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Keywords:

  • airglow;
  • equatorial thermosphere;
  • gravity wave

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observation
  5. 3. Summary and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[1] We report quasiperiodic southward moving waves, which are commonly observed in the OI 630-nm airglow images (emission altitudes of 200–300 km) near the equator, in 2-year airglow observations at Kototabang, Indonesia (0.2°S, 100.3°E, geomagnetic latitude of −10.4°). The waves have predominantly east-west phase fronts and repeatedly propagate southward with a velocity of 310 ± 110 m/s and a period of 40 ± 15 min. They are frequently observed in May–July with an occurrence rate of 53% and are also observed in other seasons with occurrences of ∼20%. The waves are observed in and to the south (geomagnetically poleward) of the equatorial ionospheric anomaly, which is identified as an airglow enhancement region moving gradually to lower geomagnetic latitudes at the premidnight local times. We suggest that gravity waves in the lower thermosphere below ∼300 km are a plausible cause of the observed quasiperiodic waves in the airglow images.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observation
  5. 3. Summary and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[2] The airglow imaging technique has been widely used to measure thermospheric and ionospheric structures using the OI (630.0 nm) emission, which has an emission layer in the bottomside of the F layer at 200–300 km. At equatorial latitudes, the 630-nm airglow intensity significantly increases in association with the equatorial ionospheric anomaly at 5°–10° north and south from the magnetic equator. These longitudinal belts of airglow enhancement are often referred to as subtropical arcs. The equatorial ionospheric anomaly is the enhancement of electron density in the subequatorial F layer due to the upward drift (fountain effect) of the equatorial ionosphere, particularly at the sunset terminator [e.g., Rishbeth and Garriot, 1969]. Recently, Sagawa et al. [2003] and Ogawa et al. [2005] reported large-scale (∼1000 km) longitudinal wavy structures drifting eastward with a velocity of ∼100 m/s in the OI 135.6 nm images obtained by the IMAGE satellite.

[3] At smaller scales, the plasma bubble is the most distinct structure in the equatorial 630-nm airglow images. The plasma bubble consists of the meridional band-like airglow depletions with zonal scale sizes of less than a few hundred kilometers [e.g., Mendillo and Baumgardner, 1982; Tinsley et al., 1997; Sahai et al., 2000; Otsuka et al., 2002; Kelley et al., 2003; Ogawa et al., 2005]. The bubble is a result of the Rayleigh-Taylor instability, for which the low-density plasma on the bottomside of the ionosphere penetrates through the dense F layer by a polarization electric field in the equatorial ionosphere at the sunset terminator.

[4] The other 630-nm airglow structure observed in the equatorial latitudes is the midnight brightness wave (MBW), which is a poleward moving wave with a latitudinal scale size of more than 1000 km [e.g., Mendillo et al., 1997; Colerico and Mendillo, 2002]. Colerico et al. [1996] and Otsuka et al. [2003] have shown that the MBW is accompanied by an enhancement of poleward neutral wind in the thermosphere, which pushes the ionosphere down along the field line to enhance the 630-nm airglow intensity. The poleward neutral wind in the MBW is probably generated from the midnight temperature maximum (MTM). Colerico et al. [1996] also reported a premidnight brightness wave (PMBW), which moves equatorward in the premidnight sector. Colerico and Mendillo [2002] suggested, however, that the PMBW may predominantly result from the relaxation of the subtropical arcs (equatorial anomaly) due to reversal of the fountain effect.

[5] In this paper, we report the observation of a new wave structure that was found in the 630-nm airglow images taken at an equatorial latitude (geomagnetic latitude of −10.4°). The waves have a predominantly east-west phase front, moving southward quasiperiodically with a period of ∼40 min, and their scale sizes are smaller than the MBW.

2. Observation

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observation
  5. 3. Summary and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

2.1. Instrumentation and Sky Condition

[6] The all-sky airglow imager used in this study is part of the Optical Mesosphere Thermosphere Imagers (OMTIs) [Shiokawa et al., 1999, 2000]. The imager uses a thinned and back-illuminated charge-coupled device (CCD) with 512 × 512 pixels, which are thermoelectrically cooled to temperatures less than −50°C. Airglow images at a wavelength of 630.0 nm (OI, emission altitude: 200–300 km) were obtained every 4.5 min with an exposure time of 105 s. Background emission was monitored at a wavelength of 572.5 nm every 30 min and with an exposure time of 105 s. The bandwidths of the band-pass filters for the measurement of the 630-nm airglow and the background emission are 2.0 nm.

[7] The airglow imager was installed at Kototabang, Indonesia (0.2°S, 100.3°E, dipole geomagnetic latitude (MLAT): −10.4°), on 26 October 2002, as one of the observation equipment components for the CPEA (Coupling Processes in the Equatorial Atmosphere) project. Figure 1 shows the occurrence rate of clear sky without clouds during the 2 years of automated measurements, from 26 October 2002 to 26 October 2004. Because of the tropical climate and surrounding mountains of Sumatra Island, the sky at Kototabang tends to be cloudy. In May and June, some clear sky intervals are available. For other months, the occurrence rate of clear sky is mostly less than 10%. The number of observation hours in June–October is less than in the other months because of instrument trouble.

image

Figure 1. Occurrence of clear sky without clouds at Kototabang, Indonesia, for the 2-year period from 26 October 2002 to 26 October 2004. The numbers of hours of clear sky (CLR) and of observation (OBS) are shown at the bottom.

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2.2. Typical Examples of Quasiperiodic Southward Moving Waves

[8] Figure 2 shows a typical example of quasiperiodic southward moving waves detected by the all-sky airglow imager at Kototabang. Figure 2b shows variations in the 630-nm airglow intensity in a north-south meridian (keogram) at the longitude of Kototabang (100.3°E). We converted the all-sky images to geographical coordinates by assuming an airglow height of 250 km, and then took a cross section in the north-south meridian at a longitude of Kototabang (100.3°E). The local time (LT) at Kototabang (shown in Figure 2a) is 7 hours ahead of the universal time (UT) in Figure 2b. This night was geomagnetically quiet, with a Kp index of 1−.

image

Figure 2. Intensity variations of 630-nm airglow in the north-south meridian (keogram), observed by an all-sky CCD imager at Kototabang (0.2°S, 100.3°E, MLAT = −10.4°), Indonesia, on 16 May 2004 for (a) deviation (in %) from 1-hour running averages and (b) absolute intensity (in Rayleigh units). The all-sky images are converted to geographical images by assuming an airglow altitude of 250 km. The intensity variations at the longitude of Kototabang (100.3°E) are shown. Southward moving waves are seen throughout the night. The narrow horizontal structures in Figure 2a (e.g., at 1600–1700 UT at −4° and 1900–2000 UT at −2°) are stars. The Kp index on this night was 1−.

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[9] In Figure 2b, an intense (∼500 R) airglow band first exists at the southern edge of the images at geographical latitudes (GLATs) of −4 to −6° at 1200–1500 UT (1900–2200 LT). It moves northward to +4 to +6° at 1500–1900 UT (2200–0200 LT) and then weakens after 1900 UT. Note that Kototabang (−0.2° GLAT) is located at −10.4° MLAT. Thus this northward motion of the bright airglow band corresponds to the motion to the geomagnetic equator. This enhanced airglow band probably corresponds to the equatorial ionospheric anomaly [Sagawa et al., 2003]. The equatorward motion of the anomaly corresponds to the reversal of the equatorial electric field from eastward to westward after sunset [Fejer et al., 1991].

[10] At 1800–1900 UT (0100–0200 LT), two band-like structures at −0.2 to −6.2° GLAT propagate southward in Figure 2b. To see such wave structures with a timescale of ∼1 hour more clearly, we made a keogram using deviations in % from 1-hour running averages in Figure 2a. This keogram was made by calculating (I(t) − Ia(t))/Ia(t), where I(t) and Ia(t) are airglow intensity at time t and average intensity over t ± 30 min, respectively, for each pixel of images. The two southward moving bands at 1800–1900 UT are clearly seen in Figure 2a. Moreover, many similar southward moving wave structures are seen in Figure 2a throughout the night. These quasiperiodic waves move poleward with respect to the geomagnetic equator, which is located at GLAT = ∼10°N. The period and southward moving velocity of these waves are ∼40 min and ∼300 m/s, respectively. The amplitude of the waves is 10–20% of the background 630-nm airglow intensity. These southward moving waves can also be recognized in the absolute intensity keogram in Figure 2b. Note that when Figures 2a and 2b are compared, the southward moving waves are observed only in the enhanced airglow band (equatorial anomaly) and south of it, and are not seen north (geomagnetically equatorward) of the anomaly. For example, the southward moving waves are seen at latitudes from ∼−2° to −6.2° at 1300–1500 UT. Some waves are seen north of −2° GLAT, but they do not propagate southward.

[11] Figure 3 presents some images of the southward moving waves shown in Figure 2, at 1812–1842 UT (0112–0142 LT). Figure 3b shows images in absolute intensity, while Figure 3a shows images in deviations from 1-hour running averages. The raw all-sky images were converted to geographical images of 12° × 12° (∼1300 km × 1300 km) in latitude and longitude by assuming an airglow height of 250 km. The height assumption of 250 km is based on a model calculation by Shiokawa et al. [2003a]. The quasiperiodic southward moving waves are seen in Figure 3a as two east-west band structures moving southward. The east-west motion (along the phase front) of the waves is not clear. The north-south wavelength is ∼700 km. Such a structure is difficult to identify in the absolute intensity images in Figure 3b, because of spatial inhomogeneity of airglow intensity. However, by looking near the western edge of the images, one can recognize that the red-to-white part moves toward the south.

image

Figure 3. The 630-nm airglow images observed at Kototabang, Indonesia, on 16 May 2004. This event is shown in the keograms of Figure 2. (a) Deviation images (in %) from 1-hour running averages. (b) Images in absolute intensity. The all-sky images are converted to geographical images by assuming an airglow altitude of 250 km.

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[12] Figure 4 shows another example of the quasiperiodic southward moving waves observed in the 630-nm keograms at Kototabang on 24 May, 2003. The format is the same as that in Figure 2. This night was geomagnetically active, with a Kp index of 3+ to 4+. In Figure 4b, the intense airglow region with a maximum intensity of more than 600 R shifts from south (1200–1500 UT) to north (1700–1900 UT). In this enhanced airglow region, the southward moving waves are observed particularly at 1400–1600 UT and 1700–1800 UT in Figure 4a. These waves are seen only in and south of the enhanced airglow region, which probably corresponds to the equatorial anomaly. These characteristics are fairly similar to those in Figure 2. The amplitude of the waves is ∼10% of the background airglow intensity.

image

Figure 4. Intensity variations of 630-nm airglow in the north-south meridian (keogram), observed by an all-sky CCD imager at Kototabang, Indonesia, on 24 May 2003 for (a) deviation (in %) from 1-hour running averages and (b) absolute intensity (in Rayleigh units). The format is the same as that in Figure 2. The Kp index on this night was 3+–4+.

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[13] Figure 5 shows an isolated southward moving wave observed during the geomagnetically quiet night (Kp = 0) of 6 October 2004. The format is the same as that in Figure 2. The intense airglow region in Figure 5b starts to move northward from −6.2° at 1300 UT. It is intensified at 1500–1600 UT at latitudes above 1°. Then a southward moving structure is launched from the intensified airglow region at 1600 UT. It reaches the southern edge of the image at 1700 UT. It is noteworthy that some tendency of repeated southward moving structures can also be recognized at 1330–1500 UT in Figure 5a.

image

Figure 5. Intensity variations of 630-nm airglow in the north-south meridian (keogram), observed by an all-sky CCD imager at Kototabang, Indonesia, on 6 October 2004 for (a) deviation (in %) from 1-hour running averages and (b) absolute intensity (in Rayleigh units). The format is the same as that in Figure 2. The sky was cloudy before 1300 UT. An intense southward moving structure was observed at 1600–1700 UT. The Kp index on this night was 0.

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[14] Figure 6 shows the images of this isolated southward moving wave at 1555–1636 UT (2255–2336 LT). The wave can be recognized in the images of both absolute intensity and deviation from 1-hour averages. Note that the wave is tilted from east-west to ESE–WNW. This wave is somewhat similar to that of the MBW reported by Colerico et al. [1996] and Otsuka et al. [2003]. However, the meridional scale size of the wave (∼700 km) is smaller than that of the MBW.

image

Figure 6. The 630-nm airglow images observed at Kototabang, Indonesia, on 6 October 2004 in the same format as that in Figure 3. This event is shown in the keograms of Figure 5. (a) Images of deviation (in %) from 1-hour running averages. (b) Images in absolute intensity.

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2.3. Simultaneous Observation With Plasma Bubbles

[15] Figure 7 shows an example of quasiperiodic southward moving waves, which were observed simultaneously with the equatorial plasma bubbles. The format is the same as that in Figure 2. The Kp indices on this night were 1+ to 2. The equatorward moving airglow enhancement region (equatorial anomaly) is not clear in this case, but it may be the weakly enhanced airglow region at ∼0° to −6.2° GLAT at 1200–1500 UT and 3°–5.8° GLAT at 1800–2000 UT in Figure 7b. The plasma bubbles are airglow depletions seen at ∼1400 UT, ∼1440 UT, and ∼1730 UT at latitudes above ∼0° GLAT. In Figure 7a, the quasiperiodic southward moving waves are recognized at 1330–1500 UT (two waves) at latitudes of −2° to −6.2° and after 1600 UT at all the plotted latitudes. The waves at 1330–1500 UT are located geomagnetically just poleward of the plasma bubbles. The waves after 1600 UT are continuously seen until the morning hours of 22 UT (0500 LT), though the airglow intensity is rather low. The amplitudes of the waves are 10–20%.

image

Figure 7. Intensity variations of 630-nm airglow in the north-south meridian (keogram), observed by an all-sky CCD imager at Kototabang, Indonesia, on 17 June 2004 for (a) deviation in (%) from 1-hour running averages and (b) absolute intensity (in Rayleigh units). The format is the same as that in Figure 2. Plasma bubbles (depletion of airglow) are observed at latitudes of 0°–5.8° at 2000–0100 UT. The Kp index on this night was 1+ to 2.

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[16] Figure 8 presents images of the bubbles and the southward moving waves shown in Figure 7, at 1325–1346 UT (2025–2046 LT). The format is the same as that in Figure 3. The bubbles are seen as north-south depletion at latitudes above ∼1° in Figure 8b. The bubble feature is more intensified in the top images, which are deviations from 1-hour averages, because the bubbles continuously move eastward. The eastward velocity of the bubble is ∼90 m/s for this case. The southward moving wave can be recognized in Figure 8a at latitudes of 0° to −6.2° as an east-west bright region (mostly in red) moving southward. These images clearly show that the plasma bubbles are basically north-south aligned structures, while the quasiperiodic southward moving wave is the east-west structure. It is noteworthy that the phase surface of the southward moving waves is slightly tilted to ESE–WNW in the image of 1346 UT in Figure 8a at latitudes of −2.2° to −4.2°.

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Figure 8. The 630-nm airglow images observed at Kototabang, Indonesia, on 17 June 2004 in the same format as that in Figure 3. This event is shown in the keograms of Figure 7. (a) Images of deviation (in %) from 1-hour running averages. (b) Images in absolute intensity. Plasma bubbles (depletion of airglow) are observed at latitudes of 0°–5.8°.

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2.4. A Case With No Southward Motion

[17] Figure 9 shows a rare case in which southward motion was not observed. The event occurred on 22 December 2003. The format is the same as that in Figure 2. The airglow enhancements are observed at latitudes of −2° to −6.2° at 1400–1600 UT and at latitudes of 2°–5.8° at 1600–2100 UT. These enhancements may correspond to the equatorward shift of the equatorial anomaly, as seen in the previous examples. As shown in Figure 9a, the airglow intensity changes with a period of ∼30–120 min. Such changes are also seen in the keogram of absolute intensity in Figure 9b. However, any latitudinal motion is not seen in the north-south keogram. We also made keograms in the east-west direction, but no longitudinal motions were seen. The airglow intensity in the whole sky varies with a period of ∼30–120 min. From the 2-year observation at Kototabang, this is the only event which shows no motion in the keogram throughout the night.

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Figure 9. Intensity variations of 630-nm airglow in the north-south meridian (keogram), observed by an all-sky CCD imager at Kototabang, Indonesia, on 22 December 2003 for (a) deviation (in %) from 1-hour running averages and (b) absolute intensity (in Rayleigh units). The format is the same as that in Figure 2. The sky was cloudy before 1400 UT. Southward moving structures are not seen on this night, although the airglow intensity changes with a period of ∼30–120 min. The Kp index on this night was 3 to 4−.

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2.5. Some Statistics

[18] On the basis of 2 years of observations of 630-nm airglow images at Kototabang, we investigated the statistical characteristics of the quasiperiodic southward moving waves. In a total of 75 clear-sky nights, we observed 32 nights with those waves. On only two nights, northward moving waves were observed together with the southward moving waves. Thus most of the observed waves propagate southward in the keogram. The averages and standard deviations of the southward moving velocity and period (timescale) are 310 ± 110 m/s and 40 ± 15 min, respectively. We also checked the zonal (east-west) motion of the waves, using east-west keograms similar to that in Figure 2. However, east-west motions are not observed for most of the waves. In a few cases, the wave phase front tends to tilt from east-west to ESE–WNW, as shown in Figures 6 and 8.

[19] Figure 10a shows the occurrence rate of the quasiperiodic southward moving waves for four seasons. The waves are frequently observed in the Northern Hemispheric summer season of May–July. The hourly rate of 91 h/172 h = 53% means that the waves are observed for most of the night. The southward moving waves are also observed for the other three seasons with a rate of ∼20%. It should be noted that the total number of hours of clear-sky nights is less for the other three seasons, as shown in Figure 1.

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Figure 10. Occurrence rate of the quasiperiodic southward moving waves observed at Kototabang, Indonesia, (a) in four seasons and (b) at three local times. The number of observation hours is shown at the top of the bars.

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[20] Figure 10b shows the occurrence rate of the quasiperiodic southward moving waves for three local time intervals. The rate tends to decrease from premidnight to postmidnight, perhaps because the southward moving waves are generated near the equatorial anomaly, which shifts to the north of the images after midnight. Although the airglow intensity significantly decreases after midnight, as shown in the previous examples, the occurrence rate is still high (∼37%).

3. Summary and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observation
  5. 3. Summary and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

3.1. Summary of the Observations

[21] We found quasiperiodic southward moving waves in the 630-nm airglow images at Kototabang, Indonesia (−10.4° MLAT). The waves have predominantly east-west phase fronts moving southward (geomagnetically poleward) in the field of view of the all-sky imager. The waves occur on both geomagnetically quiet and active nights. The amplitudes of the waves are typically 10–20%. On average, the waves have a southward velocity of 310 ± 110 m/s and a period of 40 ± 15 min. These values give a north-south wavelength of ∼700 km. In a few cases, the phase front is tilted to ESE-WNW. The waves are frequently observed in May–July with an occurrence of 53%. The waves are observed in and south of the enhanced airglow band, which probably corresponds to the equatorial ionospheric anomaly, and are not observed north (equatorward) of the southern anomaly.

3.2. The 630-nm Airglow Emission

[22] The waves are observed in the 630-nm airglow images. The major chemical reactions that generate the 630.0-nm airglow emission are:

  • equation image
  • equation image
  • equation image

Because (1) is the rate determining reaction, the production of the 630.0-nm emission is proportional to the molecular oxygen density [O2] and oxygen ion density [O+]. The oxygen ion density [O+] is nearly equal to the electron density in the F layer. The peak height of the F layer electron density is ∼400 km, while the molecular oxygen density [O2] increases with decreasing height. Thus the 630.0-nm emission is in the bottomside of the F layer at ∼200–300 km. If the F layer height decreases, the 630-nm emission would increase, because the F layer plasma (O+ ions) reacts with the high-density molecular oxygen at lower altitudes. Thus the 630-nm airglow is a sensitive indicator of the F layer height and the plasma density.

[23] There are two possible causes of the decrease in F layer height and subsequent airglow enhancement. One is the westward electric field, which causes downward E × B drift of O+ ions. The other is the poleward neutral wind, which pushes O+ ions poleward through neutral-ion collisions. Since the ions tend to move along the geomagnetic field line, which has a finite angle to the horizontal plane except for the geomagnetic equator, the poleward neutral wind pushes O+ ions down along the field line. The inclination (angle to the horizontal plane) of the geomagnetic field at an altitude of 250 km above Kototabang is −19.7° according to the IGRF-2005 magnetic field model.

3.3. Relation to the Midnight Brightness Wave

[24] One example of the poleward neutral wind causing enhancement of the 630-nm airglow is the midnight brightness wave (MBW). Colerico et al. [1996] and Otsuka et al. [2003] did observe a poleward neutral wind in association with the poleward moving MBW from a Doppler shift of the 630-nm airglow using the Fabry-Perot interferometers. These authors suggested that the MBWs are generated from the pressure bulge of the midnight temperature maximum (MTM) near the equator and move poleward as an enhancement of the 630-nm airglow [Colerico and Mendillo, 2002]. In this context, the MBW is similar to the quasiperiodic southward moving waves reported in this paper. However, the scale size of the MBW seems to be much larger (>1000 km). Moreover, the MBW propagates poleward only once per night in association with the pressure enhancement of the MTM and subsequent poleward neutral wind. The southward moving waves reported in this paper repeat continuously with a period of ∼40 min throughout the night.

3.4. Enhanced Airglow Region

[25] The quasiperiodic southward moving waves are observed in and south of the enhanced airglow region, which probably corresponds to the equatorial ionospheric anomaly. As shown in the examples in section 2, this anomaly always shifts from higher geomagnetic latitudes to lower latitudes from evening to midnight. To see this feature more clearly, we made seasonal averages of the 630-nm intensity keograms in Figure 11. Because the sky conditions were not good except for May–July, as shown in Figure 1, the plots of May–July contain ∼30 nights of data and the other three seasons contain less than 10 nights of data. Nevertheless, the shift of the enhanced airglow region from south (higher geomagnetic latitudes) to north (lower geomagnetic latitudes) is clearly seen for all the seasons. The shift begins at earlier times in May–July (at 1900–2100 LT) than in the other three seasons (at 2200–0100 LT). The airglow intensity of the anomaly is weaker in November–January than in the other seasons. This is probably because the geomagnetic equator is ∼10° north of the geographic equator at this longitude, so the plasma density on the magnetic flux tube near Kototabang must be smaller in the winter of the Northern Hemisphere, in November–January.

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Figure 11. Average nighttime variations of 630-nm airglow intensity in the north-south meridian (keogram) at Kototabang, Indonesia, in four seasons. This keogram was made from all-sky images, so the vertical axis is proportional to the zenith angle from Kototabang (−10.4° MLAT).

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[26] The equatorward shift of the enhanced airglow region probably corresponds to the reversal of electric field from eastward in the sunset terminator to westward in the night. The eastward electric field near the sunset terminator generates the equatorial ionospheric anomaly through the fountain effect (upward E × B drift). When the electric field turns westward after the sunset at the equator, the anomaly shifts to lower geomagnetic latitudes, because the westward electric field at the equator causes equatorward E × B drift at the magnetic footprint of both hemispheres.

[27] Scherliess and Fejer [1999] have shown a seasonal and longitudinal dependence of these electric field variations based on measurements of vertical plasma drift by the Jicamarca radar and the AE-E satellite. According to their Figure 5, the upward plasma drift at the sunset terminator (evening enhancement) is most intense in the equinox seasons for all longitudes. In the longitudinal sector of −20°–+180°E (including Indonesia), the time when the vertical drift turns from upward to downward is around 1930–2000 LT and does not change much with the seasons during high solar activity. The intense upward drift in the equinox seasons may cause the time delay of the anomaly shift to lower latitudes in Figure 11 compared with the solstice seasons. It is not clear why the anomaly shift is late in November–January compared with May–July.

[28] One may argue that the enhanced airglow region in Figure 11 corresponds to the neutral temperature enhancement in association with the MTM. However, the intensity of the 630-nm airglow often reaches 300–500 R in this enhanced airglow region, which is more than a few factors larger than that at midlatitudes (typically less than 100 R). On the other hand, the enhancement in neutral temperature in the MTM is less than 100 K [Herrero and Spencer, 1982], which is only ∼10% of the typical thermospheric temperature (∼1000 K). Thus the MTM is not likely to be the cause of this enhanced airglow region.

3.5. Possible Generation Mechanisms of Quasiperiodic Southward Moving Waves

[29] On the basis of the above considerations, we now discuss the possible generation mechanisms of the observed quasiperiodic southward moving waves. One of the common features of these waves is that the phase front is mostly aligned in the east-west direction. This fact suggests the generation of the waves by gravity waves in the thermosphere rather than by the oscillating electric field in the ionosphere. The oscillation of the neutral atmosphere by gravity waves causes a similar oscillation of plasma in the F layer through ion-neutral collision. Because the plasma can move only along the geomagnetic field line, the north-south oscillation in the neutral atmosphere can push plasma up/down along the field line [Hooke, 1970]. As discussed in subsection 3.2, the upward/downward motion of plasma causes reduction/enhancement of the 630-nm airglow. As a result, only the meridionally propagating gravity waves, which have a zonal (east-west) phase front, would be observed as wave structures in the 630-nm airglow images. On the other hand, the polarization electric field in the east-west direction, which pushes ionospheric plasma up/down to produce airglow variations, cannot develop in the observed east-west phase front. Thus the oscillating electric field is not likely to be the cause of the quasiperiodic southward moving waves reported in this paper.

[30] The other supporting evidence for the neutral wind perturbation is that, for some cases, the southward moving waves also show an ESE-WNW tilt of the phase front, as shown in Figures 6 and 8. This fact suggests that the waves have a westward moving component, which is opposite to the eastward plasma drift typically observed in the nighttime. Actually, when we observe the southward moving waves with plasma bubbles in Figure 8, the bubble continuously moves eastward, but the southward moving wave seems to have a component of westward motion.

[31] There have been several previous observations of gravity wave signatures in the variations of ionospheric plasma near the equator, on the basis of radio sounding techniques, such as IS radars, ionosondes, and HF Doppler signals [e.g., Sterling et al., 1971; Röttger, 1981, and references therein]. Röttger [1977] reported medium-scale traveling ionospheric disturbances (TIDs) observed through the HF backscatter and Doppler experiments in the equatorial zones of Africa and South America. These TIDs propagate out from the equatorial zone with a mean phase velocity of 210 m/s and have periods between 5 and 30 min. He suggests the source of the TIDs as the cumulonimbus activity in the equatorial troposphere. Recently, Djuth et al. [2004] reported gravity wave signatures with a timescale of 20–60 min, which is similar to the period of the waves in this paper, in the incoherent scatter power profiles in the thermosphere using the Arecibo radar in Puerto Rico (18.3°N, 293.25°E). These waves detected by the radio-sounding techniques may correspond to the waves reported in this paper. Simultaneous measurements by optical and radio techniques would be needed to fully characterize the gravity waves in the thermosphere.

[32] If we assume that the observed southward moving structures are acoustic gravity waves, they would follow the linear dispersion relation of the gravity wave [Hines, 1960], i.e.,

  • equation image

where m (=2π/λz), N, u, c, k (=2π/λh), and H are the vertical wave number, Brunt-Väisälä frequency, background wind velocity, apparent wave phase velocity, horizontal wave number, and scale height, respectively. In the thermosphere at 200–300 km, the scale height is 30–50 km. The Brunt-Väisälä frequency, which is given as N2 = 2g/5H (g: acceleration of gravity), is 8–11 × 10−3 rad/s. For typical values of the observed waves of λh = 700 km and c = 300 m/s and assuming u = 0 m/s, the vertical wavelength λz is estimated to be 200–250 km for these scale heights. λz becomes 160–200 km in case of a background southward wind u = 50 m/s. Since the thickness of the 630-nm emission layer is ∼100 km, these values of vertical wavelength are large enough to produce the observed variation in 630-nm airglow intensity by causing vertical motion of the F layer plasma. The molecular viscosity, which is not included in derivation of equation (4), may increase the actual vertical wavelength in the thermosphere.

[33] By neglecting the small term k2 in equation (4), the condition m2 > 0 gives ∣u c∣ < 2NH. The value of 2NH is less than 300 m/s throughout most of the atmosphere below 100 km. Thus the observed waves with c = 300 m/s may be evanescent or ducted in the lower atmosphere. This consideration implies two possibilities. One is that the wave may be ducted below the mesopause with some energy leaking to higher altitudes [e.g., Francis, 1973], in which case they can travel large distances horizontally between the source and the F region observation location. The other possibility is that the wave may be generated above the mesopause. One such example is the secondary waves generated in the mesopause region by dissipation of small-scale gravity waves [e.g., Vadas et al., 2003].

[34] The gravity wave scenario has, however, several unclear points that require explanation. One is that the waves almost always move southward. Northward moving waves are observed on only two nights during the 2 years of observation. From the airglow imaging measurement of the gravity waves in the mesopause region (altitude of 80–100 km), the propagation directions of small-scale gravity waves vary depending on the season due to the filtering effect of the background wind [e.g., Nakamura et al., 2001; Ejiri et al., 2003; Suzuki et al., 2004]. However, the phase velocity of the waves in this paper is ∼310 m/s, which is much faster than the background wind velocity from the troposphere to the thermosphere. Thus the wind-filtering effect does not work for the present case. The propagation direction of the gravity waves would depend only on the relative location of the gravity wave sources and the observation point.

[35] Recently, Nakamura et al. [2003] reported that small-scale (<100 km) gravity waves in the mesopause region observed by an airglow imager at the Tanjungsari observatory (6.9°S, 107.9°E), Indonesia, always move southward throughout the year. By comparing satellite cloud images, they concluded that this is because the cloud activity in the troposphere is always north of Tanjungsari due to orographic turbulences in the Sumatra Island. The airglow imager at Kototabang (0.2°S, 100.3°E) is located ∼700 km north of Tanjungsari, where the clouds are almost always in the field of view. Thus these clouds above the Sumatra Island would not be the source of the observed southward moving waves. It is known that strong tropospheric convection caused by the Asian monsoon exists north of Indonesia during Northern Hemispheric summer season. This is a possible source of the observed gravity waves, though some other source would be needed during the Northern Hemispheric winter season.

[36] The other challenge to the gravity wave scenario is that the southward moving waves are observed only in and south (geomagnetically poleward) of the equatorial ionospheric anomaly, and are not observed north (equatorward) of the anomaly. This observation may suggest a relation between the southward moving waves and the equatorial anomaly, which is the plasma structure in the ionosphere rather than the structure of the neutral atmosphere. However, this observation is probably due to the height variation of the F layer in the vicinity of the anomaly. The F layer height equatorward of the anomaly is very high (often above 500 km), particularly at premidnight local time, due to the upward drift of the plasma at the sunset terminator [e.g., Bilitza, 1990; Anderson and Roble, 1981]. Thus in the equatorward side of the anomaly the 630-nm airglow comes from higher altitudes. If the gravity waves that cause the observed wave structures in the 630-nm images are confined in the lower thermosphere, the waves would not be observed in the equatorward side of the anomaly. The observed high occurrence of the southward moving waves in May–July (Figure 10a) may partly be related to the observed early shift of the anomaly to lower latitudes in May–July (Figure 11).

[37] Recently, Vadas and Fritts [2005] derived a gravity wave anelastic dispersion relation that included molecular viscosity and thermal diffusivity. Because molecular viscosity and thermal diffusivity increase rapidly in the thermosphere, and are the primary dissipative mechanism for high-frequency gravity waves there, this dispersion relation enabled the determination of gravity wave dissipation altitudes within the thermosphere via ray tracing. As shown by S. L. Vadas and D. C. Fritts (The influence of solar variability on gravity wave structure and dissipation in the thermosphere, submitted to Journal Geophysical Research, 2005), the dissipation altitude of a gravity wave depends sensitively on its intrinsic frequency and horizontal and vertical scales, as well as on the temperature of the thermosphere. Hot thermospheres enable deeper penetration than cool thermospheres, and gravity waves with larger vertical wavelengths penetrate to higher altitudes than those with smaller vertical wavelengths. For the typical parameters of the observed waves with a period of 40 min and a horizontal wavelength of 700 km, the waves are dissipated at altitudes of 200–340 km for thermospheric temperatures of 1000–2000 K (S. Vadas, private communication, 2005). This estimation is consistent to the fact that the waves are not observed at equatorward of the anomaly.

[38] The scenario of MBW generation from the MTM is that the pressure maximum, which may correspond to the maximum airglow intensity, launches waves away from itself. This scenario may connect the enhanced airglow region and wave generation. For the present case, however, this scenario would not be applicable, because the southward moving waves are also observed just north (equatorward) of the airglow intensity peak, as shown in the example of Figure 2.

[39] Finally, we note that waves with similar periods (0.5–1.5 hours) and smaller wavelengths (100–300 km) are observed at midlatitudes (20–40° MLAT) as medium-scale traveling ionospheric disturbances (MSTIDs) [e.g., Garcia et al., 2000; Kubota et al., 2000; Shiokawa et al., 2003b]. They always propagate equatorward and westward and have a peak occurrence of more than 50% in the Northern Hemispheric summer season. If the southward moving waves are a common feature in the equatorial thermosphere at all longitudes, there must be some boundary latitude of the southward (poleward) moving waves in the equatorial latitudes and the equatorward moving MSTIDs at midlatitudes. Shiokawa et al. [2002] suggested the equatorward limit of the midlatitude MSTID propagation to be ∼18°MLAT on the basis of airglow imaging observations at a southern island of Japan. Otsuka et al. [2004] and Shiokawa et al. [2005] have shown, based on simultaneous airglow imaging observations in Japan and Australia, that the midlatitude MSTIDs have geomagnetically conjugate structures, indicating that they are generated by an oscillating electric field in the ionosphere. Similar conjugate measurements of airglow imaging may help to identify whether the electric field plays some role in the generation of the southward moving waves reported here.

4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observation
  5. 3. Summary and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[40] In 2 years of 630-nm airglow observations at Kototabang (−10.4°MLAT), Indonesia, we found quasiperiodic southward moving waves, which are commonly seen in the images. The observed features of the waves can be summarized as follows:

[41] 1. The waves have predominantly east-west phase fronts moving southward. In a few cases, the phase front is tilted to ESE-WNW.

[42] 2. The average and standard deviation of the velocity and period of the waves are 310 ± 110 m/s and 40 ± 15 min, respectively, giving a north-south wavelength of ∼700 km.

[43] 3. The amplitude of the waves is typically 10–20% in the 630-nm airglow intensity.

[44] 4. The waves are frequently observed in May–July with a high occurrence rate of 53%. In other seasons the rate is ∼20%. The rate decreases from premidnight to postmidnight. The waves occur on both geomagnetically quiet and active nights.

[45] 5. The waves are observed in and south (geomagnetically poleward) of the enhanced airglow band, which probably corresponds to the equatorial ionospheric anomaly, and are not observed north (equatorward) of the anomaly.

[46] The east-west phase front and the westward tilt of the front suggest the generation of the waves by gravity waves in the thermosphere. Fact 5 (relation to the anomaly) is probably because in the equatorward of the anomaly, the F layer height becomes very high and the waves are confined in the lower thermosphere below ∼300 km, because of gravity wave dissipations by molecular viscosity and thermal diffusivity. More investigations are needed to explain the systematic southward motion and the periodicity of the observed waves.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observation
  5. 3. Summary and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[47] We thank Y. Katoh, M. Satoh, and T. Katoh of the Solar-Terrestrial Environment Laboratory, Nagoya University, for their kind support of the development and operation of the all-sky imager. The observation at Kototabang was carried out in collaboration with the Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Japan, and the National Institute of Aeronautical and Space Science (LAPAN), Indonesia. K.S. is grateful to S. Vadas for providing a detailed estimation of gravity wave dissipation altitudes in the thermosphere. This work was supported by Grant-in-Aid for Scientific Research (13573006) and on Priority Area (764) and by the 21th Century COE Program (Dynamics of the Sun-Earth-Life Interactive System, G-4) of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

[48] Arthur Richmond thanks the reviewers for their assistance in evaluating this paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observation
  5. 3. Summary and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References