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著者人数: 28名, Accepted: 2018-12-05, 資料番号: SA1180259000

We describe performance of a new two-channel Fabry-Perot interferometer (FPI) with two thermoelectric-cooled CCD detectors to measure neutral winds for both the 558-nm (mesopause region) and 630-nm (thermosphere) airglow emissions simultaneously at Shigaraki (34.8°N, 136.1°E), Japan. The employment of the thermoelectric cooling system enables us continuous automatic operation of the FPI since October 2000. The random errors of wind measurement are estimated to be ~5–8 m/s and ~10–50 m/s for 558 nm and 630 nm, respectively. The wind velocities obtained by the FPI (558 and 630 nm) fit well with those obtained by the collocated Middle and Upper (MU) atmosphere radar.

We have developed three airglow temperature photometers with cooled-CCD detectors. The photometers measure rotational temperatures using the airglow emissions of OH and O2 near the mesopause region (altitude: 80s-100 km). The photometers also measure six other airglow and auroral lines at wavelengths of 557.7, 630.0, 777.4, 589.3, 427.8, and 486.1 nm. The CCD detectors are used to distinguish the emission lines in these airglow bands, similarly to those used by the Spectral Airglow Temperature Imagers (SATI). In this paper, we describe the configuration of the photometers, their calibration, the data processing to extract rotational temperatures and emission intensities from the measured airglow spectra, as well as the initial deployment at Platteville, Colorado (40.2°N, 255.3°E), when their observations were compared with the concurrent and nearly collocated observations by a sodium lidar. We obtain a good correlation and some systematic difference of temperatures from the photometers and the lidar, and discuss possible causes of the temperature difference.

A strong earthquake with a magnitude of 9.0 occurred at 1446:23 JST on March 11, 2011, in Japan. Ionospheric disturbances were detected at 1500 JST at four ionosonde stations. An irregular distortion of echo trace was observed at Kokubunji, which is the nearest station to the epicenter and is 440 km from it. Multiple-cusp-type trace indicating extra stratification was observed at Wakkanai and Yamagawa, which are 870 and 1410 km away from the epicenter. A small wavy fluctuation was observed at Okinawa 1910 km away from the epicenter. The real height analysis of the ionograms showed a vertical structure with a scale size of 20~30 km.

We report, for the first time, continuous observations of the nighttime F-region field-aligned irregularities (FAIs) over Indonesia. A VHF radar with operating frequency of 30.8 MHz and peak power of 20 kW has been operated at Kototabang (0.20°S, 100.32°E; dip latitude 10.4°S), Indonesia since February 2006. Five beams were allocated between ±54° in azimuth around geographic south (126°–234°). From the continuous observation from February 2006 to November 2007, we found that FAIs appeared frequently at pre-midnight between March and May and at post-midnight between May and August. The pre-midnight FAIs coincided well with GPS scintillation observed at the same site. Seasonal and local time variations of the pre-midnight FAI occurrence are consistent with those of equatorial plasma bubbles reported in previous studies (e.g., Maruyama and Matuura, 1984). These results indicate that the pre-midnight FAIs could be associated with the equatorial plasma bubbles. On the other hand, seasonal and local time variations of the post-midnight FAIs were inconsistent with those of the plasma bubbles. The features of the post-midnight FAIs can be summarized as follows: (1) The post-midnight FAIs are not accompanied by GPS scintillations. (2) Most of the post-midnight FAI regions do not show propagation, but some of them propagate westward. (3) Echo intensity of the post-midnight FAIs was weaker than that of the pre-midnight FAIs. These features are similar to those of the FAI echoes that have been observed at mid-latitude (e.g., Fukao et al., 1991). At Kototabang, Fukao et al. (2004) have firstly observed FAIs that resemble those at mid-latitude. The present paper reports statistical characteristics of the mid-latitude-type FAIs observed at Kototabang.

Two-dimensional structures of the ionospheric variations generated by the acoustic resonance between the ground surface and the lower thermosphere was observed for the first time near the epicenter after the M 9.0 Tohoku earthquake on March 11, 2011. A short-period oscillation of total electron content was observed by a GPS receiver array after the earthquake for four hours in the vicinity of the epicenter. It was centered in the east of the epicenter where the tsunami was estimated to commence. The frequency of the dominant mode of the oscillation was 4.5 mHz, 222 seconds of period, while there were minor oscillations whose frequency were 3.7 mHz and 5.3 mHz. These periods are consistent with the periods of the acoustic resonance between the ground surface and the lower thermosphere, predicted by a numerical model. The amplitude of the TEC oscillation showed a gradual change of the amplitude. The two-dimensional distributions of TEC variations generated by this resonance had wave frontal structures that extended from northwest to southeast. The resonant oscillation of the TEC was accompanied by a depletion of TEC whose duration was about 60 minutes. The area of this depletion also centered on the epicenter.

Using global positioning system (GPS) data taken from 350 dual-frequency GPS receivers in Southern California in 2002, we investigated two-dimensional maps of total electron content (TEC) perturbations with a time resolution of 30 s and a spatial resolution of 0.15°×0.15° in longitude and latitude to reveal statistical characteristics of medium-scale traveling ionospheric disturbances (MSTIDs). We found that MSTIDs can be categorized into three types. One type is daytime MSTIDs, which frequently occur in winter and equinoxes. Since most of the daytime MSTIDs propagated southeastward, we speculate that the daytime MSTIDs could be caused by atmospheric gravity waves in the thermosphere. A second type is nighttime MSTIDs, which frequently occur in summer. Nighttime MSTIDs propagate southwestward. This propagation direction is consistent with the idea that polarization electric fields could play an important role in generating nighttime MSTIDs. The third is dusk MSTIDs, which frequently occur in summer and propagate northwestward. Dusk MSTIDs could be caused by gravity waves originating from the sunset terminator because they have wavefronts almost parallel to the sunset terminator.

Ionospheric responses to the 2011 off the Pacific coast of Tohoku Earthquake are studied using the SuperDARN Hokkaido radar, which is located at (43.5°N, 143.6°E) and which monitors the ionosphere over a wide horizontal area. The radar observed an oscillation of the vertical motion of the ionosphere with a period of about 1 to 2 min. The disturbance propagated northward, away from the epicenter with the velocity of about 6.2, 4.5, 3.9 and 3.5 km/s. The latter three values are basically consistent with the propagation of the Earth’s surface waves reported in several previous studies. The propagation velocities decreased with time, which has not been reported in previous studies for this propagation velocity range. The peak-to-peak amplitudes of Doppler velocities of ground/sea scatter echoes observed by the radar were up to 200 m/s, which is considerably larger than previously-reported values using HF Doppler measurements, although they are not extremely large for this historical earthquake (M = 9.0). This is the first time that ionospheric data have been obtained with high temporal (8 s) and spatial (22.5 km) resolutions following a giant earthquake, which enables us to discuss the detailed characteristics of the propagation of coseismic ionospheric disturbances.

We report procedures to identify small-scale (20–100 km) atmospheric gravity waves from OH airglow images to estimate momentum fluxes carried by the waves. We also deduce contamination of background continuum emission in OH image, by comparing a simultaneous observation of OH lines measured by the Spectral Airglow Temperature Imager (SATI). We applied the procedures to a one-night dataset obtained at Shigaraki, Japan (34.9°N, 136.1°E) on November 19, 1999. The background wind, which is essential for deriving the intrinsic parameters of gravity waves, was measured by the Middle and Upper Atmosphere (MU) radar. Contamination of background continuum emission with the OH filter was deduced to be 30%. From these procedures, we found that the gravity waves identified in the OH images were mainly propagating southward or southeastward with horizontal wavelengths of 60–90 km and apparent phase speeds of 40–80 m/s. The estimated momentum fluxes on this night was 1–15 m2 s−2, with an average of 4.9 m2 s−2.

The Solar-Terrestrial Environment Laboratory (STEL) induction magnetometer network has been developed to investigate the propagation characteristics of high-frequency geomagnetic pulsations in the Pc1 frequency range (0.2–5 Hz). Five induction magnetometers were installed in the period 2005–2008 at Athabasca in Canada, Magadan and Paratunka in Far East Russia, and Moshiri and Sata in Japan. The sensitivity of these magnetometers is between 0.3 and 13 V/nT at turnover frequencies of 1.7–5.5 Hz. GPS time pulses are used for accurate triggering of the 64-Hz data sampling. We show examples of PiB/Pc1 magnetic pulsations observed at these five stations, as well as the harmonic structure of ionospheric Alfven resonators observed at Moshiri. We found that the Pc1 packets are slightly modulated as they propagate from high to low latitudes in the ionospheric duct. These network observations are expected to contribute to our understanding of Pc1-range magnetic pulsations and their interaction with relativistic electrons by combining the obtained results with future satellite missions that observe radiation belt particles.

In a previous work (J. Geophys. Res., 110(A01308), 1–11, 2005), the authors developed an original approach to the processing of total electron content (TEC) data obtained by GPS signals from the Japan receiver network. This approach includes removing the diurnal and seasonal variation carried by 27-day medians and the solar rotation periodicity. The relative deviations of TEC from the median—from all measured locations at a given hour—were then approximated by a regression line along the main prolongation of the Japan islands, between latitudes 24° and 45°N. The two variables of the regression line, the average value at the center and the slope were obtained as a time series, and their behavior during geomagnetic storms in the period 2000–2002 were analyzed. One interesting result was the observed enhancement of TEC at the end of the recovery phase of the storms. The slope variations clearly showed that this enhancement started from the south and was interpreted as a poleward expansion of equatorial crest. In the present paper we further analyze this post-storm phenomenon, adding foF2 data from Japanese Kokubunji and Okinawa ionosondes. We also show the latitude extension of the poleward expansion by using lat/UT contour plots. The results confirm that most of the post-storm TEC enhancements are part of the equatorial crest region which extends poleward during nighttime. In some cases, the enhanced TEC structures develop by separating from the crest region. Daytime TEC enhancements were also observed. Their structures are not confined to the equatorial crests region, but occupy the whole latitude range considered in this study. TEC post-storm enhancements were generally found to be in agreement with foF2 variations.

This paper reports the first attempt to observe the equatorward limit of medium-scale traveling ionospheric disturbances (TIDs) in the middle latitudes. The TIDs usually propagate southwestward in the northern hemisphere. An all-sky cooled-CCD imager measured 630-nm airglow at a southern island of Japan, Okinawa (26.9°N, 128.3°E, geomagnetic latitude (MLAT) = 17.0°), during the FRONT-2 campaign of August 4–15, 1999. The TIDs were detected at the mainland of Japan (∼21°–36° MLAT) by the total electron content (TEC) observations of more than 1000 GPS receivers. In the August 4 event, the TIDs moving southwestward was seen only in the northern sky of Okinawa as a depletion band in the 630-nm airglow images. In the August 6 event, the TIDs were not seen in the 630-nm images at Okinawa, although weak TID activity was observed by the GPS network at the mainland of Japan. The TEC data also showed weakening of the TID activity below 18° MLAT. Based on these observations, we suggest that there is a possible limit of medium-scale TID propagation around ∼18° MLAT.

We report the response of the ionosphere to the large earthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004. We have analyzed Global Positioning System (GPS) data obtained at two sites in Sumatra and at three sites in Thailand to investigate total electron content (TEC) variations. Between 14 and 40 min after the earthquake, TEC enhancements of 1.6–6.9 TEC units (TECU) were observed at subionospheric points located 360–2000 km north of the epicenter. From the time delays of the observed TEC enhancements, we find that the TEC enhancements propagated northward from the epicenter. The time delays between the earthquake and rapid increases in TEC, which occurred near the epicenter, are consistent with the idea that acoustic waves generated by the earthquake propagated into the ionosphere at the speed of sound to cause the TEC variations. A small TEC enhancement of 0.6 TECU was observed south of the epicenter, while no TEC enhancements were seen east of the epicenter. From a model calculation, we find that this directivity of the TEC variations with respect to the azimuth from the epicenter could be caused partially by the directivity in the response of the electron density variation to the acoustic waves in the neutral atmosphere.

We compare, for the first time, geomagnetically-conjugate plasma bubbles observed by ground-based OI 630.0-nm all-sky imagers at Shigaraki, Japan (34.8°N, 136.1°E; magnetic latitude 25.4°N) and Darwin, Australia (12.4°S, 131.0°E; magnetic latitude 22.0°S), with global-scale plasma structures (≈10,000 km in longitude) in the equatorial anomaly simultaneously detected with an OI 135.6-nm imager on the IMAGE satellite at ≈7 earth radii. As found previously, global-scale plasma structures in both hemisphere imaged by IMAGE consist of an array of geomagnetically-conjugate small- to medium-scale (a few hundreds to 1000 km in longitude) wavy structures that move to the east at ≈100 ms−1. We find the following: 1) plasma bubbles detected with the allsky imagers reach an apex altitude of ≈1800 km over the geomagnetic equator while moving to the east at ≈100 m s−1 with spacings of 200–250 km. 2) Bubbles observed with the all-sky imagers and IMAGE are embedded within the small- to medium-scale wavy structures, and some of them are located near the crest of an enhanced electron density region associated with the wavy structures. 3) The bubbles and wavy structures that are generated near sunset slant to the west with increasing latitude in both hemispheres, and tilts do not change with longitude (i.e., local time). The results suggest that the generation and evolution of plasma bubbles are closely related to those of the small- to medium-scale plasma structures.

We report the first observation of the disappearance of a plasma bubble over geomagnetically conjugate points. It was observed by airglow imagers at Darwin, Australia (magnetic latitude: −22°N) and Sata, Japan (21°N) on 8 August 2002. The plasma bubble was observed in 630-nm airglow images from 1530 (0030 LT) to 1800 UT (0300 LT) and disappeared equatorward at 1800 to 1900 UT (0300 to 0400 LT) in the field of view. The ionograms at Darwin and Yamagawa (20 km north of Sata) show strong spread-F signatures at approximately 16 to 21 UT. At Darwin, the F-layer virtual height suddenly increased from approximately 200 to approximately 260 km at the time of bubble disappearance. However, a similar F-layer height increase was not observed over the conjugate point at Yamagawa, indicating that this F-layer rise was caused not by an eastward electric field but by enhancement of the equatorward thermospheric wind over Darwin. We think that this enhancement of the equatorward neutral wind was caused by an equatorward-propagating large-scale traveling ionospheric disturbance, which was identified in the north-south keogram of 630-nm airglow images. We speculate that polarization electric field associated with this equatorward neutral wind drive plasma drift across the magnetic field line to cause the observed bubble disappearance.

We investigated the effects of the F region bottomside altitude (h’F), maximum upward E × B drift velocity, duration of pre-reversal enhancement and the integral of upward E × B drift on the latitudinal extension of equatorial plasma bubbles in the Southeast Asian sector using the observations recorded by three GPS receivers and two ionosondes. The GPS receivers are installed at Kototabang (0.2°S, 100.3°E; 9.9°S magnetic latitude), Pontianak (0.02°S, 109.3°E; 9.8°S magnetic latitude) and Bandung (6.9°S, 107.6°E; 16.7°S magnetic latitude) in Indonesia. The ionosondes are installed at magnetically equatorial stations, Chumphon (10.7°N, 99.4°E; 0.86°N magnetic latitude) in Thailand and Bac Lieu (9.3°N, 105.7°E; 0.62°N magnetic latitude) in Vietnam. We analysed those observations acquired in the equinoctial months (March, April, September and October) in 2010–2012, when the solar activity index F 10.7 was in the range from 75 to 150. Assuming that plasma bubbles are the major source of scintillations, the latitudinal extension of the bubbles was determined according to the S4 index. We have found that the peak of h’F, maximum upward E × B drift and the integral of upward E × B drift during the pre-reversal enhancement period are positively correlated with the maximum latitude extension of plasma bubbles, but that duration of pre-reversal enhancement does not show correlation. The plasma bubbles reached magnetic latitudes of 10°–20° in the following conditions: (1) the peak value of h’F is greater than 250–450 km, (2) the maximum upward E × B drift is greater than 10–70 m/s and (3) the integral of upward E × B drift is greater than 50–250 m/s. These results suggest that the latitudinal extension of plasma bubbles is controlled mainly by the magnitude of pre-reversal enhancement and the peak value of h’F at the initial phase of development of plasma bubbles (or equatorial spread F) rather than by the duration of pre-reversal enhancement.