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受賞区分：出版社・新聞社・財団等の賞  受賞国：日本国

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

To understand the mechanism of the increased frequency of intervals of pulsations of diminishing periods (IPDPs), we analyzed IPDP-type electromagnetic ion cyclotron (EMIC) waves that occurred on 19 April 2017, using ground and satellite observations. Observations by low-altitude satellites and ground-based magnetometers indicate that the increased IPDP frequency is caused by an inward (i.e., Earthward) shift of the EMIC wave source region. The EMIC wave source region moves inward along the mid-latitude trough, which we used as a proxy for the plasmapause location. A statistical analysis shows that increases in the IPDP frequency showed a positive correlation with polar cap potentials. These results suggest an enhanced convection electric field causes an inward shift of the source region. The inward shift of the source region allows EMIC waves to scatter relativistic electrons over a wide range of radial distances during the IPDP event. This mechanism suggests that IPDP-type EMIC waves are more likely to scatter relativistic electrons than other EMIC waves. We also show that the decreased phase-space density of relativistic electrons in the outer radiation belt is consistent with the extent of the source region and the resonant energy of EMIC waves, implying a possible contribution of EMIC waves to outer radiation belt loss during the main phase of geomagnetic storms.

DOI： 10.1029/2023JA031479

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

A thermospheric wind data set from a Fabry-Perot interferometer (630 nm) and the ion velocity from a Dynasonde in Tromsø, Norway, was analyzed for nine winter seasons to study the dynamics of the thermosphere and F-region ionosphere at an auroral latitude. This study focused on bifurcation in the zonal component of the neutral wind and ion velocity at midnight and its dependence on the Y component of the interplanetary magnetic field (IMF). Ionospheric plasma convection patterns are evidently imprinted on the thermospheric wind variations as aspects of the westward and eastward accelerations at dusk and late morning, respectively. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may result in the dependence because of anti-sunward plasma flow distorted in the polar cap.

DOI： 10.1029/2023GL104334

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

Equatorial plasma bubbles are a phenomenon of plasma density depletion with small-scale density irregularities, normally observed in the equatorial ionosphere. This phenomenon, which impacts satellite-based communications, was observed in the Asia-Pacific region after the largest-on-record January 15, 2022 eruption of the Tonga volcano. We used satellite and ground-based ionospheric observations to demonstrate that an air pressure wave triggered by the Tonga volcanic eruption could cause the emergence of an equatorial plasma bubble. The most prominent observation result shows a sudden increase of electron density and height of the ionosphere several ten minutes to hours before the initial arrival of the air pressure wave in the lower atmosphere. The propagation speed of ionospheric electron density variations was ~ 480–540 m/s, whose speed was higher than that of a Lamb wave (~315 m/s) in the troposphere. The electron density variations started larger in the Northern Hemisphere than in the Southern Hemisphere. The fast response of the ionosphere could be caused by an instantaneous transmission of the electric field to the magnetic conjugate ionosphere along the magnetic field lines. After the ionospheric perturbations, electron density depletion appeared in the equatorial and low-latitude ionosphere and extended at least up to ±25° in geomagnetic latitude.

DOI： 10.1038/s41598-023-33603-3

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

Equatorial plasma bubbles are a phenomenon of plasma density depletion with small-scale density irregularities, normally observed in the equatorial ionosphere. This phenomenon, which impacts satellite-based communications, was observed in the Asia-Pacific region after the largest-on-record January 15, 2022 eruption of the Tonga volcano. We used satellite and ground-based ionospheric observations to demonstrate that an air pressure wave triggered by the Tonga volcanic eruption could cause the emergence of an equatorial plasma bubble. The most prominent observation result shows a sudden increase of electron density and height of the ionosphere several ten minutes to hours before the initial arrival of the air pressure wave in the lower atmosphere. The propagation speed of ionospheric electron density variations was ~ 480–540 m/s, whose speed was higher than that of a Lamb wave (~315 m/s) in the troposphere. The electron density variations started larger in the Northern Hemisphere than in the Southern Hemisphere. The fast response of the ionosphere could be caused by an instantaneous transmission of the electric field to the magnetic conjugate ionosphere along the magnetic field lines. After the ionospheric perturbations, electron density depletion appeared in the equatorial and low-latitude ionosphere and extended at least up to ±25° in geomagnetic latitude.

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

Ion drag associated with the ionospheric plasma convection plays an important role in the high-latitude thermospheric dynamics, yet changes in the thermospheric wind with geomagnetic activity are not fully understood. We performed a statistical analysis of the thermospheric wind measurements with a Fabry–Perot interferometer (FPI; 630 nm wavelength) in Tromsø, Norway, in the winter months for 9 years. The measurements were sorted by a SuperMAG (SME) index, and a quiet-time wind pattern was defined as an hourly mean under SME ≤ 40 nT. The quiet-time wind pattern can be expected to be represented by a pressure gradient associated with the solar radiation and a geostrophic force balance. With an increase in the geomagnetic activity level, the thermospheric wind turned over from eastward to westward at dusk and increased the equatorward magnitude from midnight to dawn. Deviations from the quiet-time wind presented similar patterns in the direction with the ionospheric plasma convection but were larger in magnitude at dusk than at dawn. This is the first study to report a dawn-dusk asymmetry of the thermospheric wind acceleration feature and signatures of the eastward wind acceleration at dawn by ion drag. Graphical Abstract: [Figure not available: see fulltext.].

DOI： 10.1186/s40623-023-01829-0

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

We used the global navigation satellite system-total electron content (TEC) and Super Dual Auroral Radar Network (SuperDARN) radar to elucidate the characteristics of the plasma bubble extending to midlatitudes over North America during a geomagnetic storm on 27 and 28 May 2017. To identify plasma bubbles, we analyzed the rate of the TEC index (ROTI), which is a good indicator of the occurrence of plasma bubbles. The enhanced ROTI region expanded up to 50°N (geomagnetic latitude), and the upper limit of this region coincided with the equatorward wall of the midlatitude trough. Two-dimensional ROTI maps show that the enhanced midlatitude ROTI region moved westward at a bulk speed of approximately 310 m/s. The Fort Hays East SuperDARN radar also detected the radar echo, showing the existence of plasma density irregularities at ∼10-m scales within the midlatitude plasma bubble. The radar echo had a westward velocity of ∼300 m/s, which is almost consistent with the westward motion of the enhanced midlatitude ROTI region. We deduced that the westward propagation of the plasma bubble could be caused by a poleward sub-auroral polarization stream electric field. The observed radar echo overlapping with the enhanced ROTI region had a narrower spectral width (<50 m/s) than that of the auroral activity. This feature resembled that of the type 1 echo, although the Doppler velocity was smaller than the ion acoustic speed at the F-region height. This is the first case in which plasma density irregularities within plasma bubbles were captured by the SuperDARN radar.

DOI： 10.1029/2022JA031157

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

The X-rays and extreme ultraviolet (EUV) emitted during solar flares can rapidly change the physical composition of Earth’s ionosphere, causing space weather phenomena. It is important to develop an accurate understanding of solar flare emission spectra to understand how it affects the ionosphere. We reproduced the entire solar flare emission spectrum using an empirical model and physics-based model, and input it into the Earth’s atmospheric model, GAIA to calculate the total electron content (TEC) enhancement due to solar flare emission. We compared the statistics of nine solar flare events and calculated the TEC enhancements with the corresponding observed data. The model used in this study was able to estimate the TEC enhancement due to solar flare emission with a correlation coefficient greater than 0.9. The results of this study indicate that the TEC enhancement due to solar flare emission is determined by soft X-ray and EUV emission with wavelengths shorter than 35 nm. The TEC enhancement is found to be largely due to the change in the soft X-ray emission and EUV line emissions with wavelengths, such as Fe XVII 10.08 nm, Fe XIX 10.85 nm and He II 30.38 nm. Graphical Abstract: [Figure not available: see fulltext.].

DOI： 10.1186/s40623-023-01788-6

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その他リンク： https://link.springer.com/article/10.1186/s40623-023-01788-6/fulltext.html

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

To elucidate the effect of solar wind dynamic pressure on the activities of ionospheric plasma irregularities in both high-latitude and equatorial regions, we analyzed the long-term global navigation satellite system (GNSS) total electron content (TEC) data from 2000 to 2018 and performed a superposed epoch analysis of the GNSS rate of the TEC index (ROTI), solar wind, interplanetary magnetic field (IMF), and geomagnetic index. We found that during the main phase of geomagnetic storms, the activities of ionospheric plasma irregularities were considerably enhanced in both high-latitude and equatorial regions under high-pressure conditions, and the equatorward edge of the auroral oval moved to lower latitudes. The poleward edge of the enhanced low-latitude ROTI region (occurrence region of the plasma bubbles) in the evening sector extended to higher latitudes under high-pressure conditions. During the recovery phase of geomagnetic storms, the plasma bubble occurrence in the dusk sector was suppressed under high-pressure conditions. Our results suggest that the high-latitude convection electric field and the penetration and disturbance dynamo electric fields at low latitudes become stronger during geomagnetic storms when the solar wind dynamic pressure is enhanced. This is because the conversion from solar wind energy to electromagnetic energy in the magnetosphere is enhanced by the formation of a high plasma pressure area in the high-latitude cusp and mantle region. Therefore, not only the southward IMF but also solar wind dynamic pressure are important factors for varying global ionospheric responses during geomagnetic storms.

DOI： 10.1029/2022JA030913

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Kawai et al. (2021) reported the first ground-satellite conjugate observation of nighttime medium-scale traveling ionospheric disturbances (MSTIDs), by analyzing measurements from an airglow imager at Gakona (geographic latitude: 62.39°N, geographic longitude: 214.78°E, magnetic latitude: 63.60°N) and the Arase satellite in the magnetosphere on 3 November 2018. The Arase satellite observed variations in both the polarization electric field and the electron density as the Arase footprint passed through the MSTID structures in the ionosphere. In this study, we investigated whether these electric field and density variations associated with MSTIDs at subauroral latitudes are always observed by Arase in the magnetosphere. We used three airglow imagers installed at Gakona, Athabasca (geographic latitude: 54.60°N, geographic longitude: 246.36°E, magnetic latitude: 61.10°N), and Kapuskasing (geographic latitude: 49.39°N, geographic longitude: 277.81°E, magnetic latitude: 58.70°N) and the Arase satellite. We found eight observations of MSTIDs conjugate with Arase. They indicate that electric field and density variations associated with MSTIDs are not always observed in the magnetosphere. These variations tend to be observed in the magnetosphere during geomagnetically quiet times and when the amplitude of the MSTID is large. We categorized the MSTIDs into those caused by plasma instabilities and gravity waves and found that the electric field and density variations can be observed in the magnetosphere for both types of MSTIDs.

DOI： 10.1029/2022JA030542

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geoscience Data Journal  

This paper presents tools that help researchers implement the processes of data-led studies of upper-atmospheric phenomena. These tools were developed as a part of the activities of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) of Japan, which is a project to develop infrastructure for upper-atmospheric research data. This paper focuses on the data service named IUGONET Type-A, which was launched in October 2016 and has since evolved. In addition to being a conventional metadata catalogue, it has many other useful functions: an easy cross-searching system, a quick-look data-plotting procedure, an interactive data visualization system named UDAS web, and strong linkage with analysis software. Users can pick up relevant data from a huge number of data sets using either lists categorized by instruments/projects, observed regions and special campaigns or a world map of observatories. Users can quickly find the time, location and nature of phenomena that occurred by comparing the quick-look plots of various data displayed by the browser. UDAS web allows researchers to interactively create stacked plots of various data types that can facilitate the understanding of the relationships among phenomena observed in different regions. Furthermore, it presents a command list for software dedicated to data analysis that can smoothly lead users to perform detailed analyses. IUGONET Type-A provides a one-stop data service that can assist users in searching, examining and comprehending data for advanced analysis. It is also capable of handling old data, including analogue data and written paper documents. Thus, it will provide useful support for innovative interdisciplinary scientific research on solar–terrestrial phenomena.

DOI： 10.1002/gdj3.170

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/gdj3.170

記述言語：英語   掲載種別：研究論文（学術雑誌）  

This paper presents tools that help researchers implement the processes of data-led studies of upper-atmospheric phenomena. These tools were developed as a part of the activities of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) of Japan, which is a project to develop infrastructure for upper-atmospheric research data. This paper focuses on the data service named IUGONET Type-A, which was launched in October 2016 and has since evolved. In addition to being a conventional metadata catalogue, it has many other useful functions: an easy cross-searching system, a quick-look data-plotting procedure, an interactive data visualization system named UDAS web, and strong linkage with analysis software. Users can pick up relevant data from a huge number of data sets using either lists categorized by instruments/projects, observed regions and special campaigns or a world map of observatories. Users can quickly find the time, location and nature of phenomena that occurred by comparing the quick-look plots of various data displayed by the browser. UDAS web allows researchers to interactively create stacked plots of various data types that can facilitate the understanding of the relationships among phenomena observed in different regions. Furthermore, it presents a command list for software dedicated to data analysis that can smoothly lead users to perform detailed analyses. IUGONET Type-A provides a one-stop data service that can assist users in searching, examining and comprehending data for advanced analysis. It is also capable of handling old data, including analogue data and written paper documents. Thus, it will provide useful support for innovative interdisciplinary scientific research on solar–terrestrial phenomena.

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEE Geoscience and Remote Sensing Letters  

Space weather adversely affects satellite systems by obstructing the services of global positioning systems (GPSs), delaying the launch of space vehicles, and causing damage to basic infrastructures such as radio-communications and power distribution networks. The spatio-temporal variations of ionospheric total electron content (TEC) during geomagnetic storms can be well examined by extreme value analysis. Statistical assessment and analysis of extreme values are necessary for developing risk assessment and mitigation strategies to the affected communication systems. In this study, extreme TEC events are investigated using 25 years (1997-2021) of long-term Global Navigation Satellite System (GNSS) Earth Observation Network System (GEONET) TEC data over Japan grid point location at 34.95° N and 134.05° E, and global data. In this work, a generalized extreme value (GEV) distribution is applied to the annual maxima TEC to evaluate geomagnetic disturbances that could occur once in 44, 88, 132, and 176 years with a confidence interval of 95%. According to the reference thresholds indicated by International Civil Aviation Organization (ICAO) for safer aviation applications, the moderate and severe levels of threshold for TEC are identified as 125 and 175 TEC Units (TECU), respectively. The results illustrate that the estimated extreme TECs for one in 44, 88, 132, and 176 years are 119, 125, 128, 130 and 215, 220, 222, 225 TECU for grid point and global TEC data respectively. That is, a moderate threshold level of activity is likely to happen over considered grid point for a return period of beyond 88 years and severe threshold level of activity beyond 11 years for global TEC data.

DOI： 10.1109/LGRS.2023.3292593

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

On 15 January 2022, the submarine volcano on the southwest Pacific island of Tonga violently erupted. Thus far, the ionospheric oscillation features caused by the volcanic eruption have not been identified. Here, observations from the Super Dual Auroral Radar Network radars and digisondes were employed to analyze ionospheric oscillations in the Northern Hemisphere caused by the volcanic eruption in Tonga. Due to the magnetic field conjugate effect, the ionospheric oscillations were observed much earlier than the arrival of surface air pressure waves, and the maximum negative line-of-sight (LOS) velocity of the ionospheric oscillations exceeded 100 m/s in the F layer. After the surface air pressure waves arrived, the maximum LOS velocity in the E layer approached 150 m/s. A maximum upward displacement of 100 km was observed in the ionosphere. This work provides a new perspective for understanding the strong ionospheric oscillation caused by geological hazards observed on Earth.

DOI： 10.1029/2022GL100555

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

The thermospheric wind response to a sudden westward turning of the ion velocity at a high latitude was studied by analyzing data obtained with a Fabry–Perot interferometer (FPI; 630 nm), Dynasonde, and Swarm A & C satellites during a conjunction event. The event occurred during a geomagnetically quiet period (Kp = 0 +) through the night, but some auroral activity occurred in the north. The collocated FPI and Dynasonde measured the thermospheric wind (U) and ionospheric plasma velocity (V), respectively, in the F region at the equatorward trough edge. A notable scientific message from this study is the possible role of thermospheric wind in the energy dissipation process at F-region altitude. The FPI thermospheric wind did not instantly follow a sudden V change due to thermospheric inertia in the F region. At a pseudo-breakup during the event, V suddenly changed direction from eastward to westward within 10 min. U was concurrently accelerated westward, but its development was more gradual than that of V, with U remaining eastward for a while after the pseudo-breakup. The delay of U is attributed to the thermospheric inertia. During this transition interval, U∙V was negative, which would result in more efficient generation of frictional heating than the positive U∙V case. The sign of U∙V, which is related to the relative directions of the neutral wind and plasma drift, is important because of its direct impact on ion-neutral energy exchange during collisions. This becomes especially important during substorm events, where rapid plasma velocity changes are common. The sign of U∙V may be used as an indicator to find the times and locations where thermospheric inertia plays a role in the energy dissipation process. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01710-6

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

This study, for the first time, reports the geomagnetically conjugate structure of a plasma bubble extending to the mid-latitudes and the asymmetrical structure of the decay of the plasma bubble during a geomagnetic storm. We investigated the temporal and spatial variations of plasma bubbles in the Asian sector during a geomagnetic storm on March 1, 2013, using Global Navigation Satellite System-total electron content data with high spatiotemporal resolutions. The first important point of our data analysis results is that the plasma bubble extended from the equator to the mid-latitudes with geomagnetic conjugacy along the magnetic field lines. The total electron content data showed that the plasma bubbles appeared in the equatorial regions near 150° E after sunset during the main phase of the geomagnetic storm. From ionosonde data over both Japan and Australia, they suggest that a large eastward electric field existed in the Asian sector. Finally, the plasma bubbles extended up to the mid-latitudes (~ 43° geomagnetic latitude) in both hemispheres, maintaining geomagnetic conjugacy. The second point is that the mid-latitude plasma bubble disappeared 1–2 h earlier in the northern hemisphere than in the southern hemisphere at close to midnight. In the northern hemisphere, the ionospheric virtual height decreased near midnight, followed by a rapid decrease in the total electron content and a rapid increase in the ionospheric virtual height. These results imply that the mid-latitude plasma bubble disappeared as the background plasma density decreased after midnight due to the recombination resulting from the descent of the F layer. Therefore, we can conclude that mid-latitude plasma bubbles can be asymmetric between the northern and southern hemispheres because of the rapid decay of plasma bubbles in one of the hemispheres. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01682-7

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

To elucidate the characteristics of electromagnetic conjugacy of traveling ionospheric disturbances just after the 15 January 2022 Hunga Tonga-Hunga Ha’apai volcanic eruption, we analyze Global Navigation Satellite System-total electron content data and ionospheric plasma velocity data obtained from the Super Dual Auroral Radar Network Hokkaido pair of radars. Further, we use thermal infrared grid data with high spatial resolution observed by the Himawari 8 satellite to identify lower atmospheric disturbances associated with surface air pressure waves propagating as a Lamb mode. After 07:30 UT on 15 January, two distinct traveling ionospheric disturbances propagating in the westward direction appeared in the Japanese sector with the same structure as those at magnetically conjugate points in the Southern Hemisphere. Corresponding to these traveling ionospheric disturbances with their large amplitude of 0.5 – 1.1 × 1016 el/m2 observed in the Southern Hemisphere, the plasma flow direction in the F region changed from southward to northward. At this time, the magnetically conjugate points in the Southern Hemisphere were located in the sunlit region at a height of 105 km. The amplitude and period of the plasma flow variation are ~ 100–110 m/s and ~ 36–38 min, respectively. From the plasma flow perturbation, a zonal electric field is estimated as ~ 2.8–3.1 mV/m. Further, there is a phase difference of ~ 10–12 min between the total electron content and plasma flow perturbations. This result suggests that the external electric field variation generates the traveling ionospheric disturbances observed in both Southern and Northern Hemispheres. The origin of the external electric field is an E-region dynamo driven by the neutral wind oscillation associated with atmospheric acoustic waves and gravity waves. Finally, the electric field propagates to the F region and magnetically conjugate ionosphere along magnetic field lines with the local Alfven speed, which is much faster than that of Lamb mode waves. From these observational facts, it can be concluded that the E-region dynamo electric field produced in the sunlit Southern Hemisphere is a main cause of the two distinct traveling ionospheric disturbances appearing over Japan before the arrival of the air pressure disturbances. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01665-8

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

A strong volcanic eruption caused a clear vertical acoustic resonance between the sea surface and the thermosphere. Its effects are observed as geomagnetic and GPS-TEC oscillations near the volcano and its geomagnetic conjugate area. The geomagnetic oscillations are observed at Apia and Honolulu geomagnetic observatories with amplitude of about 2 nT and 0.2 nT, respectively. The volcanic eruption started around 04:14 UT on January 15, 2022. The oscillations appeared at 04:21UT at Apia, Samoa, only about 7 min after the start of eruption. Because the distance between the volcano and Apia is about 841 km, it takes about 40 min for a sound wave to propagate from the volcano to Apia. Therefore, it is more plausible to assume that the magnetic oscillation observed at Apia about 7 min after the eruption is caused by the sound waves propagated vertically upward to the ionosphere and generated an electric current. The coherent appearance of geomagnetic oscillation at Honolulu located near the geomagnetic conjugate point of the volcano strongly support the idea that the ionospheric current generated over the volcano diverted as a field-aligned current which flew to the opposite hemisphere and caused the geomagnetic oscillation at Honolulu. The earliest start of GPS-TEC oscillation was around 04:15UT near the volcanic eruption, and it was around 04:20 UT at KOKV station in Hawaii. The time-lag of the TEC variations between Samoa and Hawaii obtained by a cross-correlation analysis is 4.5 min or 8.5 min. These time differences are much smaller than the travel time of the seismic waves from the volcano to Hawaii islands. Therefore, it is suggested that the electric field transmitted along geomagnetic field caused the TEC variation observed over Hawaii Islands. A sawtooth waveform of geomagnetic oscillation observed at Apia and Honolulu is analyzed and a possible generation mechanism is discussed. Graphical Abstract: [Figure not available: see fulltext.]

DOI： 10.1186/s40623-022-01653-y

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Simultaneous observations from a 630 nm all-sky airglow imager, GNSS-TEC receivers, and an ionosonde are used to investigate the role of E- and F-region coupling on the generation of medium-scale traveling ionospheric disturbances (MSTIDs). The primary observations are OI 630 nm airglow images taken by an all-sky imager in Kühlungsborn (54.07°N; 11.46°E, 53.79°N Mlat.), a site in northern Germany. Out of 226 nights of observations, MSTIDs were found only in 18 nights, demonstrating the low occurrence rate over Kühlungsborn. We focused on four MSTIDs events: two during the vernal equinox and two during summer. Coincident measurements of detrended GNSS-TEC supported the presence of MSTIDs during the selected events, and simultaneous observations from the ionosonde in Juliusruh (54.60°N, 13.4°E, 54.02°N Mlat.) showed sporadic-E (Es) layer and spread-F activity in the E- and F-region, respectively. We observed the onset of the observed MSTIDs to be around the 15°–20°E longitude and 60–45°N latitude belts. Additionally, we found that in each case, the onset of MSTIDs coincides with the presence of an Es layer with sporadic-E trace is observed (foEs) exceeding 4 MHz. This suggests that an Es layer with foEs ≥ 4MHz was a source of the generation of these MSTIDs. Altitude of the Es layer could be another important factor in generating MSTIDs. The Es layer should exist at an altitude where Hall conductivity is large, as happened in the present study.

DOI： 10.1029/2021JA030159

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

The equatorial ionosphere endured plasma density irregularities during a geomagnetic storm on 21–22 December 2014. To understand the underlying mechanism, we analyzed the rate of the total electron content change (ROTI) data obtained from a global navigation satellite system, along with solar wind, interplanetary magnetic field (IMF), geomagnetic indices, Jicamarca incoherent scatter radar, and magnetometer data. The results indicate that the ROTI enhancement related to plasma density irregularities (plasma bubbles) occurred three times in the equatorial and low latitude regions of the American sector during the geomagnetic storm. The first, second, and third enhancements which have a longitudinal extent of ∼20° appeared in the post-sunset, pre-midnight, and post-midnight sectors, respectively. The second enhancement occurred during the recovery phase of the storm-time substorm even though the IMF remained southward. During this period, the direction of the dayside equatorial electrojet (EEJ) changed from eastward to westward, while the nightside upward plasma velocity at Jicamarca increased to 28.8 m/s. The response of the EEJ and upward ion drift implies that the westward and eastward electric fields were intensified on the dayside and nightside, respectively. Therefore, these results suggest that an over-shielding electric field penetrates the dayside/nightside equator simultaneously in association with a substorm recovery phase, and that the electric field generates plasma bubbles by the Rayleigh-Taylor instability mechanism. Plasma bubbles induced by the penetration of an over-shielding electric field due to substorm activity have not previously been reported.

DOI： 10.1029/2021JA030240

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

To establish the statistical behavior of ionospheric TEC variations from high latitudes to mid-latitudes during the main and recovery phases of geomagnetic storms, we conducted a superposed epoch analysis of interplanetary magnetic field, solar wind, geomagnetic indices (AE and SYM-H), and global navigation satellite system (GNSS)-total electron content (TEC) data for 20 yr (2000–2019). In this study, we identify 663 geomagnetic storm events with the minimum SYM-H value of less than −40 nT and investigate the characteristics of the TEC variations for the weak (−60 ≤ SYM-Hmin < −40 nT), moderate (−100 ≤ SYM-Hmin < −60 nT), and strong (−150 ≤ SYM-Hmin < −100 nT) geomagnetic storms. The main results obtained from the present study are as follows: (a) The TEC enhancements related to the tongue of ionization (TOI), auroral oval, and storm-enhanced density (SED) plume are more dominant in winter than in summer during the main phase of geomagnetic storms. (b) The structure of the mid-latitude trough in the nighttime sector becomes unclear in winter. (c) The TEC depletion at auroral and mid-latitudes (40°–70° GMLAT: geomagnetic latitude) starts to appear in the morning sector (8–10 hr GMLT: geomagnetic local time) during the main phase of geomagnetic storms and the decreased region extends in the lower latitude and GMLT directions with time. The negative storm activity tends to be enhanced significantly as the storm intensity becomes larger. The activity of the TEC depletion is dominant in summer than in winter, which is agreement with the classical ionospheric storm scenario.

DOI： 10.1029/2021JA029687

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

We applied a novel three-dimensional spectral analysis method to GPS-TEC perturbation (GPS-TECP) maps to study the propagation direction of daytime and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) over North America. By this method, we can automatically calculate the phase velocity spectrum and directionality of MSTIDs. We focused on the periods of high MSTIDs occurrence, namely, June–July 2006 and November–December 2006, to study nighttime and daytime MSTIDs. We divided North America into the west (100°–130°W, 25°–55°N) and east (70°–100°W, 25°–55°N) parts. Our results show that both daytime and nighttime MSTID propagations exhibit strong longitudinal variations as a function of local time and day-to-day variations. The power peaks of daytime MSTIDs are from 10:00–16:00 LT in the west part and 10:00–14:00 LT in the east part. The predominant propagation directions of daytime MSTIDs are southward (southeastward) in the west (east). The daytime local time variations demonstrate that the MSTIDs display directional change in the west part; however, a similar directional change is not very pronounced in the east part. The local time variations of nighttime MSTIDs shows the power peaks from 22:00–02:00 LT in the west and 20:00–00:00 LT in the east. We found that the predominant propagation direction in the west part is westward with a wider azimuthal band (∼210°–300°) than the east part (∼210°–240°). By comparing nighttime propagation directions between the western and eastern parts, we reached the conclusion that the magnetic declination angle affects the propagation direction.

DOI： 10.1029/2020JA028791

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

A new type of terrestrial line spectra found by the Arase satellite was reported in 2018. These spectra are called “hectometric line spectra (HLS)”. They primarily consist of constant frequency narrowband components at frequencies between 525 kHz and 1,700 kHz, which originate and are sometimes amplified from AM broadcasting waves. In addition to these, other generated emissions are observed. Entrances and the mode conversion of the AM broadcasting waves into the equatorial and low-latitude ionosphere with plasma density depletion called plasma bubbles are observed. Electron density profiles and equatorial plasma bubbles are examined through Global Positioning System (GPS)-Total electron content (TEC) analyses. As a result, the Arase satellite which observed the HLS passed through the TEC depression region near the equatorial ionosphere associated with plasma bubbles. The scenario based on the mode conversion of the L-O mode to the Z mode and vice versa was confirmed with observations and GPS-TEC analyses. Another entrance when foF2 estimated from TEC is lower than 1,000 kHz instead of a plasma bubble is also found. No mode conversion is necessary, then.

DOI： 10.1029/2021JA029813

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

The Radiation Belt Storm Probes (RBSP) and the Arase satellites have different inclinations and sometimes they fly both near the equator and off the equator on the same magnetic field line simultaneously. Such conjunction events give us opportunities to compare the electron density at different latitudes. In this study, we analyzed the plasma waves observed by Arase and RBSP during the three conjunction events during and after the September 7, 2017 storm. The electron number density at the satellite positions was estimated from frequencies of the Upper Hybrid Resonance emissions obtained by the High Frequency Analyzer of the Plasma Wave Experiment onboard the Arase and the Waves instrument onboard the RBSP, respectively. During the three conjunction events, the satellites passed through the plume, inner trough (the narrow region with low electron density between the main body of the plasmasphere and the plume), plasmatrough with variable electron density, and partially refilled plasmasphere. The power-law index m for the inner trough and plume was inferred to be 4–7 and ∼0, respectively. This is interpreted to mean that the trough was close to collisionless and the plume was relatively near diffusive equilibrium. In the plasmatrough with the varying density, both the high-density and low-density regions had m ∼ 0. The low-density portion of this region may have a different origin from the inner trough, because of the different m indices. For the partially refilled plasmasphere in the storm recovery phase, the power-law index m showed negative values, meaning that the density in the equatorial plane was higher than at higher latitudes.

DOI： 10.1029/2020JA029073

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA029073

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Physics  

This paper reports that plasma density depletions appearing at middle latitudes near sunrise survived until afternoon on 29 May 2017 during the recovery phase of a geomagnetic storm. By analyzing GPS data collected in Japan, we investigate temporal variations in the horizontal two-dimensional distribution of total electron content (TEC) during the geomagnetic storm. The SYM-H index reached −142 nT around 08 UT on 28 May 2017. TEC depletions extending up to approximately 38°N along the meridional direction appeared over Japan around 05 LT (LT = UT + 9 hours) on 29 May 2017, when TEC rapidly increased at sunrise due to the solar extreme ultraviolet (EUV) radiation. The TEC depletions appeared sequentially over Japan for approximately 8 hours in sunlit conditions. At 06 LT on 29 May, when the plasma depletions first appeared over Japan, the background TEC was enhanced to approximately 17 TECU, and then decreased to approximately 80% of the TEC typical of magnetically quiet conditions. We conclude that this temporal variation of background plasma density in the ionosphere was responsible for the persistence of these plasma depletions for so long in daytime. By using the Naval Research Laboratory: Sami2 is Another Model of the Ionosphere (SAMI2), we have evaluated how plasma production and ambipolar diffusion along the magnetic field may affect the rate of plasma depletion disappearance. Simulation shows that the plasma density increases at the time of plasma depletion appearance; subsequent decreases in the plasma density appear to be responsible for the long-lasting persistence of plasma depletions during daytime. The plasma density depletion in the top side ionosphere is not filled by the plasma generated by the solar EUV productions because plasma production occurs mainly at the bottom side of the ionosphere.

DOI： 10.26464/epp2021046

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

On a geomagnetic quiet night of October 29, 2018, we captured an observational evidence of the onset of dark band structures within the field-of-view of an all-sky airglow imager operating at 630.0 nm over a geomagnetic low-mid latitude transition region, Hanle, Leh Ladakh. Simultaneous ionosonde observations over New Delhi shows the occurrence of spread-F in the ionograms. Additionally, virtual and peak height indicate vertical upliftment in the F layer altitude and reduction in the ionospheric peak frequency were also observed when the dark band pass through the ionosonde location. All these results confirmed that the observed depletions are indeed associated with ionospheric F region plasma irregularities. The rate of total electron content index (ROTI) indicates the absence of plasma bubble activities over the equatorial/low latitude region which confirms that the observed event is a mid-latitude plasma depletion. Our calculations reveal that the growth time of the plasma depletion is ∼2 h if one considers only the Perkins instability mechanism. This is not consistent with the present observations as the plasma depletion developed within ∼25 min. By invoking possible Es layer instabilities and associated E-F region coupling, we show that the growth rate increases roughly by an order of magnitude. This strongly suggests that the Cosgrove and Tsunoda mechanism may be simultaneously operational in this case. Furthermore, it is also suggested that reduced F region flux-tube integrated conductivity in the southern part of onset region created conducive background conditions for the growth of the plasma depletion on this night.

DOI： 10.1029/2020JA028837

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Relationship between the locations of the midlatitude trough minimum in the ionosphere and plasmapause in the inner magnetosphere has been statistically investigated using global navigation satellite system (GNSS)-total electron content (TEC) and electron density data obtained from the Arase satellite from March 23, 2017 to May 31, 2020. In this analysis, we identify the midlatitude trough minimum as a minimum value of GNSS-TEC at subauroral and midlatitude regions, and determine the plasmapause as an electron density decrease by a factor of 5 or more within ΔL < 0.5 in the inner magnetosphere. As a result, the plasmapause does not always coincide with the midlatitude trough minimum in all magnetic local time (MLT) sectors under all geomagnetic conditions. During the geomagnetically quiet periods, the midlatitude trough minimum is located at higher and lower geomagnetic latitudes (GMLATs) of the plasmapause in the MLT ranges of 5–21 and 21–5 h, respectively. This implies that both the features could not be on the same magnetic field line. On the other hand, during the storm main phase, the midlatitude trough minimum and plasmapause move toward a low-latitude region with day-night and dawn-dusk asymmetries and the correlation becomes highest, compared with that under other geomagnetic conditions. Especially, both the features mapped on the ionosphere at a height of 300 km exist near GMLAT in the afternoon-midnight sectors. This suggests that the midlatitude trough and plasmapause are formed at almost the same location due to an enhanced subauroral polarization stream during the storm main phase.

DOI： 10.1029/2020JA028943

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028943

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

We performed a superposed epoch analysis of solar wind, interplanetary magnetic field, geomagnetic index, and the rate of total electron content (TEC) index (ROTI) derived from global navigation satellite system-TEC data during 652 geomagnetic storm events (minimum SYM-H < −40 nT), to clarify the occurrence features and causes of storm-time plasma bubbles in the equatorial to mid-latitude ionosphere. In this analysis, we defined the time of the SYM-H minimum as the zero epoch. As a result, the ROTI enhancement started at the duskside magnetic equator and expanded to higher latitudes during the main phase. Approximately 1 h after the onset of the recovery phase, the ROTI values at the magnetic equator in the dusk-to-midnight sectors decreased while those in the dawn sector increased. This situation persisted for at least 12 h. The ratio of the ROTI during the main phase to that during the quiet period in the dusk sector is the largest in May–July. The ratio of the ROTI during the recovery phase decreased during dusk with increasing solar activity. Considering the requirement of the Rayleigh-Taylor instability, the difference in the magnetic local time of the ROTI signature, between the main and recovery phases, can be explained by a local time distribution of storm-time electric fields associated with a prompt penetration electric field and disturbance dynamo. This implies that the occurrence feature of the plasma bubble is different from that during quiet times when the input of solar wind energy to the magnetosphere and ionosphere increases significantly.

DOI： 10.1029/2020JA029010

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Isolated proton auroras (IPAs) appearing at subauroral latitudes are generated by energetic protons precipitating from the magnetosphere through interaction with electromagnetic ion cyclotron (EMIC) waves. An IPA thus indicates the spatial scale and temporal variation of wave-particle interactions in the magnetosphere. In this study, a unique event of simultaneous ground and magnetospheric satellite observations of two IPAs were conducted on March 16, 2015, using an all-sky imager at Athabasca, Canada and Van Allen Probes. The Van Allen Probes observed two isolated EMIC waves with frequencies of ∼1 and 0.4 Hz at L ≈ 5.0 when the satellite footprint crossed over the two IPAs. This suggests that the IPAs were caused by localized EMIC waves. Proton flux at 5–20 keV increased locally when the EMIC waves appeared. Electron flux at energies below ∼500 eV also increased. Temperature anisotropy of the energetic protons was estimated at 1.5–2.5 over a wide L-value range of 3.0–5.2. Electron density gradually decreased from L = 3.5 to 5.4, suggesting that the EMIC wave at L ≈ 5.0 was located in the gradual plasmapause. From these observations, we conclude that the localized IPAs and associated EMIC waves took place because of localized enhancement of energetic proton flux and plasma density structure near the plasmapause. The magnetic field observed by the satellite showed small variation during the wave observation, indicating that the IPAs were accompanied by the weak field-aligned current.

DOI： 10.1029/2020JA029078

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15–35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside Atmosphere and Ionosphere model for Aeronomy), it was found that EUV radiation of approximately 15–35 nm affects a wide altitude range of 120–300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules.[Figure not available: see fulltext.]

DOI： 10.1186/s40623-021-01376-6

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その他リンク： https://link.springer.com/article/10.1186/s40623-021-01376-6/fulltext.html

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Stable auroral red (SAR) arcs are optical events with dominant 630.0-nm emission caused by low-energy electron heat flux into the topside ionosphere from the inner magnetosphere. SAR arcs are observed at subauroral latitudes and often occur during the recovery phase of magnetic storms and substorms. Past studies concluded that these low-energy electrons were generated in the spatial overlap region between the outer plasmasphere and ring-current ions and suggested that Coulomb collisions between plasmaspheric electrons and ring-current ions are more feasible for the SAR-arc generation mechanism rather than Landau damping by electromagnetic ion cyclotron waves or kinetic Alfvén waves. This work studies three separate SAR-arc events with conjunctions, using all-sky imagers and inner magnetospheric satellites (Arase and Radiation Belt Storm Probes [RBSP]) during non-storm-time substorms on December 19, 2012 (event 1), January 17, 2015 (event 2), and November 4, 2019 (event 3). We evaluated for the first time the heat flux via Coulomb collision using full-energy-range ion data obtained by the satellites. The electron heat fluxes due to Coulomb collisions reached ∼109 eV/cm2/s for events 1 and 2, indicating that Coulomb collisions could have caused the SAR arcs. RBSP-A also observed local enhancements of 7–20-mHz electromagnetic wave power above the SAR arc in event 2. The heat flux for the freshly detached SAR arc in event 3 reached ∼108 eV/cm2/s, which is insufficient to have caused the SAR arc. In event 3, local flux enhancement of electrons (<200 eV) and various electromagnetic waves were observed, these are likely to have caused the freshly detached SAR arc.

DOI： 10.1029/2020JA029081

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA029081

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

The total electron content (TEC) data derived from the GAIA (Ground-to-topside model of Atmosphere Ionosphere for Aeronomy) is used to study the seasonal and longitudinal variation of occurrence of medium-scale traveling ionospheric disturbances (MSTIDs) during daytime (09:00–15:00 LT) for the year 2011 at eight locations in northern and southern hemispheres, and the results are compared with ground-based Global Positioning System (GPS)-TEC. To derive TEC variations caused by MSTIDs from the GAIA (GPS) data, we obtained detrended TEC by subtracting 2-h (1-h) running average from the TEC, and calculated standard deviation of the detrended TEC in 2 h (1 h). MSTID activity was defined as a ratio of the standard deviation to the averaged TEC. Both GAIA simulation and GPS observations data show that daytime MSTID activities in the northern and southern hemisphere (NH and SH) are higher in winter than in other seasons. From the GAIA simulation, the amplitude of the meridional wind variations, which could be representative of gravity waves (GWs), shows two peaks in winter and summer. The winter peak in the amplitude of the meridional wind variations coincides with the winter peak of the daytime MSTIDs, indicating that the high GW activity is responsible for the high MSTID activity. On the other hand, the MSTID activity does not increase in summer. This is because the GWs in the thermosphere propagate poleward in summer, and equatorward in winter, and the equatorward-propagating GWs cause large plasma density perturbations compared to the poleward-propagating GWs. Longitudinal variation of daytime MSTID activity in winter is seen in both hemispheres. The MSTID activity during winter in the NH is higher over Japan than USA, and the MSTID activity during winter in the SH is the highest in South America. In a nutshell, GAIA can successfully reproduce the seasonal and longitudinal variation of the daytime MSTIDs. This study confirms that GWs cause the daytime MSTIDs in GAIA and amplitude and propagation direction of the GWs control the noted seasonal variation. GW activities in the middle and lower atmosphere cause the longitudinal variation. [Figure not available: see fulltext.].

DOI： 10.1186/s40623-021-01369-5

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

In order to reveal solar activity dependence of the medium-scale traveling ionospheric disturbances (MSTIDs) at midlatitudes, total electron content (TEC) data obtained from a Global Positioning System (GPS) receiver network in Japan during 22 years from 1998 to 2019 were analyzed. We have calculated the detrended TEC by subtracting the 1-h running average from the original TEC data for each satellite and receiver pair, and made two-dimensional TEC maps of the detrended TEC with a spatial resolution of 0.15° × 0.15° in longitude and latitude. We have investigated MSTID activity, defined as δI/ I¯ , where δI and I¯ are standard deviation of the detrended TEC and the average vertical TEC within the area of 133.0°–137.0° E and 33.0°–37.0° N for 1 h, respectively. From each 2-h time series of the detrended TEC data within the same area as the MSTID activity, auto-correlation functions (ACFs) of the detrended TEC were calculated to estimate the horizontal propagation velocity and direction of the MSTIDs. Statistical results of the MSTID activity and propagation direction of MSTIDs were consistent with previous studies and support the idea that daytime MSTIDs could be caused by atmospheric gravity waves, and that nighttime MSTIDs were caused by electro-dynamical forces, such as the Perkins instability. From the current long-term observations, we have found that the nighttime MSTID activity and occurrence rate increased with decreasing solar activity. For the daytime MSTID, the occurrence rate increased with decreasing solar activity, whereas the MSTID activity did not show distinct solar activity dependence. These results suggest that the secondary gravity waves generated by dissipation of the primary gravity waves propagating from below increase under low solar activity conditions. The mean horizontal phase velocity of the MSTIDs during nighttime did not show a distinct solar activity dependence, whereas that during daytime showed an anticorrelation with solar activity. The horizontal phase velocity of the daytime MSTIDs was widely distributed from 40 to 180 m/s under high solar activity conditions, whereas it ranged between 80 and 200 m/s, with a maximum occurrence at 130 m/s under low solar activity conditions, suggesting that gravity waves with low phase velocity could be dissipated by high viscosity in the thermosphere under low solar activity conditions. [Figure not available: see fulltext.]

DOI： 10.1186/s40623-020-01353-5

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Various subauroral optical features have been studied by analyzing data collected during periods of geomagnetic disturbances. Most events have been typically found at geomagnetic latitudes of 45–60°. In this study, however, we present a red arc event found at geomagnetic 68° north (L ≈ 7.1) in the Scandinavian sector during a period of geomagnetically quiet conditions within a short intermission between two high-speed solar wind events. The red arc appeared to coincide with a pseudo breakup at geomagnetic 71–72°N and a rapid equatorward expansion of the polar cap. However, the red arc disappeared in approximately 7 min. Simultaneous measurements with the Swarm A/C satellites indicated the appearance of the red arc at the ionospheric trough minimum and a conspicuous enhancement of the electron temperature, suggesting the generation of the arc by heat flux. Since there are meaningful differences in the red arc features from already-known subauroral optical features such as the stable auroral red (SAR) arc, we considered that the red arc is a new phenomenon. We suggest that the ephemeral red arc may represent the moment of SAR arc birth associated with substorm particle injection, which is generally masked by bright dynamic aurorae.

DOI： 10.1029/2020JA028468

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028468

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

A stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring-current ions. In the overlap region, plasmaspheric electrons are heated by ring-current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all-sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring-current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR-arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring-current ions is the most plausible mechanism for the SAR-arc generation.

DOI： 10.1029/2020JA028068

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028068

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Temporal and spatial evolutions of total electron content (TEC) and electron density in the ionosphere during a geomagnetic storm that occurred on 27 and 28 September 2017 have been investigated using global TEC data obtained from many Global Navigation Satellite System stations together with the ionosonde, geomagnetic field, Jicamarca incoherent scatter and Super Dual Auroral Radar Network (SuperDARN) radar data. Our analysis results show that a clear enhancement of the ratio of the TEC difference (rTEC) first occurs from noon to afternoon at high latitudes within 1 hr after a sudden increase and expansion of the high-latitude convection and prompt penetration of the electric field to the equator associated with the southward excursion of the interplanetary magnetic field. Approximately 1–2 hr after the onset of the hmF2 increase in the midlatitude and low-latitude regions associated with the high-latitude convection enhancement, the rTEC and foF2 values begin to increase and the enhanced rTEC region expands to low latitudes within 1–2 hr. This signature suggests that the ionospheric plasmas in the F2 region move at a higher altitude due to local electric field drift, where the recombination rate is smaller, and that the electron density increases due to additional production at the lower altitude in the sunlit region. Later, another rTEC enhancement related to the equatorial ionization anomaly appears in the equatorial region approximately 1 hr after the prompt penetration of the electric field to the equator and expands to higher latitudes within 3–4 hr.

DOI： 10.1029/2019JA026873

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

The characteristics of global electron density variations in the ionosphere during a geomagnetic storm on 7 and 8 November 2004 were investigated using total electron content (TEC) obtained from the global navigation satellite system (GNSS). The regions of enhanced TEC over North America, Europe, and Japan first appeared in the middle-latitude regions. The TEC enhancements over North America showed a rapid longitudinal expansion and reached a wide longitudinal extent during the initial and main phases of the geomagnetic storm. TEC enhancements were simultaneously observed in both North America and Japan at 05:00 UT on 8 November. Observation data from the Defense Meteorological Satellite Program showed a slight enhancement of electron density at 850 km below the equatorward boundary of the middle-latitude trough (45–48°N in geomagnetic latitude) over the Pacific Ocean. This electron density variation may correspond to the TEC enhancements observed in both Japan and North America. These results imply that an enhanced TEC region existed between North America and Japan. The TEC enhancement in Japan appeared with a magnetic conjugacy in the Southern Hemisphere, indicating one of the characteristics of storm-enhanced density (SED). Moreover, TEC enhancements simultaneously appeared from Japan to central Asia at 11:00 UT on 8 November, corresponding to the early recovery phase of the geomagnetic storm. From the above results, it is suggested that SED phenomena can be simultaneously generated over a wide longitudinal width (~100°). The longitudinal extent of this SED event is 2.5–5.0 times longer than those reported by previous studies.

DOI： 10.1029/2019JA026713

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Atmospheric and Solar-Terrestrial Physics  

We present the results of a comprehensive study of traveling ionospheric disturbances (TIDs) occurring over Europe during the total solar eclipse of 20 March 2015. For detection of wave structures and estimation of TID parameters, two remote sensing techniques were combined: incoherent scatter (IS) radars and European and Finnish dense GPS receiver networks. Similar procedures were applied for processing both IS and GPS data. We developed a new method enabling to analyze TEC data separately in the temporal and spatial domain. For the first time, we produced maps of band-pass filtered TEC variations and reported both large- and medium-scale prevailing TIDs observed during this solar eclipse, both having similar periods of about 50 – 60 min. The downward phase progression indicates that TIDs were induced by atmospheric gravity waves generated at lower altitudes. The variations in IS power attained peak relative amplitudes of 0.22 (22%) at 220 km over Tromsø and of 0.17 (17%) at 200 km over Kharkiv. The vertical phase velocity was about 57 m/s over Tromsø. It increased from 25 to 170 m/s over Kharkiv with altitude increasing from 120 to 310 km. Over Western Europe, large-scale TIDs (LSTIDs) had prevailing north-east direction over the region from 45°to 50°N and 2°W to 8°E. Here, their average horizontal phase velocity Vm was 803±281m∕s and the absolute amplitudes of TEC variations usually do not exceed 0.17 TECU. For this region, we found strong differences in LSTID propagation azimuth between the solar eclipse day and the two adjacent days of 19 and 21 March 2015, used as reference. This most likely indicates that these LSTIDs were directly caused by the solar eclipse through local heating/cooling processes occurring during the passing of the Moon penumbra. Over another region, limited by 44°–50°N and 13°–19°E, the LSTIDs had south-east propagation. Over Finland, the LSTIDs also propagated southeastward having Vm=774±202m∕s and TEC amplitudes up to 0.6 TECU. A possible evidence of LSTID generation at high latitudes indirectly by this solar eclipse through an excitation of slow magnetosonic waves was experimentally detected. Medium-scale TIDs (MSTIDs) propagated southeastward over both regions having Vm values of 144±54m∕s over Western Europe and of 104±43m∕s over Finland. Over Northern Europe, the maximum MSTID amplitudes were greater by a factor of 5 compared to those over Western Europe and reached 0.4 TECU. We did not detect a clear difference in MSTID propagation between solar eclipse and reference days. The IS and GPS results are in consistency with each other. The detected TID parameters of predominant periods, relative amplitudes, altitude range and MSTID horizontal propagation direction generally correspond to the results of other studies.

DOI： 10.1016/j.jastp.2019.05.015

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

The solar radiation focusing effect is related to the specific phenomenon of propagation of the Sun-emitted HF and VHF waves through terrestrial ionosphere. This natural effect is observed with ground-based radio instruments running within the 10-200 MHz range as distinctive patterns - the spectral caustics (SCs) - on the solar dynamic spectra. It has been suggested that SCs are associated with medium-scale traveling ionospheric disturbances (MSTIDs). In this paper, we present the first direct observations of SCs induced by MSTIDs, using solar dynamic spectra with SCs obtained by different European radio telescopes on 2014 January 8 and simultaneous two-dimensional detrended total electron content (dTEC) maps over Europe. Spatial examination of dTEC maps as well as precise timing analysis of the maps and the dynamic spectra have been performed. First, we found several pairs of one-to-one (TID-SC) correspondences. The study provides strong observational evidence supporting the suggestion that MSTIDs are the cause of SCs.

DOI： 10.3847/1538-4357/ab1b52

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Space Weather  

We report an extreme erosion of the plasmasphere arising from the September 2017 storm. The cold electron density is identified from the upper limit frequency of upper hybrid resonance waves observed by the Plasma Wave Experiment instrument onboard the Exploration of energization and Radiation in Geospace/Arase satellite. The electron density profiles reveal that the plasmasphere was severely eroded during the recovery phase of the storm and the plasmapause was located at L = 1.6–1.7 at 23 UT 8 September 2017. This is the first report of deep erosion of the plasmasphere (LPP < 2) with the in situ observation of the electron density. The degree of the severity is much more than what is expected from the relatively moderate value of the SYM-H minimum (−146 nT). We attempt to find a possible explanation for the observed severe depletion by using both observational evidence and numerical simulations. Our results suggest that the middle latitude electric field had penetrated from the high-latitude storm time convection for several hours. Such an unusually long-lasting penetration event can cause this observed degree of severity.

DOI： 10.1029/2019SW002168

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

In order to statistically investigate the relationship between the low-latitude coronal holes (CHs), the solar wind speed, and the geomagnetic activity in solar cycles 23 (1996–2008) and 24 (2009–2016), we conducted a superposed epoch analysis of the variations in CH area, solar winds, the interplanetary magnetic field (IMF), and geomagnetic indices (AL, AU, and SYM-H) for the period from 1996 to 2016. We further divided the temporal variations of the IMF into four types and then investigated the variations in solar winds, the IMF, and the geomagnetic indices before and after the corotating interaction region (CIR) reached Earth’s magnetosphere in each case. As a result, we observed a north–south asymmetry in the CH area, which shows that the CH area was much larger in the southern hemisphere than in the northern hemisphere during solar cycles 23 and 24. In addition, the CH area for solar cycle 24 tended to appear in a wider latitude region compared with that for solar cycle 23. The maximum values of the CH area and the solar wind speed in solar cycle 24 tended to be smaller than those in solar cycle 23. The relationship between these maximum values showed a positive correlation for both solar cycles. The distribution was larger for solar cycle 23 than for solar cycle 24. The variations in solar wind speed and the geomagnetic indices (AE and SYM-H) associated with CIRs in solar cycle 24 tended to be smaller than those in solar cycle 23. We conclude that the geomagnetic activity for solar cycle 24 associated with CIRs was slightly lower compared with that for solar cycle 23. This decrease in geomagnetic activity was due to a decrease in the dawn-to-dusk solar wind electric field intensity, which is obtained as the product of the solar wind speed and the north–south component of the solar wind magnetic field.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-019-1005-y

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Space Science Reviews  

With the advent of the Heliophysics/Geospace System Observatory (H/GSO), a complement of multi-spacecraft missions and ground-based observatories to study the space environment, data retrieval, analysis, and visualization of space physics data can be daunting. The Space Physics Environment Data Analysis System (SPEDAS), a grass-roots software development platform (www.spedas.org), is now officially supported by NASA Heliophysics as part of its data environment infrastructure. It serves more than a dozen space missions and ground observatories and can integrate the full complement of past and upcoming space physics missions with minimal resources, following clear, simple, and well-proven guidelines. Free, modular and configurable to the needs of individual missions, it works in both command-line (ideal for experienced users) and Graphical User Interface (GUI) mode (reducing the learning curve for first-time users). Both options have “crib-sheets,” user-command sequences in ASCII format that can facilitate record-and-repeat actions, especially for complex operations and plotting. Crib-sheets enhance scientific interactions, as users can move rapidly and accurately from exchanges of technical information on data processing to efficient discussions regarding data interpretation and science. SPEDAS can readily query and ingest all International Solar Terrestrial Physics (ISTP)-compatible products from the Space Physics Data Facility (SPDF), enabling access to a vast collection of historic and current mission data. The planned incorporation of Heliophysics Application Programmer’s Interface (HAPI) standards will facilitate data ingestion from distributed datasets that adhere to these standards. Although SPEDAS is currently Interactive Data Language (IDL)-based (and interfaces to Java-based tools such as Autoplot), efforts are under-way to expand it further to work with python (first as an interface tool and potentially even receiving an under-the-hood replacement). We review the SPEDAS development history, goals, and current implementation. We explain its “modes of use” with examples geared for users and outline its technical implementation and requirements with software developers in mind. We also describe SPEDAS personnel and software management, interfaces with other organizations, resources and support structure available to the community, and future development plans.

DOI： 10.1007/s11214-018-0576-4

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/s1121

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：2018 2nd URSI Atlantic Radio Science Meeting, AT-RASC 2018  

The Earth's plasmasphere is the vast 'doughnut' shaped dense plasma region of the magnetosphere that is filled with cold ions and electrons of ionospheric origin. The plasmaspheric plasma density shows an abrupt drop by a factor of 5 or more around 4-6 Re (Re: Earth's radius). The boundary has been called 'plasmapause'. Since the plasmaspheric cold plasma controls generation of plasma waves, their propagation features, and particle acceleration via wave-particle interaction, detailed investigation of the temporal and spatial variations of the plasmasphere and plasmapause location during a geomagnetic storm is important for understanding a change in plasma wave environments in the inner magnetosphere. Recent studies showed a good correlation between the mid-latitude ionospheric trough and the plasmapause for both geomagnetically quiet and disturbed conditions [1]. However, they did not reach the detailed investigation of the characteristics of spatial variation of the mid-latitude ionospheric trough and its temporal variation with high resolution due to limitation of a usage of global GIM of TEC. In this study, we investigate characteristics of temporal and spatial variations of the mid-latitude ionospheric trough during a geomagnetic storm which occurred on April 4, 2017 using the 5-min average Global Navigation Satellite System (GNSS) Total Electron Content (TEC) data together with solar wind, interplanetary magnetic field, geomagnetic field, and Arase High Frequency Analyzer (HFA) (subcomponent of Plasma Wave Experiment (PWE)) observation data. As a result, the location of the mid-latitude ionospheric trough moves equatorward from 60 to 48 degrees within 4 hours after the onset of the storm main phase. The movement speed increases from 1.3 to 3.5 degrees of geomagnetic latitude per hour after the onset of storm-time substorm. The increasing speed means an abrupt shrink of the plasmasphere due to a sudden enhancement of convection electric field in the inner magnetosphere associated with the substorm onset. The location of the mid-latitude ionospheric trough identified from the minimum value of the GNSS-TEC data from the auroral to mid-latitude regions is almost in good agreement with that of an abrupt drop of electron density derived from the upper limit frequency of the upper hybrid resonance (UHR) waves detected by the HFA instrument onboard the Arase satellite. In this case, the electron density profile along the Arase orbit shows an irregular variation of the electron density near the plasmapause. During the main phase of the geomagnetic storm, the geomagnetic longitude distribution of the mid-latitude trough location shows a wavy structure with its scale of 1000-2500 km. The shape of the wavy structure varies with time during the storm main phase. This phenomenon has not yet been reported before due to the limitation of ground dense GNSS receiver networks. After the start of the recovery phase of the geomagnetic storm, the mid-latitude ionospheric trough quickly moves poleward from 48 to 60 degrees in geomagnetic latitude within 4 hours in a geomagnetic longitude range of 310-360 degrees in geomagnetic longitude. The average speed of the poleward movement is 2.3 degrees of geomagnetic latitude per hour.

DOI： 10.23919/URSI-AT-RASC.2018.8471490

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

Temporal and spatial variations of the midlatitude ionospheric trough during a geomagnetic storm on 4 April 2017 have been investigated using Global Navigation Satellite System total electron content data together with Arase satellite observations. After the geomagnetic storm commencement, the trough minimum location moves equatorward from 60 to 48° in geomagnetic latitude within 4 hr. The trough minimum location identified from the Global Navigation Satellite System total electron content data is located near the footprint of an abrupt drop of electron density detected by the Arase High-Frequency Analyzer instrument. The longitudinal variation of the trough minimum location shows a significant variation with a scale of 1,000–2,500 km during both storm and quiet times. This phenomenon has not yet been reported by previous studies. After the onset of the storm recovery phase, the trough minimum location rapidly moves poleward back to the quiet time location within 4 hr.

DOI： 10.1029/2018GL078723

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Disaster Research  

Two-dimensional ionospheric total electron content (TEC) maps have been derived from ground-based Global Navigation Satellite System (GNSS) receiver networks and applied to studies of various ionospheric disturbances since the mid-1990s. For the purpose of monitoring and researching ionospheric conditions and ionospheric space weather phenomena, we have developed TEC maps of areas over Japan using the dense GNSS network, GNSS Earth Observation NETwork (GEONET), which consists of about 1300 stations and is operated by the Geospatial Information Authority of Japan (GSI). Currently, we are providing high-resolution, two-dimensional maps of absolute TEC, detrended TEC, rate of TEC change index (ROTI), and loss-of-lock on GPS signal over Japan on a real-time basis. Such high-resolution TEC maps using dense GNSS receiver networks are one of the most effective ways to observe, on a scale of several 100 km to 1000 km, ionospheric variations caused by traveling ionospheric disturbances and/or equatorial plasma bubbles, which can degrade single-frequency and differential GNSS positioning/navigation. We have collected all the available GNSS receiver data in the world to expand the TEC observation area. Currently, however, dense GNSS receiver networks are available in only limited areas, such as Japan, North America, and Europe. To expand the two-dimensional TEC observation with high resolution, we have conducted the Dense Regional and Worldwide International GNSS TEC observation (DRAWING-TEC) project, which is engaged in three activities: (1) standardizing GNSS-TEC data, (2) developing a new high-resolution TEC mapping technique, and (3) sharing the standardized TEC data or the information of GNSS receiver network. We have developed a new standardized TEC format, GNSS-TEC EXchange (GTEX), which is included in the Formatted Tables of ITU-R SG 3 Data-banks related to Recommendation ITU-R P.311. Sharing the GTEX TEC data would be easier than sharing the GPS/GNSS data among those in the international ionospheric researcher community. The DRAWING-TEC project would promote studies of medium-scale ionospheric variations and their effect on GNSS.

DOI： 10.20965/jdr.2018.p0535

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CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

This paper summarizes the specifications and initial evaluation results of Wire Probe Antenna (WPT) and Electric Field Detector (EFD), the key components for the electric field measurement of the Plasma Wave Experiment (PWE) aboard the Arase (ERG) satellite. WPT consists of two pairs of dipole antennas with ~ 31-m tip-to-tip length. Each antenna element has a spherical probe (60 mm diameter) at each end of the wire (15 m length). They are extended orthogonally in the spin plane of the spacecraft, which is roughly perpendicular to the Sun and enables to measure the electric field in the frequency range of DC to 10 MHz. This system is almost identical to the WPT of Plasma Wave Investigation aboard the BepiColombo Mercury Magnetospheric Orbiter, except for the material of the spherical probe (ERG: Al alloy, MMO: Ti alloy). EFD is a part of the EWO (EFD/WFC/OFA) receiver and measures the 2-ch electric field at a sampling rate of 512 Hz (dynamic range: ± 200 mV/m) and the 4-ch spacecraft potential at a sampling rate of 128 Hz (dynamic range: ± 100 V and ± 3 V/m), with the bias control capability of WPT. The electric field waveform provides (1) fundamental information about the plasma dynamics and accelerations and (2) the characteristics of MHD and ion waves in various magnetospheric statuses with the magnetic field measured by MGF and PWE–MSC. The spacecraft potential provides information on thermal electron plasma variations and structure combined with the electron density obtained from the upper hybrid resonance frequency provided by PWE–HFA. EFD has two data modes. The continuous (medium-mode) data are provided as (1) 2-ch waveforms at 64 Hz (in apoapsis mode, L > 4) or 256 Hz (in periapsis mode, L < 4), (2) 1-ch spectrum within 1–232 Hz with 1-s resolution, and (3) 4-ch spacecraft potential at 8 Hz. The burst (high-mode) data are intermittently obtained as (4) 2-ch waveforms at 512 Hz and (5) 4-ch spacecraft potential at 128 Hz and downloaded with the WFC-E/B datasets after the selection. This paper also shows the initial evaluation results in the initial observation phase.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-017-0760-x

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

The plasmas (electrons and ions) in the inner magnetosphere have wide energy ranges from electron volts to mega-electron volts (MeV). These plasmas rotate around the Earth longitudinally due to the gradient and curvature of the geomagnetic field and by the co-rotation motion with timescales from several tens of hours to less than 10 min. They interact with plasma waves at frequencies of mHz to kHz mainly in the equatorial plane of the magnetosphere, obtain energies up to MeV, and are lost into the ionosphere. In order to provide the global distribution and quantitative evaluation of the dynamical variation of these plasmas and waves in the inner magnetosphere, the PWING project (study of dynamical variation of particles and waves in the inner magnetosphere using ground-based network observations, http://www.isee.Nagoya-u.ac.jp/dimr/PWING/) has been carried out since April 2016. This paper describes the stations and instrumentation of the PWING project. We operate all-sky airglow/aurora imagers, 64-Hz sampling induction magnetometers, 40-kHz sampling loop antennas, and 64-Hz sampling riometers at eight stations at subauroral latitudes (~ 60° geomagnetic latitude) in the northern hemisphere, as well as 100-Hz sampling EMCCD cameras at three stations. These stations are distributed longitudinally in Canada, Iceland, Finland, Russia, and Alaska to obtain the longitudinal distribution of plasmas and waves in the inner magnetosphere. This PWING longitudinal network has been developed as a part of the ERG (Arase)-ground coordinated observation network. The ERG (Arase) satellite was launched on December 20, 2016, and has been in full operation since March 2017. We will combine these ground network observations with the ERG (Arase) satellite and global modeling studies. These comprehensive datasets will contribute to the investigation of dynamical variation of particles and waves in the inner magnetosphere, which is one of the most important research topics in recent space physics, and the outcome of our research will improve safe and secure use of geospace around the Earth.[Figure not available: see fulltext.]

DOI： 10.1186/s40623-017-0745-9

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Geophysical Union ({AGU})  

Characteristics of seasonal variation and solar activity dependence of the X and Y components of the geomagnetic solar quiet (Sq) daily variation at Memambetsu in midlatitudes and Guam near the equator have been investigated using long-term geomagnetic field data with 1 h time resolution from 1957 to 2016. The monthly mean Sq variation in the X and Y components (Sq-X and Sq-Y) shows a clear seasonal variation and solar activity dependence. The amplitude of seasonal variation increases significantly during high solar activities and is proportional to the solar F10.7 index. The pattern of the seasonal variation is quite different between Sq-X and Sq-Y. The result of the correlation analysis between the solar F10.7 index and the Sq-X and Sq-Y shows an almost linear relationship, but the slope of the linear fitted line varies as a function of local time and month. This implies that the sensitivity of Sq-X and Sq-Y to the solar activity is different for different local times and seasons. The pattern of the local time and seasonal variations of Sq-Y at Guam shows good agreement with that of a magnetic field produced by interhemispheric field-aligned currents (FACs), which flow from the summer to winter hemispheres in the dawn and dusk sectors and from the winter to summer hemispheres in the prenoon to afternoon sectors. The direction of the interhemispheric FAC in the dusk sector is opposite to the concept of Fukushima's model.

DOI： 10.1002/2017JA024342

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

We investigate spatial and temporal evolution of large-scale electric fields in the magnetosphere and ionosphere associated with sudden commencements (SCs) using multipoint equatorial magnetospheric (THEMIS, RBSP, and GOES) and ionospheric (C/NOFS) satellites with radars (SuperDARN). A distinct SC event on 17 March 2013 shows that the magnetospheric electric field in the equatorial plane propagates from dayside toward nightside as a fast-mode wave. The ionospheric electric field responds ~41 s after the onset of dayside magnetospheric electric field, which can be explained by the propagation of the Alfvén wave along magnetic field lines. The wavelet analysis shows that the Alfvén wave is dominant in the plasmasphere. Poynting fluxes toward the ionosphere support these propagations. From a statistical analysis of response time, tailward propagation speed is estimated at about 1000–1100 km/s. We also statistically derive a spatial distribution and time evolution of the magnetospheric electric field in the dawn-dusk direction (Ey). Our result shows that negative Ey (dawnward) propagates from noon toward the magnetotail, followed by positive Ey (duskward). The propagation characteristics of electric fields in the equatorial plane depend on magnetic local time. At noon, negative Ey lasts for about 1 min, and positive Ey becomes dominant about 2 min after the SC onset. Negative Ey soon attenuates in the nightside region, while the positive Ey propagates fairly well to the premidnight or postmidnight regions while maintaining a certain amplitude. The enhancement of positive Ey is due to the enhancement of magnetospheric convection associated with the main impulse of SCs.

DOI： 10.1002/2017JA023990

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その他リンク： http://orcid.org/0000-0002-8160-3553

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：論文集(書籍)内論文   出版者・発行元：John Wiley & Sons, Inc.  

DOI： 10.1002/9781119066880.ch7

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD  

The statistics of gravity wave momentum flux estimation are investigated using data from the MU radar at Shigariki, Japan (136 degrees E, 35 degrees N). The radar has been operating during campaign periods since 1986. The first part of the paper focuses on a multi-day campaign during October 13-31, 1986. The second part of the paper investigates data after 2006 when the radar was operated in a meteor scatter mode. Momentum fluxes are derived from both the turbulent scatter and the meteor scatter measurements, but the techniques are quite different. Probability Distribution Functions are formed using turbulent scatter data. These show that wave packets sometimes have momentum flux magnitudes in excess of 100 m(2) s(-2). The technique for meteor radars, introduced by Hocking (2005), has been widely adopted by the radar community in recent years. The momentum flux estimated using this technique is found to be anti-correlated with the background tidal winds. A validation investigation is carried out for periods with a high meteor echo data rate. The conclusion was that the method can be used to calculate the sign of momentum flux, but does not accurately specify the magnitude. (C) 2016 The Authors. Published by Elsevier Ltd.

DOI： 10.1016/j.jastp.2016.01.013

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER HEIDELBERG  

The local time variation of geomagnetic sudden commencements (SCs) has not been taken into account in the Siscoe's linear relationship which connects the SC amplitude with the corresponding dynamic pressure variation of the solar wind. By considering the physical background of SC, we studied which local time is best to extract the information of the solar wind dynamic pressure and concluded that the SC amplitude at 4-5 h local time of middle and low-latitude stations most directly reflects the dynamic pressure effect. This result is used to re-check the order of magnitude of the largest 3 SCs observed since 1868.

DOI： 10.1186/s40623-016-0444-y

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA022101

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

The ionospheric plasma in midlatitude moves upward/downward during the geomagnetic sudden commencement causing the HF Doppler frequency changes; SCF (+ -) and (- +) on the dayside and nightside, respectively, except for the SCF (+ -) in the evening as found by Kikuchi et al. (1985). Although the preliminary and main frequency deviations (PFD, MFD) of the SCF have been attributed to the dusk-to-dawn and dawn-to-dusk potential electric fields, there still remain questions if the positive PFD can be caused by the compressional magnetohydrodynamic (MHD) wave and what causes the evening anomaly of the SCF. With the HF Doppler sounder, we show that the dayside ionosphere moves upward toward the Sun during the main impulse (MI) of the SC, when the compressional wave is supposed to push the ionosphere downward. The motion of the ionosphere is shown to be correlated with the equatorial electrojet, matching the potential electric field transmitted with the ionospheric currents from the polar ionosphere. We confirmed that the electric field of the compressional wave is severely suppressed by the conducting ionosphere and reproduced the SC electric fields using the global MHD simulation in which the potential solver is employed. The model calculations well reproduced the preliminary impulse and MI electric fields and their evening anomaly. It is suggested that the electric potential is transmitted from the polar ionosphere to the equator by the zeroth-order transverse magnetic (TM0) mode waves in the Earth-ionosphere waveguide. The near-instantaneous transmission of the electric potential leads to instantaneous global response of the incompressible ionosphere.

DOI： 10.1002/2015JA022166

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：COPERNICUS GESELLSCHAFT MBH  

Two nearly identical meteor radars were operated at Koto Tabang (0.20 degrees S, 100.32 degrees E), West Sumatra, and Biak (1.17 degrees S, 136.10 degrees E), West Papua, in Indonesia, separated by approximately 4000 km in longitude on the Equator. The zonal and meridional momentum flux, u'w' and v'w', where u, v, and w are the eastward, northward, and vertical wind velocity components, respectively, were estimated at 86 to 94 km altitudes using the meteor radar data by applying a method proposed by Hocking (2005). The observed u'w' at the two sites agreed reasonably well at 86, 90, and 94 km during the observation periods when the data acquisition rate was sufficiently large enough. Variations in v'w' were consistent between 86, 90, and 94 km altitudes at both sites. The climatological variation in the monthly averaged u'w' and v'w' was investigated using the long-term radar data at Koto Tabang from November 2002 to November 2013. The seasonal variations in u'w' and v'w' showed a repeatable semiannual and annual cycles, respectively. u'w' showed eastward values in February-April and July-September and v'w' was northward in June to August at 90-94 km, both of which were generally anti-phase with the mean zonal and meridional winds, having the same periodicity. Our results suggest the usefulness of the Hocking method.

DOI： 10.5194/angeo-34-369-2016

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

A statistical study using a large dataset of Super Dual Auroral Radar Network (SuperDARN) observations is conducted for transient ionospheric plasma flows associated with sudden impulses (SI) recorded on ground magnetic field. The global structure of twin vortex-like ionospheric flows is found to be consistent with the twin vortices of ionospheric Hall current deduced by the past geomagnetic field observations. An interesting feature, which is focused on in this study, is that the flow structures show a dawn-dusk asymmetry depending on the combination of the polarity of SI and interplanetary magnetic field (IMF)-By. Detailed statistics of the SuperDARN observations reveal that the dawn-dusk asymmetry of flow vortices due to IMF-By appears during negative SIs, while such asymmetric characteristics are not seen during positive SIs. On the basis of the upstream observations, we suggest that this particular dawn-dusk asymmetry is caused by the interaction between the pre-existing round convection cell and a pair of the transient convection vortices associated with SIs.

DOI： 10.1186/s40623-015-0360-6

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Using in situ observations from the Republic of China Satellite-1 spacecraft, we investigated the time response and local time dependence of the ionospheric electric field at mid-low latitudes associated with geomagnetic sudden commencements (SCs) that occurred from 1999 to 2004. We found that the ionospheric electric field variation associated with SCs instantaneously responds to the preliminary impulse (PI) signature on the ground regardless of spacecraft local time. Our statistical analysis also supports the global instant transmission of electric field from the polar region. In contrast, the peak time detected in the ionospheric electric field is earlier than that of the equatorial geomagnetic field (~20s before in the PI phase). Based on the ground-ionosphere waveguide model, this time lag can be attributed to the latitudinal difference of ionospheric conductivity. However, the local time distribution of the initial excursion of ionospheric electric field shows that dusk-to-dawn ionospheric electric fields develop during the PI phase. Moreover, the westward electric field in the ionosphere, which produces the preliminary reverse impulse of the geomagnetic field on the dayside feature, appears at 18-22h LT where the ionospheric conductivity beyond the duskside terminator (18h LT) is lower than on the dayside. The result of a magnetohydrodynamic simulation for an ideal SC shows that the electric potential distribution is asymmetric with respect to the noon-midnight meridian. This produces the local time distribution of ionospheric electric fields similar to the observed result, which can be explained by the divergence of the Hall current under nonuniform ionospheric conductivity. Key Points Ionospheric electric fields respond instantaneously and globally Instant transmission of electric field is explained by the waveguide model Nonuniform ionospheric conductivity affects LT dependence of electric field

DOI： 10.1002/2015JA021309

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

Background: This paper presents an overview of the capacity-building activities and science-enabling services of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project. This Japanese program, which started in 2009, is building a metadata database (MDDB) of ground-based observations and is developing an analysis software to handle the data linked to the MDDB system for use by the solar-terrestrial physics community. Because the institutional members of the IUGONET are mainly universities in Japan, we explore tools that can contribute to advanced education as well as promote research activities. Findings: In this paper, we describe the utilities of the IUGONET for education, including our capacity-building activities in developing countries. We have regularly facilitated training seminars for Japanese students on the use of our tools (IUGONET MDDB and the software), and we have held capacity-building seminars for young scientists in developing countries. In addition to the MDDB, we have prepared various 'gateway' tools for users who are unfamiliar with 'keywords' to search for data. One of these is a geographical display tool that uses Google Earth (KML file), which is included as supplemental material to this paper. The usefulness of the IUGONET has been proven over its first 5 years of operation by the increasing number of its users, which has led to the production of approximately 500 scientific papers, including 42 thesis papers. Conclusions: The IUGONET community collaborates with the Scientific Committee on Solar-Terrestrial Physics program, not only in its scientific activities, but also in the establishment of E-infrastructure and capacity building.

DOI： 10.1186/s40623-014-0170-2

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

Characteristics of long-term variation in the amplitude of solar quiet (Sq) geomagnetic field daily variation have been investigated using 1-h geomagnetic field data obtained from 69 geomagnetic observation stations within the period of 1947 to 2013. The Sq amplitude observed at these geomagnetic stations showed a clear dependence on the 10- to 12-year solar activity cycle and tended to be enhanced during each solar maximum phase. The Sq amplitude was the smallest around the minimum of solar cycle 23/24 in 2008 to 2009. The relationship between the solar F10.7 index and Sq amplitude was approximately linear but about 53% of geomagnetic stations showed a weak nonlinear relation to the solar F10.7 index. In order to remove the effect of solar activity seen in the long-term variation of the Sq amplitude, we calculated a linear or second-order fitting curve between the solar F10.7 index and Sq amplitude during 1947 to 2013 and examined the residual Sq amplitude, which is defined as the deviation from the fitting curve. As a result, the majority of trends in the residual Sq amplitude that passed through a trend test showed negative values over a wide region. This tendency was relatively strong in Europe, India, the eastern part of Canada, and New Zealand. The relationship between the magnetic field intensity at 100-km altitude and residual Sq amplitude showed an anti-correlation for about 71% of the geomagnetic stations. Furthermore, the residual Sq amplitude at the equatorial station (Addis Ababa) was anti-correlated with the absolute value of the magnetic field inclination. This implies movement of the equatorial electrojet due to the secular variation of the ambient magnetic field.

DOI： 10.1186/s40623-014-0155-1

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Computer Physics Communications  

The Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project focuses on handling ground-based observational data of the upper atmosphere. To this end, the project members have been developing a data analysis software package which is based on Interactive Data Language (IDL). Filling the spatial gaps in observational data requires the use of numerical models. In this paper,wediscuss an IDL software package for global ionospheric conductivity by integration of 3rd party numerical models. The model can be used to create further derived models.

DOI： 10.1016/j.cpc.2014.08.011

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Data Science Journal  

An overview of the Interuniversity Upper atmosphere Global Observation NETwork (IUGONET) project is presented. This Japanese program is building a meta-database for ground-based observations of the upper atmosphere, in which metadata connected with various atmospheric radars and photometers, including those located in both polar regions, are archived. By querying the metadata database, researchers are able to access data file/information held by data facilities. Moreover, by utilizing our analysis software, users can download, visualize, and analyze upper-atmospheric data archived in or linked with the system. As a future development, we are looking to make our database interoperable with others.

DOI： 10.2481/dsj.IFPDA-07

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

Background: The Interuniversity Upper atmosphere Global Observation NETwork (IUGONET) project is a 6-year research project which started in 2009. The objective of this project is to establish a metadata database of various ground-based observation data covering a wide region from the Sun to the Earth; this will encourage more studies on the mechanisms of long-term variations in the upper atmosphere. Findings: For archiving purposes, the metadata database system for cross-searching various data distributed across many universities and institute was developed based on the existing repository software called DSpace as the core component and the Space Physics Archive Search and Extract (SPASE) data model as the metadata format. The IUGONET metadata database is still in operation since it was released in March 2012. The system is continuously examined, tested, and updated to improve its quality. The OpenSearch interface in the IUGONET metadata database allows the user to use external applications easily for exchanging metadata and/or for analyzing data. Conclusions: We conducted self-examination of our product, which was added for planning future directions of the IUGONET project.

DOI： 10.1186/1880-5981-66-133

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

超高層大気に於ける地球規模の物理現象の機構を解明する為には,様々なデータを用いた分野横断的な研究推進が重要である.インターネット上の様々なデータを直ちに取得する為の基盤が,分野横断的な研究の1つの鍵となる.IUGONETプロジェクトは,複数の研究機関によって分散管理されている,様々なデータの URL等の所在情報等を提供可能なメタデータ・データベースを構築することによって,この課題に対処した.しかしながら,このメタデータ・データベースは広範な研究領域を対象としている為,専門分野外のユーザーにとって単語検索時における適切な検索語句の選択が容易でないことが指摘された.これを補助するべく,検索語句の関連語の取得とそれらを用いた再検索クエリ文字列の自動生成を行う為に,そのメタデータ・データベースと連想検索エンジン GETAssocを連携させた.そして,この連想検索用の辞書を作成する為に, SAO/NASA Astrophysics Data Systemからメタデータを抽出した.再クエリ文字列を生成する為に用意した辞書を用いることで,期待通りの連想検索結果を得ることが出来た.

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Data Science Journal  

An overview of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project is presented with a brief description of the products to be developed. This is a Japanese inter-university research program to build the metadata database for ground-based observations of the upper atmosphere. The project also develops the software to analyze the observational data provided by various universities/institutes. These products will be of great help to researchers in efficiently finding, obtaining, and utilizing various data dispersed across the universities/institutes. This is expected to contribute significantly to the promotion of interdisciplinary research, leading to more a comprehensive understanding of the upper atmosphere.

DOI： 10.2481/dsj.WDS-030

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Ubiquity Press Ltd  

An overview of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project is presented with a brief description of the products to be developed. This is a Japanese inter-university research program to build the metadata database for ground-based observations of the upper atmosphere. The project also develops the software to analyze the observational data provided by various universities/institutes. These products will be of great help to researchers in efficiently finding, obtaining, and utilizing various data dispersed across the universities/institutes. This is expected to contribute significantly to the promotion of interdisciplinary research, leading to more a comprehensive understanding of the upper atmosphere.

DOI： 10.2481/dsj.WDS-030

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

本論文では,大学間連携プロジェクト「超高層大気長期変動の全球地上ネットワーク観測・研究」IUGONET)において、作成されたメタデータの受付やメタデータ・データベースへの登録を行っている「メタデータ登録・管理システム」のメタデータ登録処理に関する性能試験の結果について報告する.メタデータ登録・管理システムは,フリーのバージョン管理ソフトウェアであるGit を利用したメタデータ受付部と,そこからメタデータ・データベースへメタデータを登録するメタデータインポート部の,2つの処理部で構成される.この2つの処理部の性能を評価するために,実際に作成・登録されたメタデータを用いて,物理マシンおよび仮想マシン上に構築した Linux環境のそれぞれについて,メタデータの処理速度を計測した.その結果,Gitによるメタデータ受付部は物理マシンか仮想マシンかに依らず,メタデータ総数が500万件レベルまで増えても数分以内で処理が完了するのに対して,メタデータインポート部は処理時間が数時間から数十時間とかなり長くなり,特に検索エンジンであるLuceneのインデックス更新については,物理マシンにおける実行速度が仮想マシンのそれより約 4倍高速であることがわかった.メタデータインポート部を物理マシン上に実装した場合,メタデータ総数 500万件では,新規登録メタデータ数10万件につき5時間弱プラスLuceneインデックス更新で21時間ほど処理時間がかかると見積もられる.

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

大学間連携プロジェクト「超高層大気長期変動の全球地上ネットワーク観測・研究」(IUGONET)では,IUGONET参加機関である東北大学,名古屋大学,京都大学,九州大学および国立極地研究所の5 機関が所有する多種多様な超高層大気データを可視化・解析できるデータ解析ソフトウェアUDAS(iUgonet Data Analysis Software)を開発している.UDAS は,IDL 言語で書かれたTHEMIS ミッションのデータ解析ソフトウェアTDAS(THEMIS Data Analysis Software suite)のプラグインソフトであり,TDAS が持つ様々な時系列データの可視化・解析ツールやGUI 機能を利用することができる.我々は,2011 年5 月よりIUGONET ウェブベージにおいてUDAS のβ バージョンの公開を開始し,2012 年2月に本公開を開始した.2012 年10 月以降には,UDAS をTDAS に組み込み,THEMIS ソフトウェアウェブサイトから公開することを計画している.UDAS を提供するにあたり,プログラムの動作確認作業を軽減するためのテストツールの開発も行った.さらに,IDL ライセンスを持たない研究者のために,IDL Virtual Machine 環境で走るTDAS の実行ファイルを開発し,テスト公開を開始した.これらの新しい展開により,UDAS 及びIUGONET 所属機関が所有する地上観測データの利用が促進されることが期待される.

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

大学間連携プロジェクト「超高層大気長期変動の全球地上ネットワーク観測・研究」(IUGONET)では,IUGONET参加機関である東北大学,名古屋大学,京都大学,九州大学および国立極地研究所の5 機関が所有する多種多様な超高層大気データを可視化・解析できるデータ解析ソフトウェアUDAS(iUgonet Data Analysis Software)を開発している.UDAS は,IDL 言語で書かれたTHEMIS ミッションのデータ解析ソフトウェアTDAS(THEMIS Data Analysis Software suite)のプラグインソフトであり,TDAS が持つ様々な時系列データの可視化・解析ツールやGUI 機能を利用することができる.我々は,2011 年5 月よりIUGONET ウェブベージにおいてUDAS のβ バージョンの公開を開始し,2012 年2月に本公開を開始した.2012 年10 月以降には,UDAS をTDAS に組み込み,THEMIS ソフトウェアウェブサイトから公開することを計画している.UDAS を提供するにあたり,プログラムの動作確認作業を軽減するためのテストツールの開発も行った.さらに,IDL ライセンスを持たない研究者のために,IDL Virtual Machine 環境で走るTDAS の実行ファイルを開発し,テスト公開を開始した.これらの新しい展開により,UDAS 及びIUGONET 所属機関が所有する地上観測データの利用が促進されることが期待される.

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：CODATA  

An overview of the Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project is presented with a brief description of the products to be developed. This is a Japanese inter-university research program to build the metadata database for ground-based observations of the upper atmosphere. The project also develops the software to analyze the observational data provided by various universities/institutes. These products will be of great help to researchers in efficiently finding, obtaining, and utilizing various data dispersed across the universities/institutes. This is expected to contribute significantly to the promotion of interdisciplinary research, leading to more a comprehensive understanding of the upper atmosphere.

DOI： 10.2481/dsj.WDS-030

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：日本語   掲載種別：研究論文（研究会，シンポジウム資料等）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

The spatial evolution of vortex-like flow structures induced by a negative sudden impulse (SI-) is studied on the basis of SuperDARN King Salmon HF radar (KSR) with other ground and satellite data. A large dip in the solar wind density induced a fairly large SI-with a SYM-H amplitude of ∼40 nT. The SI-induced ionospheric flow signatures in the evening sector (MLT ∼ 19 h) were observed by KSR as a westward flow associated with the preliminary impulse (PI) followed by a more intense eastward flow with the main impulse (MI) in the sub-auroral region of the magnetic latitude ∼60-70 deg, consistent with the local ground magnetic field observations. Following the first PI-MI flow sequence, KSR saw a second and possibly third sequence of flow variation which were much smaller in flow amplitude than the first pair but showed qualitatively very similar flow variations and latitudinal/longitudinal propagation characteristics. These observations can be interpreted as aftershocks of the first PI-MI; the same sequence of vortices and field-aligned currents were generated and then drifted anti-sunward with the same mechanism, namely the pumping motion of the dayside magnetosphere. These results are qualitatively consistent with predictions suggested by recent numerical simulations. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA018093

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

To determine the characteristics and origin of observed storm-time electron density enhancements in the polar cap, and to investigate the spatial extent (noon-midnight direction) of associated O⁺ ion outflows, we analyzed nearly simultaneous observations of such electron density enhancements from the Akebono satellite and ion upflows from the Polar satellite during a geomagnetic storm occurring on 6 April 2000. The Akebono satellite observed substantial electron density enhancements by a factor of ∼10-90 with a long duration of ∼15 h at ∼2 R<inf>E</inf> in the southern polar region. The Polar satellite outflow measurements in the northern polar cap at ∼7-4 R<inf>E</inf> exhibited velocity filtering of the ∼100 eV to ∼0 eV (from the spacecraft potential) ion outflow from the cleft ion fountain, with resultant temperatures declining from ∼3 eV to 0.03 eV with increasing distance from the cusp. Similar velocity filtering was detected in the southern polar cap at ∼1.8-3.5 R<inf>E</inf>. The region of O⁺ ion outflows with fluxes exceeding 5×10⁸/cm²/s (mapped to 1000 km altitude) extended ∼10 MLAT (∼1000 km) at the ionosphere from the cusp/cleft into the dayside polar cap at ∼2.5 R<inf>E</inf>. These coordinated Akebono-Polar observations are consistent with the development of storm-time electron density enhancements in the polar cap as a result of the bulk outflow of low-energy plasma as part of the cleft ion fountain. The large spatial scale, large ion fluxes, and the long duration indicate significant supply of very-low-energy O⁺ ions to the magnetosphere through this region. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017900

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

The magnetic local time and latitude dependence of amplitude of the main impulse (MI) of geomagnetic sudden commencements (SCs) and its seasonal variation have been investigated using high time resolution (1-3 sec) geomagnetic data in the latitudinal range 27-70 degrees for the period 1996-2010. The daytime distribution of the SC-MI amplitude in the sub-auroral and middle latitudes (35-60 degrees) is similar to the DP-2 type geomagnetic variation which shows negative and positive changes in the morning and afternoon, respectively. The magnetic field variation is reversed around the magnetic latitude of 63-65 degrees. This suggests that a pair of field-aligned currents (FACs), resembling the region-1 (R-1) FACs, is located near the magnetic latitude of 63-65 degrees. The nighttime SC amplitude is enhanced significantly in the low and middle latitudes (27-60 degrees). The enhancement is due to the magnetic effect produced by the SC-MI FACs. In the nighttime auroral latitude (63-65 degrees), the SC amplitude decreases steeply due to the enhanced westward auroral electrojet associated with the compression of the magnetosphere. The size of the diurnal variation tends to increase significantly during the summer, compared with that during the winter. This seasonal variation suggests that the DP-2 type ionospheric currents (ICs) and FACs generated during the SC-MI phase are intensified by increased ionospheric conductivities during the summer. It can be concluded that the large-scale MI current system in the ionosphere and magnetosphere is voltage generator. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA018006

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

In order to clarify the global distribution of ionospheric currents during a geomagnetic storm, we analyzed ground magnetic disturbances from high latitudes to the magnetic equator for the storm on September 7-8, 2002, with the minimum SYM-H value of -168 nT. In this analysis, we investigated magnetic field deviations in the northward component from the SYM-H, as functions of the dipole magnetic latitude (DMLAT) and the magnetic local time (MLT). During the main phase of the storm, the deviations at the low latitudes (10 35 in DMLAT) were positive/negative in the dawn/dusk (0-9/11-24 h MLT) sector. On the other hand, the deviations at the dayside middle latitudes (35-55 in DMLAT) were negative/positive in the morning/afternoon (6-12/13-15 h MLT) sector. The local time distribution at the low latitudes may represent the dawn-dusk asymmetry of the storm time ring current, while that at the dayside middle latitudes coincides with the DP2 currents due to the convection electric field associated with the Region 1 field-aligned currents (R1 FACs). All over the nightside middle latitude, the deviations were positive. This implies the direct effect of the R1 FACs through the Biot-Savart's law. At the geomagnetic equator, the eastward and westward electrojets were intensified on the day and nightside, respectively, being caused by the penetrated dawn-to-dusk convection electric field. We found that the MLT distribution of the magnetic deviations during the recovery phase was in opposite sense to that during the main phase at the dayside middle latitudes. The reversed magnetic disturbances must be due to the overshielding electric field associated with the Region 2 field-aligned currents (R2 FACs). Similarly, the deviations at the dayside and nightside equator were reversed, indicating penetration of the dusk-to-dawn overshielding electric field into the equatorial ionosphere. Based on the above results, we propose a current system including the ionospheric currents at middle latitudes caused by the R1/R2 FACs, equatorial EEJ/CEJ, and asymmetric ring current, during the main/recovery phase of the geomagnetic storm. © 2012. American Geophysical Union.

DOI： 10.1029/2012JA017566

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

J-GLOBAL

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

J-GLOBAL

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

著者一同は,平成21 年度から6 年計画で行っている「超高層大気長期変動の全球地上ネットワーク観測・研究(略称IUGONET)」プロジェクトの開発者として,超高層大気長期変動に関する地上観測データのメタデータ・データベースを開発中である.観測データの所在情報に代表される様々なメタデータをデータベース化することにより,複数の機関によって分散管理されている多様な観測データに対するアクセシビリティの向上をもたらす事がIUGONET プロジェクトの目的のひとつである.我々は,SPASE コンソーシアムによって策定されたSPASE データ・モデル/メタデータ・フォーマットをベースに,さらに拡張を施したIUGONET 共通メタデータ・フォーマットを策定した.そして,フリーのリポジトリ・ソフトウェアであり,デフォルトではDublin Core メタデータ・フォーマットのみを取り扱うDSpace に対し,IUGONET 共通メタデータ・フォーマットを取り扱えるようにカスタマイズを施した.本論文では,サイエンスの為のメタデータ・データベースの適用事例のひとつとして,IUGONET メタデータ・データベースについて述べる.

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

本論文では,大学間連携プロジェクト「超高層大気長期変動の全球地上ネットワーク観測・研究」(IUGONET) で策定・公開されたメタデータフォーマットと,そのフォーマットに沿って策定されたメタデータの登録・管理を行うシステムの概要について報告する.IUGONETでは,米国やヨーロッパの研究者で構成されるコンソーシアムが策定したSpace Physics Archive Search and Extract (SPASE) データモデル/メタデータフォーマットをベースにして,超高層大気分野の様々な地上観測データに対応できるように拡張を施すことにより,IUGONET共通メタデータフォーマットを策定した.またIUGONET研究機関で作成されるメタデータを登録・管理するために,フリーのバージョン管理ソフトウェアであるGIT を用いた,メタデータXMLファイルの登録・履歴管理のシステムを構築した.これらを用いることにより,IUGONETでは,参加機関が生産し続ける観測データについてのメタデータの,作成・収集・アーカイブを行っている.

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

本論文では,大学間連携プロジェクト「超高層大気長期変動の全球地上ネットワーク観測・研究」(略称:IUGONET)の概要と主要な開発プロダクトについて述べる.IUGONETプロジェクトでは,国立極地研究所,東北大学,名古屋大学,京都大学,および九州大学が連携し,これまでに蓄積された多様な地上観測データに関するメタデータのデータベースを構築する. メタデータ・データベースによって,各機関・組織に分散して存在する観測データの横断的な検索・取得を可能にし,観測データの効率的な流通,さらには分野横断型の総合解析による新しい超高層大気研究を促進する.プロジェクトによる開発では,メタデータのフォーマットとしてSPASE (Space Physics Archive Search and Extract)を採用し,超高層大気の地上観測に即した変更を加えることでIUGONET共通メタデータフォーマットを策定する. メタデータ・データベースについては,DSpaceをベースにIUGONETのメタデータに対応したカスタマイズを施すことでシステムを構築する.一方,データ解析ソフトウェアに関しては,IDL (Interactive Data Language)で書かれたTDAS (THEMIS Data Analysis Software suite)をベースに開発を行う.

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：宇宙航空研究開発機構  

本論文では,大学間連携プロジェクト「超高層大気長期変動の全球地上ネットワーク観測・研究」(IUGONET)で開発された解析ソフトウェア(UDAS)の概要について報告する.UDASは,IUGONET参加機関である東北大学,名古屋大学,京都大学,九州大学および国立極地研究所の5機関が所有する超高層大気データの可視化・解析ソフトウェアである.UDASは,IDL (Interactive Data Language) で書かれたTHEMISミッションの統合データ解析ツールTDAS (THEMIS Data Analysis Software suite)のプラグインソフトであり,TDASの機能を利用することで,IUGONET所属機関が所有する種類の異なる複数の時系列データの可視化・解析を容易にする.また,初心者でも利用し易いように,GUI (Graphical User Interface) も提供する.UDASは,2011年5月よりIUGONETウェブベージでβバージョンがリリースされている.さらに,UDAS開発の今後の展望について述べる.

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Toward the understanding of the effect of the magnetosphere originated disturbances on the global ionospheric electric field and current system, we developed a two-dimensional ionospheric potential solver based on the so-called "thin shell model." The important extension from the previous studies is that our model covers the pole-to-pole ionosphere without placing any boundary at the equator. By using this solver, we investigate how the ionospheric electric field changes from undershielding condition to overshielding condition as the field aligned current (FAC) distribution changes. Calculations are performed by changing IR2/IR1 (the ratio of current intensities of region 2 (R2) and region 1 (R1) FACs) and by moving R2-FAC relative to the fixed R1-FAC. The results are summarized as follows: (1) The turning point, at which the ionosphere turns from undershielding to overshielding is IR2/IR1 = 0.7 ∼ 0.8. (2) With increasing the local time deference between the R1 and R2-FAC peaks, the efficiency of the shielding by R2-FAC increases but the associated potential skews to the nightside. (3) At the same time the shielding effect is weakened around noon, where the R1-potential intrudes to the low latitude region instead, but the R2-potential remains dominant at other local times. The result suggests that the overshielding or undershielding should be identified by observations not only in a limited local time sector but also in the overall ionosphere as much as possible. In order to accurately describe the ionospheric condition, we suggest new classification terms, "complete-overshielding" and "incomplete-overshielding. " © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017669

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

We constructed an empirical model of the electron density profile with solar zenith angle (SZA) dependence in the polar cap during geomagnetically quiet periods using 63 months of Akebono satellite observations at solar maximum. The electron density profile exhibits a transition at ∼2000 km altitude only under dark conditions. The electron density and scale height at low altitudes change drastically, by factors of 25 (at 2300 km altitude) and 2.0, respectively, as the SZA increases from 90 to 120. The SZA dependence of the ion and electron temperatures is also investigated statistically on the basis of data obtained by the Intercosmos satellites and European Incoherent Scatter (EISCAT) Svalbard radar (ESR). A drastic change in the electron temperature occurs near the terminator, similarly to that in the electron density profile obtained by the Akebono satellite. The sum of the ion and electron temperatures obtained by the ESR (∼6500 K at ∼1050 km altitude under sunlit conditions and ∼3000 K at ∼750 km altitude under dark conditions) agrees well with the scale height at low altitudes obtained from the Akebono observations, assuming that the temperature is constant and that O + ions are dominant. Comparisons between the present statistical results (SZA dependence of the electron density and ion and electron temperatures) and modeling studies of the polar wind indicate that the plasma density profile (especially of the O+ ion density) in the polar cap is strongly controlled by solar radiation onto the ionosphere by changing ion and electron temperatures in the ionosphere during geomagnetically quiet periods. Copyright © 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016631

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/978-94-007-0326-1_34

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Observations of Poynting fluxes associated with onset of convection electric fields are essential for understanding of electromagnetic energy transport from the solar wind toward the magnetosphere leading to changes in the convection electric field, which is one of the most fundamental parameters in the magnetosphere-ionosphere coupled system. We present Cluster multispacecraft observations of Poynting fluxes associated with abrupt changes in large-scale electric fields during sudden commencements and southward turning of the interplanetary magnetic field (IMF). The Cluster spacecraft detected Poynting fluxes dominated by the field-aligned upward component during the preliminary impulse of sudden commencements and in the initial period after southward turning of the IMF. The upward Poynting flux indicates existence of Alfvn waves transporting electromagnetic energy from the ionosphere toward the magnetosphere leading to magnetospheric convection changes. The waveguide model and global magnetohydrodynamic (MHD) simulation calculating evolution of the Poynting flux following solar wind pressure enhancements also show upward Poynting fluxes propagating from the ionosphere toward the magnetosphere faster than the propagation of compressional waves. We conclude that the ionosphere acts as a channel to transmit electromagnetic energy supplied as field-aligned currents toward a wide region in the magnetosphere-ionosphere system instantaneously, leading to changes in magnetospheric convection electric fields. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2010JA015491

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Velocity distributions of upflowing ions in the polar ionosphere are crucial to understand their destinations. Natural plasma wave observations by the plasma wave and sounder experiments and thermal ion observations by the suprathermal ion mass spectrometer onboard the Akebono satellite at ∼9000 km altitude in the polar magnetosphere during the geomagnetic storms showed that ions in the region of enhanced electron density in the polar cap were dominated by very-low-energy O+ ions (∼85%) with upward velocities of 4-10 km s-1, corresponding to streaming energies of 1.3-8.4 eV. The fluxes of very-low-energy upflowing O+ ions exceeded 1 × 10 9 cm-2 s-1 (mapped to 1000 km altitude) across wide regions. These signatures are consistent with high-density plasma supplied by the cleft ion fountain mechanism. Trajectory calculations of O+ ions based on the Akebono observations as the initial condition showed the transport paths and accelerations of the O+ ions and indicated that the velocities of the very-low-energy upflowing O+ ions through the dayside polar cap are enough to reach the magnetosphere under strong convection. The calculations suggest the importance of the very-low-energy upflowing O + ions with large fluxes in the total O+ ion supply toward the magnetosphere, especially the near-Earth tail region and inner magnetosphere. The initially very-low-energy O+ ions can contribute significantly to the ring current formation during geomagnetic storms since some of the O+ ions were transported into the ring current region with typical energies of ring current ions (several tens of keV) in the trajectory calculations. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2010JA015601

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Occurrence features of the preliminary impulse (PI) of geomagnetic sudden commencement (SC) both in the Pacific Ocean and South Atlantic Anomaly (SAA) regions were investigated using the long-term magnetic field data obtained from the Circum-pan Pacific Magnetometer Network (CPMN) and NICT Space Weather Monitoring (NSWM) magnetometer networks. The low-latitude preliminary reverse impulse (PRI) at Okinawa (OKI: dip latitude = 37.97°) in the Pacific Ocean region appeared in all the magnetic local time (MLT) sectors with the peak occurrence rate of 40% around noon. On the other hand, the PRI occurrence rate at Santa Maria (SMA: dip latitude = -34.35°) near the center of the SAA region showed a significant enhancement in the daytime sector (0800-1600 h, MLT) with the peak value of 80%, which resembles the occurrence feature of the equatorial PRI. Moreover, the PRI amplitude around noon at SMA was about 3.0 times larger than that at OKI. From the calculation of the ionospheric conductivity derived from the IRI-2007 and NRLMSISE-00 models, it is shown that the height-integrated conductivity was more enhanced in the SAA region (SMA), where the ambient magnetic field intensity is weak, compared with that in the Pacific Ocean region (OKI). Therefore, the anomalous increase of the PRI occurrence and amplitude is caused by the significant enhancement of the ionospheric conductivity due to the weakness of the ambient magnetic field intensity in the SAA region. Copyright © 2010 by the American Geophysical Union.

DOI： 10.1029/2009JA015035

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2009JA014499

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2009JA014562

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

The transient response of convection electric fields in the inner magnetosphere to southward turning of the interplanetary magnetic field (IMF) is investigated using in-situ electric field observations by the CRRES and Akebono spacecraft. Electric fields earthward of the inner edge of the electron plasma sheet show quick responses simultaneously with change in ionospheric electric fields, which indicates the arrival of the first signal related to southward turning. A coordinated observation of the electric field by the CRRES and Akebono spacecraft separated by 5 RE reveals a simultaneous increase in the dawn-dusk electric field in a wide region of the inner magnetosphere. A quick response associated with the southward turning of the IMF is also identified in in-situ magnetic fields. It indicates that the southward turning of the IMF initiates simultaneous (less than 1 min) enhancements of ionospheric electric fields, convection electric fields in the inner magnetosphere, and the ring or tail current and region 2 FACs. In contrast, a quick response of convection electric fields is not identified in the electron plasma sheet. A statistical study using 161 events of IMF orientation change in 1991 confirms a prompt response within 5 min for 80% of events earthward of the electron plasma sheet, while a large time lag of more than 30 min is identified in electric fields in the electron plasma sheet. The remarkable difference in the response of electric fields indicates that electric fields in the electron plasma sheet are weakened by high conductance in the magnetically conjugated auroral ionosphere. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2009JA014277

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2008JA013871

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Statistical analysis of the main impulse (MI) amplitude of geomagnetic sudden commencements (SCs) in a region from the middle latitudes to equator has been made using the long-term geomagnetic field data obtained from the Yap (geomagnetic latitude, θ = 0.38°), Guam (θ = 5.22°), Okinawa (θ = 16.54°), Kakioka (θ = 27.18°), Memanbetsu (θ = 35.16°), and St. Paratunka (θ = 45.58°) stations. The magnetic local time (MLT) dependence of SC amplitude in the middle latitudes showed magnetic field variations produced by two-cell ionospheric currents (DP 2-type currents) which are driven by the dawn-to-dusk electric field accompanying a pair of field-aligned currents (FACs). The effect of the DP 2-type currents at least expands to the low latitude (θ = 16.54°). In this region, the DL part of SC produced by the enhanced Chapman-Ferraro currents can be dominant, but the DP part of SC contaminated 7% of the DL one. On the other hand, at the daytime equator between 8:00 and 16:00 (MLT), the SC amplitude is considerably enhanced with its peak amplitude of 3.24 (normalized SYM-H value) around 11:00 (MLT) due to the Cowling effect. Another interesting point is that the SC amplitude in the nighttime sector was enhanced at all the stations again, and its peak value increases with increasing magnetic latitude. This result suggests that the effect of the FACs associated with the MI phase of SC expands to the equator. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013871

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Meridional electron density distributions above 45 invariant latitude (ILAT) during geomagnetically quiet periods are statistically studied. Electron density data were obtained from plasma waves observed by the Akebono satellite from March 1989 to February 1991 (near solar maximum) in an altitude range of 274-10,500 km. Field-aligned electron density profiles were fitted by the sum of exponential and power law functions. The transition height, where the power law term equals the exponential term, is highest in the summer (at low solar zenith angle (SZA)) at ∼4000 km and lowest in the winter (at high SZA) at ∼1800 km in a region of ILAT ≥ 70; this is caused by the larger scale height in the summer (∼550 km) than that in the winter (∼250 km). The largest seasonal variation and SZA dependence of the electron density are found at an altitude of ∼2000 km with a factor of ∼50 (∼104 /cc in the summer, ∼103 /cc in the winter) in the trough, auroral, and polar cap regions. The seasonal variation and SZA dependence are smaller, about a factor of 5-10, above ∼5000 km. Day-night asymmetries in each season (within a factor of 5) are smaller than the seasonal variation. The scale height is larger in the dayside than in the nightside in each season. These results indicate that photoionization processes in the ionosphere strongly control electron density distributions up to at least ∼5000 km in the trough, auroral, and polar cap regions. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013288

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2008JA013871

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Faculty of Science, Kyushu University  

We present the low-latitude ionospheric electric field during the main impulse (MI) phase of two geomagnetic sudden commencements (SCs), observed for the first time by an FM-CW radar. From our observation, the direction of the ionospheric electric field is eastward in the daytime and westward in the nighttime sectors. This difference indicates the penetration of the dawn-to-dusk polar electric field associated with the SCs into the low-latitude ionosphere.

DOI： 10.5109/11811

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Terra Scientific Pub. Co.  

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Space Physics  

Electric field and potential distributions in the inner magnetosphere during geomagnetic storms have been investigated using the Akebono/EFD data. Using this electric field, we study injection of ring current particles and acceleration of radiation belt electrons by single-particle calculations. During the main phase, the dawn-dusk electric field is intensified especially in a range of 2 < L < 5 with a maximum amplitude of 6 mV/m on the duskside, and a two-cell convection pattern with a potential difference of 180 kV is identified. The convection pattern on the equatorial plane is significantly distorted with a large potential drop of 70 kV on the dawn and dusk sectors, indicating an intrinsic source of large-scale electric field in the inner magnetosphere. The plasma sheet ions are gathered into the dusk to premidnight sector in the inner magnetosphere in the region of enhanced electric field due to the strong E × B drift. The ions are transported into around 4 R E with an acceleration of more than 1 order of magnitude within 40 min, conserving the first adiabatic invariants. Relativistic electrons with initial energy of some hundreds of kiloelectron volts at 5 RE are energized to more than 100 keV for 3 hours. The energy spectrum during the recovery phase of 9 October 1990 geomagnetic storm observed by the CRRES satellite is reproduced without the radial diffusion or nonadiabatic acceleration by plasma waves. It is possible that this acceleration process is the inhomogeneity of the large-scale electric field, which corresponds to the ∇ × E term along orbits of electrons around the Earth. Copyright 2007 by the American Geophysical Union.

DOI： 10.1029/2006JA012177

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Terra Scientific Pub. Co.  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

Analysis of the plasma wave observation data provided by the plasma waves and sounder experiment (PWS) on board the Akebono satellite frequently reveals the presence of electrostatic electron cyclotron harmonic (ESCH) waves in the low-latitude region (MLAT < 45°) of the plasmasphere within an altitude range from about 3000 km to the apogee of the satellite (initial apogee was 10,500 km). Even at moderate or low geomagnetic activity, intense ESCH waves often appear near the equatorial region of the plasmasphere above the upper hybrid resonance (UHR) frequency at the lowest harmonic number branch of the fQn ESCH waves. We identified these plasma waves as the equatorial plasmasphere fQn waves (EP-fQn). The spectra of the EP-fQn waves are characterized by a narrow band structure and by a strong nature, with a wave intensity that ranges from 3.46 × 10-8 to 3.31 × 10-4 V/m. The maximum intensity is nearly coincident with the upper limit of the PWS receiver in the low-gain mode. Statistical analysis results reveal that the EP-fQn waves are observable in all the local time sectors; however, the occurrence probability shows a clear enhancement in the early morning sector of 01-03 MLT in the plasmasphere. The EP-fQn wave activities are suppressed within a period of strong magnetic disturbances as well as solar minimum phase. The linear dispersion relation analysis using a two-component plasma model reveals that supra-thermal plasma with the energy of about 750 eV and with a large temperature anisotropy (A = T-perp/T-parallel - 1 > 40) must be present in order to realize an appearance of a positive growth rate at the observed frequency and propagation angle of the ESCH waves. Since the hot plasma with such a high anisotropy has not been detected, the validity of the present two-component plasma model remains an open question. The occurrence feature of the ESCH waves showed that there is a constant activation or a constant flow-in of free energy to generate the strong plasma instability of ESCH waves near the post-midnight sector of the plasmasphere. The existence of ESCH waves revealed that the nature of the plasmaspheric plasma is more turbulent and active than has been believed. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.

DOI： 10.1186/BF03352723

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記述言語：日本語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

In order to clarify the generation mechanism of Z-mode waves observed in the equatorial plasmasphere, the growth rate of Z-mode electromagnetic waves has been calculated under the higher-order cyclotron interaction process. Z-mode waves can interact with some tens of keV electrons with large pitch angles even in the dense cold background, and the amplitude is consistent with the Akebono plasma wave measurements. UHR and whistler mode waves are also excited by the same electron distribution, and this is also consistent with observations. The origin of these energetic electrons are identified as the ring current electrons injected into the plasmasphere by the intense large-scale electric field during geomagnetic storms, accelerated perpendicular to the ambient magnetic field and confined around the geomagnetic equator conserving the first and second adiabatic invariants. Since the intensity of Z-mode and UHR waves is associated with the development and decay of the ring current, ring current particles are most possible candidate for the free energy source of these waves. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.

DOI： 10.1186/BF03352043

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

The distribution of the storm-time electric field in the inner magnetosphere has been investigated by using the Akebono/EFD data within a period from 1989 to 1995. During the main phase of geomagnetic storms, the strong electric field appears between L = 2 and L = 6 in both dawn and dusk sectors with the magnitude from 2 to 4 mV/m, and the maximum value appears at L = 3 in the dusk sector. During the recovery and weakly disturbed period, the region with intense electric field moves outward with decreasing the amplitude. This structure of the localized electric field is quite different from the Volland-Stern electric field model, suggesting the existence of another electric field source other than the large-scale convection electric field. During quiet periods, although the electric field fairly follows the corotation electric field, the distribution at L = 3.5 indicates a small deviation from pure corotation. Copyright 2006 by the American Geophysical Union.

DOI： 10.1029/2006GL027510

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.asr.2005.05.082

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCI LTD  

Electric field variations in the inner magnetosphere and plasmasphere regions associated with sudden commencements (SCs) are investigated by using the observation data of the Akebono satellite which has been carried out more than 15 years since 1989. 117 of 153 SC events in the low-latitude (MLAT &lt; 45 degrees) region, which occurred within a period from March 1989 to January 1996, showed a shift of the magnetospheric convection electric field with the magnitude of 0.1-3.2 mV/m about 1 min after the electric field signature with a bi-polar waveform due to the passage of fast-mode hydromagnetic (HM) waves. The increase of the convection electric field takes place in the entire magnetic local time sector in the inner magnetosphere. The amplitude does not depend on L-value and magnetic local time but is proportional to the SC amplitude measured at Kakioka. The majority of the electric field enhancements persist for about 4-14 min. The origin of the convection electric field in the inner magnetosphere is a plasma motion caused by the compression of the magnetosphere due to the solar wind shock and discontinuity. (C) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2005.05.082

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Berlin  

In order to investigate spatial and temporal variations of fast and slow Z-mode waves frequently observed in the equatorial plasmasphere, statistical studies have been performed by using plasma wave observation data obtained by the Akebono satellite within a period from 1989 to 1995. It has been clarified that fast and slow Z-mode waves are intensified within ±5° of geomagnetic latitudes in an altitude range from 6000 km to the apogee (10500 km) of the satellite without obvious local time dependence. Long-term averaged intensity of fast Z-mode waves has almost the same orders of magnitude as that of slow Z-mode waves. These results indicate that significant part of fast Z-mode waves are not produced by the linear mode conversion process from slow Z-mode waves, but excited by more direct process. Furthermore, the region of intensified fast and slow Z-mode waves has been spread in a wider geomagnetic latitude range of ±10° during geomagnetic storms. These evidences suggest that one of the possible free energy sources is ring current particles injected into the equatorial region of the plasmasphere during geomagnetic storms. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS)
The Seismological Society of Japan
The Volcanological Society of Japan
The Geodetic Society of Japan
The Japanese Society for Planetary Sciences
TERRAPUB.

DOI： 10.1186/BF03351930

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

Variations of cold plasma density distribution and large-scale electric field in the inner magnetosphere and plasmasphere during a geomagnetic storm were investigated by using the observation data of the Akebono satellite which has been carried out for more than 15 yeas since March, 1989. We focus on the super geomagnetic storm on March 13-15, 1989, for which the maximum negative excursion of the Dst index was -589 nT. During the main phase of the magnetic storm, the strong convection electric field with a spatially inhomogeneous structure appears in the inner magnetosphere between L = 2.0 and 7.0. The averaged intensity of the electric field was in a range of about 2.5-9.2 mV/m. The spatial distribution in the magnetic equatorial region indicates that the magnitude within an L-value range of 2.2-7.0 is much larger than that observed at L = 7.0-10.0. Associated with the appearance of the strong convection electric field, the cold plasma density near the trough region around L = 3.0-6.0 was enhanced with one or two order magnitude, compared with that in the magnetically quiet condition. This implies that a mount of the ionospheric plasma may be supplied from the topside ionosphere into the trough and plasmasphere regions by the frictional heating due to the fast plasma convection in the ionosphere as pointed out by previous studies on the enhancements of plasma density in these regions, based on incoherent scatter radar and total electron content (TEC) observations (e.g., Yeh and Foster, 1990; Foster et al., 2004). During the recovery phase of the magnetic storm, the convection electric field observed in the inner magnetosphere and plasmasphere regions recovers within 3-4 days almost up to the level of the magnetically quiet condition.

DOI： 10.1186/BF03351843

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：TERRA SCIENTIFIC PUBL CO  

Electric and magnetic field variations inside the plasmasphere associated with SCs identified on the ground are analyzed based on the Akebono satellite observations which have been carried out more than 13 years since March 1989. 126 electric field observation data corresponding to SCs show abrupt change of intensity as well as direction within a few minutes inside the plasmasphere. Temporal variations of the electric field showed a bipolar waveform with the amplitude range of 0.2-38 mV/m. The electric field signature is followed by a dumping oscillation with the period of Pc3-4 ranges. The magnetic field variations of 33 SCs also show an abrupt increase of 0.2-65 nT within a few minutes, which indicate the compression of the magnetosphere due to the discontinuity of solar wind. The initial excursion of the electric field during SCs tends to be directed westward. The amplitude does not show a dependence on magnetic local time that has been observed outside the plasmasphere. The magnitude of the electric field variations tends to be proportional with the power of 0.6 to the magnetic field variation in the plasmasphere. The Poynting vector of the initial SC impulse is directed toward the earth, which suggests that energy of magnetic disturbances associated with SCs propagates toward the earth inside the plasmasphere with the refraction due to the plasma density gradient. One of the most interesting results from the present study is that a DC offset of the Ey component of the electric field appears after the initial electric field impulse associated with SCs. This signature is interpreted to be a magnetospheric convection electric field penetration into the inner plasmasphere (L=2.5). The intensity of the offset of the Ey field gradually increases by 0.5-2.0 mV/m about 1-2 minutes after the onset of the initial electric field impulse and persists about 10-30 minutes.

DOI： 10.1186/BF03353409

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：AMER GEOPHYSICAL UNION  

Plasma wave phenomena associated with sudden commencements (SC) are analyzed using the database of the Akebono satellite observations that have been carried out for more than 13 years since March 1989. All the 719 data sets simultaneously observed in the periods of SC events show that plasma waves are enhanced with one-to-one correspondence to SCs in entire regions of the polar cap, auroral zone, and plasmasphere within a response time of +/-90 s. In the middle latitude and equatorial regions of the plasmasphere, intensification and frequency shift of electromagnetic whistler mode, LHR waves, and ion cyclotron harmonic waves are found. The electric field variations in this region also show clear response to the onset of SCs with the amplitude of 0.2-30 mV/m. The variations are observed in the nightside as well as dayside sectors, and no clear dependence of magnetic latitude, local time, and L shell is found. On the other hand, electrostatic whistler mode waves and electromagnetic ion cyclotron waves which show broadband spectra are generated in the high-latitude region. Spectra of low-energy particles observed simultaneously with the enhancement of these plasma waves show that electron fluxes are enhanced in all of pitch angle bins of the low-energy particle detector onboard the Akebono satellite. Near the cusp region, the ion fluxes are more enhanced in the upward direction than in the downward direction along the magnetic field lines. In about half of the cases of the high-latitude events, sudden appearance and intensification of AKR are also found after the onsets of SC. The delay time between the onsets of SC and AKR enhancement shows several minutes with the average time of 5.7 min. Time differences between the onsets of SC measured at Kakioka Magnetic Observatory and plasma wave enhancements observed by the Akebono satellite show positive correlation with possible delay time according to the propagation route of SC disturbances. Propagation character of SC disturbances shows two group signatures: one group takes a route which crosses the geomagnetic equator region with an average speed of 389.5 km/s. The speed is almost consistent with plasmaspheric fast-mode MHD wave velocity. The other group takes a route which starts from the dayside cusp region. In the second case, the SC disturbances propagate through the polar ionosphere region from the dayside to the nightside sectors with an average speed of 47 km/ s in the X-GSM coordinate corresponding to the ionospheric fast-mode MHD wave velocity.

DOI： 10.1029/2003JA009964

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

Plasma wave phenomena associated with sudden commencements (SCs) were analyzed using the database of Akebono satellite observations that has been contributed to for more than 13 years, since March 1989. The plasma wave data from 263 satellite passages covering the onsets of SCs included 85 cases of auroral kilometric radiation (AKR) enhancement within a frequency range of 100kHz to 1.2MHz. The majority of the spectra of the SC-related AKR exhibited a two-banded structure with a harmonic relationship. The start time of the AKR enhancements tended to occur after the onset of the SCs, determined using the geomagnetic records of the Kakioka Magnetic Observatory, within a time range of 3 to 8min, with an average delay time of 5.26min. Based on this delay time feature, the magnetic disturbances associated with SCs were thought to propagate from the dayside magnetosphere to the nightside tail region where they compressed the plasma sheet. On the other hand, the data set reveals 19 cases of terrestrial hectometric radiation (THR) that were also associated with SCs appearing within a frequency range of 900kHz to 4MHz. The THR onset tended to occur 1 to 9min after the SC onset, with an average delay time of 5.84min.

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立極地研究所  

Plasma wave phenomena associated with sudden commencements (SCs) were analyzed based on observations conducted with the Akebono satellite, which has been collecting data for more than 13 years (since March 1989). Simultaneous plasma wave observation data for 257 SCs reveal that enhanced plasma waves are observed with an exact one-to-one correspondence with the SCs throughout the entire observation region, including the polar and plasmasphere regions. Electromagnetic whistler mode and ion cyclotron waves are enhanced in the low latitude plasmasphere, while electrostatic whistler mode and electromagnetic ion cyclotron waves are generated in the polar region. The onset times of the SC-triggered plasma waves exhibit a delay or lead time characteristic, compared with the onset times of SCs identified by the Kakioka Magnetic Observatory, with a time resolution of 1 s. By comparing the difference in SCs and enhanced electron plasma waves onset times, the propagation route of the SC disturbances can be identified in the plasmasphere.

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記述言語：英語  

J-GLOBAL

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記述言語：英語  

記述言語：英語  

DOI： 10.1186/s40623-016-0444-y

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記述言語：英語  

DOI： 10.5194/angeo-34-369-2016

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記述言語：英語  

DOI： 10.1002/2015JA022101

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記述言語：日本語  

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記述言語：日本語  

記述言語：英語   掲載種別：講演資料等（セミナー，チュートリアル，講習，講義他）  

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記述言語：日本語   掲載種別：講演資料等（セミナー，チュートリアル，講習，講義他）  

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記述言語：日本語   掲載種別：講演資料等（セミナー，チュートリアル，講習，講義他）  

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記述言語：英語  

DOI： 10.1016/j.jastp.2016.01.013

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記述言語：英語  

The ionospheric plasma in midlatitude moves upward/downward during the geomagnetic sudden commencement causing the HF Doppler frequency changes; SCF (+-) and (-+) on the dayside and nightside, respectively, except for the SCF (+-) in the evening as found by Kikuchi et al. (1985). Although the preliminary and main frequency deviations (PFD, MFD) of the SCF have been attributed to the dusk-to-dawn and dawn-to-dusk potential electric fields, there still remain questions if the positive PFD can be caused by the compressional magnetohydrodynamic (MHD) wave and what causes the evening anomaly of the SCF. With the HF Doppler sounder, we show that the dayside ionosphere moves upward toward the Sun during the main impulse (MI) of the SC, when the compressional wave is supposed to push the ionosphere downward. The motion of the ionosphere is shown to be correlated with the equatorial electrojet, matching the potential electric field transmitted with the ionospheric currents from the polar ionosphere. We confirmed that the electric field of the compressional wave is severely suppressed by the conducting ionosphere and reproduced the SC electric fields using the global MHD simulation in which the potential solver is employed. The model calculations well reproduced the preliminary impulse and MI electric fields and their evening anomaly. It is suggested that the electric potential is transmitted from the polar ionosphere to the equator by the zeroth-order transverse magnetic (TM0) mode waves in the Earth-ionosphere waveguide. The near-instantaneous transmission of the electric potential leads to instantaneous global response of the incompressible ionosphere.

DOI： 10.1002/2015JA022166

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記述言語：日本語   掲載種別：講演資料等（セミナー，チュートリアル，講習，講義他）  

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J-GLOBAL

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記述言語：日本語   出版者・発行元：京都大学生存圏研究所  

地球を取り巻く大気圏(対流圏・成層圏・中間圏・熱圏)、およびその周辺の宇宙圏(電離圏・プラズマ圏・磁気圏・惑星間空間)の基本構造と環境は、太陽から放射されるエネルギー(太陽放射、太陽風)を入力とした平衡状態によって規定される。しかしながら、太陽放射エネルギーは常に一定ではなく、太陽活動に応じて変動するため、大気圏・宇宙圏で観測される長期的な環境変動が太陽活動によるものか、人為起源によるものかを見極めるには、長期にわたる様々な地上・衛星観測データを統合解析することによって、太陽活動に対するこれらの領域の応答過程を解明し、定量的に一つのシステムとして捉える必要がある。このような研究を促進させるためには、様々な地上・衛星観測データを一元的に取り扱うデータ解析システムが必要不可欠となるが、2009 年5 月から開始された大学間連携プロジェクト「Inter-university Upper atmosphere Global Observation NETwork: IUGONET」において、様々な地上観測データを相互参照し、可視化・相関解析を行う研究インフラを開発してきた。現在では、多くの地上観測データが公開され、このデータ解析システムを駆使して大気圏・宇宙圏の短期・長期変動に関する研究を行える環境にある。本解説では、IUGONET プロジェクトの概要について述べ、これまで行ってきた大気圏・宇宙圏の太陽活動に対する応答とそのメカニズムに関する研究を紹介する。

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その他リンク： http://hdl.handle.net/2433/225787

記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：英語  

DOI： 10.1186/s40623-015-0360-6

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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Using in situ observations from the Republic of China Satellite-1 spacecraft, we investigated the time response and local time dependence of the ionospheric electric field at mid-low latitudes associated with geomagnetic sudden commencements (SCs) that occurred from 1999 to 2004. We found that the ionospheric electric field variation associated with SCs instantaneously responds to the preliminary impulse (PI) signature on the ground regardless of spacecraft local time. Our statistical analysis also supports the global instant transmission of electric field from the polar region. In contrast, the peak time detected in the ionospheric electric field is earlier than that of the equatorial geomagnetic field (similar to 20s before in the PI phase). Based on the ground-ionosphere waveguide model, this time lag can be attributed to the latitudinal difference of ionospheric conductivity. However, the local time distribution of the initial excursion of ionospheric electric field shows that dusk-to-dawn ionospheric electric fields develop during the PI phase. Moreover, the westward electric field in the ionosphere, which produces the preliminary reverse impulse of the geomagnetic field on the dayside feature, appears at 18-22h LT where the ionospheric conductivity beyond the duskside terminator (18h LT) is lower than on the dayside. The result of a magnetohydrodynamic simulation for an ideal SC shows that the electric potential distribution is asymmetric with respect to the noon-midnight meridian. This produces the local time distribution of ionospheric electric fields similar to the observed result, which can be explained by the divergence of the Hall current under nonuniform ionospheric conductivity.

DOI： 10.1002/2015JA021309

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：英語  

記述言語：日本語   出版者・発行元：日本気象学会  

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記述言語：英語  

記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

DOI： 10.2481/dsj.WDS-030

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：英語  

DOI： 10.2481/dsj.IFPDA-07

記述言語：英語  

DOI： 10.1186/1880-5981-66-133

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：英語  

DOI： 10.1016/j.cpc.2014.08.011

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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記述言語：日本語  

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記述言語：英語   掲載種別：研究発表ペーパー・要旨（国際会議）  

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J-GLOBAL

記述言語：日本語  

大学間連携プロジェクト「超高層大気長期変動の全球地上ネットワーク観測・研究」(IUGONET)では,IUGONET参加機関である東北大学,名古屋大学,京都大学,九州大学および国立極地研究所の5 機関が所有する多種多様な超高層大気データを可視化・解析できるデータ解析ソフトウェアUDAS(iUgonet Data Analysis Software)を開発している.UDAS は,IDL 言語で書かれたTHEMIS ミッションのデータ解析ソフトウェアTDAS(THEMIS Data Analysis Software suite)のプラグインソフトであり,TDAS が持つ様々な時系列データの可視化・解析ツールやGUI 機能を利用することができる.我々は,2011 年5 月よりIUGONET ウェブベージにおいてUDAS のβ バージョンの公開を開始し,2012 年2月に本公開を開始した.2012 年10 月以降には,UDAS をTDAS に組み込み,THEMIS ソフトウェアウェブサイトから公開することを計画している.UDAS を提供するにあたり,プログラムの動作確認作業を軽減するためのテストツールの開発も行った.さらに,IDL ライセンスを持たない研究者のために,IDL Virtual Machine 環境で走るTDAS の実行ファイルを開発し,テスト公開を開始した.これらの新しい展開により,UDAS 及びIUGONET 所属機関が所有する地上観測データの利用が促進されることが期待される.

J-GLOBAL

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記述言語：日本語   出版者・発行元：日本気象学会  

CiNii Books

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記述言語：英語  

The spatial evolution of vortex-like flow structures induced by a negative sudden impulse (SI-) is studied on the basis of SuperDARN King Salmon HF radar (KSR) with other ground and satellite data. A large dip in the solar wind density induced a fairly large SI- with a SYM-H amplitude of similar to 40 nT. The SI- induced ionospheric flow signatures in the evening sector (MLT similar to 19 h) were observed by KSR as a westward flow associated with the preliminary impulse (PI) followed by a more intense eastward flow with the main impulse (MI) in the sub-auroral region of the magnetic latitude similar to 60-70 deg, consistent with the local ground magnetic field observations. Following the first PI-MI flow sequence, KSR saw a second and possibly third sequence of flow variation which were much smaller in flow amplitude than the first pair but showed qualitatively very similar flow variations and latitudinal/longitudinal propagation characteristics. These observations can be interpreted as aftershocks of the first PI-MI; the same sequence of vortices and field-aligned currents were generated and then drifted anti-sunward with the same mechanism, namely the pumping motion of the dayside magnetosphere. These results are qualitatively consistent with predictions suggested by recent numerical simulations. Citation: Hori, T., A. Shinbori, N. Nishitani, T. Kikuchi, S. Fujita, T. Nagatsuma, O. Troshichev, K. Yumoto, A. Moiseyev, and K. Seki (2012), Evolution of negative SI- induced ionospheric flows observed by SuperDARN King Salmon HF radar, J. Geophys. Res., 117, A12223, doi:10.1029/2012JA018093.

DOI： 10.1029/2012JA018093

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記述言語：日本語  

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記述言語：英語  

To determine the characteristics and origin of observed storm-time electron density enhancements in the polar cap, and to investigate the spatial extent (noon-midnight direction) of associated O+ ion outflows, we analyzed nearly simultaneous observations of such electron density enhancements from the Akebono satellite and ion upflows from the Polar satellite during a geomagnetic storm occurring on 6 April 2000. The Akebono satellite observed substantial electron density enhancements by a factor of similar to 10-90 with a long duration of similar to 15 h at similar to 2 R-E in the southern polar region. The Polar satellite outflow measurements in the northern polar cap at similar to 7-4 R-E exhibited velocity filtering of the similar to 100 eV to similar to 0 eV (from the spacecraft potential) ion outflow from the cleft ion fountain, with resultant temperatures declining from similar to 3 eV to 0.03 eV with increasing distance from the cusp. Similar velocity filtering was detected in the southern polar cap at similar to 1.8-3.5 RE. The region of O+ ion outflows with fluxes exceeding 5 x 10(8) /cm(2)/s (mapped to 1000 km altitude) extended similar to 10 degrees MLAT (similar to 1000 km) at the ionosphere from the cusp/cleft into the dayside polar cap at similar to 2.5 RE. These coordinated Akebono-Polar observations are consistent with the development of storm-time electron density enhancements in the polar cap as a result of the bulk outflow of low-energy plasma as part of the cleft ion fountain. The large spatial scale, large ion fluxes, and the long duration indicate significant supply of very-low-energy O+ ions to the magnetosphere through this region.

DOI： 10.1029/2012JA017900

Web of Science

記述言語：英語   掲載種別：速報，短報，研究ノート等（学術雑誌）  

DOI： 10.1029/2012JA017669

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The magnetic local time and latitude dependence of amplitude of the main impulse (MI) of geomagnetic sudden commencements (SCs) and its seasonal variation have been investigated using high time resolution (1-3 sec) geomagnetic data in the latitudinal range 27-70 degrees for the period 1996-2010. The daytime distribution of the SC-MI amplitude in the sub-auroral and middle latitudes (35-60 degrees) is similar to the DP-2 type geomagnetic variation which shows negative and positive changes in the morning and afternoon, respectively. The magnetic field variation is reversed around the magnetic latitude of 63-65 degrees. This suggests that a pair of field-aligned currents (FACs), resembling the region-1 (R-1) FACs, is located near the magnetic latitude of 63-65 degrees. The nighttime SC amplitude is enhanced significantly in the low and middle latitudes (27-60 degrees). The enhancement is due to the magnetic effect produced by the SC-MI FACs. In the nighttime auroral latitude (63-65 degrees), the SC amplitude decreases steeply due to the enhanced westward auroral electrojet associated with the compression of the magnetosphere. The size of the diurnal variation tends to increase significantly during the summer, compared with that during the winter. This seasonal variation suggests that the DP-2 type ionospheric currents (ICs) and FACs generated during the SC-MI phase are intensified by increased ionospheric conductivities during the summer. It can be concluded that the large-scale MI current system in the ionosphere and magnetosphere is voltage generator.

DOI： 10.1029/2012JA018006

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In order to clarify the global distribution of ionospheric currents during a geomagnetic storm, we analyzed ground magnetic disturbances from high latitudes to the magnetic equator for the storm on September 7-8, 2002, with the minimum SYM-H value of -168 nT. In this analysis, we investigated magnetic field deviations in the northward component from the SYM-H, as functions of the dipole magnetic latitude (DMLAT) and the magnetic local time (MLT). During the main phase of the storm, the deviations at the low latitudes (10 degrees-35 degrees in DMLAT) were positive/negative in the dawn/dusk (0-9/11-24 h MLT) sector. On the other hand, the deviations at the dayside middle latitudes (35 degrees-55 degrees in DMLAT) were negative/positive in the morning/afternoon (6-12/13-15 h MLT) sector. The local time distribution at the low latitudes may represent the dawn-dusk asymmetry of the storm time ring current, while that at the dayside middle latitudes coincides with the DP2 currents due to the convection electric field associated with the Region 1 field-aligned currents (R1 FACs). All over the nightside middle latitude, the deviations were positive. This implies the direct effect of the R1 FACs through the Biot-Savart's law. At the geomagnetic equator, the eastward and westward electrojets were intensified on the day and nightside, respectively, being caused by the penetrated dawn-to-dusk convection electric field. We found that the MLT distribution of the magnetic deviations during the recovery phase was in opposite sense to that during the main phase at the dayside middle latitudes. The reversed magnetic disturbances must be due to the overshielding electric field associated with the Region 2 field-aligned currents (R2 FACs). Similarly, the deviations at the dayside and nightside equator were reversed, indicating penetration of the dusk-to-dawn overshielding electric field into the equatorial ionosphere. Based on the above results, we propose a current system including the ionospheric currents at middle latitudes caused by the R1/R2 FACs, equatorial EEJ/CEJ, and asymmetric ring current, during the main/recovery phase of the geomagnetic storm.

DOI： 10.1029/2012JA017566

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J-GLOBAL

記述言語：日本語  

J-GLOBAL

記述言語：日本語  

J-GLOBAL

記述言語：日本語  

J-GLOBAL

記述言語：日本語  

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記述言語：日本語  

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記述言語：日本語  

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記述言語：日本語   出版者・発行元：日本気象学会  

CiNii Books

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記述言語：日本語   出版者・発行元：京都大学大学院理学研究科附属天文台  

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We constructed an empirical model of the electron density profile with solar zenith angle (SZA) dependence in the polar cap during geomagnetically quiet periods using 63 months of Akebono satellite observations at solar maximum. The electron density profile exhibits a transition at similar to 2000 km altitude only under dark conditions. The electron density and scale height at low altitudes change drastically, by factors of 25 (at 2300 km altitude) and 2.0, respectively, as the SZA increases from 90 degrees to 120 degrees. The SZA dependence of the ion and electron temperatures is also investigated statistically on the basis of data obtained by the Intercosmos satellites and European Incoherent Scatter (EISCAT) Svalbard radar (ESR). A drastic change in the electron temperature occurs near the terminator, similarly to that in the electron density profile obtained by the Akebono satellite. The sum of the ion and electron temperatures obtained by the ESR (similar to 6500 K at similar to 1050 km altitude under sunlit conditions and similar to 3000 K at similar to 750 km altitude under dark conditions) agrees well with the scale height at low altitudes obtained from the Akebono observations, assuming that the temperature is constant and that O(+) ions are dominant. Comparisons between the present statistical results (SZA dependence of the electron density and ion and electron temperatures) and modeling studies of the polar wind indicate that the plasma density profile (especially of the O(+) ion density) in the polar cap is strongly controlled by solar radiation onto the ionosphere by changing ion and electron temperatures in the ionosphere during geomagnetically quiet periods.

DOI： 10.1029/2011JA016631

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記述言語：英語  

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Observations of Poynting fluxes associated with onset of convection electric fields are essential for understanding of electromagnetic energy transport from the solar wind toward the magnetosphere leading to changes in the convection electric field, which is one of the most fundamental parameters in the magnetosphere-ionosphere coupled system. We present Cluster multispacecraft observations of Poynting fluxes associated with abrupt changes in large-scale electric fields during sudden commencements and southward turning of the interplanetary magnetic field (IMF). The Cluster spacecraft detected Poynting fluxes dominated by the field-aligned upward component during the preliminary impulse of sudden commencements and in the initial period after southward turning of the IMF. The upward Poynting flux indicates existence of Alfven waves transporting electromagnetic energy from the ionosphere toward the magnetosphere leading to magnetospheric convection changes. The waveguide model and global magnetohydrodynamic (MHD) simulation calculating evolution of the Poynting flux following solar wind pressure enhancements also show upward Poynting fluxes propagating from the ionosphere toward the magnetosphere faster than the propagation of compressional waves. We conclude that the ionosphere acts as a channel to transmit electromagnetic energy supplied as field-aligned currents toward a wide region in the magnetosphere-ionosphere system instantaneously, leading to changes in magnetospheric convection electric fields.

DOI： 10.1029/2010JA015491

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Velocity distributions of upflowing ions in the polar ionosphere are crucial to understand their destinations. Natural plasma wave observations by the plasma wave and sounder experiments and thermal ion observations by the suprathermal ion mass spectrometer onboard the Akebono satellite at similar to 9000 km altitude in the polar magnetosphere during the geomagnetic storms showed that ions in the region of enhanced electron density in the polar cap were dominated by very-low-energy O+ ions (similar to 85%) with upward velocities of 4-10 km s(-1), corresponding to streaming energies of 1.3-8.4 eV. The fluxes of very-low-energy upflowing O+ ions exceeded 1 x 10(9) cm(-2) s(-1) (mapped to 1000 km altitude) across wide regions. These signatures are consistent with high-density plasma supplied by the cleft ion fountain mechanism. Trajectory calculations of O+ ions based on the Akebono observations as the initial condition showed the transport paths and accelerations of the O+ ions and indicated that the velocities of the very-low-energy upflowing O+ ions through the dayside polar cap are enough to reach the magnetosphere under strong convection. The calculations suggest the importance of the very-low-energy upflowing O+ ions with large fluxes in the total O+ ion supply toward the magnetosphere, especially the near-Earth tail region and inner magnetosphere. The initially very-low-energy O+ ions can contribute significantly to the ring current formation during geomagnetic storms since some of the O+ ions were transported into the ring current region with typical energies of ring current ions (several tens of keV) in the trajectory calculations.

DOI： 10.1029/2010JA015601

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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The penetration of magnetospheric electric fields to the magnetic equator has been investigated for two intense magnetic storms that occurred on 31 March 2001 and 6 November 2001. The digital ground magnetic data from equatorial station Tirunelveli (TIR, 0.17 degrees S geomagnetic latitude (GML)) and low-latitude station Alibag (ABG, 10.17 degrees N GML) have been used to identify the storm time electrojet index, EEJ(Dis), which is the difference of the magnetic field variations between TIR and ABG after removing the quiet day variations. The appearance of enhanced DP 2 currents and counterelectrojets (CEJ) during the main and recovery phases of the magnetic storms is possibly due to prompt penetration of electric fields from the high latitudes. These signatures can be interpreted as a clear indicator of the eastward and westward electric fields at the equator. The observed results suggest that the magnitude of the equatorial ionospheric currents driven by the penetrating electric fields is very sensitive to ionospheric conductivity (which depends on local time). Moreover, the intensity of the DP 2 currents started decreasing during the end of the main phase of the storm despite the large negative southward IMF Bz, indicating the dominance of a well-developed shielding electric field for 1 h. As an effect of penetrating electric fields at the equator, the equatorial ionization anomaly is enhanced during the main phase (because of strong eastward electric field) and is inhibited or reduced due to the strong CEJ (because of westward electric field) during the recovery phase.

DOI： 10.1029/2009JA014562

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J-GLOBAL

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The transient response of convection electric fields in the inner magnetosphere to southward turning of the interplanetary magnetic field (IMF) is investigated using in-situ electric field observations by the CRRES and Akebono spacecraft. Electric fields earthward of the inner edge of the electron plasma sheet show quick responses simultaneously with change in ionospheric electric fields, which indicates the arrival of the first signal related to southward turning. A coordinated observation of the electric field by the CRRES and Akebono spacecraft separated by 5 RE reveals a simultaneous increase in the dawn-dusk electric field in a wide region of the inner magnetosphere. A quick response associated with the southward turning of the IMF is also identified in in-situ magnetic fields. It indicates that the southward turning of the IMF initiates simultaneous (less than 1 min) enhancements of ionospheric electric fields, convection electric fields in the inner magnetosphere, and the ring or tail current and region 2 FACs. In contrast, a quick response of convection electric fields is not identified in the electron plasma sheet. A statistical study using 161 events of IMF orientation change in 1991 confirms a prompt response within 5 min for 80% of events earthward of the electron plasma sheet, while a large time lag of more than 30 min is identified in electric fields in the electron plasma sheet. The remarkable difference in the response of electric fields indicates that electric fields in the electron plasma sheet are weakened by high conductance in the magnetically conjugated auroral ionosphere.

DOI： 10.1029/2009JA014277

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記述言語：日本語  

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：英語   出版者・発行元：AMER GEOPHYSICAL UNION  

Statistical analysis of the main impulse (MI) amplitude of geomagnetic sudden commencements (SCs) in a region from the middle latitudes to equator has been made using the long-term geomagnetic field data obtained from the Yap (geomagnetic latitude, theta = 0.38 degrees), Guam (theta = 5.22 degrees), Okinawa (theta = 16.54 degrees), Kakioka (theta = 27.18 degrees), Memanbetsu (theta = 35.16 degrees), and St. Paratunka (theta = 45.58 degrees) stations. The magnetic local time (MLT) dependence of SC amplitude in the middle latitudes showed magnetic field variations produced by two-cell ionospheric currents (DP 2-type currents) which are driven by the dawn-to-dusk electric field accompanying a pair of field-aligned currents (FACs). The effect of the DP 2-type currents at least expands to the low latitude (theta = 16.54 degrees). In this region, the DL part of SC produced by the enhanced Chapman-Ferraro currents can be dominant, but the DP part of SC contaminated 7% of the DL one. On the other hand, at the daytime equator between 8: 00 and 16: 00 (MLT), the SC amplitude is considerably enhanced with its peak amplitude of 3.24 ( normalized SYM-H value) around 11: 00 (MLT) due to the Cowling effect. Another interesting point is that the SC amplitude in the nighttime sector was enhanced at all the stations again, and its peak value increases with increasing magnetic latitude. This result suggests that the effect of the FACs associated with the MI phase of SC expands to the equator.

DOI： 10.1029/2008JA013871

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Meridional electron density distributions above 45 degrees invariant latitude (ILAT) during geomagnetically quiet periods are statistically studied. Electron density data were obtained from plasma waves observed by the Akebono satellite from March 1989 to February 1991 (near solar maximum) in an altitude range of 274-10,500 km. Field-aligned electron density profiles were fitted by the sum of exponential and power law functions. The transition height, where the power law term equals the exponential term, is highest in the summer (at low solar zenith angle (SZA)) at similar to 4000 km and lowest in the winter (at high SZA) at similar to 1800 km in a region of ILAT &gt;= 70 degrees; this is caused by the larger scale height in the summer (similar to 550 km) than that in the winter (similar to 250 km). The largest seasonal variation and SZA dependence of the electron density are found at an altitude of similar to 2000 km with a factor of similar to 50 (similar to 10(4) /cc in the summer, similar to 10(3) /cc in the winter) in the trough, auroral, and polar cap regions. The seasonal variation and SZA dependence are smaller, about a factor of 5-10, above similar to 5000 km. Day-night asymmetries in each season (within a factor of 5) are smaller than the seasonal variation. The scale height is larger in the dayside than in the nightside in each season. These results indicate that photoionization processes in the ionosphere strongly control electron density distributions up to at least similar to 5000 km in the trough, auroral, and polar cap regions.

DOI： 10.1029/2008JA013288

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J-GLOBAL

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J-GLOBAL

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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DOI： 10.1186/BF03352043

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Analysis of the plasma wave observation data provided by the plasma waves and sounder experiment (PWS) on board the Akebono satellite frequently reveals the presence of electrostatic electron cyclotron harmonic (ESCH) waves in the low-latitude region (MLAT &lt; 45 degrees) of the plasmasphere within an altitude range from about 3000 km to the apogee of the satellite (initial apogee was 10,500 km). Even at moderate or low geomagnetic activity, intense ESCH waves often appear near the equatorial region of the plasmasphere above the upper hybrid resonance (UHR) frequency at the lowest harmonic number branch of the f(Qn) ESCH waves. We identified these plasma waves as the equatorial plasmasphere f(Qn) waves (EP-f(Qn)). The spectra of the EP-f(Qn) waves are characterized by a narrow band structure and by a strong nature, with a wave intensity that ranges from 3.46 x 10(-8) to 3.31 X 10(-4) V/m. The maximum intensity is nearly coincident with the upper limit of the PWS receiver in the low-gain mode. Statistical analysis results reveal that the EP-f(Qn) waves are observable in all the local time sectors; however, the occurrence probability shows a clear enhancement in the early morning sector of 01-03 MLT in the plasmasphere. The EP-f(Qn) wave activities are suppressed within a period of strong magnetic disturbances as well as solar minimum phase. The linear dispersion relation analysis using a two-component plasma model reveals that supra-thermal plasma with the energy of about 750 eV and with a large temperature anisotropy (A = T-perp/T-parallel - 1 &gt; 40) must be present in order to realize an appearance of a positive growth rate at the observed frequency and propagation angle of the ESCH waves. Since the hot plasma with such a high anisotropy has not been detected, the validity of the present two-component plasma model remains an open question. The occurrence feature of the ESCH waves showed that there is a constant activation or a constant flow-in of free energy to generate the strong plasma instability of ESCH waves near the post-midnight sector of the plasmasphere. The existence of ESCH waves revealed that the nature of the plasmaspheric plasma is more turbulent and active than has been believed.

DOI： 10.1186/BF03352723

Web of Science

記述言語：英語  

Electric field and potential distributions in the inner magnetosphere during geomagnetic storms have been investigated using the Akebono/EFD data. Using this electric field, we study injection of ring current particles and acceleration of radiation belt electrons by single-particle calculations. During the main phase, the dawn-dusk electric field is intensified especially in a range of 2 &lt; L &lt; 5 with a maximum amplitude of 6 mV/m on the duskside, and a two-cell convection pattern with a potential difference of 180 kV is identified. The convection pattern on the equatorial plane is significantly distorted with a large potential drop of 70 kV on the dawn and dusk sectors, indicating an intrinsic source of large-scale electric field in the inner magnetosphere. The plasma sheet ions are gathered into the dusk to premidnight sector in the inner magnetosphere in the region of enhanced electric field due to the strong E x B drift. The ions are transported into around 4 R-E with an acceleration of more than 1 order of magnitude within 40 min, conserving the first adiabatic invariants. Relativistic electrons with initial energy of some hundreds of kiloelectron volts at 5 R-E are energized to more than 100 keV for 3 hours. The energy spectrum during the recovery phase of 9 October 1990 geomagnetic storm observed by the CRRES satellite is reproduced without the radial diffusion or nonadiabatic acceleration by plasma waves. It is possible that this acceleration process is the inhomogeneity of the large-scale electric field, which corresponds to the del x E term along orbits of electrons around the Earth.

DOI： 10.1029/2006JA012177

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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The distribution of the storm-time electric field in the inner magnetosphere has been investigated by using the Akebono/EFD data within a period from 1989 to 1995. During the main phase of geomagnetic storms, the strong electric field appears between L = 2 and L = 6 in both dawn and dusk sectors with the magnitude from 2 to 4 mV/m, and the maximum value appears at L = 3 in the dusk sector. During the recovery and weakly disturbed period, the region with intense electric field moves outward with decreasing the amplitude. This structure of the localized electric field is quite different from the Volland-Stern electric field model, suggesting the existence of another electric field source other than the large-scale convection electric field. During quiet periods, although the electric field fairly follows the corotation electric field, the distribution at L = 3.5 indicates a small deviation from pure corotation.

DOI： 10.1029/2006GL027510

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記述言語：日本語   掲載種別：研究発表ペーパー・要旨（全国大会，その他学術会議）  

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