We report for the first time simultaneous observations of medium-scale traveling ionospheric disturbances (MSTIDs) at geomagnetic conjugate points in both hemispheres, using two all-sky airglow imagers at midlatitudes. A 630-nm all-sky CCD imager at Sata, Japan, detected MSTIDs with a wavefront elongated from NW to SE on the night of August 9, 2002. During this event, MSTIDs with a wavefront elongated from SW to NE were observed at the geomagnetic conjugate point, Darwin, Australia. To investigate geomagnetic conjugacy of the MSTID structures, the Darwin images were mapped The MSTID structures mapped from Darwin to its magnetic conjugate points along the geomagnetic field lines (B) coincide closely with those in the Sata images. This result suggests that polarization electric field (Ep) plays an important role in the generation of MSTIDs. Ep maps along B and moves the F region plasma upward or downward by E × B drifts, causing plasma density perturbations with structures mirrored in the northern and southern hemispheres.
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 Wave-like perturbations of the F-region plasma, called the Traveling Ionospheric Disturbance (TID), have been observed by various techniques since the 1950s (see Hunsucker  for a review). Since 1960s, Medium-Scale TIDs (MSTIDs), which have horizontal wavelengths of several hundred kilometers, have been thought to be signatures of atmospheric gravity waves that propagate upward from the lower atmosphere, or are created in conjunction with auroral activity [Hines, 1960; Hooke, 1968; Hunsucker, 1982]. Recent observations of airglow images using highly-sensitive cooled-CCD cameras have revealed propagation features of MSTIDs [e.g., Mendillo et al., 1997; Kubota et al., 2000]. Most of MSTIDs observed at night over Japan propagate southwestward in all seasons [Shiokawa et al., 2003a]. This preferred propagation direction cannot be explained by the classical theory of gravity waves [Miller et al., 1997; Kelley and Miller, 1997]. An electrical force related to electric field perturbation, which could be caused by a plasma instability, is another candidate for source of MSTIDs [Perkins, 1973; Saito et al., 1998; Kelley and Makela, 2001; Shiokawa et al., 2003b]. An electric field can easily be transmitted along geomagnetic field (B) from one hemisphere to the other without attenuation, because of high electrical conductivity parallel to B, and would force the ionospheric plasma in both hemispheres to move in directions perpendicular to B. Therefore, if electric fields are involved in the generation of MSTIDs, the spatial and temporal structures of MSTID are expected to be mirrored in both hemispheres. To test the electric-field hypothesis, we have used two all-sky airglow imagers one at Sata, Japan, in the northern hemisphere and the other at Darwin, Australia, in the southern hemisphere. These two sites are nearly connected by geomagnetic field lines.
 All-sky airglow 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 optically to enable determination of absolute intensity, in units of Rayleigh [Shiokawa et al., 2000]. Two-dimensional images of 630-nm airglow intensity were obtained every 5.5 min and 6 min at Sata and Darwin, respectively.
 The geomagnetic conjugate point of Darwin (28.8°N, 131.3°E) is located 250 km southeast of Sata. Figure 1 shows location of observational sites and field-of-view (FOV) of the imagers for the 630-nm airglow observations. Since FOV of the imager covers an area of about 1,000 km in diameter, the conjugate area covered by the Darwin imager FOV, overlaps largely that of the Sata imager FOV. This setup allowed us to investigate the geomagnetic conjugacy of airglow structures [Otsuka et al., 2002].
 Simultaneous MSTID images of oxygen airglow (630 nm) were obtained on the night of August 9, 2002. Geomagnetic conditions were quiet on this day (Kp = 0 ∼ 1+). Figure 2a shows the MSTID images of 630-nm airglow obtained at Sata for 1448–1518 UT (2348–2418 LT) on August 9, 2002. The original all-sky images were converted into geographical coordinates by assuming that the airglow layer existed at 250 km altitude. The absolute intensity in units of Rayleigh was obtained using the sensitivity data of the imager and by subtracting the contamination from the background continuum emission [Shiokawa et al., 2000]. The emission intensity was about 80 and 70 Rayleighs at Sata and Darwin, respectively. To investigate perturbations in airglow intensity, percentage of airglow intensity deviations from 1-hour average to the background is shown in the images of Figure 2. The MSTIDs appear as several band-like structures of low and high airglow intensities with a horizontal wavelength of about 200 km. The structure can be seen to be elongated from northwest to southeast, and to have moved toward the southwest at an azimuth of 210° from the north, at 80 m/s. During this event, MSTIDs with a wavefront elongated from southwest to northeast was observed at the geomagnetic conjugate point, Darwin (Figure 2b). The MSTIDs propagated northwestward at the almost same velocity as the MSTIDs observed at Sata. Amplitudes of the airglow intensity perturbations were approximately 20% and 40% of the background at Sata and Darwin, respectively.
 However, the sky over Sata was hazy. This probably causes relatively large errors in estimation of the absolute airglow intensity over Sata. For this reason, in the present paper, we do not discuss amplitudes of the airglow perturbations and focus on comparison of the spatial structures of the airglow perturbations between the Sata and Darwin images.
 In order to investigate geomagnetic conjugacy of the MSTID structures between the northern and southern hemispheres, we mapped the Darwin images shown in Figure 2b into the northern hemisphere along the geomagnetic field lines using the International Geomagnetic Reference Field 2000 model (IGRF2000) [Mandea and Macmillan, 2000]. The eastern half of the mapped images was superimposed on the corresponding Sata images (see Figure 2c). Two band-like structures of high and low airglow intensities in the left-half of the image can be seen to be smoothly connected to those in the right-half side of the image. It is, therefore, apparent that the airglow structures over Sata coincide closely with those mapped onto the northern hemisphere from Darwin along B. The good correspondence in structural features between the two hemispheres can be seen in all MSTID images for 1448–1520 UT. This result clearly indicates that the 630-nm airglow structures associated with MSTIDs are mapped along B and mirrored between the northern and southern hemispheres. Airglow images were not obtained at Sata before and after the period shown in Figure 2, because the sky was cloudy. We, therefore, do not know whether the MSTID started and ceased simultaneously in both hemispheres, or not.
Figure 3 shows the neutral wind velocity in the thermosphere measured with a Fabry-Perot Interferometer (FPI) at Shigaraki (34.9°N, 134.8°E; ∼500 km northeast from Sata) on the night of August 9, 2002. The FPI measures the wind velocities for the first (solid curve) and second (dashed curve) fringes independently. Details of the FPI measurements are described by Shiokawa et al. [2003c]. The differences of the wind velocities between the two fringes are less than 50 m/s, indicating an accuracy of the wind measurement. The wind was directed almost southward with a velocity of 130 m/s at around 15 UT when the MSTIDs were observed at Sata and Darwin.
 The MSTIDs were observed simultaneously at geomagnetic conjugate points in the northern and southern hemispheres. The observed MSTID structures were mirrored in both hemispheres connected by geomagnetic field B. This mirrored conjugacy of the MSTID structures can be explained by an idea that MSTIDs are generated through electric field variations. Since 630-nm airglow intensity is almost proportional to field-line-integrated Pedersen conductivity (ΣP) in the F region, airglow structures represent spatial variations of ΣP [Kelley et al., 2000; Makela and Kelley, 2003].
 The field-line integrated ionospheric current J is given by ΣP(E + U × B), where E is electric field and U is neutral wind velocity. During this event, U measured with a Fabry-Perot airglow interferometer at Shigaraki was directed southward with a velocity of 130 m/s (Figure 3). This indicated that the electric current driven by thermospheric neutral wind U × B flowed eastward. E was not measured during this event. However, E is expected to be smaller than and anti-parallel to U × B because E is generally generated by the F-region dynamo mechanism [Rishbeth, 1971]. Consequently, J probably flows to the almost same direction as U × B. Since J traverses the perturbations of ΣP (wavefront of MSTIDs over Sata), polarization electric fields (Ep) should be generated to maintain a divergence-free of ionospheric currents. For this reason, Ep should be perpendicular to the wavefronts of MSTIDs, and northeastward (southwestward) in the regions of low (high) airglow intensity.
Shiokawa et al. [2003b] have presented that electric field perturbations, measured by a satellite, have been associated with MSTID structures in airglow images and that the direction of the electric fields is consistent with that of the polarization electric fields which are expected to be generated by the spatial perturbations of ΣP (inferred from the airglow variation). In the northern hemisphere, the eastward component of Ep in the low airglow-intensity regions, causes northward and upward plasma drift which transports the F region plasma to a higher altitude. This causes further reduction of the airglow intensity because the airglow emission rate is proportional to the product of the O2 and O+ densities.
Ep generated in one hemisphere is transmitted along B to the other hemisphere. Saito et al.  estimated the transmission efficiency of E along B at 25° invariant latitude from the parallel and Pedersen conductivities. They found that E 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 E with larger scale sizes is transmitted almost without attenuation. Therefore, in the present event, the northeastward (southwestward) Ep in the low (high) airglow intensity regions in the northern hemisphere is transmitted to the conjugate ionosphere. The transmitted Ep moves the plasma upward (downward) by E × B drifts and causes decrease (increase) in the airglow intensity. Such a scenario explains the appearance of mirrored structures of the airglow perturbations in the two hemispheres connected by B.
 The observed structures of airglow perturbations may develop as consequence of an ionospheric instability that acts through electrical processes involving E. The Perkins instability is one of possible mechanisms [Perkins, 1973]. This mechanism explains wave vector direction of the observed MSTIDs, while gravity wave theories do not. According to the theory of this instability, the perturbations in both ΣP (airglow) and E grow with time under the condition that the wave vector of ΣP perturbations lies between the direction of J and the eastward direction. In the present event, the geometry is not suitable for the Perkins instability, because J is expected to flow eastward, as described above. In the time just before the observations shown in Figure 2, when the MSTISs were probably formed, the wind was directed to the southeast so that direction of J was expected to be northeast. This geometry can drive the Perkins instability.
 In reported features of the Perkins inability, the F region in only one hemisphere is considered [Perkins, 1973; Hamza, 1999]. The mirrored conjugacy of the MSTID structures shown in the present paper suggests that the interaction between the F regions in both hemispheres needs to be included in the theories. Numerical simulations of the Perkins instability including effect of the conjugate F region show that the coupling with the conjugate ionosphere is important in the evolution of a MSTIDs and that neutral wind velocity in the both hemispheres largely affect the growth rate of the instability [Saito et al., 1998]. However, we have no observations of the neutral wind in the southern hemisphere for the present event. Further observations including neutral winds and electric fields in the both hemispheres, and a new theory that includes the electrical coupling between the two hemispheres, as shown in this paper, are needed to explain generation mechanisms of MSTIDs.
 We have conducted geomagnetic conjugate observations of 630-nm airglow at Sata, Japan, and Darwin, Australia, with two all-sky CCD imagers. The airglow perturbations caused by MSTIDs were simultaneously observed at both sites at around midnight of August 9, 2002. The observed MSTID structures were mirrored in the northern and southern hemispheres connected by geomagnetic field lines. This result indicates that polarization electric fields play an important role in generation of MSTIDs. The polarization electric fields are expected to be generated to maintain a divergence-free of electric currents in the F region where Pedersen conductivity is perturbed by the MSTIDs. The electric fields map along the geomagnetic field lines and move the F region plasma upward or downward by E × B drifts, causing perturbations of the plasma density. Such a scenario explains mirrored structures of the MSTIDs in hemispheres. Direction of the electric current expected from thermospheric neutral wind simultaneously measured in the northern hemisphere is consistent with this scenario. However, further observations of neutral winds in the conjugate hemisphere will be needed to clarify generation mechanisms of the MSTIDs.
 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 R. Tsunoda for his helpful comments on the present work. This work is supported by Grant-in-Aid for Scientific Research (11440145, 13573006) and on Priority Area (764) of the Ministry of Education, Culture, Sports, Science and Technology of Japan.