We report for the first time large-scale equatorial F-region airglow depletions extending to low-midlatitudes in both hemispheres. The observational sites were located at low-midlatitude geomagnetic conjugate points. Clear depletions of 630.0-nm airglow intensity due to equatorial plasma bubbles were simultaneously observed with two all-sky imagers at Sata, Japan (magnetic latitude 24°N), and its geomagnetic conjugate point, Darwin, Australia (magnetic latitude 22°S), on the night of November 12, 2001. Airglow depletion regions with east-west scale sizes of 40–100 km extend poleward. The maximum apex altitude of the plasma bubbles is about 1,700 km over the geomagnetic equator. The depletions move eastward at about 100 m/s, without changing their structures. The Darwin depletion structures mapped onto the northern hemisphere along the geomagnetic field coincide closely with structures in the Sata images, even for the 40-km structure. These observations indicate that plasma depletions in the equatorial ionosphere elongate along the geomagnetic field lines.
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 Plasma bubbles are depletions in the equatorial F region plasma. The altitude gradient of the bottomside F region at the magnetic equator becomes steep after sunset due to the chemical recombination of lower F region plasma. An eastward electric field generated by coupling processes between E and F regions at the terminator pushes the F region plasma vertically upward [Farley et al., 1986]. Under these conditions, a perturbation at the F region bottomside plasma density profile becomes unstable by the Rayleigh-Taylor instability mechanism (see Kelley ). Bottomside F-region magnetic flux tubes with low-density plasma rise to higher altitudes to become elongated plasma depleted flux tubes.
 Nighttime airglow depletions caused by equatorial plasma bubbles have been observed with all-sky airglow imagers at equatorial regions and low latitudes [e.g., Sahai et al., 2000; Abalde et al., 2001]. The advantage of airglow imagers is their ability to observe the two-dimensional structures of plasma bubbles. At Arecibo, Puerto Rico, at mid-latitude, Mendillo et al.  observed airglow depletions caused by plasma bubbles that reached very high altitudes (>1,500 km) over the magnetic equator. These airglow observations, however, were conducted only in one hemisphere. Weber et al.  compared all-sky images of airglow depletions with simultaneous in-situ electron density measured by the AE-E satellites (approximately 430 km altitude) on the same geomagnetic field in the same and opposite hemispheres and investigated the conjugacy and structure of the plasma depletions. The current paper presents the first geomagnetic conjugate observations of The current paper presents the first simultaneous observations of 630.0-nm airglow depletions caused by equatorial plasma bubbles using two all-sky imagers installed at low-midlatitude geomagnetic conjugate points in both hemispheres. In order to investigate geomagnetic conjugacy of the depletions, the depletion structures are mapped to their magnetic conjugate points using the IGRF-95 model [Barton, 1997] which is sufficiently accurate for the purposes of this paper.
 All-sky imagers have been operated at Sata (31.0°N, 130.7°E; magnetic latitude 24°N) in Japan since July 2000 and at Darwin (12.4°S, 131.0°E; magnetic latitude 22°S) in Australia since October 2001 as part of the Optical Mesosphere Thermosphere Imagers (OMTIs) [Shiokawa et al., 1999]. The imagers were calibrated using a 2-m integrating sphere to determine absolute intensity in units of Rayleigh (R) [Shiokawa et al., 2000]. Two-dimensional images of 630.0-nm airglow intensity were obtained every 5.5 min and 6 min at Sata and Darwin, respectively. At Darwin, 777.4-nm images were also obtained every 30 min. The exposure time was 165 s for both the 630.0-nm and 777.4-nm images.
 The geomagnetic conjugate point of Darwin (28.8°N, 131.3°E) is located 250 km southeast of Sata. Since the field of view (FOV) of the imager covers an area of about 1,000 km, the conjugate points of the Darwin imager FOV mostly overlap with the Sata imager FOV. This setup allows us to investigate the geomagnetic conjugacy of airglow structures produced by plasma bubbles.
Figure 1a shows an all-sky image of the 630.0-nm airglow observed at Sata at 1544 UT (0044 LT) on November 12, 2001. Geomagnetic conditions were quiet on this day (Kp = 0 ∼ 1+). The center of the image corresponds to the location of Sata. Upward is to the north and left is to the east. Clear airglow depletions are observed in the image. The depletion regions extend from the southern edge of the image to the north, with bifurcation at several points. The depletions are the signature of the plasma bubbles rising to high altitudes over the geomagnetic equator. Figures 1b and 1c show 630.0-nm and 777.4-nm images, respectively, observed at Darwin. The time at which the two images were taken is nearly the same as that of the Sata image in Figure 1a. Note that upward is to the north and left is to the east. Clear airglow depletions can also be seen in the Darwin images. The 630.0-nm depletion structures at Darwin (Figure 1b) are similar to those at Sata (Figure 1a), indicating that the plasma depletions elongate along the geomagnetic field. The 777.4-nm depletion shifts to the north as compared to the 630.0-nm depletion. This shift is probably due to the difference in altitude of the airglow layers; the 630.0-nm layer exists at the bottomside F region (about 250 km), while the 777.4-nm emission comes from around the F region peak altitude (about 400 km).
Figure 2a shows a two-dimensional map of the absolute 630.0-nm intensity at Sata (Figure 1a) in geographical coordinates. In Figure 2b, the Darwin 630.0-nm image (Figure 1b) is mapped onto the northern hemisphere along the geomagnetic field using IGRF-95 [Barton, 1997]. The mapping area is 125.7°–135.7°E in longitude and 26.0°–36.0°N in latitude for both Figures 2a and 2b. The peak airglow layer is assumed to exist at a 250-km altitude. The center of each map corresponds to the location of Sata. In Figure 2a, the airglow intensity is enhanced to an unusual level (beyond 1,000 R) at lower latitudes. This indicates that the crest of the equatorial anomaly has expanded to higher latitudes near Sata. On the other hand, the intensity at Darwin is not enhanced so much (≤400 R), suggesting a strong north-south asymmetry of the equatorial anomaly.
 In Figures 2a and 2b, the main depletion region extends from the equatorward edge of the map (27.0°N, 131.8°E) toward higher latitudes and bifurcates at (28.4°N, 131.6°E). A small branch of the depletion around (27.5°N, 130.7°E) can also be seen in both maps. Depletion structures mapped from Darwin (Figure 2b) coincide very well with those obtained at Sata (Figure 2a). The apex altitude at the geomagnetic equator is also shown at the left of the figure. The plasma bubbles rise to a 1,700-km altitude above the equator.
 To investigate the geomagnetic conjugacy of the depletions in more detail, longitudinal cross-sections of the 630.0-nm images in Figures 2a and 2b are displayed in Figure 3. The heavy and light curves show the longitudinal variations of the 630.0-nm intensity at Sata and Darwin, respectively, along latitudes of 27.4°N (Figure 3a) and 29.5°N (Figure 3b) between 129.0° and 134.0°E. The corresponding horizontal distances are shown at the bottom of the figures. The east-west scale sizes of the depletions are 40–100 km. (Figures 3a and 3b) shows that the depletions at Sata that exist at 130.7°E and 131.7°E (130.6°E and 131.8°E) coincide fairly well with those at Darwin.
Figure 4 shows a time sequence of the Sata intensity map during 1457–1627 UT (2357–0127 LT) on the night of November 12. The depletions move eastward at about 100 m/s and the depletion structures are almost unchangeable for about 90 min during passage through the field of view.
 Our conjugate airglow observations show that plasma depletion structures with scale sizes of 40–100 km are almost identical in both hemispheres. According to the Rayleigh-Taylor instability theory, a polarization electric field is generated within the plasma depletion region to maintain current continuity. The polarization electric field is transmitted along the geomagnetic field line, which is an equipotential because of the high electrical conductivity parallel to the geomagnetic field. However, due to a finite parallel conductivity, small-scale electric fields are partially short-circuited and only large-scale electric fields are transmitted over long distances along the geomagnetic fields. Saito et al.  estimated the transmission efficiency of the electric field along the magnetic field at 25° invariant latitude. They found that the electric field with a scale size of 30 km is attenuated only 10% for transmission from the ionosphere at a 200-km altitude to the opposite hemisphere, and that electric fields with larger scale sizes are almost completely transmitted to the opposite hemispheres. The present results indicate that not only electric fields but also plasma structures with scale sizes of 40–100 km are mapped between the two hemispheres.
Huang and Kelley  have suggested that since the electric field and the movement of plasma within a wide bubble are inhomogeneous, the central part of the bubble may migrate toward the two sides of the bubble while they are moving to higher altitudes, resulting in bifurcation of the plasma bubble, as we observed.
 A plasma bubble is generated at the solar terminator in the evening. Keskinen et al. , using a magnetic-flux-tube integrated model, reported that the time-scale for a bubble to reach altitudes of 700–800 km is approximately 25–30 min. Our plasma bubbles were observed at around midnight and moved eastward, without changing their structures for more than 90 min (Figure 4). This suggests that the plasma bubbles have stopped their evolution and maintain their structures while moving toward the east by E × B drift.
 The vertical plasma drift velocity is the most important parameter for the generation and development of plasma bubbles [Fejer et al., 1999; Mendillo et al., 2001] and the growth rate of the Rayleigh-Taylor instability depends on the vertical drift (eastward electric field). A higher drift velocity lifts the plasma to higher altitudes and enhances the equatorial anomaly. In fact, the observed plasma bubbles were well developed reaching an apex of 1,700 km on November 12, and the equatorial anomaly expanded toward higher latitudes, particularly in the northern hemisphere, increasing the F region plasma density south of Sata. These facts suggest that a strong eastward electric field was generated at the solar terminator.
Figures 2a and 2b show that the 630.0-nm intensity at Sata is much stronger than that at Darwin, suggesting a highly asymmetric distribution of the F region plasma density with respect to the geomagnetic equator (asymmetry of the equatorial anomaly). The asymmetry was observed during 10–16 UT (19–01 LT). This asymmetry may be caused by transequatorial thermospheric winds [Rishbeth, 1972], namely, the northward winds push the F region plasma downward (upward) in the northern (southern) hemisphere, resulting in the higher (lower) 630.0-nm airglow intensity. On the other hand, Maruyama and Matuura  have proposed that transequatorial winds suppress the growth of plasma bubbles. The northward winds increase the field-line-integrated Pedersen conductivity because they lower the F region altitude, increasing the Pedersen conductivity in the northern hemisphere. The high Pedersen conductivity causes short-circuiting of the polarization electric field and hence suppresses the growth of the Rayleigh-Taylor instability. This wind suppression mechanism explains the seasonal-longitude patterns of the equatorial spread F occurrence [Maruyama, 1988]. Mendillo et al.  have tested the hypothesis that the wind suppression mechanism plays a critical role in the day-to-day variability of equatorial spread F. Their results, however, show no convincing evidence for the wind suppression mechanism. Our observational results of the plasma bubbles with the strong asymmetry of airglow intensity between the conjugate points show the opposite tendency to the wind suppression mechanism. Similar asymmetry was also observed during an event of plasma bubbles on October 18, 2001.
 The wind suppression mechanism may work at the onset of the plasma bubble which occurs at the solar terminator in the evening (around 19 LT). Since the plasma bubbles we observed around local midnight moved eastward at about 100 m/s, their onset would occur approximately 4 hours earlier at 1,500 km west of our observational sites. We do not have observations of the equatorial anomaly and meridional neutral wind at the onset region. However, because the ambient F region plasma moves eastward by E × B drift with almost the same velocity as the plasma bubbles [Valladares et al., 1996], we expect that the observed asymmetry of the equatorial anomaly existed at the onset region in the evening. Alternatively, the transequatorial northward wind might strengthen the asymmetry of the equatorial anomaly between the onset region (evening) and the observation part (midnight).
 We have conducted geomagnetic conjugate observations of 630.0-nm airglow at Sata, Japan, and Darwin, Australia, with all-sky imagers. Clear depletions of the airglow intensity caused by equatorial plasma bubbles were simultaneously observed at both stations at around midnight of November 12, 2001. The airglow depletions extended poleward and bifurcated at several points. Our observations suggest that the plasma bubbles rose to an altitude of about 1,700 km over the geomagnetic equator. A time sequence of the image reveals that the depletions, moving eastward at about 100 m/s, maintained their structure during their passage through the field of view of the all-sky imager (about 90 min). The depletion structures over Sata coincide closely with those mapped onto the northern hemisphere from Darwin along the geomagnetic field. This fact clearly indicates that plasma depletion regions with east-west scale sizes of 40–100 km elongate along the geomagnetic field lines in both hemispheres.
 We thank Y. Katoh, M. Satoh, and T. Katoh of the Solar-Terrestrial Environment Laboratory, Nagoya University, for their kind support of airglow imaging observations. We are grateful to N. Balan for his helpful comments on the present work. This work is supported by Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan (11440145 and 13573006).