height is caused by the upward propagation of DE3 and SE2. In particular, the upward propagating SE2 is important for the generation of the wave-4 structure of neutral density at middle latitudes. Furthermore, our simulation predicts that the 4-peak structure in the longitudinal direction is evident at the height range from 150 km to 500 km. We examined the day-to-day variation of wave-4 amplitude of neutral density at 400 km height and its relation with SE2 and DE3 amplitude at various heights. The day-to-day variation of the wave-4 amplitude of neutral density at low latitudes (middle latitudes) in the upper thermosphere is correlated with the temporal variations of DE3 (SE2) amplitudes in the lower atmosphere. This result indicates a clear relationship between the variation of the wave-4 structure of thermospheric density and the tropospheric and stratospheric variabilities.
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Geophysical Research Letters 30, 1789. doi:10.1029/2003GL017695. Miyoshi, Y., Fujiwara, H., 2006. Excitation mechanism of intraseasonal oscillation in the equatorial mesosphere and lower thermosphere. Journal of Geophysical Research 111, D14108. doi:10.1029/2005JD006993. Miyoshi, Y., Fujiwara, H., 2008. Gravity waves in the thermosphere simulated by a general circulation model. Journal of Geophysical Research 113, D01101. doi:10.1029/2007JD008874. Miyoshi, Y., Fujiwara, H., Jin, H., Shinagawa, H., Liu, H., Terada, K., 2011. Model study on the formation of the equatorial mass density anomaly in the thermosphere. Journal of Geophysical Research 116, A05322. doi:10.1029/ 2010JA016315. Oberheide, J., Forbes, J.M., H¨ausler, K., Wu, Q., Bruinsma, S.L., 2009. Tropospheric tides from 80 to 400 km: propagation, interannual variability, and solar cycle effects. Journal of Geophysical Research 114, D00I05. doi:10.1029/2010JD012388. Oberheide, J., Forbes, J.M., Zhang, X., Bruinsma, S.L., 2011. Wave driven variability in the ionosphere–thermosphere–mesosphere system from TIMED observations: what contributes to the ‘‘wave 4’’? Journal of Geophysical Research 116, A01306. doi:10.1029/2010JA015911. Rishbeth, H., 2000. The equatorial F-layer: problems and puzzles. Annales Geophysicae 18, 730–739. Shinagawa, H., 2009. Ionospheric simulation. Journal of National Institute of Information and Communication Technology 56, 199–207. Shepherd, G.G., 2011. Thermospheric observations of equatorial wavenumber 4 density perturbations from WINDII data. Geophysical Research Letters 38, L08801. doi:10.1029/2011GL046986.
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Wave-4 structure of the neutral density in the thermosphere and its relation to atmospheric tides Yasunobu Miyoshi a,n, Hidekatsu Jin b, Hitoshi Fujiwara c, Hiroyuki Shinagawa b, Huixin Liu a a
Department of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan National Institute of Information and Communication Technology, Koganei, Japan c Faculty of Science and Technology, Seikei University, Musashino, Japan b
a r t i c l e i n f o
abstract
Article history: Received 1 August 2011 Received in revised form 25 November 2011 Accepted 1 December 2011 Available online 20 January 2012
The generation mechanism for the 4-peak longitudinal structure of the neutral density in the upper thermosphere is examined using an atmosphere–ionosphere coupled model. Our result indicates that the wave-4 structure of the neutral density in the upper thermosphere is caused by the upward propagation of the eastward diurnal tide with zonal wavenumber 3 (DE3) and the eastward semidiurnal tide with zonal wavenumber 2 (SE2) from the troposphere. The wave-4 structure of the neutral density in the equatorial region is mainly generated by the DE3, while the SE2 is important for the generation of the wave-4 structure in middle latitudes. Our simulation demonstrates that the wave4 structure is evident at the height range from 150 km to 500 km. Furthermore, we examine the day-today variation of the wave-4 amplitude of the neutral density at 400 km height and its relation with the SE2 and DE3 amplitudes at various heights. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Thermosphere Atmospheric tide Modeling
1. Introduction Behaviors of the equatorial thermosphere/ionosphere are unique and interesting. These structures in the ionosphere and thermosphere are known as the Equatorial Ionization Anomaly (EIA) and the Equatorial Mass density Anomaly (EMA), respectively. The EIA is explained by a fountain effect, which is driven by a large-scale eastward electric field during daytime. Many observational and theoretical studies have revealed various behaviors of the EIA (e.g., Rishbeth, 2000; Lin et al., 2007). As for the EMA, detailed behaviors are revealed by satellite observations (Hedin and Mayr, 1973; Liu et al., 2007). Using an atmosphere– ionosphere coupled model, Miyoshi et al. (2011) studied the generation mechanism of the EMA. They showed that the density trough along the dip equator is basically generated by the ion drag force, which is prominent at higher solar activity levels. The 4-peak (wave-4) longitudinal structure of the ionosphere/ thermosphere has recently been revealed by quasi-Sun-synchronous satellites. The wave-4 structure of the electron density near the EIA is a well-known phenomenon (e.g., Immel et al., 2006; Lin et al., 2007). This wave-4 in the ionosphere is explained by modulation of the day-time E-region electric field generated by the eastward diurnal tide with zonal wavenumber 3 (DE3; Immel n
Corresponding author. Tel.: þ81 92 642 2683; fax: þ 81 92 642 2684. E-mail addresses: [email protected] (Y. Miyoshi), [email protected] (H. Jin), [email protected] (H. Fujiwara), [email protected] (H. Shinagawa), [email protected] (H. Liu). 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.12.002
et al., 2006; Hagan et al., 2007). The wave-4 structure is also found in neutral parameters in the thermosphere, such as density ¨ (Liu et al., 2009; Lei et al., 2010), zonal wind (Luhr et al., 2007) and temperature (Forbes et al., 2009). Liu et al. (2009) investigated the generation mechanism of the wave-4 structure of the EMA. They suggested that the upward propagation of the DE3 to the upper thermosphere is one of plausible candidates for the generation of the wave-4 structure of neutral density. However, the wave-4 structure of the EMA has an antisymmetric component about the equator (e.g., Figure 1 of Liu et al., 2009). This antisymmetric component cannot be explained by the DE3, because the DE3 in the upper thermosphere is almost the symmetric structure about the equator. Bruinsma and Forbes (2010), England et al. (2010) and Oberheide et al. (2011) demonstrated the importance of the eastward semidiurnal tide with zonal wavenumber 2 (SE2) for generation of the wave-4 structure. They predicted the vertical and latitudinal structures of the SE2 using the Hough function extension method. Using the CHAMP and GRACE satellites, Forbes et al. (2009) showed the wave-4 structure of the temperature component due to the semidiurnal tide in the upper thermosphere. However, effects of the SE2 on the wave-4 structure of neutral density in the upper thermosphere are not well known. Thus the question of the generation mechanism for the wave-4 structure of neutral density is still open. The purpose of this study is to investigate the generation mechanism of the wave-4 structure of neutral density using an atmosphere–ionosphere coupled model. Jin et al. (2011) have recently developed a new whole
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atmosphere–ionosphere coupled model, which is called Groundto-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). This model solves the ionosphere–thermosphere interaction self-consistently, including the electrodynamics. Using this model, we study the mechanism for the wave-4 longitudinal modulation of neutral density. The descriptions of the GAIA model used in this study and numerical simulation are presented in Section 2. Results and discussion are presented in Section 3. Concluding remark follows in Section 4.
2. Descriptions of the GCM and numerical simulation A new whole atmosphere–ionosphere coupled model (GAIA) has been recently developed by coupling three models: a whole atmosphere general circulation model (GCM; Miyoshi and Fujiwara, 2003), an ionosphere model (Shinagawa, 2009) and an electrodynamics model (Jin et al., 2008). This model solves the ionosphere–thermosphere interaction self-consistently, including the electrodynamics. The electrodynamics model treats the closure of global ionospheric currents induced both by the neutral wind and by the setup of a polarization electric field under the assumption of equipotential geomagnetic field lines. The present model assumes a tilted dipole for the geomagnetic field configuration. A detailed description of the GAIA model is found in Jin et al. (2011). For neutral atmospheric part the GAIA model covers the whole range from the ground surface to the exobase and contains a full set of physical processes appropriate for investigating vertical coupling processes in the troposphere, stratosphere, mesosphere and thermosphere as well as the interaction with the ionosphere (Miyoshi and Fujiwara, 2003, 2006, 2008; Fujiwara and Miyoshi, 2006, 2010). Effects of the surface topography and land–sea contrast are also taken into account. A more detailed description of these physical processes is given by Miyoshi and Fujiwara (2003) and Miyoshi (1999). Using this model, we can simulate the excitation of atmospheric waves (such as tides, gravity waves and planetary waves) in the whole atmosphere, including their upward propagation into the upper atmosphere self-consistently without posing any boundaries between the lower and upper regime. The wave-4 structure and the amplitude of the DE3 peak during the period from August to October at low solar activity (e.g., Forbes et al., 2008). Therefore, in this study, the solar F10.7 cm flux is fixed at 70 10 22 W/m2/Hz, and geomagnetically quiet condition is assumed. The data are sampled at 1-h intervals from 1 September to 31 October.
3. Result and discussion 3.1. Wave-4 structure of the neutral density The longitude–geographic latitude distribution of neutral density at 400 km height in a fixed local time frame is shown in Fig. 1a. This is averaged over September and October at 12–13 LT, when the EMA is most evident. The density trough is located near the dip equator, while the density maximum appears around 25–351 geomagnetic latitudes. However, the equatorial trough of the neutral density along the dip equator obtained in this study is weaker than the CHAMP observation (Liu et al., 2009). The solar flux level during the CHAMP observation is high, while that in this study is low. Thus, the difference of the EMA structure is due to the difference of the solar activity level. The wave-4 longitudinal structure of neutral density is prominent in the northern hemisphere, but less obvious in the southern hemisphere. The density maximum in the southern hemisphere is found around 110–1701E, and other peaks appear at 201E and 2701E. In order to compare the longitudinal variation of neutral density with that of electron density, the global distribution of electron density at 12–13 LT is presented in Fig. 1b. The wellknown EIA structure and the density trough aligned with the dip equator are visible. Fig. 1b also shows a wave-4 longitudinal variation of electron density at the south crest of EIA peaking at 201E, 1101E, 1901E and 2601E. On the other hand, the 4-peak structure at the north crest of EIA is less obvious. However, the distinct peaks are located at 2001E and 2701E, and other weak peaks are found around 01E and 1101E. It is noteworthy that the location of the four peaks of EMA is somewhat displaced from that of EIA. Furthermore, the wave-4 structure of the electron density disappears at 25–401 geomagnetic latitudes, where the prominent wave-4 structure of neutral density has its maximum. These features of the wave-4 structures of EIA and EMA are in good agreement with those obtained by the CHAMP satellite (Liu et al., 2009; Huang et al., 2011). The amplitude of the wave-4 component of neutral density along 301 geomagnetic latitude is 3.80 10 14 kg m 3, while that along 301 geographic latitude is 3.78 10 14 kg m 3. The difference of the amplitude between the geomagnetic and geographic coordinates is small. The horizontal structures of the tides are generally described in geographic coordinate. Therefore, geographical latitudes are used to extract the wave-4 feature in neutral density. In order to examine the wave-4 structure of neutral density, the zonal wavenumber 4 component of neutral density is extracted. Fig. 2a shows the global distribution of the zonal wavenumber 4 component of neutral density at 12–13 LT. The amplitude of the wave-4 component at 12–13 LT is larger in the northern hemisphere than in the southern hemisphere. The phase difference between the
Fig. 1. (a) Longitude–geographic latitude distribution of the neutral mass density during 12–13 LT at a height of 400 km averaged over September and October. Units are in 10 12 kg m 3. (b) Same as in (a) except for electron density. Units are in 1012 m 3.
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Fig. 2. (a) Longitude–latitude distribution of the zonal wavenumber 4 component of neutral density at 12–13 LT. Units are in 10 14 kg m 3. (b) Longitude–latitude distribution of the zonal wavenumber 4 component due to the diurnal and semidiurnal variations. (c) The distribution of the zonal wavenumber 4 component due to the diurnal variation. (d) The distribution of the zonal wavenumber 4 component due to the semidiurnal variation.
northern and southern hemispheres increases with increasing latitude and the phase at 401N is almost out of phase with that at 401S. However, the phase difference in this study is larger than that obtained by the CHAMP observations (e.g., Figure 1 of Liu et al., 2009). The longitudinal distribution of neutral density in Liu et al. (2009) was average during the period from 14 LT to 18 LT (4 h average), while that in this study was average during the period from 12 LT to 13 LT (1 h average). The 4 h average is not appropriate, because the semidiurnal tide plays an important role on the longitudinal structure of the neutral density.
3.2. Local time variation of the wave-4 structure The local time variation of the wave-4 structure of the neutral density at 400 km height is examined here. The upper row of Fig. 3 shows the longitude–local time distributions of the wave-4 components at the equator, 301N and 301S. The wave-4 component at the equator moves eastward with a period of 24 h, indicating the importance of the DE3. The wave-4 components at 7301 latitudes move eastward with time and have semidiurnal variations. The middle and lower rows of Fig. 3 display the distributions of the wave-4 components due to the diurnal and semidiurnal tides, respectively. The wave-4 structure at the equator is determined by the diurnal tide (Fig. 3b), and the wave-4 component due to the semidiurnal tide at the equator (Fig. 3c) is negligibly small. The diurnal variation of the wave-4 component at 301N (Fig. 3e) is nearly in phase with that at 301S (Fig 3h), while the semidiurnal variation of the wave-4 component at 301N (Fig. 3f) is out of phase with that at 301S (Fig. 3i). At 7301 latitudes, the amplitude of the wave-4 component due to the semidiurnal tides is 3 10 14 kg/m3, which is larger than that due to the diurnal tides by a factor of 2. However, the diurnal tides modulate the local time variation of the wave-4 structure. For example, the amplitude of the wave-4 component at 301N (301S) is relatively small during the period from 23 LT to 03 LT (from 12 LT to 16 LT).
The global distribution of the wave-4 component due to the diurnal tide (the semidiurnal tide) at 12–13 LT is shown in Fig. 2c (Fig. 2d). The wave-4 component due to the diurnal tide is almost symmetric about the equator and has a maximum at the equator. The wave-4 component due to the semidiurnal tide is antisymmetric about the equator and has maxima at 730–351 latitudes. These features are characteristics of the SE2 (Oberheide et al., 2011). At 710–151 latitudes, the amplitude of the wave-4 component due to the diurnal tide is comparable to that due to the semidiurnal tide. Fig. 2b shows the distribution of the wave-4 component due to superposition of the diurnal and semidiurnal tides. In the northern hemisphere, the longitudinal variation of the wave-4 component due to the semidiurnal tide at 12–13 LT is nearly in phase with that due to the diurnal tide, so that the longitudinal variation due to the diurnal plus semidiurnal tides is enhanced. On the other hand, at 12–13 LT in the southern hemisphere, the longitudinal variation of the wave-4 component due to the semidiurnal tide is almost out of phase with that due to the diurnal tide. This is the reason for the larger amplitude of the wave-4 component in the northern hemisphere. The wave-4 structure due to superposition of the diurnal and semidiurnal tides (Fig. 2b) is almost the same as the wave-4 structure due to all the tidal components (Fig. 2a) except for the poleward of 601 latitude. This result indicates that the wave-4 structure of the neutral density is generated by both the diurnal and semidiurnal tides. 3.3. Vertical structures of DE3 and SE2 We showed that the SE2 and DE3 play an important role on the generation of the wave-4 structure at 400 km. In this subsection, we will show the vertical structures of SE2 and DE3 and their influence on the wave-4 structure of neutral density at various heights in the thermosphere. In order to compare the present result with the results by previous studies, Fig. 4 shows the vertical structures of amplitude and phase of the temperature component due to DE3. In the thermosphere, the amplitude has a peak at the equator, and the phase structure is symmetric about the equator. This means
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Fig. 3. (a) Longitude–local time distribution of the wave-4 component of neutral density at the equator at 400 km height. Units are in 10 14 kg m 3. (b) Same as in (a) except for the component due to the diurnal variation. (c) Same as in (a) except for the component due to the semidiurnal variation. (d) Longitude–local time distribution of the wave-4 component of neutral density at 301N. (e) Same as in (d) except for the component due to the diurnal variation. (f) Same as in (d) except for the component due to the semidiurnal variation. (g) Longitude–local time distribution of the wave-4 component of neutral density at 301S. (h) Same as in (g) except for the component due to the diurnal variation. (i) Same as in (g) except for the component due to the semidiurnal variation.
Fig. 4. (a) Latitude–height distribution of the DE3 amplitude in the temperature component. Units are in K. (b) Same as in (a) except for the phase distribution. Units are in h. Phase is defined as the universal time of maximum at 01E.
that the symmetric mode is dominant above 80 km height. The vertical structure of DE3 in the thermosphere indicates the upward propagation of the symmetric mode of DE3 from the troposphere to the thermosphere. The amplitude in the mesosphere has a peak at 151N, and the phase below 80 km height is not symmetric about the equator. In the mesosphere and stratosphere, both the symmetric and antisymmetric modes exist, and superposition of the symmetric and antisymmetric modes makes the latitudinal phase structure complicated (Forbes et al., 2008). Fig. 5 shows the distributions of amplitude and phase of temperature due to SE2. The amplitude in the thermosphere maximizes around 725–351 latitudes, with the largest amplitude of 7 K. The equatorial minimum of the amplitude and the
latitudinal phase structure indicate the dominance of the antisymmetric mode in the thermosphere. Obviously, the vertical structure of SE2 in the thermosphere shows the upward propagation of the antisymmetric mode of SE2 from the troposphere to the thermosphere. The latitudinal phase structure below 80 km height is very complicated, and it is due to superposition of the symmetric and antisymmetric modes. The discussion concerning the behaviors of the symmetric and antisymmetric modes in the mesosphere and thermosphere was found in Oberheide et al. (2011). In the lower thermosphere, second symmetric (HME3) and antisymmetric (HME4) modes, which cannot propagate into the upper thermosphere, have considerable amplitudes. Furthermore, the first symmetric (HME1) mode is not negligibly small below the lower
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Fig. 5. (a) Same as in Fig. 4a except for SE2. (b) Same as in Fig. 4b except for SE2.
Fig. 6. (a) Longitude–geographic latitude distribution of the neutral mass density during 12–13 LT at a height of 200 km. Units are in 10 10 kg m 3. (b) As in (a) except for 150 km height. Units are 10 9 kg m 3.
thermosphere. The features of DE3 and SE2 obtained in this study are quite similar to those using the Hough mode extensions of the TIMED observation (Oberheide et al., 2011). The propagation of DE3 and SE2 from the lower atmosphere to the thermosphere implies that the longitudinal wave-4 structure in a fixed local time frame is considered to be evident not only in the upper thermosphere but also in the lower thermosphere. The longitudinal variation of neutral density at various heights is examined here. Fig. 6 shows the global distributions of neutral density at 12–13 LT at altitudes of 150 and 200 km. A prominent 4-peak structure is evident not only at low latitudes but also at middle latitudes of the northern hemisphere. The weak wave-3 and wave-4 longitudinal variations are visible in the southern hemisphere. Thus, the longitudinal wave-4 structure of neutral density is prominent above 150 km height although the wave-4 longitudinal structure of the electron density is weak in the 150–200 km height region (Lin et al., 2007). Using the WINDII data, Shepherd (2011) showed the wave-4 structure of neutral density at an altitude of 250 km, which is consistent with the present result. 3.4. Day-to-day variation of the wave-4 amplitude Forbes et al. (1997) indicated that the seasonal variation of the convective activity in the tropics affects seasonal variation of the tidal amplitude in the mesosphere and lower thermosphere using a Global Scale Wave Model (GSWM). Using a whole atmosphere GCM, Miyoshi and Fujiwara (2003) showed that day-to-day variations in tropical convective activity produced day-to-day variations in the intensity of diurnal tides that are propagating into the upper atmosphere. The DE3 and SE2 in the upper thermosphere, which produce the wave-4 structure of neutral
density, are considered to be excited by latent heating in the troposphere. Therefore in this subsection, we examine day-to-day variation of the wave-4 amplitude of neutral density at 400 km height and its relation to the day-to-day variations of DE3 and SE2 amplitudes at various heights. The day-to-day variation of amplitude of the zonal wavenumber 4 component of neutral density at 12–13 LT in September and October is estimated. The upper row of Fig. 7 shows the time variations of wave-4 amplitudes at 301N and the equator. Fluctuations of the wave-4 amplitude with periods of 10–20 days are evident both at 301N and at the equator. However, the variations at 301N and at the equator do not correlate with each other. Namely, the wave-4 amplitude at 301N has peaks on 10–15 September, 4–5 October and 18–19 October, while the wave-4 amplitude at the equator attenuates on 11–12 September, 21–23 September, 3–4 October and 21–22 October. The lower row of Fig. 7 shows the time variations of amplitudes of SE2 and DE3 temperature components. Significant fluctuations of the SE2 and DE3 amplitudes with periods of 10–20 days are found. It is clearly seen that the day-to-day variation of wave-4 amplitude of neutral density at 301N (equator) is correlated with the variation of SE2 (DE3) amplitude. Thus, the temporal variation of SE2 amplitude produces the day-to-day variation of wave-4 amplitude of neutral density at 301N, while the day-to-day variation of wave-4 amplitude at the equator is determined by the temporal variation of DE3 amplitude. Next, we examine the day-to-day variations of SE2 and DE3 amplitudes at various heights. Fig. 8a and b shows the variations of DE3 amplitudes at 120 and 400 km heights, respectively. There is a good correlation between the amplitude at 400 km height and the amplitude at 120 km height. The day-to-day variations of the
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Fig. 7. (a) Temporal variation of the amplitude of wave-4 component of neutral density at 301N during 12–13 LT. 5-day running mean is performed. Units are in 10 14 kg m 3. (b) As in (a) except for the variation at the equator. (c) The temporal variation of amplitude of temperature due to SE2 at 301N. Units are in K. (d) As in (c) except for the DE3 at the equator.
shows the variations of the symmetric component due to the DE3 at 40 and 70 km heights, respectively. The variations of DE3 amplitudes at 40 and 70 km heights are similar to those at 120 and 400 km heights. Namely, the amplitude minima around 10–12 September, 20–23 September, 3–4 October and 20–22 October are evident at all heights. This result indicates that the day-to-day variation of DE3 amplitude at 400 km height is caused by the DE3 variation of the lower atmospheric origin. The day-to-day variations of SE2 amplitudes at 120 and 400 km heights are shown in Fig. 9a and b, respectively. The temporal variation of SE2 amplitude at 120 km height is different from that at 400 km height. The variation of the amplitude due to the antisymmetric mode at 120 km height is examined (Fig. 9c). The antisymmetric component has minima on 25 September and on 12 October, and these minima are also evident at 400 km height. The variation of the antisymmetric component at 120 km height is similar to the variation of the SE2 amplitude at 400 km height. Thus, the difference of the variations at 400 km and at 120 km is explained by the fact that the symmetric modes have considerable amplitudes in the lower thermosphere. Fig. 9d and e shows the day-to-day variations of amplitudes of the antisymmetric mode at 40 and 70 km heights, respectively. Again, the amplitude minima at 40 and 70 km heights are evident around 25 September and 12 October. Thus, the temporal variations of antisymmetric amplitude of SE2 at 40 and
Fig. 8. (a) Temporal variations of the DE3 amplitude in the temperature at the equator at a height of 400 km. Five-day running mean is performed. Units are in K. (b) As in (a) except for 120 km height. (c) Temporal variations of the DE3 amplitude due to the symmetric component at the equator at 70 km height. (d) As in (c) except for 40 km height.
symmetric component due to DE3 in the stratosphere and mesosphere are examined, because the antisymmetric modes have considerable amplitudes below the mesopause region. Fig. 8c and d
Fig. 9. (a) Temporal variations of the SE2 amplitude in the temperature at 351N at an altitude of 400 km. Units are in K. (b) Same as in (a) except for 120 km height. (c) Temporal variations of the SE2 amplitude due to the antisymmetric component at 351N at 120 km height. (d) As in (c) except for 70 km height. (e) As in (d) except for 40 km height.
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70 km heights are somewhat similar to those at 120 and 400 km heights. This result shows the day-to-day variation of SE2 amplitude in the upper thermosphere is closely related to the variations of antisymmetric amplitude of SE2 in the lower atmosphere. The previous studies (e.g., Forbes et al., 1997) showed that latent heat release in the troposphere is the main source of the SE2 and DE3 in the thermosphere. In the next step, the day-to-day variation of convective activity in the tropics and its impact on the day-to-day variations of DE3 and SE2 amplitudes should be evaluated. This will be the subject of a future study.
4. Concluding remarks Using the GAIA model, the generation mechanism for the wave-4 structure of neutral density and its day-to-day variation are studied. Liu et al. (2009) discussed the generation mechanism of the wave-4 structure of neutral density at 400 km height. They concluded that the ion drag force associated with the EIA’s wave4 structure is not a major cause for the wave-4 structure of neutral density. They suggested that the upward propagation of DE3 to the upper thermosphere is one of the plausible candidates for the generation of the wave-4 formation of neutral density, because the phase relation between the neutral density and zonal ¨ ¨ wind at 400 km height (Hausler and Luhr, 2009) is in good agreement with the theoretical predictions by Oberheide et al. (2009). Our result confirms that the wave-4 structure of neutral density at 400 km height is caused by the upward propagation of DE3 and SE2. In particular, the upward propagating SE2 is important for the generation of the wave-4 structure of neutral density at middle latitudes. Furthermore, our simulation predicts that the 4-peak structure in the longitudinal direction is evident at the height range from 150 km to 500 km. We examined the day-to-day variation of wave-4 amplitude of neutral density at 400 km height and its relation with SE2 and DE3 amplitude at various heights. The day-to-day variation of the wave-4 amplitude of neutral density at low latitudes (middle latitudes) in the upper thermosphere is correlated with the temporal variations of DE3 (SE2) amplitudes in the lower atmosphere. This result indicates a clear relationship between the variation of the wave-4 structure of thermospheric density and the tropospheric and stratospheric variabilities.
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