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 first investigation of Equatorial Plasma Bubble (EPB) intensities across longitudinal sectors of the globe using observations from global navigation satellite system (GNSS) receivers. GNSS data from a total of 93 receiver stations located within ±20 degrees of the geomagnetic equator across the globe were used. The data covered periods of years 2014 and 2019 which are respectively years of high and low solar activity in solar cycle 24. We define a parameter known as the Standard deviation of Residual TEC (SRT) to characterize the EPB intensities. The EPB occurrence was defined by day-night differences of the rate of change of TEC index (ROTI). We observed a high correlation (r ∼ 0.80) between the magnitudes of the SRT and ROTI during the EPB occurrence, but the correlation is low (r ∼ 0.37) during non occurrence of EPB. The EPB intensities are greater during seasons with high occurrence rates. The EPB intensities and occurrence rates are also greater during the high solar activity. We found that the post-sunset intensities are greatest in the Atlantic region, followed by the African region, then the American, Australian, Asian, and Pacific regions in that order. The post-midnight intensities are greatest in the African region, followed by the Atlantic, American, Australian, Asian, and Pacific regions in that order.

DOI： 10.1016/j.jastp.2023.106097

Web of Science

Scopus

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

Web of Science

Scopus

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  

Atmospheric gravity waves (AGWs) and medium-scale traveling ionospheric disturbances (MSTIDs) in the upper atmosphere can be observed in nocturnal airglow images. Spectral analysis of airglow images provides propagation direction and intensity of these waves. However, the spectral analysis of airglow images has not been done yet for stations in the southern hemisphere except for Antarctica. In this study, we calculated the horizontal phase velocity spectra of AGWs in the mesosphere-lower-thermosphere (MLT) region near the mesopause and MSTIDs in the F-region ionosphere from airglow images at wavelengths of 557.7 and 630.0 nm, respectively, by applying the 3-dimensional Fourier spectral analysis to the airglow images obtained at Darwin (12.4°S, 131.0°E) in Australia for 13.75 years from 2001 to 2007 and from 2011 to 2019. The spectra of AGWs in the MLT region show clear characteristics that the southward power spectral density (PSD) is stronger in summer and weaker in winter. Tropospheric convection was located at north of Darwin in summer and above Darwin in winter, suggesting that the tropospheric convection is a possible source of AGWs in the mesopause region. The spectra of MSTIDs in the F-region ionosphere show that the dominant northwestward PSD is stronger in winter and during solar quiet periods. These features can be explained if the observed MSTIDs are caused by the ionospheric instabilities. A weak positive correlation was observed between PSDs of AGWs in the mesosphere and MSTIDs. We propose that the MSTIDs propagating other than northwestward may be generated by AGWs.

DOI： 10.1029/2022JA030769

Web of Science

Scopus

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

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2022JD038249

Web of Science

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

Web of Science

Scopus

その他リンク： https://link.springer.com/article/10.1186/s40623-023-01829-0/fulltext.html

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.3389/fspas.2023.1147531

Web of Science

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 report the first statistical study of stable auroral red (SAR) arcs detached from the main auroral oval during non-storm time, using multi-event conjugate measurements by the Defense Meteorological Satellite Program (DMSP) satellites (F13–F19) and a ground-based all-sky imager at Athabasca (Canada) (54.6°W, 246.36°Е, MLAT = 61.5°, MLON = 308.3°, L = 4.4). We found 63 events of detached SAR arc conjunctions with the DMSP satellites in the northern hemisphere and 18 events in the opposite southern hemisphere from 2006 to 2018. Measurements aboard DMSP satellites show that detached SAR arcs are in general associated with enhancements of electron temperature (60 cases) and electron density troughs (58 cases). Only 14 cases show strong horizontal flow associated with the detached SAR arcs, indicating that the strong plasma flow is not a necessary condition to cause the detached SAR arcs. The electron temperature measured by DMSP associated with detached SAR arcs positively correlates with F10.7 solar activity index. The measured emission intensities at 630.0 nm in the SAR arcs show a good correlation with the electron temperature. These results indicate that the detached SAR arcs during non-storm time are caused by heat flux from the magnetosphere associated with substorms, and their intensity depends on the background plasma condition in the ionosphere.

DOI： 10.1029/2022JA030544

Web of Science

Scopus

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

Scopus

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  

Pc1 geomagnetic pulsations correspond to electromagnetic ion cyclotron (EMIC) waves in the magnetosphere and are excited there with frequencies of 0.2–5 Hz. The instantaneous longitudinal extent of Pc1 waves on the ground has not been estimated yet. In this study, we analyze the Pc1 pulsations observed at seven longitudinally-distributed ground stations at subauroral latitudes at ∼60° magnetic latitude for 1 year from July 2018 to June 2019. The hourly occurrence rates of Pc1 pulsations at all 7 stations have a peak (14%–39.6%) in the post-noon sector and a local minimum (4.1%–8.1%) at midnight. The average frequencies become highest (0.6–1.1 Hz) after midnight and lowest (0.3–0.5 Hz) after noon at all 7 stations. An increasing tendency of total Pc1 occurrence with respect to magnetic latitude was observed. Based on these observations, we obtained a peak of probability distribution of the instantaneous Pc1 longitudinal extent as ∼82.5° with a half maximum at ∼114°, though this probability distribution can be affected by the limitation of the number of the stations. We also made model calculations on the possible longitudinal extent using artificial random Pc1 waves with fixed extents. The comparison of the model results with the observation suggests longitudinal extent of 70°–86° comparable to the peak of probability distribution (∼82.5°). A superposed epoch analysis shows that the longitudinal extent of Pc1 waves tends to increase during recovery phase of geomagnetic storms.

DOI： 10.1029/2021ja029987

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JA029987

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  

Protons of tens of keV can be resonantly scattered by electromagnetic ion cyclotron (EMIC) waves excited in the magnetosphere, resulting in proton precipitation down to the upper atmosphere. In this study, we report for the first time the ionospheric height-dependent ionization in response to EMIC-associated isolated proton aurora (IPA) using simultaneous space-borne and ground-based measurements. On 06 March 2019, the Polar Orbiting Environmental Satellites observed significant proton precipitation in the dusk sector (MLT ∼ 19), while ground-based magnetometers detected a clear signature of EMIC waves. Meanwhile, the conjugated all sky imager captured an IPA and the nearby Poker Flat incoherent scatter radar (PFISR) showed enhanced electron density in the E region, suggesting a potential consequence of the EMIC wave-driven proton precipitation. The Global Airglow model simulations confirmed the dominant impact of proton precipitation on the ionosphere and agreed well with PFISR observations. This study confirmed physical links from the magnetosphere to the ionosphere through EMIC-driven proton precipitation.

DOI： 10.1029/2022JA030983

Scopus

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

The eruption of the Tonga volcano on January 13 and 15, 2022 and related intense lightning activity led to the excitation of a number of specific electromagnetic oscillations in different frequency ranges. We examine properties of these oscillations, using data from magnetometers of various types located in Kamchatka and in the Pacific region. We confirmed that there might have been a geomagnetic response to the formation of an acoustic resonance between the Earth surface and the ionosphere: localized harmonic oscillations with a frequency 3.5-4.0 mHz, which lasted for ~1.5 hr, were detected ~15 min after the beginning of the eruption at distance of ~800 km. An increase was observed in the intensity of the Schumann resonance at stations in the Far East. Broadband emission stimulated by intense volcanic lightning was detected to occur in the Pc1 range (2-5 Hz). The emission presumably results from the excitation of the magnetosonic waveguide in the upper ionosphere by lightning activity.

DOI： 10.12737/stp-91202306

Web of Science

Scopus

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  

Numerous publications have appeared in the literature describing a new seismo-electromagnetic effect – the appearance of magnetic pulses with a duration of 1–40 s, which are detected at distances up to ten thousand km several minutes before earthquakes of even low magnitude. An assumption was made about the universality of the processes of impulse precursor generation and the fundamental possibility of a short-term (several minutes) warning of an approaching earthquake. In the presented article, the possibility of the appearance of ultra-low-frequency pulses that precede seismic events with magnitudes M>5.0, according to the data of a network of induction magnetometers in the Far East, is investigated. In the records of these highly sensitive magnetometers, impulse disturbances are constantly encountered, some of which are observed synchronously at several stations, which eliminates the influence of local noise. The spectral maximum of about 7–8 Hz of the oscillatory structure, which manifests itself in many pulses, corresponds to the fundamental frequency of the Schumann resonator. A comparison of magnetic observations with data from the WWLLN (World Wide Lightning Location Network) showed that some of the pulses were caused by lightning discharges in the 650 km vicinity of magnetic station. Despite the fact that some of the impulses are observed immediately before earthquakes, it is impossible to speak with confidence about their connection with seismic activity. The calculation of the number of impulses in the 5-min interval before and after the seismic shock shows that they are randomly distributed relative to the moment of the earthquake. Apparently, the pulses are predominantly due to the ionospheric response to a far lightning discharge.

DOI： 10.21455/gr2023.2-1

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：International Union of Radio Science (URSI)  

DOI： 10.46620/22-0049

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Frontiers in Astronomy and Space Sciences  

The study of ionospheric irregularities is important since many technological systems might be influenced by the ionosphere. In this work, we use data from the Global Navigation Satellite Systems (GNSS) receiver installed in Abuja, Nigeria, GPS Scintillation Network Decision Aid (SCINDA) TEC data from the Air Force Research Laboratory (AFRL) data archive, and the geomagnetic data from the World Data Center (WDC) in Kyoto, Japan, to investigate the relationship between geomagnetic storm and ionospheric irregularity occurrences using the rate of change of total electron content (TEC) index (ROTI), with a validation using the S4 indices, during the peak of the 24th solar cycle. The occurrences of irregularities were investigated on day-to-day and seasonal bases. The nighttime ionospheric irregularities, which are attributed to ionospheric plasma irregularities in the equatorial ionospheric F-region, were found to be prevalent. To investigate the relationship between the strength of ionospheric irregularities (ROTI) and the geomagnetic storm (Dst), the periodogram power spectral density (PSD) and regression analysis were used. The results showed that there was no correlation, cc = 0.073, between the Dst and ROTI, implying that the strengths of ionospheric irregularities occurring during geomagnetic storms are not strictly decided by the magnitudes of the storms; this was also confirmed using the S4 index. The impact of geomagnetic storms caused enhanced development or inhibition of ionospheric irregularities. We observed that the bulk of the storms occurring during the period of this study is not associated with ionospheric irregularities. Finally, the investigation showed that the correlation between the ROTI and Dst observed during the coronal mass ejection (CME)-driven geomagnetic storms was higher than that during the corotating interaction region (CIR)-driven geomagnetic storms, during the peak of the 24th solar cycle. The results of this work confirm the findings by other researchers.

DOI： 10.3389/fspas.2022.969235

Web of Science

Scopus

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  

Auroral brightening is one of the most common phenomena that occur during substorm onset and is usually recognized as a projection of the substorm-associated magnetospheric plasma dynamics to the ionosphere. However, electromagnetic fields and plasma features associated with the substorm brightening arc have not been well understood. In this study, we present a comprehensive observation of the source plasma and field variations of a substorm brightening aurora in the inner magnetosphere. We performed a unique conjugate observation of a substorm brightening auroral arc observed by a ground-based camera and by the Arase satellite in the magnetospheric source region at L ∼ 6. The event was observed at Tromsø (69.6°N, 19.2°E), Norway, on 12 October 2017. The brightening arc indicates east-west structures with longitudinal scales of ∼0.5°–2.0°. Field-aligned bi-directional electrons with an energy range between 66 and 1,800 eV were detected by the satellite, simultaneously with the appearance of the brightening arc in the camera. These electrons were probably supplied from the auroral brightening region in the ionosphere, indicating that the satellite was on the same field line of the brightening aurora. The magnetic and electric field data show characteristic fluctuations and earthward Poynting flux around the time that the satellite crossed the aurora. Anti-phase oscillations between the thermal pressure and the magnetic pressure are also reported. Based on these observations, we suggest the possibility that a ballooning instability occurred in the source region of the substorm brightening arc in the inner magnetosphere at L ∼ 6.

DOI： 10.1029/2021JA030072

Web of Science

Scopus

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

Web of Science

Scopus

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  

Relativistic electron precipitation (REP) from the Earth’s radiation belt plays an important role in mesospheric ozone loss as a connection between space weather and the climate system. However, the rapid (tens of minutes) destruction of mesospheric ozone directly caused by REP has remained poorly understood due to the difficulty of recognizing its location and duration. Here we show a compelling rapid correspondence between localized REP and ozone destruction during a specific auroral phenomenon, the called an isolated proton aurora (IPA). The IPA from the Earth’s radiation belt becomes an important spatial and temporal proxy of REP, distinct from other auroral phenomena, and allowing visualizing micro-ozone holes. We found ozone destruction of as much as 10–60% within 1.5 h of the initiation of IPA. Electromagnetic ion cyclotron waves in the oxygen ion band observed as the driver of REP likely affect through resonance with mainly ultra-relativistic (> 2 mega-electron-volts) energy electrons. The rapid REP impact demonstrates its crucial role and direct effect on regulating the atmospheric chemical balance.

DOI： 10.1038/s41598-022-20548-2

Web of Science

Scopus

PubMed

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  

Three-dimensional Fourier spectral analysis of airglow images is a useful technique to provide propagation characteristics of atmospheric gravity waves (AGWs) in the mesopause region and medium-scale traveling ionospheric disturbances (MSTIDs) in the thermosphere observed at emission wavelengths of 557.7 nm (altitudes: 90–100 km for AGWs) and 630.0 nm (200–300 km for MSTIDs). However, a statistical study of spectral analysis of airglow images over years has not been done yet at Hawaii. Here, we report the first spectral analysis of AGWs and MSTIDs using airglow images obtained at Haleakala (20.71°N, 203.74°E), Hawaii, during the three years from 2013 to 2016. The power spectral density (PSD) of AGWs tends to be larger toward north to northeast in summer and from north to west and southward in winter. The eastward (summer) and westward (winter) preferences are consistent with a wind-filtering mechanism of AGWs by a mesospheric jet. Some correspondence is found between the locations of tropospheric convection activities and AGW propagation directions, suggesting that these mesospheric AGWs are originated from the troposphere. The PSDs of MSTIDs are larger in solstice than in equinox. This PSD enhancement in solstice is consistent with a sporadic-E (Es) layer occurrence in either northern or southern hemisphere, suggesting that the MSTIDs over Hawaii could be caused by a coupling process between the Perkins and Es layer instabilities. A small correlation was found between the PSDs of AGWs in the mesopause region and the PSDs of MSTIDs, suggesting that the MSTIDs are partly caused by AGWs from the mesopause region.

DOI： 10.1029/2022JA030346

Web of Science

Scopus

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. Thus, an IPA is the ionospheric projection of the spatial and temporal variation of wave-particle interaction regions in the magnetosphere. In this study, we conducted unique multi-event analysis of simultaneous observations of IPAs and their source regions on 22 April, 7 September, and 22 March 2018, using all-sky imagers at subauroral latitudes and the Van Allen Probes. When the satellite footprint passed over the IPAs associated with ground Pc1 geomagnetic pulsations, locally generated He+-band EMIC waves with the same frequencies as the ground Pc1 pulsations were observed in all events. The IPAs and EMIC waves had comparable narrow widths in the latitudinal direction. The EMIC waves appeared during the rapid enhancement of the ring current proton flux at energy range of ∼10–50 keV, while they disappeared at the rapid decrease of the electron density. From these results, we conclude that the boundaries of the localized IPAs and EMIC waves were determined by the overlap region of energetic proton enhancement and the plasmasphere. This overlap of ring-current protons and plasmasphere is a favorable condition for the pitch-angle scattering of protons by the EMIC waves. Characteristic magnetic and electric field variations with the IPAs were not observed by the satellite, indicating that the IPAs were not accompanied by field-aligned currents comparable to that of oval auroral arcs.

DOI： 10.1029/2022JA030455

Web of Science

Scopus

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 present the first comparative statistical study of subauroral arc detachment from the main auroral oval at Athabasca (magnetic latitude = 61.5°N), Canada, for three different types of subauroral arcs: pure red arc, red arc with simultaneous emission in green-line (red + green arc), and STEVE (strong thermal emission velocity enhancement). Based on 15-years (2006–2020) of all-sky imaging observations, this study not only uncovers the occurrence characteristics of different arcs but also provides important insights into the specific geomagnetic conditions under which STEVE develops. Red arc was the most common subauroral arc (139 events), followed by red + green arc (42 events), and STEVE (26 events) was a rare phenomenon. The detachment rate of red and red + green arcs exhibits dependence on both the solar flux and geomagnetic activity. The detachment rate of STEVE was higher during premidnight, whereas that of red and red + green arcs was higher around the midnight sector. The geomagnetic activity was relatively higher for STEVE, the decrease in the AL index and local X-component magnetic variations being ∼2–3 times higher for STEVE as compared to other arcs. STEVE shows a strong association with asymmetric ring current in terms of prominent bay-like enhancement in ASY-H index prior to its detachment. Such bay-like enhancement was ∼4 times higher for STEVE as compared to other arcs. STEVE events were accompanied by dispersionless injection for both electron and proton flux at geosynchronous orbit. These results unambiguously suggest that STEVE develops after substorm associated energy injection and significant intensification of the asymmetric ring current.

DOI： 10.1029/2021JA029856

Web of Science

Scopus

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  

Microbursts are impulsive (<1 s) injections of electrons into the atmosphere, thought to be caused by nonlinear scattering by chorus waves. Although attempts have been made to quantify their contribution to outer belt electron loss, the uncertainty in the overall size and duration of the microburst region is typically large, so that their contribution to outer belt loss is uncertain. We combine datasets that measure chorus waves (Van Allen Probes [RBSP], Arase, ground-based VLF stations) and microburst (>30 keV) precipitation (FIREBIRD II and AC6 CubeSats, POES) to determine the size of the microburst-producing chorus source region beginning on 5 December 2017. We estimate that the long-lasting (∼30 hr) microburst-producing chorus region extends from 4 to 8 (Formula presented.) MLT and 2–5 (Formula presented.) L. We conclude that microbursts likely represent a major loss source of outer radiation belt electrons for this event.

DOI： 10.1029/2022GL099655

Web of Science

Scopus

PubMed

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2022GL099655

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Frontiers in Astronomy and Space Sciences  

This paper reports a story of developing the idea of Earthward ion flow braking in the near-Earth plasma sheet and its relationship with substorm onset processes. This idea and the data that support it are the basis for today’s two competing models for substorms: the near-Earth neutral-line model and the current disruption model. The idea was developed when the author was staying at the Max Planck Institute for Extraterrestrial Physics (MPE) from July 1996 to June 1997. The story addresses the colleagues and mentors who had contributed to the development of this idea. The lessons learned from this story are also summarized for students and early-career scientists for their development of new scientific ideas.

DOI： 10.3389/fspas.2022.957776

Web of Science

Scopus

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  

This paper presents the highlights of joint observations of the inner magnetosphere by the Arase spacecraft, the Van Allen Probes spacecraft, and ground-based experiments integrated into spacecraft programs. The concurrent operation of the two missions in 2017–2019 facilitated the separation of the spatial and temporal structures of dynamic phenomena occurring in the inner magnetosphere. Because the orbital inclination angle of Arase is larger than that of Van Allen Probes, Arase collected observations at higher L-shells up to L∼ 10. After March 2017, similar variations in plasma and waves were detected by Van Allen Probes and Arase. We describe plasma wave observations at longitudinally separated locations in space and geomagnetically-conjugate locations in space and on the ground. The results of instrument intercalibrations between the two missions are also presented. Arase continued its normal operation after the scientific operation of Van Allen Probes completed in October 2019. The combined Van Allen Probes (2012-2019) and Arase (2017-present) observations will cover a full solar cycle. This will be the first comprehensive long-term observation of the inner magnetosphere and radiation belts.

DOI： 10.1007/s11214-022-00885-4

Web of Science

Scopus

PubMed

その他リンク： https://link.springer.com/article/10.1007/s11214-022-00885-4/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：Geophysical Research Letters  

The auroral acceleration region plays an important role in the magnetosphere-ionosphere coupling system. In this study, signatures of an auroral U-shaped potential structure were found for the first time in the near-equatorial inner magnetosphere by the Arase satellite at ∼6.0 RE geocentric distance and 11° magnetic latitude. The observed magnetic and electric field variations corresponded to the equatorward motion of the upward field-aligned current and converging perpendicular electric field. Examining the three-dimensional velocity distribution function of H+ and O+ ions, we demonstrate that upflowing ion beams were significantly deflected in an east-west direction with a perpendicular velocity up to ∼80 km/s, which is consistent with the ExB drift velocity. A simple particle drift model with the inferred auroral perpendicular potential presents a new kink-like drift path of ions from the magnetotail, implying that the auroral potential structure has a great impact on particle dynamics in the near-earth plasma sheet.

DOI： 10.1029/2022GL098105

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2022GL098105

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  

Flash aurora driven by an isolated chorus element can be a useful ionospheric indicator for identifying the source wave properties via wave-particle interactions. Using ground observation and modeling approaches, here we report the temporal characteristics of flash aurora that depend on the chorus frequency width and the sweep rate. We found that the contraction time increases more than the expansion time in patchy auroral variations, due to the difference in the minimum electron energies resonated with the chorus wave packet away from the equatorial source to higher latitudes. Especially, the contraction time strongly depends on the higher-frequency chorus waves due to cyclotron resonance with lower-energy electrons. The model calculations support that the chorus element ranges from lower-band to upper-band frequencies with respect to half the gyrofrequency at the exact generation region. Our study provides the prompt (milliseconds) chorus-driven electron dynamics through the spatiotemporal characteristics of flash aurora in the ionosphere.

DOI： 10.1029/2021GL097597

Web of Science

Scopus

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  

During disturbed geomagnetic conditions, the energetic particles in the inner magnetosphere are known to undergo precipitation loss due to interaction with various plasma waves. This study, investigates the energetic particle precipitation events statistically using coordinate observations from the ground riometer network and the inner-magnetospheric satellite mission, Arase. We have compared cosmic noise absorption (CNA) data obtained from the Finnish ground riometer network located in the auroral/sub-auroral latitudes with the comprehensive data set of omnidirectional electron/proton flux and plasma waves in ELF/VLF frequency range from the Arase satellite during the overpass intervals. The study period includes one and a half years of data between March 2017 and September 2018 covering Arase conjunctions with the riometer stations from all magnetic local time sectors. The relation between the plasma flux/waves observed at the satellite with the riometer absorptions are investigated statistically for CNA (absorption >0.5 dB) and non-CNA (absorption <0.5 dB) cases separately. During CNA events, Arase observed elevated electron flux in the medium energy range (2–100 keV), and plasma wave activity in the whistler-mode frequency range (0.5–3 kHz) of the spectra. Our study provides an estimate of the statistical dependence of the electron flux and plasma wave observations at Arase with the ground reality of actual precipitation.

DOI： 10.1029/2022JA030271

Web of Science

Scopus

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  

We present an observation of rapid flux depressions in relativistic electrons, which is referred to as “EMIC-induced drifting electron holes (EDEHs).” The Arase, Van Allen Probes, and THEMIS detected simultaneously electron flux fluctuations. The time variation of flux shows depressions of 1-min scale with energy dispersion, which appear only in the relativistic energy range and small pitch angles. These characteristics of the flux depression indicate that electromagnetic ion cyclotron waves caused pitch angle scattering on a short time scale in a longitudinally limited region. The Arase satellite detected the local depression of the phase space density of 1,000 MeV/G electron, indicating that EMIC waves cause the true loss of electrons. Tracing the energy dispersion profile of EDEHs, we show that EDEHs are formed at localized region in the dusk side. Multisatellite observations demonstrate that a series of EDEHs eventually cause a substantial depression of the radiation belt on 1-hr time scale.

DOI： 10.1029/2021GL095194

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021GL095194

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Lecture Notes in Networks and Systems  

This paper proposes initially to apply convolutional neural network (CNN) for detecting the equatorial plasma bubbles on the ASI images. The considered CNN model is the YOLO v3 tiny model under a deep learning API (Keras), running on top of the machine learning platform (TensorFlow). Our program for EPB detection is written in Python that is extended easily to combine into a space weather web site for detecting and notifying EPBs in our next step. The results show that the YOLO v3-based CNN can detect the EPBs in ASI images with different intensities obtained from many countries. The threshold is tested and selected to be 0.40 suitably for detecting the anomaly (EPB existence). The maximum anomalous value is selected to decide the EPB occurrence.

DOI： 10.1007/978-981-19-2130-8_38

Scopus

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）  

DOI： 10.1142/9789811260100_0073

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：International Union of Radio Science (URSI)  

DOI： 10.46620/22-0050

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：Lecture Notes in Electrical Engineering  

In this paper lightning stroke charges are estimated remotely by using combinations of Extremely Low Frequency (ELF) radio waves, conventional Lightning Locating System (LLS) and local meteorological information for the first time. Each lightning stroke location from LLS and corresponding transient signature of ELF magnetic field observed at a long distance (−1000 km between lightning stroke and observation site) are utilized to estimate the charge moment change (CMC) of the lightning stroke. The lightning stroke charge Q is then derived by dividing the CMC by the altitude of the neutralized lightning charge. The corresponding altitude of the neutralized lightning charge for each stroke is determined by assuming local −20 °C altitude for a positive stroke (−10 °C altitude for a negative strokes) from MesoScale Model (MSM). Spatial and temporal frequency distributions of energetic lightning (large amount of stroke charge) and cumulative frequency distributions of stroke charge in northwestern part of Japan in local summer and winter seasons for 4 years were derived. The spatial occurrence frequency distributions indicate that lightning strokes with a large amount of neutralized charge occur more frequently in inland areas during summertime and in coastal areas in winter. The cumulative frequency distribution indicates that positive polarity lightning strokes in winter tend to have a large charge. Our statistical results are in good agreement with the lightning properties inferred from local observations, and are considered to be useful information not only to study electrical properties of thunderstorms but also for future maintenance planning of power grid systems in wide areas and mitigation of lightning damages.

DOI： 10.1007/978-981-16-8690-0_30

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）  

The region of Southeast Asia is known as one of the lightning-prone areas in the world. However, spatial and temporal distributions of high-energy lightning discharges, which cause natural disasters e.g. significant damages to local power grid systems and forest fires, are still unknown. This paper reports initial results on the statistical characteristics of energetic lightning around Malay Peninsula. Highly accurate timing and location as well as lightning peak current (Ip ) of lightning strokes were obtained by ENTLN (Earth Networks Total Lightning Network), while the vertical charge moment change Qds as a proxy of high-energy lightning was derived by using ground-based ELF magnetic field observations in Pahang, Malaysia with a small threshold of less than 10 C·km. The characteristics of energetic lightning for both rainy and dry seasons were successfully derived for the first time in this region. The results indicate that highly energetic lightning occur frequently throughout the rainy and dry seasons, mainly over the Malacca Strait. Long-term statistical analysis with an extended data analysis period is necessary to understand further the seasonal and yearly dependences of energetic lightning and their relations with local climate conditions.

DOI： 10.1007/978-981-16-8690-0_29

Scopus

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

In October 2017, the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP) Bureau established a committee for the design of SCOSTEP's Next Scientific Programme (NSP). The NSP committee members and authors of this paper decided from the very beginning of their deliberations that the predictability of the Sun-Earth System from a few hours to centuries is a timely scientific topic, combining the interests of different topical communities in a relevant way. Accordingly, the NSP was christened PRESTO -PREdictability of the variable Solar-Terrestrial cOupling. This paper presents a detailed account of PRESTO; we show the key milestones of the PRESTO roadmap for the next 5 years, review the current state of the art and discuss future studies required for the most effective development of solar-terrestrial physics.

DOI： 10.5194/angeo-39-1013-2021

Web of Science

Scopus

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  

This study exploits the volumetric sampling capabilities of the Resolute Bay Incoherent Scatter Radar (RISR-N) in collaboration with all-sky imagery and in situ measurements to examine the interplay between cold plasma transport and auroral precipitation during a high-latitude lobe reconnection event on the dawn side. Solar wind IMF preceding the event was characterized by an impulsive negative excursion in Bz embedded within a period of Bz > 0 and By < 0. The combined effects of transport and magnetic stress release associated with a high-latitude reconnection pulse drove a co-mingling between patches and soft electron precipitation, creating common regions of elevated electron density and temperature. Vertical ionospheric profiles extracted in the rest frame of the drifting patch showed a contemporaneous increase in Te above 200 km and Ne below 250 km while at the same time showing only a small impact in Ne near the F-region peak. The observations suggest a new mechanism for creating a “hot patch” wherein the density enhancement is not generated by the precipitation but is warmed by it. The physics-based GEMINI model was used to explore the response to the observed precipitation as a function of altitude and time. Model results suggest that a correlated enhancement in Ne and Te at DMSP altitudes (∼800 km), that is, hot patch, can be produced by auroral heating and upward diffusion, irrespective of lower altitude density structure. The study highlights the need for densely distributed observations in space and time for understanding both mesoscale and small-scale ionospheric dynamics in regions subject to complex forcing.

DOI： 10.1029/2021JA029657

Web of Science

Scopus

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 present observations of an equatorward detachment of the auroral arc from the main oval and magnetically conjugate measurements made by the Arase satellite in the inner magnetosphere. The all-sky imager at Gakona (magnetic latitude = 63.6°N), Alaska, shows the detachment of the auroral arc in both red and green lines at local midnight (∼0130–0230 MLT) on 30 March 2017. The electron density derived from the Arase in-situ observations shows that this arc occurred outside the plasmapause. At the arc crossing, the electron flux of energies ∼0.1–2 keV is found to be locally enhanced at L∼4.3–4.5. We estimated auroral intensities for both red and green lines by using the Arase low-energy (0.1–19 keV) electron flux data. The peak latitude of the estimated intensity shows reasonably good correspondence with the observed intensity mapped at the ionospheric footprints of the Arase satellite. These findings indicate that the observed arc detachment at Gakona was associated with the localized enhancement of low-energy electrons (∼0.1–2 keV) at the inner edge of the electron plasma sheet. Further, we employ the simulation results of the Community Coordinated Modeling Center (CCMC), the BATS-R-US–CIMI 3-D MHD code to understand the conditions in the inner magnetosphere around the time of detachment. Although the simulation could not reproduce the lower-energy component responsible for the arc detachment, it successfully reproduced two earthward convection events at the lower radial distance (R) (R ≤ ∼4) around the time of arc detachment and the features of enhanced convection in similarity with the observations.

DOI： 10.1029/2020JA029080

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA029080

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  

We examined the growth and propagation of fine-structured electromagnetic ion cyclotron (EMIC) waves related to time-varying density irregularities using multipoint measurement data observed by Arase, Van Allen Probe A, and two ground-based induction magnetometers (Gakona and Dawson) during a field line conjunction event on April 18, 2019. We analyzed the wave data obtained by the aforementioned spacecraft and stations, and found that the appearance of fine structures in the observed EMIC waves clearly coincided with the ambient electron density irregularities in the magnetosphere, which can cause periodic wave growth and waveguiding on their propagation. Furthermore, we found that the latitudinal widths of the EMIC wave activity region and the wave propagation duct were ∼185 km and less than 80 km at an auroral altitude of 100 km, respectively. We also found thermal ion heating ((Formula presented.) eV/q) during the EMIC wave activity.

DOI： 10.1029/2021GL096488

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021GL096488

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 investigated the spatial characteristics of pulsating auroras (PsA) using digital camera measurements from the International Space Station. These measurements covered PsAs in (Formula presented.) 5-h wide regions in the magnetic local time (MLT) direction in short time intervals ((Formula presented.) 10 min). Analyses of two events suggested that the periodicity of the main pulsation of a PsA does not exhibit any clear dependence on the magnetic latitude (63– (Formula presented.)) and MLT (00–05). These results suggest that the periods are not controlled by the bounce time of trapped electrons but by local conditions such as gradients of temperature and/or density of electrons near the magnetospheric source region. We also analyzed the colors of PsAs as proxies for the energies of precipitating electrons. The blue and green channels of the digital camera are sensitive to the band emission of molecular nitrogen ions and the green line of oxygen atoms, respectively. Because the energy bands of precipitating electrons producing those two emissions are different, ratios of blue to green (B/G ratios) can be used as proxies for the energies of PsA electrons. The B/G ratio tends to be higher in the morning sector than in the midnight sector, which is consistent with the results of previous studies showing the MLT dependence of the energies of PsA electrons. This capability of estimating the energies of auroral electrons from digital camera images is expected to provide more opportunities for citizen scientists to contribute more deeply to auroral science.

DOI： 10.1029/2021JA029564

Web of Science

Scopus

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  

We report the concurrent observations of F-region plasma changes and field-aligned currents (FACs) above isolated proton auroras (IPAs) associated with electromagnetic ion cyclotron Pc1 waves. Key events on March 19, 2020 and September 12, 2018 show that ground magnetometers and all-sky imagers detected concurrent Pc1 wave and IPA, during which NOAA POES observed precipitating energetic protons. In the ionospheric F-layer above the IPA zone, the Swarm satellites observed transverse Pc1 waves, which span wider latitudes than IPA. Around IPA, Swarm also detected the bipolar FAC and localized plasma density enhancement, which is occasionally surrounded by wide/shallow depletion. This indicates that wave-induced proton precipitation contributes to the energy transfer from the magnetosphere to the ionosphere.

DOI： 10.1029/2021GL095090

Web of Science

Scopus

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  

There is increasing interest to understand thermospheric neutral wind and ionized plasma coupling, especially by using empirical observations. This is the first study that reports simultaneous measurements of thermospheric neutral wind and equatorial plasma bubble (EPB) speeds over the African equatorial region. Neutral wind speeds were obtained using a Fabry-Perot interferometer (FPI), while EPB speeds were obtained using an all-sky optical imager. Both instruments are co-located in Abuja (Geographic: 8.99°N, 7.38°E; Geomagnetic: 1.60°S). Data for 11 nights of coincident neutral wind and EPB speed observations were used. The neutral wind speeds were in the range of about −30 to 300 m/s, while EPB speeds were in the range of about 15–400 m/s. The speeds of both neutral winds and EPBs were generally observed to be faster during the early night hours and to slow down towards midnight. In some instances, the neutral wind and EPB speeds were comparable, an observation that is in agreement with full development of the F region dynamo. There were also instances in which the EPBs were faster or slower than the neutral winds. In one of the nights, a southward turning of the IMF-Bz was observed to cause a sudden increase in the EPB speed. For some nights, the neutral winds were faster than the EPBs during earlier hours of the nights, and the disparity reduced during the later hours of the nights. This was attributed to the possibility that the F region dynamo was yet to be fully developed during the earlier hours.

DOI： 10.1016/j.jastp.2021.105663

Web of Science

Scopus

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  

Medium-scale traveling ionospheric disturbances (MSTIDs) are a phenomenon widely and frequently observed over the ionosphere from high to low latitudes. Night time MSTIDs are caused generally by the polarization electric field in the ionosphere. However, propagation of this polarization electric field to the magnetosphere has not yet been identified. Here, we report the first observation of the polarization electric field and associated density variations of a night time MSTID in the magnetosphere. The MSTID event was observed by an all-sky airglow imager at Gakona (geographical latitude: 62.39°N, geographical longitude: 214.78°E, magnetic latitude: 63.20°N), Alaska. The Arase satellite passed over the MSTID in the inner magnetosphere at 0530–0800 UT (2030–2300 LT) on November 3, 2018. This MSTID, observed in 630 nm airglow images, was propagating westward with a horizontal wavelength of ∼165 km, a north–south phase front, and a phase velocity of ∼80 m/s. The Arase satellite footprint on the ionosphere crossed the MSTID in the direction nearly perpendicular to the MSTID phase fronts. The electric field and electron density observed by the Arase satellite showed periodic variation associated with the MSTID structure with amplitudes of ∼2 mV/m and ∼150 cm−3, respectively. The electric field variations projected to the ionosphere are mainly in the east-west direction and are consistent with the direction of the polarization electric field expected from MSTID growth by E × B drift. This observation indicates that the polarization electric field associated with the MSTID in the ionosphere is projected onto the magnetosphere, causing plasma density fluctuations in the magnetosphere.

DOI： 10.1029/2020JA029086

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA029086

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  

Large Scale Wave Structures (LSWS) in the equatorial ionospheric F-region were observed by measuring spatial and temporal variations within detrended total electron content (dTEC) data obtained by ground-based GNSS receivers over the South American continent. By using dTEC-maps, we have been able to produce, for the first-time, two-dimensional representations of LSWS. During the period from September to December, the LSWS frequently occurred starting a few hours prior to Equatorial Plasma Bubble (EPB) development. From 17 events of LSWS observed in 2014 and 2015, wave characteristics were obtained: the observed wavelengths, periods, and the phase speeds are respectively, ~900 km, ~41 min and ~399 m/s; the waves propagated from the northeast to southeast. In some cases the front of the oscillation was meridionally aligned, extending to more than 1600 km, the first time such large extension of the wavefront has been reported. From F-layer bottom height oscillation data, measured by ionosonde, LSWS exhibit two different vertical phase propagation modes, in-phase and downward phase. The former mode indicates the presence of a polarization electric field in the F-layer bottom side; the latter suggests propagation of atmospheric gravity waves. The presence of LSWS near the solar terminator, followed by the development of EPBs, suggests that the upwelling of the F-layer bottom height produces a condition favorable to the development of Rayleigh–Taylor instability.

DOI： 10.26464/epp2021047

Web of Science

Scopus

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  

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.[Figure not available: see fulltext.]

DOI： 10.1186/s40623-021-01486-1

Web of Science

Scopus

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：公益財団法人 日本学術協力財団  

<p>　拡大する人類の宇宙利用の安全・安心な運用と太陽活動変動の地球気候への影響を理解・予測するために、国際学術会議傘下のSCOSTEPを中心としてPRESTO（変動する太陽地球結合系の予測可能性）プログラムが2020-2024年に推進されている。PRESTOでは、太陽・惑星間空間・ジオスペース、宇宙天気と地球大気、太陽活動とその気候影響、という3つの課題に対して国際共同研究が進められている。SCOSTEPは太陽地球系科学の発展のために、国際スクールやオンラインセミナー・講義、若手派遣事業を実施してきた。日本学術会議は日本の代表としてSCOSTEPに加盟し、分担金の拠出やSCOSTEPに対する国内対応の議論・情報交換を行ってきた。日本は、歴代のプログラムの国際会議を主催したり、SCOSTEP会長、副会長、理事やプログラム議長を輩出したりするなど、SCOSTEPの運営に大きく貢献している。</p>

DOI： 10.5363/tits.26.8_18

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  

In-situ observations of spatial variations of the wave-particle interaction region require a large number of satellite probes. As an alternative, flash-type auroras, a kind of pulsating aurora, driven by discrete chorus elements, can be used to investigate the interaction region with a high spatial resolution. We estimated the spatial extent of wave-particle interaction region from ground-based observations of flash aurora at Gakona (62.39°N, 214.78°E), Alaska at subauroral latitudes, and found that the auroral expansion was predominantly to the low-latitude side. The spatial displacement is thought to be caused by the propagation effects of chorus waves in the magnetosphere. Using ray tracing analysis to take into account chorus wave propagation, we reconstructed the spatiotemporal evolution of the volume emission rate and confirmed that the predominant expansion is toward the lower-latitude side in the ionosphere. This study shows that chorus wave propagation in the magnetosphere gives new insight for characterizing the transverse size (across the geomagnetic field line) of wave-particle interaction regions. The calculated spatial scale of the column auroral emission shows a correlation with the magnetic latitude of the resonance region at magnetic latitudes within 10° of the equatorial plane of the magnetosphere. Our results suggest that the spatial scale of a flash aurora is indirectly related to the chorus amplitude because the latitudinal range of the wave-particle interaction is important for the growth of wave amplitude.

DOI： 10.1029/2021JA029254

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JA029254

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

Polar cap patches are known as islands of enhanced electron density in the polar cap F region ionosphere. To observe the airglow signature of polar cap patches continuously at a fixed point near the center of the MLAT/MLT coordinate system, we started operating an all-sky imager in Eureka, Canada since 2015, where the magnetic latitude is ~87°. By statistically analyzing the 630-nm airglow images from Eureka, it was identified that the luminosity of patches, which is proportional to the electron density in the F region, increases during specific UT interval from 16 to 22 UT. This outstanding UT variation of patch luminosity can be explained simply by the systematic shift of the terminator in the MLAT/MLT coordinate system due to the offset between the geographic and geomagnetic pole. That is, the spatial relationship between the dense source plasma in the sunlit area and the high-latitude convection system controls the UT variation. We also found that when the IMF By is positive, the number of polar cap patches was twice of that in the negative IMF By. By deriving average convection patterns from the archived SuperDARN map potential data, we confirmed that the configuration of plasma convection is more appropriate for patches to be transported toward the magnetic pole during the positive IMF By condition.

DOI： 10.1016/j.polar.2020.100608

Web of Science

Scopus

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 present study aims to investigate the influence of the zonal wind velocity on equatorial plasma bubble (EPB) occurrences over Southeast Asia. The observation of the EPB occurrence is obtained from the GPS Rate of TEC change index. Meanwhile, the zonal winds were measured using a Fabry-Perot interferometer located at Kototabang and Chiang Mai stations, and the height of F layer was acquired using an ionosonde at Chumphon station near the magnetic equator. This is the first study to report the influence of zonal wind velocity variation on EPB occurrences with the presence and absence of EPB using GPS data in the Southeast Asian sector. The results illustrated that the average magnitude of zonal wind velocity during the presence of EPB (78 ± 23 m/s) was higher than that of its absence (68 ± 21 m/s). It was observed using long-term data analyses which led to in-depth analyses. The analysis of temporal variation of zonal wind variation demonstrated that the zonal winds during EPB were higher in the evening compared to midnight and postmidnight periods from medium to high solar activities. The dependence of zonal wind velocity on EPB over local time was obtained based on the analysis which utilized the data collected during equinox in high solar activity. Besides that, a positive correlation was obtained between the zonal wind velocity and EPB occurrences during pre-reversal enhancement (PRE) corroborated the effects of zonal wind influence on PRE, and thus EPB occurrences.

DOI： 10.1029/2020JA028994

Web of Science

Scopus

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

Web of Science

Scopus

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

Web of Science

Scopus

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：Journal of Atmospheric and Solar-Terrestrial Physics  

DOI： 10.1016/j.jastp.2021.105593

Web of Science

Scopus

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 Space Weather and Space Climate  

This paper presents an international cooperation which has successfully developed research capacities in the scientific disciplines of sun-earth relations and space weather in many countries over the world during the past decades. This success was based on the deployment of scientific instruments in countries that did not have them, on the sharing of knowledge and research tools, on thesis supervision and on the integration of researchers trained in their country. This article will only focus on aspects of training conducted by ICTP, Boston College, ICG, SCOSTEP and GIRGEA. We will highlight what has been enhanced in international cooperation to achieve this success and what remains to be done.

DOI： 10.1051/swsc/2021006

Web of Science

Scopus

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  

Pc1 geomagnetic pulsations, equivalent to electromagnetic ion cyclotron waves in the magnetosphere, display a specific amplitude modulation, though the region of the modulation remains an open issue. To classify whether the amplitude modulation has a magnetospheric or ionospheric origin, an isolated proton aurora (IPA), which is a proxy of Pc1 wave-particle interactions, is compared with the associated Pc1 waves for a geomagnetic conjugate pair, Halley Research Base in Antarctica and Nain in Canada. The temporal variation of an IPA shows a higher correlation coefficient (0.88) with Pc1 waves in the same hemisphere than that in the opposite hemisphere. This conjugate observation reveals that the classic cyclotron resonance is insufficient to determine the amplitude modulation. We suggest that direct wave radiation from the ionospheric current by IPA should also contribute to the amplitude modulation.

DOI： 10.1029/2020GL091384

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020GL091384

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

The Sun is a variable active-dynamo star, emitting radiation in all wavelengths and solar-wind plasma to the interplanetary space. The Earth is immersed in this radiation and solar wind, showing various responses in geospace and atmosphere. This Sun–Earth connection variates in time scales from milli-seconds to millennia and beyond. The solar activity, which has a ~11-year periodicity, is gradually declining in recent three solar cycles, suggesting a possibility of a grand minimum in near future. VarSITI—variability of the Sun and its terrestrial impact—was the 5-year program of the scientific committee on solar-terrestrial physics (SCOSTEP) in 2014–2018, focusing on this variability of the Sun and its consequences on the Earth. This paper reviews some background of SCOSTEP and its past programs, achievements of the 5-year VarSITI program, and remaining outstanding questions after VarSITI. [Figure not available: see fulltext.]

DOI： 10.1186/s40645-021-00410-1

Web of Science

Scopus

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 unique example of a polar cap arc producing clear amplitude and phase scintillations in GPS L-band signals is presented using observations from an all-sky imager and a GPS receiver and a digital ionosonde at Resolute Bay and the SuperDARN Inuvik radar. During the southward interplanetary magnetic field (IMF) condition, the polar cap arc moved quickly from the dusk-side to the midnight auroral oval at a speed of ∼700 m/s, as revealed by all-sky 557.7 and 630.0 nm images. When it intersected the raypath of GPS signals, both amplitude and phase scintillations appeared, which is very different from previous results. Moreover, the scintillations were precisely determined through power spectral analysis. We propose that the strong total electron content (TEC) enhancement (∼6 TECU) and flow shears in association with the polar cap arc under the southward IMF condition were creating the scintillations. It provides evidence for the existence of polar cap arc scintillations that may be harmful for satellite applications even through L-band signals.

DOI： 10.1029/2020JA028968

Web of Science

Scopus

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 situ electron density profiles obtained from Arase in the night magnetic local time (MLT) sector and from RBSP-B covering all MLTs are used to study the small-scale density irregularities present in the plasmasphere and near the plasmapause. Electron density perturbations with amplitudes >10% from background density and with time-scales less than 30-min are investigated here as the small-scale density irregularities. The statistical survey of the density irregularities is carried out using nearly 2 years of density data obtained from RBSP-B and 4 months of data from Arase satellites. The results show that density irregularities are present globally at all MLT sectors and L-shells both inside and outside the plasmapause, with a higher occurrence at L > 4. The occurrence of density irregularities is found to be higher during disturbed geomagnetic and interplanetary conditions. The case studies presented here revealed: (1) The plasmaspheric density irregularities observed during both quiet and disturbed conditions are found to coexist with the hot plasma sheet population. (2) During quiet periods, the plasma waves in the whistler-mode frequency range are found to be modulated by the small-scale density irregularities, with density depletions coinciding well with the decrease in whistler intensity. Our observations suggest that different source mechanisms are responsible for the generation of density structures at different MLTs and geomagnetic conditions.

DOI： 10.1029/2020JA027917

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA027917

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  

Transverse Pc1 waves propagating from magnetospheric source regions undergo mode conversion to the compressional mode in the ionosphere due to the induced Hall current. Mode converted Pc1 waves propagate across the magnetic field through the ionospheric waveguide. This process is called Pc1 wave ducting (PWD). PWDs have been observed by magnetometers on both ground and low Earth orbit satellites over a wide latitudinal and longitudinal range. In this work, we present the statistical analysis results of PWD exploiting Swarm satellites from 2015 to 2019. Spatial distributions show that the PWDs are mainly observed over the South Atlantic Anomaly longitudes, possibly due to the high Hall conductivity and F-region density, and at subauroral/auroral latitudes ((Formula presented.) MLAT). The occurrence rate of PWD increases with increasing AE and (Formula presented.) SYM-H (Formula presented.) indices. Seasonal dependence shows that PWD exhibits a high occurrence rate during equinox and local summer while local winter hosts only a low occurrence. The asymmetry between summer and winter can be explained by the ionospheric plasma density. The high occurrence rate in equinox may result from intense geomagnetic activity during the equinox, probably due to the Russell-McPherron effect. From our statistical analysis, we conclude that the occurrence of PWD is controlled by both ionospheric plasma conditions and geomagnetic activity, and that the mode conversion and PWD occur more efficiently as plasma density increases.

DOI： 10.1029/2020JA029016

Web of Science

Scopus

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

We present a detailed investigation of the formation of an additional sodium density peak at altitudes of 79-85 km below the main peak of the sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromsø, Norway, on the night of 19 December 2014. The airglow imager observations of OH emissions revealed four passing frontal systems that resembled mesospheric bores, which typically occur in ducting regions of the upper mesosphere. For about 1.5 h, the lower-altitude sodium peak had densities similar to that of the main peak of the layer around 90 km. The lower-altitude sodium peak weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column-integrated sodium densities increased gradually during the formation of the lower-altitude sodium peak. Temperatures measured with the lidar indicate that there was a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in the OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km, wherein the temperature was also higher. Both factors would have liberated sodium atoms from the reservoir species and suppressed the reconversion of atomic sodium into reservoir species so that the lower-altitude sodium peak could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals.

DOI： 10.5194/acp-21-2343-2021

Web of Science

Scopus

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 present the first multi-wavelength imaging observations of strong thermal emission velocity enhancement (STEVE) using an all-sky imager at Athabasca (magnetic latitude = 61.5°N), Canada. This study is based on three STEVE events which were accompanied by picket fence structures in the green-line. Although the STEVE arc was dominant in 630 and 557.7-nm, weak emissions were also found in other wavelengths including OI at 844.6, Hβ, Na, and the nominal background filter at 572.5-nm. As observed at 630 and 557.7-nm, the STEVE arc started as a faint arc close to the auroral oval and moved equatorward. The 557.7-nm arc exhibited picket fence structure at later times after it moved equatorward. The picket fence was sometimes found to persist even after the 630-nm arc had disappeared. During a particular event, the STEVE arcs in both the 630 and 557.7-nm were found to carry a ribbon-like motion moving along the arc. We have found that STEVE arcs are embedded in a region of weak diffuse auroral emissions. The STEVE arcs have sharp boundaries and these boundaries are different in red- and green-line. The sharp decrease in the intensity at the immediate poleward edge of the STEVE arc appears as a “dark-band” in the green-line images. Based on the horizontal component of the geomagnetic field at Fort Smith (magnetic latitude 67.28°N), we find that the STEVE arc detachment, its equatorward motion, and its brightness coincided with changes in the magnetic activity during the recovery phase of a substorm.

DOI： 10.1029/2020JA028622

Web of Science

Scopus

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 report the properties of the ionospheric exit point and characteristics of several types of very low frequency waves, including chorus and quasiperiodic emissions, based on a comprehensive dataset of simultaneous observations between ground and space. Whistler-mode waves were observed at Kannuslehto (L = 5.5, KAN), Finland, and in the inner magnetosphere by the Japanese Arase satellite. During the 2017–2018 winter campaign, we found 13 cases showing one-to-one correspondence of wave spectra between KAN and Arase. This is the first time that such a large number of conjugated events have been reported at once. The duration of the events ranged from a few minutes up to 3 h, with 90% of events detected in the afternoon sector. While the occurrence rate is higher during daytime, this can also be related to a majority of the detected waves being quasiperiodic emissions, a known dayside phenomenon. Arase was usually located within 30° of the equator, at L ∼4–5, and detected mostly waves propagating at oblique angles (≥20°). Frequently, the ionospheric magnetic footprint of Arase was located equatorwards (south) from KAN, often in the same geographical area. We investigated the probable location of the ionospheric exit point of the waves from the location of the footprint of Arase and the angle of arrival of waves detected at KAN. Using density measurements at Arase we discuss magnetospheric wave propagation; we find that, in most cases, waves were unducted in their propagation from the satellite to the ground.

DOI： 10.1029/2020JA028682

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028682

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  

Magnetospheric Extremely Low-Frequency/Very Low-Frequency (ELF/VLF) waves have an important role in the acceleration and loss of energetic electrons in the magnetosphere through wave-particle interaction. It is necessary to understand the spatiotemporal development of magnetospheric ELF/VLF waves to quantitatively estimate this effect of wave-particle interaction, a global process not yet well understood. We investigated spatiotemporal development of magnetospheric ELF/VLF waves using 6 PWING ground-based stations at subauroral latitudes, Exploration of energization and Radiation in Geospace and RBSP satellites, POES/MetOp satellites, and the RAM-SCB model, focusing on the March and November 2017 storms driven by corotating interaction regions in the solar wind. Our results show that the ELF/VLF waves are enhanced over a longitudinal extent from midnight to morning and dayside associated with substorm electron injections. In the main to early storm recovery phase, we observe continuous ELF/VLF waves from ∼0 to ∼12 MLT in the dawn sector. This wide extent seems to be caused by frequent occurrence of substorms. The wave region expands eastward in association with the drift of source electrons injected by substorms from the nightside. We also observed dayside ELF/VLF wave enhancement, possibly driven by magnetospheric compression by solar wind, over an MLT extent of at least 5 h. Ground observations tend not to observe ELF/VLF waves in the post-midnight sector, although other methods clearly show the existence of waves. This is possibly due to Landau damping of the waves, the absence of the plasma density duct structure, and/or enhanced auroral ionization of the ionosphere in the post-midnight sector.

DOI： 10.1029/2020JA028216

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028216

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  

Bright, discrete, thin auroral arcs are a typical form of auroras in nightside polar regions. Their light is produced by magnetospheric electrons, accelerated downward to obtain energies of several kilo electron volts by a quasi-static electric field. These electrons collide with and excite thermosphere atoms to higher energy states at altitude of ~ 100 km; relaxation from these states produces the auroral light. The electric potential accelerating the aurora-producing electrons has been reported to lie immediately above the ionosphere, at a few altitudes of thousand kilometres1. However, the highest altitude at which the precipitating electron is accelerated by the parallel potential drop is still unclear. Here, we show that active auroral arcs are powered by electrons accelerated at altitudes reaching greater than 30,000 km. We employ high-angular resolution electron observations achieved by the Arase satellite in the magnetosphere and optical observations of the aurora from a ground-based all-sky imager. Our observations of electron properties and dynamics resemble those of electron potential acceleration reported from low-altitude satellites except that the acceleration region is much higher than previously assumed. This shows that the dominant auroral acceleration region can extend far above a few thousand kilometres, well within the magnetospheric plasma proper, suggesting formation of the acceleration region by some unknown magnetospheric mechanisms.

DOI： 10.1038/s41598-020-79665-5

Web of Science

Scopus

PubMed

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

Web of Science

Scopus

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

Web of Science

Scopus

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  

Although the occurrence rate of electromagnetic ion cyclotron (EMIC) waves is high in the outer magnetosphere (L > 7), it has been suggested in the past that a steep plasma density gradient region of the plasmapause is a preferred location for the generation of EMIC waves. To examine spectral properties of the EMIC waves occurred near the nominal location of the plasmapause (L = 4–5), we focus on Pc1-Pc2 waves observed at subauroral latitude Athabasca station (magnetic latitude: ~62°, and L ~ 4.6). A statistical study of 10,494 wave samples identified from Athabasca data for 2007–2008 reveals the following wave characteristics. (1) Wave frequencies are higher in the postmidnight-to-dawn sector and lower in the late afternoon sector. (2) They mostly appear to be in frequency band between helium and oxygen gyrofrequencies (i.e., He-band) calculated from the dipole field model magnetic field intensity at L = 4.6. (3) The occurrence rate of Pc1-Pc2 waves has a peak in the prenoon sector at 0900–1100 LT under quiet geomagnetic conditions (Kp ≤ 1), but is peaked in the afternoon sector under moderate and disturbed geomagnetic conditions (Kp ≥ 2). (4) The Pc1-Pc2 waves observed at Athabasca were composed of a mixture of left-hand, right-hand, and linearly polarized waves. By comparing previous and recent studies with these observations, we suggest that the subauroral latitude Pc1-Pc2 waves are associated with EMIC waves generated near the plasmapause and discuss the EMIC wave properties in a region of cold and dense plasmas containing heavy ions in the inner magnetosphere.

DOI： 10.1016/j.jastp.2020.105292

Web of Science

Scopus

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 observed a signature of equatorial plasma bubbles (EPBs) followed by a medium-scale traveling ionospheric disturbance (MSTID) crossing the solar terminator over the geomagnetic equator during the period of post-sunset rise (PSSR) of the F layer. Simultaneous observations of the EPBs and the MSTIDs by ground-based Global Navigation Satellite System receivers and OI 630-nm imagers, ionospheric parameters by ionosondes, and mesosphere–lower thermosphere (MLT) gravity waves (MLT-GWs) by OH airglow imagers have been carried out in the equatorial and low-latitude region of Brazil. On 16–17 September 2015, MSTIDs with very fast horizontal phase speeds, propagating toward northeast (NE), were observed over the latitude region of 0–30°S in the afternoon to evening time zone. It was geomagnetically quiet time (Kp: 2–3). An EPB development was observed after one of the MSTIDs crossed the solar terminator at 22:00 UT. Large-scale MLT-GWs with slower phase speeds were also observed both at São João do Cariri (7.5°S, 35.0°W) propagating from southwest (SW) to NE and Cachoeira Paulista (22.7°S, 45.0°W) propagating northward. This is to report on what we consider as a signature of direct seeding of EPBs by an MSTID that was likely induced by a secondary GW from tropospheric deep cloud convections to the south. This study highlights a possible dynamical process from the troposphere to ionosphere via this vertical coupling process over the 2,500 km of horizontal distance.

DOI： 10.1029/2019JA027566

Web of Science

Scopus

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  

Recent availability of a considerable amount of satellite and ground-based data has allowed us to analyze rare conjugated events where extremely low and very low frequency waves from the same source region are observed in different locations. Here, we report a quasiperiodic (QP) emission, showing one-to-one correspondence, observed by three satellites in space (Arase and the Van Allen Probes) and a ground station. The main event was on 29 November 2018 from 12:06 to 13:08 UT during geomagnetically quiet times. Using the position of the satellites we estimated the spatial extent of the area where the one-to-one correspondence is observed. We found this to be up to 1.21 Earth's radii by 2.26 hr MLT, in radial and longitudinal directions, respectively. Using simple ray tracing calculations, we discuss the probable source location of these waves. At ∼12:20 UT, changes in the frequency sweep rate of the QP elements are observed at all locations associated with magnetic disturbances. We also discuss temporal changes of the spectral shape of QP observed simultaneously in space and on the ground, suggesting the changes are related to properties of the source mechanisms of the waves. This could be linked to two separate sources or a larger source region with different source intensities (i.e., electron flux). At frequencies below the low hybrid resonance, waves can experience attenuation and/or reflection in the magnetosphere. This could explain the sudden end of the observations at the spacecraft, which are moving away from the area where waves can propagate.

DOI： 10.1029/2020JA028126

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028126

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  

Prompt penetration of convection/overshielding electric fields to equatorial and low latitudes during the southward/northward turnings of interplanetary magnetic field (IMF Bz) have been widely studied in the literature. The other types of penetration electric fields due to sudden changes in the solar wind dynamic pressure, IMF By and during the onset of substorms have also been recently reported. In this paper, we present the exclusive role of solar wind density changes on the prompt equatorial electric field disturbances using the long-term observations of equatorial electrojet (EEJ) from the Indian sector. In response to the sharp increases in the solar wind density, prompt increases/decreases in the EEJ indicating the eastward/westward prompt penetration electric field (PPEF) of ~20 min periods have been consistently observed on the dayside/nightside. The prompt equatorial electric field disturbances of the opposite polarity have also been observed when the density decreases sharply. Further, the polarity of these PPEF disturbances does not show any clear dependency on the direction of IMF Bz and By. This paper is the first report with a statistically significant number of observations on the characteristics of equatorial electric field disturbances in response to the sudden enhancements/decreases in the solar wind density alone on both dayside and nightside. The underlying physical mechanisms for the prompt equatorial electric field disturbances have been discussed in light of enhanced high-latitude convection and additional field-aligned currents due to sudden enhancement of solar wind density.

DOI： 10.1029/2020JA027869

Web of Science

Scopus

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  

We report the first observation of plasma density oscillations coherent with magnetic Pc1 waves. Swarm satellites observed compressional Pc1 wave activity in the 0.5–3 Hz band, which was coherent with in situ plasma density oscillations. Around the Pc1 event location, the Antarctic Neumayer Station III (L ~ 4.2) recorded similar Pc1 events in the horizontal component while NOAA-15 observed isolated proton precipitations at energies above 30 keV. All these observations support that the compressional Pc1 waves at Swarm are oscillations converted from electromagnetic ion cyclotron (EMIC) waves coming from the magnetosphere. The magnetic field and plasma density oscillate in-phase. We compared the amplitudes of density and magnetic field oscillations normalized to background values and found that the density power is much larger than the magnetic field power. This difference cannot be explained by a simple magnetohydrodynamic (MHD) model, although steep horizontal/vertical gradients of background ionospheric density can partly reconcile the discrepancy.

DOI： 10.1029/2020GL089000

Web of Science

Scopus

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 investigate the longitudinal structure of the oxygen torus in the inner magnetosphere for a specific event found on 12 September 2017, using simultaneous observations from the Van Allen Probe B and Arase satellites. It is found that Probe B observed a clear enhancement in the average plasma mass (M) up to 3–4 amu at L = 3.3–3.6 and magnetic local time (MLT) = 9.0 h. In the afternoon sector at MLT ~ 16.0 h, both Probe B and Arase found no clear enhancements in M. This result suggests that the oxygen torus does not extend over all MLT but is skewed toward the dawn. Since a similar result has been reported for another event of the oxygen torus in a previous study, a crescent-shaped torus or a pinched torus centered around dawn may be a general feature of the O+ density enhancement in the inner magnetosphere. We newly find that an electromagnetic ion cyclotron (EMIC) wave in the H+ band appeared coincidently with the oxygen torus. From the lower cutoff frequency of the EMIC wave, the ion composition of the oxygen torus is estimated to be 80.6% H+, 3.4% He+, and 16.0% O+. According to the linearized dispersion relation for EMIC waves, both He+ and O+ ions inhibit EMIC wave growth and the stabilizing effect is stronger for He+ than O+. Therefore, when the H+ fraction or M is constant, the denser O+ ions are naturally accompanied by the more tenuous He+ ions, resulting in a weaker stabilizing effect (i.e., larger growth rate). From the Probe B observations, we find that the growth rate becomes larger in the oxygen torus than in the adjacent regions in the plasma trough and the plasmasphere.[Figure not available: see fulltext.]

DOI： 10.1186/s40623-020-01235-w

Web of Science

Scopus

PubMed

その他リンク： http://link.springer.com/article/10.1186/s40623-020-01235-w/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  

Auroral arcs and diffuse auroras are common phenomena at high latitudes, though characteristics of their source plasma and fields have not been well understood. We report the first observation of fields and particles including their pitch-angle distributions in the source region of auroral arcs and diffuse auroras, using data from the Arase satellite at L ~ 6.0–6.5. The auroral arcs appeared and expanded both poleward and equatorward at local midnight from ~0308 UT on 11 September 2018 at Nain (magnetic latitude: 66°), Canada, during the expansion phase of a substorm, while diffuse auroras covered the whole sky after 0348 UT. The top part of auroral arcs was characterized by purple/blue emissions. Bidirectional field-aligned electrons with structured energy-time spectra were observed in the source region of auroral arcs, while source electrons became isotropic and less structured in the diffuse auroral region afterwards. We suggest that structured bidirectional electrons at energies below a few keV were caused by upward field-aligned potential differences (upward electric field along geomagnetic field) reaching high altitudes (~30,000 km) above Arase. The bidirectional electrons above a few keV were probably caused by Fermi acceleration associated with the observed field dipolarization. Strong electric-field fluctuations and earthward Poynting flux were observed at the arc crossing and are probably also caused by the field dipolarization. The ions showed time-pitch-angle dispersion caused by mirror reflection. These results indicate a clear contrast between auroral arcs and diffuse auroras in terms of source plasma and fields and generation mechanisms of auroral arcs in the inner magnetosphere.

DOI： 10.1029/2019JA027310

Web of Science

Scopus

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 present observations of broadband (∼10–50 mHz) Pc3–Pc4 waves on 4 January 2014. The waves were detected on the dayside simultaneously in a compressional component (δBz) at the Radiation Belt Storm Probes A (RBSP-A) in the inner magnetosphere and in the north-south component (δH) on the ground at a low-latitude Bohyun (BOH) station (L = 1.3) during an interval of small interplanetary magnetic field cone angle, suggesting that upstream ultralow frequency (ULF) waves were the source of the magnetospheric Pc3–Pc4 waves. We observed the ground-satellite high coherence between RBSP-A δBz and BOH δH for a prolonged time interval lasting 6.5 hr, which has not been reported previously, during which RBSP-A was on the inbound and outbound legs moving from L = ∼3.3 to ∼6.3. In order to understand the spatial mode structure of the compressional waves, we examined the ground-satellite cross phase for the high-coherence interval and found that the waves observed in and out of the plasmasphere propagated earthward at the average fast-mode speed of ∼700–1,000 km/s. We also observed cross-phase values smoothly changing with the radial distance of the spacecraft across the plasmapause. This indicates that the presence of the plasmapause has little effect on our fast-mode waves propagating into the inner magnetosphere and to the ground low-latitude station.

DOI： 10.1029/2020JA028074

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：IEEE  

DOI： 10.23919/ursigass49373.2020.9232442

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  

Simultaneous eastward and westward traveling surges were observed at Tjörnes, Iceland, and Syowa station, Antarctica, respectively. Several remarkable differences were identified. (1) The position of the initial bright spot was shifted by 1.7 to 2.3 MLT between both hemispheres. (2) The surges differ in traveling speed between the eastward traveling surge (6.5 km s−1) and the westward traveling surge (1.3 km s−1). (3) The Arase satellite was located on a geomagnetic field line connecting both ground stations and observed a significant excess in westward component of the magnetic field, which is consistent with the large shifts of the initial bright spots in both hemispheres. (4) The background Hall current flows eastward (Northern Hemisphere) and westward (Southern Hemisphere). The observed north-south asymmetry of the traveling surges suggests that the ionosphere can play an essential role in controlling the fundamental spatiotemporal development of auroras in both hemispheres.

DOI： 10.1029/2020GL088750

Web of Science

Scopus

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 equatorial plasma bubbles, once developed, grow nonlinearly into topside ionosphere, and simultaneous secondary instabilities lead to the development of shorter scale irregularities. The altitudinal growth and generation of smaller scale irregularities determine the spatio-temporal occurrence and the intensity of ionospheric scintillations at wide spectrum of radio waves and have significant implications on the GNSS/Satellite Based Augmentation Systems. In this letter, we present a unique equatorial plasma bubble observation from equatorial atmosphere radar that provides hitherto undisclosed evidence for the smaller (3-m) scale irregularities initially developing at higher altitudes and subsequently developing to lower altitudes in a narrow funnel-like structure. The responsible mechanisms for early development of shorter scale irregularities in the topside and their subsequent development at lower altitudes are discussed in light of difference between the time scales of altitudinal growth and cascading rate of secondary instabilities through high-resolution bubble model simulations.

DOI： 10.1029/2020GL087256

Web of Science

Scopus

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  

Magnetic fluctuations in the near-Earth magnetotail are an important signature of substorm onset. In a previous statistical study, we reported their occurrence rates, spatial distributions, and relationship with plasma flows. In the present study, we investigated their spectral properties using 11 years of measurements from the Time History of Events and Macroscale Interactions during Substorms mission for 2008–2018. We found 10,848 severe magnetic fluctuation events with (Formula presented.), where σB and (Formula presented.) are the standard deviation and average, respectively, of the magnetic field intensity for the local proton gyroperiod. The occurrence rates of severe magnetic fluctuations show no clear dependence on the F10.7 index in one solar cycle. We extracted 36 dipolarization events with severe magnetic fluctuations. In the power spectral density (PSD) of the magnetic fluctuations during dipolarizations, the steepness of the spectral slope increased with increasing frequency in almost all the events. The average PSDs are shown sorted by (a) distance to the neutral sheet and (b) ambient magnetic field intensity. In all groups, the slopes of the average PSDs increased abruptly from below ∼10−1.3 Hz (0.05 Hz) to above ∼10−1.3 Hz, which is close to the gyrofrequency of O+ ions. It is the first time that a change of slope near the proton gyrofrequency (frequency range: 0.05–1 Hz) was found in cases of larger ambient magnetic field intensity, implying that the magnetic fluctuations were relatively strong near the proton gyrofrequency. These results suggest that the magnetic fluctuations contribute to the nonmagnetohydrodynamic effect in the ion motion.

DOI： 10.1029/2020JA027834

Web of Science

Scopus

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  

Ducting Pc1 waves are observed over a wide latitudinal range by the low-Earth orbit satellites as compressional mode. In this paper, we present the first observation of the modulation of ducting Pc1 waves by equatorial plasma bubbles (EPBs) based on the Swarm satellites. We show two ducting Pc1 events propagating across EPBs that occurred on 7 April 2016 and on 27 September 2017. We found that the EPBs modulate the Pc1 wave propagation by setting up reflection boundaries and leakage holes in the ionospheric waveguide. We also found that changes of Pc1 wave intensities generally follow the electron density variation and that the intensity is stronger at higher density region. From the comparison between Swarm-A and Swarm-C observations, we conclude that ionospheric plasma plays an important role for Pc1 waveguide even though their density is significantly low in EPBs.

DOI： 10.1029/2020GL088054

Web of Science

Scopus

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  

Here, we introduce a low-cost airglow imaging system developed for observing plasma bubble signatures in 630.0-nm airglow emission from the F region of the ionosphere. The system is composed of a small camera, optical filter, and fish-eye lens, and is operated using free software that automatically records video from the camera. A pilot system was deployed in Ishigaki Island in the southern part of Japan (Lat 24.4, Lon 124.4, Mlat 19.6) and was operated for ~ 1.5 years from 2014 to 2016 corresponding to the recent solar maximum period. The pilot observations demonstrated that it was difficult to identify the plasma bubble signature in the raw image captured every 4 s. However, the quality of the image could be improved by reducing the random noise of instrumental origin through an integration of 30 consecutive raw images obtained in 2 min and further by subtracting the 1-h averaged background image. We compared the deviation images to those from a co-existing airglow imager of OMTIs, which is equipped with a back-illuminated cooled CCD camera with a high quantum efficiency of ~ 90%. It was confirmed that the low-cost airglow imager is capable of imaging the spatial structure of plasma bubbles, including their bifurcating traces. The results of these pilot observations in Ishigaki Island will allow us to distribute the low-cost imager in a wide area and construct a network for monitoring plasma bubbles and their space weather impacts on satellite navigation systems.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-020-01187-1

Web of Science

Scopus

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

This paper introduces a new system that can monitor aurora and atmospheric airglow using a low-cost Watec monochromatic imager (WMI) equipped with a sensitive camera, a filter with high transmittance, and the non-telecentric optics. The WMI system with 486-nm, 558-nm, and 630-nm band-pass filters has observable luminosity of about ~200–4000 Rayleigh for 1.07-sec exposure time and about ~40–1200 Rayleigh for 4.27-sec exposure time, for example. It is demonstrated that the WMI system is capable of detecting 428-nm auroral intensities properly, through comparison with those measured with a collocated electron-multiplying charge-coupled device (EMCCD) imager system with narrower band-pass filter. The WMI system has two distinct advantages over the existing system: One makes it possible to reduce overall costs, and the other is that it enables the continuous observation even under twilight and moonlight conditions. Since 2013 a set of multi-wavelength WMIs has been operating in northern Scandinavia, Svalbard, and Antarctica to study meso- and large-scale aurora and airglow phenomena. Future development of the low-cost WMI system is expected to provide a great opportunity for constructing a global network for multi-wavelength aurora and airglow monitoring.

DOI： 10.1016/j.polar.2019.100501

Web of Science

Scopus

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 present a comprehensive investigation on the propagation characteristics of duskside medium-scale traveling ionospheric disturbances (MSTIDs) using 630.0-nm airglow emissions over Tromsø (69.6°N, 19.2°E; magnetic latitude: 66.7°N). The unique points of our observation are (1) duskside MSTIDs primarily exhibited eastward motion under quiet conditions but turned to the westward direction associated with geomagnetic disturbances, (2) the westward moving MSTIDs again turned to the eastward direction when the geomagnetic disturbance ceased, (3) the turning of MSTIDs to the westward direction was invariably associated with an increase of the northward component of the magnetic field observed by the local ground-based magnetometers and with the equatorward expansion of the auroral oval, and (4) the Super Dual Auroral Radar Network convection maps revealed that the location of Tromsø was inside (outside) the duskside convection cell during the time of appearance of westward (eastward) moving MSTIDs. The average eastward and westward velocities of MSTIDs were ~25–80 and ~40–140 m/s, respectively. The Doppler shift measurement of the 630-nm airglow by a Fabry-Perot interferometer at Tromsø showed that northeastward winds were predominant during the appearance of eastward moving MSTIDs. These experimental evidences suggest that the oscillatory motion of MSTIDs over high latitudes is driven by the convection electric field. The MSTIDs tend to move eastward under geomagnetically quiet conditions but show westward motion under the influence of convection electric field associated with auroral activities in the duskside of two-cell convection pattern.

DOI： 10.1029/2019JA027598

Web of Science

Scopus

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 full-color photographs of aurora have been taken with digital single-lens reflex cameras mounted on the International Space Station (ISS). Since these photographs do not have accurate time and geographical information, in order to use them as scientific data, it is necessary to calibrate the imaging parameters (such as looking direction and angle of view of the camera) of the photographs. For this purpose, we calibrated the imaging parameters using a city light image taken from the Defense Meteorological Satellite Program satellite following the method of Hozumi et al. (2016, https://doi.org/10.1186/s40623-016-0532-z). We mapped the photographs onto the geographic coordinate system using the calibrated imaging parameters. To evaluate the accuracy of the mapping, we compared the aurora taken simultaneously from ISS and ground. Comparing the spatial structure of discrete aurora and the temporal variation of pulsating aurora, the accuracy of the data set is less than 0.3 s in time and less than 5 km in space in the direction perpendicular to the looking direction of the camera. The generated data set has a wide field of view ((Formula presented.) 1,100 (Formula presented.) 900 km), and their temporal resolution is less than 1 s. Not only that, the field of view can sweep a wide area ((Formula presented.) 3,000 km in longitude) in a short time ((Formula presented.) 10 min). Thus, this new imaging capability will enable us to capture the evolution of fine-scale spatial structure of aurora in a wide area.

DOI： 10.1029/2019JA027729

Web of Science

Scopus

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  

This work investigated the zonal drift velocity, VE, of equatorial plasma bubbles (EPBs) which was calculated from the zonal keogram of the rate of total electron content index obtained using the Global Positioning System data from the Malaysia Real-Time Kinematics GNSS Network over Southeast Asia across the period 2008 until 2013 (498 nights). In general, a long-term study on the VE of EPBs and its dependence on the season, local time, and solar activity over Southeast Asia by using Global Positioning System multipoint receivers has never been conducted before. The case study showed that the VE estimated from Malaysia Real-Time Kinematics GNSS Network was basically within a similar range to that of the 630-nm airglow images obtained at Kototabang, Indonesia. Furthermore, the VE of EPBs was higher during high solar-activity period, which was in line with past studies. The time range of the EPB VE peak is more accurately determined compared to previous studies. In high solar activity conditions, a peak of VE was observed at 2130 LT (equinox), 2200 LT (June solstice), and after 2300 LT (December solstice). After this peak, the EPB VE steadily decreased with time during the equinox and was varied throughout the night during June solstice. Moreover, it was found to differ with time from the evening to the postmidnight period during December solstice.

DOI： 10.1029/2019JA027521

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2019JA026807

Web of Science

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  

The brightness of aurorae in Earth's polar region often beats with periods ranging from sub-second to a few tens of a second. Past observations showed that the beat of the aurora is composed of a superposition of two independent periodicities that co-exist hierarchically. However, the origin of such multiple time-scale beats in aurora remains poorly understood due to a lack of measurements with sufficiently high temporal resolution. By coordinating experiments using ultrafast auroral imagers deployed in the Arctic with the newly-launched magnetospheric satellite Arase, we succeeded in identifying an excellent agreement between the beats in aurorae and intensity modulations of natural electromagnetic waves in space called "chorus". In particular, sub-second scintillations of aurorae are precisely controlled by fine-scale chirping rhythms in chorus. The observation of this striking correlation demonstrates that resonant interaction between energetic electrons and chorus waves in magnetospheres orchestrates the complex behavior of aurora on Earth and other magnetized planets.

DOI： 10.1038/s41598-020-59642-8

Web of Science

Scopus

PubMed

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

The present work displays the observations of an afternoon detached aurora along with ionospheric high-latitude trough. The event was observed by DMSP F17 on 19 September 2014. The afternoon detached aurora was isolated from the auroral oval and was located between 12:00–18:00 magnetic local time (MLT) and 65–70° geomagnetic latitude (MLAT). Particle observations indicate that the afternoon detached aurora was produced by energetic ring current ions with energies above ~10 keV where the main ion energy was likely to be above the upper limit of DMSP measurement (~30 keV). Magnetometer observation from the ground implies that the energetic ions were likely scattered by EMIC waves. Both the detached aurora and the auroral oval are found to be well inside the high-latitude trough with MLAT between ~64° and ~76° (68–80° GLAT). The auroral oval corresponds to a westward (sunward) plasma drift. It is expected that the westward drift transports the low-density plasma in the nightside toward the dayside, leading to the high-latitude trough formation. The afternoon detached aurora was well equatorward of the high-latitude trough, and the corresponding plasma drift was nearly zero. The plasma associated with the detached aurora is expected to be stagnant, and broaden the high-latitude trough equatorward.

DOI： 10.1016/j.asr.2019.10.003

Web of Science

Scopus

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 compare for the first time two conjugate events showing simultaneous very low frequency (VLF) wave observations between the same ground station and spacecraft, at different geomagnetic conditions and on opposite sides of the magnetosphere. Waves were observed at Kannuslehto (MLAT = 64.4°N, L=5.46), Finland, and on board Arase (Exploration of energization and Radiation in Geospace, ERG) in the inner magnetosphere. Case 1 on 28 March 2017 shows quasiperiodic (QP) emissions and chorus simultaneously observed on the postmidnight side during the recovery phase of a storm, with sustained high solar wind speed and AE index. Case 2 on 30 November 2017 shows clear one-to-one correspondence of QP elements on the noonside during geomagnetic quiet time (Dst>10 nT and AE<100 nT). We present the characteristics of both cases, focusing on coherence and spatial extent of the waves, electron density, and magnetic field variations. We report that the magnetic field gradient plays a role in the changes of spectral features of the waves.

DOI： 10.1029/2019JA026663

Web of Science

Scopus

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 paper investigates the capability of geomagnetic storm parameters in the disturbance storm-time (Dst), Kp, and AE indices to distinguish between severe space weather (SvSW) that causes the reported electric power outages and/or telecommunication failures and normal space weather (NSW) that does not cause these severe effects in a 50 yr period (1958-2007). The parameters include the storm intensities DstMin (minimum Dst during the main phase, MP, of the storm), (dDst/dt)MPmax, Kpmax, and AEmax. In addition, the impulsive parameter is derived for the storms that are automatically identified in the Kyoto Dst and USGS Dst.∫TMP| DstMP| dt is the integral of the modulus of the Dst from onset of the MP (MPO) to the DstMin. T MP is the MP duration from MPO to DstMin. The corresponding mean values KpMP and AEMP are also calculated. Regardless of the significant differences in the storm parameters between the two Dst indices, the IpsDst in both indices seems to identify four of the five SvSW events (and the Carrington event) in more than 750 NSW events that have been reported to have occurred in 1958-2007, while all other parameters separate one or two SvSWs from the NSWs. Using the Kyoto IpsDst threshold of -250 nT, we demonstrate a 100% true SvSW identification rate with only one false NSW. Using the false NSW event (1972 August 4), we investigate whether using a higher resolution Dst might result in a more accurate identification of SvSWs. The mechanism of the impulsive action leading to large IpsDst and SvSW involves the coincidence that the fast interplanetary coronal mass ejection velocity V contains its shock (or front) velocity ΔV and large interplanetary magnetic field Bz southward covering ΔV.

DOI： 10.3847/1538-4357/ab5113

Web of Science

Scopus

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 first regional total electron content (TEC) model over the entire African region (known as AfriTEC model) using empirical observations is developed and presented. Artificial neural networks were used to train TEC observations obtained from Global Positioning System receivers, both on ground and onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites for the African region from years 2000 to 2017. The neural network training was implemented using inputs that enabled the networks to learn diurnal variations, seasonal variations, spatial variations, and variations that are connected with the level of solar activity, for quiet geomagnetic conditions (−20 nT ≤ Dst ≤ 20 nT). The effectiveness of three solar activity indices (sunspot number, solar radio flux at 10.7-cm wavelength [F10.7], and solar ultraviolet [UV] flux at 1 AU) for the neural network trainings was tested. The F10.7 and UV were more effective, and the F10.7 was used as it gave the least errors on the validation data set used. Equatorial anomaly simulations show a reduced occurrence during the June solstice season. The distance of separation between the anomaly crests is typically in the range from about 11.5 ± 1.0° to 16.0 ± 1.0°. The separation is observed to widen as solar activity levels increase. During the December solstice, the anomaly region shifts southwards of the equinox locations; in year 2012, the trough shifted by about 1.5° and the southern crest shifted by over 2.5°.

DOI： 10.1029/2019JA027065

Web of Science

Scopus

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  

Magnetospheric extremely low frequency/very low frequency (ELF/VLF) waves are plasma waves emitted from high-energy electrons in the magnetosphere. These waves have received much attention, as they contribute to the acceleration and loss of relativistic electrons in the radiation belts through wave-particle interactions. The longitudinal extent of ELF/VLF waves has not been well-understood, although the extent is important in quantitative evaluation of relativistic electron variations. In this study, we analyzed data from continuous ground-based simultaneous observations of ELF/VLF waves over a 2-month period in November and December of 2017, using six loop antennas located at roughly equal intervals around the north geomagnetic pole at ∼60° magnetic latitudes. We estimated the longitudinal extent of magnetospheric ELF/VLF waves based on their occurrence rate. Our results showed that the ELF/VLF wave occurrence rate differed by twofold to threefold, depending on the longitudes of the observation sites. We explain this difference in terms of longitudinal differences in the ionosphere's magnetic field intensity, possibly due to the electron loss that occurs during the bounce motion at longitudes of small magnetic field intensity. Based on our statistical analysis, we estimated the typical longitudinal extent of ELF/VLF waves as ∼76°. Time series analysis results showed that the large longitudinal extent of the ELF/VLF waves occurs frequently during the main phase of geomagnetic storms and is also associated with substorms represented by the auroral electrojet index.

DOI： 10.1029/2019JA026810

Web of Science

Scopus

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  

Three different episodes of prompt penetration electric field (PPEF) disturbances are observed during the main phase of the St. Patrick's Day storm on 17 March 2015 under steady southward interplanetary magnetic field (IMF) Bz conditions unlike the conventional PPEF associated with southward or northward turnings of IMF Bz. These PPEF events took place during the period when strong disturbance dynamo fields are prevailing in the background. The first event is triggered by a solar wind dynamic pressure pulse that caused a sharp eastward PPEF and strong enhancement of equatorial electrojet current in Brazilian dayside. The second event caused another short but strong westward PPEF on dayside due to the reversal of IMF By from duskward to dawnward under steady IMF Bz. The third event caused a longer eastward PPEF in association with a solar wind dynamic pressure pulse followed by the onset of a substorm, which has led to strong enhancement of equatorial electrojet, quick rejuvenation and symmetric redistribution of equatorial ionization anomaly in the Brazilian sector. The signatures of the PPEF with opposite polarity and smaller magnitudes are also observed in the Asian sector on the nightside. The possible mechanisms for the observed PPEF events under steady IMF Bz are discussed in terms of changes in the high-latitude field-aligned currents and reconfiguration of high-latitude convection fields using Active Magnetosphere and Planetary Electrodynamics Response Experiment and Super Dual Auroral Radar Network high-frequency radar observations.

DOI： 10.1029/2019JA027069

Web of Science

Scopus

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  

Conventionally, the minimum value of Dst (DstMin) and maximum values of Kp and AE (Kpmax and AEmax) representing the geomagnetic storm intensities have been used for investigating space weather in Earth's environment. The present paper uses the derived parameters (IpsDst, IpsKp, and IpsAE) giving the mean values of Dst, Kp, and AE during the main phase (MP) of Dst storms for investigating ionosphere-thermosphere storms and low-latitude (630 nm) aurora. The derived parameters (IpsDst, IpsKp, and IpsAE) representing the impulsive strength of geomagnetic storms seem to have more systematic dependence among themselves than among the intensities (DstMin, Kpmax, and AEmax). The ionosphere-thermosphere storms observed by the CHAMP (Challenging Minisatellite Payload) satellite and low-latitude auroras observed by optical imagers are much more intense during high impulsive storms than high intensity storms. In a statistical sense, over 175 positive ionospheric storms (△NmF2) observed in 1985–2005 and the intensity of 20 red auroras observed in 1989–2004 at midlatitudes correlate better with the impulsive parameters than the intensity parameters, with the best correlation being with IpsDst. The mechanism of the impulsive action (high-energy input over a short duration) leading to large IpsDst arises from the impact of fast solar storms (interplanetary coronal mass ejections) with large IMF Bz southward at their front (or shock). The impulsive action results in bright low-latitude auroras and strong ionosphere-thermosphere storms.

DOI： 10.1029/2019JA027080

Web of Science

Scopus

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  

Auroral emission at 427.8-nm from N2+ ions is caused by precipitation of energetic electrons, or by resonant scattering of sunlight by auroral N2+ ions. The latter often causes impressive purple aurora at high altitudes. However, statistical characteristics of auroral 427.8-nm emission have not been well studied. In this paper we report occurrence characteristics of high 427.8-nm emission intensities (more than 100 R) at subauroral latitudes, based on measurements by a filter-tilting photometer over 14 years (2005–2018) at Athabasca, Canada (magnetic latitude: ~62°). We divided the data set into solar elevation angles (θs) more than and less than −24° (shadow height of sunlight: 600 km) to separate the 427.8-nm emissions caused by resonant scattering of sunlight and those excited by auroral electrons, respectively. The occurrence rate of 427.8-nm emissions of more than 100 R is 10.6% and 7.65% for θs more than and less than −24°, respectively, confirming that resonant scattering of sunlight by N2+ ions is a cause of the strong 427.8-nm emissions of more than 100 R in the sunlit ionosphere. The occurrence rate is high in the postmidnight sector and increases with increasing geomagnetic activity, solar wind speed, and density. The occurrence rate is lowest in winter. A high occurrence rate was observed in 2015–2018, during the declining phase of the 11-year solar activity. Superposed epoch analysis indicates that the 427.8-nm emission exceeds 100 R when solar wind speed increases and solar wind density concurrently decreases, though the standard deviation of the data is rather large.

DOI： 10.1029/2019JA026704

Web of Science

Scopus

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 average winds in the thermosphere during geomagnetically quiet times are important because they provide a baseline wind in the upper atmosphere, but they remain insufficiently understood at high latitudes. This paper reports the first direct ground-based wind measurements of the quiet-time thermospheric wind pattern at Tromsø in Norway using 2009–2015 data from a Fabry–Perot interferometer. We analyzed red-line wind measurements (630.0 nm; altitude: 200–300 km). On average, the zonal wind shows a decrease of eastward wind compared with diurnal tidal wind before midnight. A maximum speed of 100 m/s occurs at both the dusk and dawn sides. The meridional wind has a diurnal tide structure with a minimum value of − 130 m/s around midnight. We also found occasional large wind deviations (> 100 m/s) from the averages, even during geomagnetically quiet times. We suggest that these large wind deviations are caused by the plasma convection associated with weak substorm activities with auroral electrojet (AE) index values of less than 100 nT that occurred at local times different from that at Tromsø.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-019-1093-8

Web of Science

Scopus

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

We present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar at Syowa Station (SYO; −69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03–07 UT on 21 March 2017, just after the arrival of corotating interaction region in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐ to 80‐km altitude. Cosmic noise absorption measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55–70 km measured with an medium‐frequency radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, cosmic noise absorption, and PMWE, can be explained by the interaction between the high‐speed solar wind containing corotating interaction regions and the magnetosphere.

DOI： 10.1029/2019JA026891

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 studied atmospheric gravity waves (AGWs) and nighttime medium-scale traveling ionospheric disturbances (MSTIDs) using a three-dimensional spectral analysis technique for airglow images at wavelengths of 557.7 nm (altitude: 90–100 km for AGWs) and 630.0 nm (200–300 km for MSTIDs), obtained from Athabasca (ATH), Canada (55°N, 247°E, 2005–2017), and Magadan (MGD), Russia (60°N, 151°E, 2008–2017), over 10–13 years. The AGW propagation direction in summer was from northwestward to northward in ATH and northeastward in MGD with phase speeds of 20–60 m/s. In winter at ATH, they are more omnidirectional with weak preference from northwestward to southward with a speed less than 40 m/s, while another weaker power exists from northeastward to southeastward from 70 to 120 m/s. In winter at MGD, there was no dominant direction in the phase-velocity spectra with spectral power an order smaller than ATH. We suggest that these AGW characteristics were caused by wind filtering and intensity and locations of tropospheric sources. The MSTIDs at ATH propagated southwestward in spring and winter and northeastward in summer and fall. The MSTIDs at MGD propagated northeastward, eastward, and westward in spring, fall, and winter, respectively, with weaker power than that at ATH. The phase speeds are mostly less than 100 m/s except for fall. The propagation direction tends to change from south-southwestward in the evening to north-northeastward after the midnight at both ATH and MGD. We discuss possible reasons for these MSTID characteristics at high latitudes based on Perkins and E-F coupling instabilities, high-latitude plasma convection, and thermospheric neutral winds.

DOI： 10.1029/2019JA026783

Web of Science

Scopus

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  

Polar cap ionospheric plasma flow studies often focus on large-scale averaged properties and neglect the mesoscale component. However, recent studies have shown that mesoscale flows are often found to be collocated with airglow patches. These mesoscale flows are typically a few hundred meters per second faster than the large-scale background and are associated with major auroral intensifications when they reach the poleward boundary of the nightside auroral oval. Patches often also contain ionospheric signatures of enhanced field-aligned currents and localized electron flux enhancements, indicating that patches are associated with magnetosphere-ionosphere coupling on open field lines. However, magnetospheric measurements of this coupling are lacking, and it has not been understood what the magnetospheric signatures of patches on open field lines are. The work presented here explores the magnetospheric counterpart of patches and the role these structures have in plasma transport across the open field-line region in the magnetosphere. Using red-line emission measurements from the Resolute Bay Optical Mesosphere Thermosphere Imager, and magnetospheric measurements made by the Cluster spacecraft, conjugate events from 2005 to 2009 show that lobe measurements on field lines connected to patches display (1) electric field enhancements, (2) Region 1 sense field-aligned currents, (3) field-aligned enhancements in soft electron flux, (4) downward Poynting fluxes, and (5) in some cases enhancements in ion flux, including ion outflows. These observations indicate that patches highlight a localized fast flow channel system that is driven by the magnetosphere and propagates from the dayside to the nightside, most likely being initiated by enhanced localized dayside reconnection.

DOI： 10.1029/2019JA026611

Web of Science

Scopus

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  

Energy input from the magnetosphere during substorms can strongly affect the high-latitude thermosphere. The ionospheric current caused by thermospheric wind variations may also provide a feedback to the magnetosphere. In this study, we investigate the characteristics of high-latitude thermospheric wind variations at local substorm onsets at Tromsø, Norway, as well as the possibility of such feedback mechanism. A Fabry–Perot interferometer (FPI) at Tromsø provided wind measurements estimated from the Doppler shift of red-line emission (630.0 nm) of aurora and airglow. We analyzed wind data in 2009 with a time resolution of ~ 13 min. We first carefully identified the onset times of isolated local substorms at Tromsø and extracted four wind measurements from red-line emission. All these events showed increases of eastward components at local substorm onsets. For northward components, these events showed decreases except for those at midnight. The observed wind variations at local substorm onsets were less than 49 m/s. These values are much smaller than the typical plasma convection speed in the auroral zone. We speculate that the ionospheric current caused by thermospheric wind variations at local substorm onsets does not provide strong feedback to the development of substorm expansion phase in the magnetotail. We discuss the possible causes of these wind variations in the context of plasma convection, diurnal tides, and arc-associated electric field.

DOI： 10.1186/s40623-019-1072-0

Web of Science

Scopus

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  

In recent years, experimental results have consistently shown evidence of electromagnetic ion cyclotron (EMIC) wave-driven electron precipitation down to energies as low as hundreds of keV. However, this is at odds with the limits expected from quasi-linear theory. Recent analysis using nonlinear theory has suggested energy limits as low as hundreds of keV, consistent with the experimental results, although to date this has not been experimentally verified. In this study, we present concurrent observations from Polar-orbiting Operational Environmental Satellite, Radiation Belt Storm Probes, Global Positioning System, and ground-based instruments, showing concurrent EMIC waves and sub–MeV electron precipitation, and a global dropout in electron flux. We show through test particle simulation that the observed waves are capable of scattering electrons as low as hundreds of keV into the loss cone through nonlinear trapping, consistent with the experimentally observed electron precipitation.

DOI： 10.1029/2019GL082401

Web of Science

Scopus

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  

Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low-frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q = 2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q = 2 ions is ~10% below 3,000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere.

DOI： 10.1029/2019GL083024

Web of Science

Scopus

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 spatial features of low-latitude Pi2 (6.6–25 mHz) pulsations, a comprehensive study is carried out for the first time using magnetic field measurements from a global network of low-latitude ground stations (Mlat: ± 2° − 51°) and the Swarm multisatellites located simultaneously at day and night local times. We have investigated 1-year data from 2014 and found 15 Pi2 events with coherent oscillations at satellite and ground. The Pi2 oscillations in the compressional, toroidal, and poloidal components at satellite and H, D, and Z components at ground are investigated by estimating its coherence, amplitude, and cross phase with respect to midnight ground H variations. The analogous pairs of magnetic field components (satellite compressional with ground H and satellite toroidal with ground D) above and below the ionosphere are found to have identical phase during night and opposite phase during day, indicating the magnetospheric and ionospheric sources for nighttime and daytime Pi2s, respectively. During nighttime, Pi2 oscillations identified in the poloidal component are found to oscillate in phase (out of phase) in the Southern (Northern) Hemisphere. At ground, the phase and amplitude of H showed significant change near the dawn terminator, whereas H oscillates mostly in phase with respect to midnight ground H at other local times. The oscillations in D component have phase reversal near midnight, dawn, dusk, and noon meridians with opposite hemispheres having opposite phase. These Pi2 characteristics observed globally at ground and at the topside ionosphere suggest that the sources for nighttime and daytime low-latitude Pi2s are oscillating field-aligned currents and ionospheric currents, respectively.

DOI： 10.1029/2018JA026094

Web of Science

Scopus

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 report the first comparison of ground and satellite measurements of low-latitude traveling ionospheric disturbances (TIDs). Three TID events were simultaneously observed by a 630.0-nm airglow imager and the CHAllenging Minisatellite Payload (CHAMP) satellite at Kototabang, Indonesia (geographic coordinates: 0.2° S, 100.3° E, geomagnetic latitude: 10.6° S). In 630.0-nm airglow images of all three events, there are clear southward-moving structures. Events 1 and 2 are a single pulse with horizontal scales of ∼500–1,000 km. Event 3 shows five wave fronts with a horizontal scale size of 500–1,000 km. All three TIDs are medium-scale TIDs. Horizontal wavelengths of both airglow intensity at an average emission altitude of 250 km and CHAMP neutral density variations measured at 400 km are estimated by fitting a sinusoidal function to the observed data. The estimated horizontal wavelengths for airglow and neutral density data are 1,031 and 880 km for event 1 and 560 and 420 km for event 3, respectively. These values between airglow and CHAMP are comparable, suggesting both instruments are observing the same wave. For event 1, the CHAMP electron density mapped along the geomagnetic field line onto the airglow altitude does not show wave structure similar to the airglow variation. For events 2 and 3, the plasma density did not show wavy structures similar to the waves seen in the airglow image and CHAMP neutral density. These results suggest that the TIDs observed in airglow images are not caused by ionospheric plasma instability but by gravity waves in the thermosphere.

DOI： 10.1029/2018JA025634

Web of Science

Scopus

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  

This paper presents a comprehensive study of geomagnetic storm time currents using magnetic field recorded by multispacecraft polar-orbiting mission, Swarm. During geomagnetic storm period, the magnetic field variations obtained after removing the internal geomagnetic field and quiet time contributions can be considered as a proxy for storm time currents and are found to follow the temporal profile of Dst index very closely. These variations at the equatorial crossings recorded by multiple-spacecraft are used to estimate the Dst values and are found to have a good match with the ground-based Dst index. The average deviation between these two is around 4–13%. We have estimated the asymmetry by taking the difference between the magnetic field variations at two local time sectors separated by 12 hr. The estimated asymmetry shows a good match with the AsyH-index, especially when satellite traverses in the dawn-dusk sector. In general, the magnetic field variations are stronger in the night-to-dusk sector than day-to-dawn sector, which could be due to the larger pressure-gradients near night-to-dusk caused by ion movements. The important advantage of Swarm mission is that it provides an opportunity to investigate the longitudinal gradients in the storm time magnetic fields. It is observed that in general, the gradients are stronger during the main phase of the storm, centered near the equator with a latitudinal width of ~20–30° in both the hemispheres, and are supportive to the scenario of particle-injection from the magnetotail. The stronger gradients are observed at higher latitudes (~40°) during the episodes of substorms and might be associated with the ionospheric/field-aligned currents.

DOI： 10.1029/2018JA025692

Web of Science

Scopus

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  

Transient mesospheric echo in the VHF range was detected at an altitude of 65–70 km during the auroral breakup that occurred from 2220 to 2226 UT on June 30, 2017. During this event, the footprint of the Arase satellite was located within the field of view of the all-sky imagers at Syowa Station in the Antarctic. Auroral observations at Syowa Station revealed the dominant precipitation of relatively soft electrons during the auroral breakup. A corresponding spike in cosmic noise absorption was also observed at Syowa Station, while the Arase satellite observed a flux enhancement of > 100 keV electrons and a broadband noise without detecting chorus waves or electromagnetic ion cyclotron waves. A general-purpose Monte Carlo particle transport simulation code was used to quantitatively evaluate the ionization in the middle atmosphere. Results of this study indicate that the precipitation of energetic electrons of > 100 keV, rather than X-rays from the auroral electrons, played a dominant role in the transient and deep (65–70 km) mesospheric ionization during the observed auroral breakup.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-019-0989-7

Web of Science

Scopus

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

Chorus waves, among the most intense electromagnetic emissions in the Earth’s magnetosphere, magnetized planets, and laboratory plasmas, play an important role in the acceleration and loss of energetic electrons in the plasma universe through resonant interactions with electrons. However, the spatial evolution of the electron resonant interactions with electromagnetic waves remains poorly understood owing to imaging difficulties. Here we provide a compelling visualization of chorus element wave–particle interactions in the Earth’s magnetosphere. Through in-situ measurements of chorus waveforms with the Arase satellite and transient auroral flashes from electron precipitation events as detected by 100-Hz video sampling from the ground, Earth’s aurora becomes a display for the resonant interactions. Our observations capture an asymmetric spatial development, correlated strongly with the amplitude variation of discrete chorus elements. This finding is not theoretically predicted but helps in understanding the rapid scattering processes of energetic electrons near the Earth and other magnetized planets.

DOI： 10.1038/s41467-018-07996-z

Web of Science

Scopus

PubMed

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.12737/stp-52201906

Web of Science

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

The paper presents the results on first synchronous observations of variations in auroral luminosity and geomagnetic field, made with high temporal resolution at the ISTP SB RAS high-latitude station Istok (70° N, 88° E) in September–December 2018. Auroras were recorded with all-sky camera, pulsations in the auroras were recorded by a photometer in four spectral ranges with silicon photomultipliers. Continuous monitoring of geomagnetic pulsations was performed using a LEMI-30 three-component induction magnetometer. Both synchronous bursts of auroras and magnetic field pulsations, as well as disturbances of auroras, not accompanied by disturbances in the geomagnetic field, were observed. We note that the photometer clearly recorded short-period (~20 min) variations in in auroral luminosity. At the same time, some instability of the photometer signal level occurred at sufficiently long time intervals. In the photometer data, there are powerful signal bursts, probably of a hardware nature. Nevertheless, the temporary distribution analysis of the registration moments (registration frequency) of signal bursts indicates the possible dependence of the burst registration frequency on the geomagnetic activity level.

DOI： 10.12737/szf-52201906

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.23919/ursiap-rasc.2019.8738444

Web of Science

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  

Electromagnetic ion cyclotron (EMIC) waves potentially cause precipitation loss of relativistic electrons from the outer radiation belt to the atmosphere through pitch angle scattering. However, the direct evidence of each EMIC wave element and burst of precipitation has not yet been reported. Here we show the temporal and spatial correspondence of the EMIC waves with relativistic electron precipitation (REP) during the geomagnetic storm of 27 March 2017. EMIC waves were observed at several stations in North America. REP was detected as a decrease of subionospheric radio amplitudes observed at Athabasca, Canada. When isolated proton aurora, observed at Athabasca, appeared on the radio propagation path, we found a good correspondence between the temporal variations of REP and EMIC waves, and REP preceded EMIC waves by 24 s. This time lag is consistent with the travel time difference between relativistic electrons and EMIC waves from the magnetospheric equatorial plane to the ionosphere.

DOI： 10.1029/2018GL080126

Web of Science

Scopus

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  

Pulsating auroras (PsAs) are thought to be generated by precipitating electrons scattered by lower-band chorus (LBC) waves near the magnetic equator. One-to-one correlation between the LBC intensity and the PsA intensity has been reported. Electrostatic electron cyclotron harmonic (ECH) waves can also scatter electrons. However, direct correlation between ECH and PsA has not been reported yet. In this study, using a coordinated Exploration of energization and Radiation in Geospace (Arase) satellite and ground-based imager observation, we report that not only LBC but also ECH have correlation with PsA. We estimated the precipitating electron energy by assuming that the time lag when the cross-correlation coefficient became the highest was travel time of electrons from the modulation region. We found that the estimated energies show reasonable values as the cyclotron resonance energy of each wave.

DOI： 10.1029/2018GL080145

Web of Science

Scopus

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  

There has been increasing evidence for pitch angle scattering of relativistic electrons by electromagnetic ion cyclotron (EMIC) waves. Theoretical studies have predicted that the loss time scale of MeV electrons by EMIC waves can be very fast, suggesting that MeV electron fluxes rapidly decrease in association with the EMIC wave activity. This study reports on a unique event of MeV electron loss induced by EMIC waves based on Arase, Van Allen Probes, and ground-based network observations. Arase observed a signature of MeV electron loss by EMIC waves, and the satellite and ground-based observations constrained spatial-temporal variations of the EMIC wave activity during the loss event. Multisatellite observation of MeV electron fluxes showed that ~2.5-MeV electron fluxes substantially decreased within a few tens of minutes where the EMIC waves were present. The present study provides an observational estimate of the loss time scale of MeV electrons by EMIC waves.

DOI： 10.1029/2018GL080262

Web of Science

Scopus

その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2018GL080262

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  

Low Earth orbit satellites frequently encounter Pc1 pulsations, but most have been observed with limited latitudinal extent or short lifetime. In this study we analyze two large-scale Pc1 pulsations (both latitudinally wide and long-lasting) generated by ionospheric ducting effect using Swarm and ground magnetometers on 25 June and 3 September 2015. Swarm observed the 25 June pulsations on both dayside and nightside during the storm time substorm (a strong geomagnetic storm on 23 June with D st  = − 204 nT). We found the Pc1 pulsations were pervasive in both magnetic local time sectors of dayside and nightside for at least 2 hr. Another large Pc1 pulsation on 3 September was observed during a nonstorm substorm period. We conclude that (1) ionospheric ducting can transmit Pc1 waves to a wide range of L shells, (2) geomagnetic storm is not a prerequisite for such large-scale ducting, and (3) wave intensity can abruptly decrease across sharp gradients in the ionospheric plasma density.

DOI： 10.1029/2018GL080693

Web of Science

Scopus

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  

Recent studies suggest that electrons with energies up to several hundred keV precipitate into the atmosphere associated with pulsating aurora (PsA). It is debated the highest energy of precipitating electrons associated with PsA. Here we report for the first time that the energy extends to relativistic energies. PsA was observed by THEMIS all-sky imagers during a substorm that occurred on 27 March 2017. Energetic electron precipitation was detected by very low frequency subionospheric propagation. We found similar time variations between the auroral intensity and perturbation of the received radio signal intensity when the PsA occurred on the radio path. The perturbation showed a short recovery time of ~2 s. The recovery time indicates relaxation from ionospheric modification due to energetic electron precipitation and depends on the stopping altitude of the electrons. The recovery time required a stopping altitude of 50–60 km and indicates that the PsA is accompanied by relativistic electron precipitation.

DOI： 10.1029/2018GL080222

Web of Science

Scopus

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  

© 2018, The Author(s). The fluxgate magnetometer for the Arase (ERG) spacecraft mission was built to investigate particle acceleration processes in the inner magnetosphere. Precise measurements of the field intensity and direction are essential in studying the motion of particles, the properties of waves interacting with the particles, and magnetic field variations induced by electric currents. By observing temporal field variations, we will more deeply understand magnetohydrodynamic and electromagnetic ion-cyclotron waves in the ultra-low-frequency range, which can cause production and loss of relativistic electrons and ring-current particles. The hardware and software designs of the Magnetic Field Experiment (MGF) were optimized to meet the requirements for studying these phenomena. The MGF makes measurements at a sampling rate of 256 vectors/s, and the data are averaged onboard to fit the telemetry budget. The magnetometer switches the dynamic range between ± 8000 and ± 60,000 nT, depending on the local magnetic field intensity. The experiment is calibrated by preflight tests and through analysis of in-orbit data. MGF data are edited into files with a common data file form

DOI： 10.1186/s40623-018-0800-1

Web of Science

Scopus

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

We developed user-friendly software based on Matsuda et al.'s (2014) 3D-FFT method (Matsuda-transform, M-transform) for airglow imaging data analysis as a function of Interactive Data Language (IDL). Users can customize the range of wave parameters to process when executing the program. The input for this function is a 3-D array of a time series of a 2-D airglow image in geographical coordinates. We applied this new function to mesospheric airglow imaging data with slightly different observation parameters obtained for the period of April-May at three different latitudes: Syowa Station, the Antarctic (69- S, 40- E); Shigaraki, Japan (35- N, 136- E); and Tomohon, Indonesia (1- N, 122- E). The day-to-day variation of the phase velocity spectrum at the Syowa Station is smaller and the propagation direction is mainly westward. In Shigaraki, the day-to-day variation of the horizontal propagation direction is larger than that at the Syowa Station; the variation in Tomohon is even larger. In Tomohon, the variation of the nightly power spectrum magnitude is remarkable, which indicates the intermittency of atmospheric gravity waves (AGWs). The average nightly spectrum obtained from April-May shows that the dominant propagation is westward with a phase speed < 50ms-1 at the Syowa Station and east-southeastward with a phase speed of up to - 80ms-1 in Shigaraki. The day-to-day variation in Tomohon is too strong to discuss average characteristics; however, a phase speed of up to - 100ms-1 and faster is observed. The corresponding background wind profiles derived from MERRA-2 indicate that wind filtering plays a significant role in filtering out waves that propagate eastward at the Syowa Station. On the other hand, the background wind is not strong enough to filter out relatively high-speed AGWs in Shigaraki and Tomohon and the dominant propagation direction is likely related to the distribution and characteristics of the source region, at least in April and May.

DOI： 10.5194/angeo-36-1597-2018

Web of Science

Scopus

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  

Rapid (<1 s) intensity modulation of pulsating auroras is caused by successive chorus elements as a response to wave-particle interactions in the magnetosphere. Here we found that a pulsating auroral patch responds to the time spacing for successive chorus elements and possibly to chorus subpacket structures with a time scale of tens of milliseconds. These responses were identified from coordinated Arase satellite and ground (Gakona, Alaska) observations with a high-speed auroral imager (100 Hz). The temporal variations of auroral intensity in a few-hertz frequency range exhibited a spatial concentration at the lower-latitude edge of the auroral patch. The spatial evolution of the auroral patch showed repeated expansion/contraction with tens of kilometer scales in the ionosphere, which could be spatial behaviors in the wave-particle interactions. These observations indicate that chorus elements evolve coherently within the auroral patch, which is approximately 900 km in the radial and longitudinal directions at the magnetic equator.

DOI： 10.1029/2018GL079812

Web of Science

Scopus

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  

We report the first statistical analysis of stable auroral red (SAR) arc detachments from the main auroral oval, using 630-nm all-sky cooled-Charge Coupled Device images obtained at Athabasca (magnetic latitude = 61.7°N), Canada. SAR arc detachments from the main oval can be an important way of monitoring the characteristics of ring current particle injection in the inner magnetosphere. We analyzed all-sky images obtained for 11 years from 2006 to 2016 and found 163 SAR arc detachment events. The SAR arc detachments tend to occur in the premidnight sector, indicating the ring current ion drift to the dusk sector. The SAR arc detachments also tend to occur at the beginning of the substorm recovery phase, suggesting that the SAR arcs detach from the main oval as the main auroral oval returns to higher latitudes. The equatorward velocities of detached SAR arcs are from −100 m/s (poleward) to +200 m/s (equatorward), corresponding to magnetospheric electric fields from −1 to +2 mV/m.

DOI： 10.1029/2018GL079615

Web of Science

Scopus

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  

DOI： 10.1016/j.jastp.2018.09.008

Web of Science

Scopus

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  

Distributions of plasma and magnetic pressures are the basic information to investigate macroscopic dynamics of the Earth's magnetosphere. Several studies have been made on magnetic and plasma pressures and macroscopic plasma instabilities in the magnetosphere. However, correlation between magnetic and plasma pressure variations has not been statistically investigated. In this paper, we analyze the statistical characteristics of the phase relationships between variations of magnetic and plasma pressures at frequencies of 4–15 mHz using 2 years of the THEMIS-E satellite data in the nightside magnetosphere. Spectral peaks with coherence greater than 0.85 between magnetic and plasma pressures for 1-hr time segments were selected. The average occurrence rates of the phase relationships are antiphase (within ±10∘ from 180°), 39.75%; in-phase (within ±10∘ from 0°), 0.73%; and other phases (10–170°), 49.83%. For the other-phase events, the phase differences are much closer to antiphase rather than to in-phase. Thus, we conclude that the two pressure variations tend to be antiphase. The antiphase and in-phase relationships are observed mainly at radial distances outside 8 RE and inside 8 RE, respectively. The high occurrence region of antiphase relationship is in the dawnside during magnetically quiet times and shifts to dusk side at active times defined as Dst <−10 nT. The occurrence rates of the phase relationships do not change significantly depending on the AE and Dst indices, plasma β, and IMF-Bz. Based on these results, we discuss the correspondence between the phase relationships and the possible magnetohydrodynamic force balances that can produce these phase relationships.

DOI： 10.1029/2018JA025846

Web of Science

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

放射線帯外帯の相対論的電子は、磁気圏内で発生するプラズマ波動との相互作用により散乱され地球大気に降下する。この過程は、磁気嵐の主相における外帯消失の要因の一つと考えられており、降下した相対論的電子は、国際宇宙ステーションや低軌道衛星への悪影響、下部電離圏での電子密度増加に伴う通信障害を引き起こし得る。本研究では、放射線帯の相対論的電子を降下消失させる電磁イオンサイクロトロン（EMIC）波動に着目した。電子降下は、VLF/LF帯の人工電波を用いて観測した。地上と電離圏下端の間を長距離伝搬する電波の信号は、電子降下に伴う電離圏D領域高度での局所的な擾乱により振幅・位相変調をうける。本研究では、複合地上観測により、2017年3月27日の磁気嵐主相時のEMIC波動とそれに伴う相対論的電子降下の時空間対応を調べた。その結果、数10秒から数分の時間スケールで変化するEMIC波動強度の増大時に、相対論的電子が散乱され大気に降下消失することが明らかとなった。

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  

This paper reports two unique auroral features: postmidnight purple auroral rays and global Pc1 geomagnetic pulsations, observed before the onset of the corotating interaction region (CIR) storm of 21 March 2017, at the beginning of the first campaign of the new Particles and Waves in the Inner magnetosphere using Ground-based network observation (PWING) longitudinal ground network with the Arase satellite. The purple auroral rays were observed from ~0315 to 0430 UT (~03–04 magnetic local time) in the northeastern sky at Husafell, Iceland (magnetic latitude: 64.9°N). We newly propose that the entry of high-density CIR plasma into the magnetotail created purple auroral rays in the sunlit ionosphere. Pc1 geomagnetic pulsations at frequencies of 0–0.5 Hz were observed after ~00 UT over a wide local time range, of 13 hr, from midnight to afternoon sectors at subauroral latitudes associated with CIR arrival. These results indicate preconditioning of the magnetosphere due to crossing of a CIR.

DOI： 10.1029/2018GL079103

Web of Science

Scopus

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  

During 26–29 March 2017 magnetic storm, the Arase satellite observed typical ultra low frequency (ULF) waves and acceleration of relativistic electrons. We simulate the global distribution of these ULF waves using CRCM with BATSRUS global magnetospheric magnetohydrodynamic model. The simulation can qualitatively reproduce the ULF waves observed by Arase at frequencies of 2–3 mHz. However, the simulated ULF wave power is 1–2 orders of magnitude smaller than the observation. The simulated ULF wave activity has a good correlation with the solar-wind dynamic pressure variation, while the wave activity on the ground is enhanced even during the recovery phase, possibly due to the Kelvin-Helmholtz instability and/or substorms. We also study the 3–6 April 2017 magnetic storm, in which low ULF wave activity and weak acceleration of relativistic electrons are seen. We suggest that the existence of ULF waves plays an important role in accelerating electrons up to relativistic energies.

DOI： 10.1029/2018GL078857

Web of Science

Scopus

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  

Different types of medium-scale traveling ionospheric disturbances (MSTIDs) have been observed at Cachoeira Paulista (22.4°S; 45.0°W), Brazil, from June 2013 to December 2015, using airglow OI 630.0-nm images. During the period, 58 MSTIDs were identified and classified as follows: dark band MSTIDs (around 10 events) and periodic MSTIDs (48 events). Dark band MSTIDs present phase velocity between 50 and 200 m/s and propagation direction to northwestward. On the other hand, periodic MSTIDs have phase velocity of 50 to 200 m/s, horizontal wavelengths from 80 to 160 km, periods ranging between 5 and 45 min, and propagation directions are mainly north-northeastward. The wave parameters indicate that periodic MSTIDs have different characteristics when compared to dark band MSTIDs, suggesting that periodic MSTIDs are not generated through the well-known Perkins and E-F coupling instability. In addition to it, the present study indicates that the spectral characteristics found in Brazil are different from other regions such as Japan and Indonesia. Therefore, we intend to do the statistics of the wave parameters (wavelength, phase velocity, period, propagation direction, and time occurrence) and investigate the generation mechanisms of periodic MSTIDs at low to middle latitude for the first time. Furthermore, the anisotropy observed in periodic MSTID propagation direction can be explained by different mechanisms.

DOI： 10.1029/2018JA025438

Web of Science

Scopus

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  

Auroral finger-like structures appear equatorward of the auroral oval in the diffuse auroral region and contribute to the auroral fragmentation into patches. A previous report of the first conjugate observation of auroral finger-like structures using a Time History of Events and Macroscale Interactions during Substorms (THEMIS) Ground-Based Observatories camera and the THEMIS-E satellite at a radial distance of ∼8 RE showed antiphase oscillations of magnetic and plasma pressures in the dawnside plasma sheet. In the present study, we report another simultaneous observation of auroral finger-like structures at Gillam, Canada, at ∼0900 UT (0230 magnetic local time) on 14 November 2014 with the Radiation Belt Storm Probes satellites at 5.8 RE in the inner magnetosphere. From this simultaneous observation event, we obtained the following observations. (1) Auroral finger-like structures developed poleward in the equatorward moving auroral arc at the equatorward edge of the auroral oval. (2) Both the electron and ion OMNI fluxes measured by HOPE increased at ∼0900 UT as the satellite footprint entered the auroral region, indicating that the satellite was crossing the observed auroral finger-like structures. (3) The absolute value of magnetic pressure was several times that of the plasma pressure, and no systematic phase relationship was identified between the magnetic and plasma pressures, unlike that in the THEMIS case. Based on these observations, we discuss two possible causes of the observed finger-like structures, namely, pressure-driven instability in the magnetosphere and gradient drift instability in the ionosphere. In this paper, the latter possibility is newly suggested to develop in the equatorward moving aurora associated with the westward electric field in the equatorward ionospheric density gradient.

DOI： 10.1029/2018JA025480

Web of Science

Scopus

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 {GmbH}  

DOI： 10.5194/angeo-2018-107

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2018JA025585

Web of Science

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  

This paper presents observations of electromagnetic ion cyclotron (EMIC) waves from multiple data sources during the four Geospace Environment Modeling challenge events in 2013 selected by the Geospace Environment Modeling Quantitative Assessment of Radiation Belt Modeling focus group: 17 and 18 March (stormtime enhancement), 31 May to 2 June (stormtime dropout), 19 and 20 September (nonstorm enhancement), and 23–25 September (nonstorm dropout). Observations include EMIC wave data from the Van Allen Probes, Geostationary Operational Environmental Satellite, and Time History of Events and Macroscale Interactions during Substorms spacecraft in the near-equatorial magnetosphere and from several arrays of ground-based search coil magnetometers worldwide, as well as localized ring current proton precipitation data from low-altitude Polar Operational Environmental Satellite spacecraft. Each of these data sets provides only limited spatial coverage, but their combination shows consistent occurrence patterns and reveals some events that would not be identified as significant using near-equatorial spacecraft alone. Relativistic and ultrarelativistic electron flux observations, phase space density data, and pitch angle distributions based on data from the Relativistic Electron-Proton Telescope and Magnetic Electron Ion Spectrometer instruments on the Van Allen Probes during these events show two cases during which EMIC waves are likely to have played an important role in causing major flux dropouts of ultrarelativistic electrons, particularly near L* ~4.0. In three other cases, identifiable smaller and more short-lived dropouts appeared, and in five other cases, these waves evidently had little or no effect.

DOI： 10.1029/2018JA025505

Web of Science

Scopus

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 electrified medium-scale traveling ionospheric disturbances (EMSTIDs) occurring as geomagnetically conjugate features in the middle latitude F region ionosphere are studied using multi-instrument observations. Airglow imaging of OI 630 nm emission over Sata, Japan, and Darwin, Australia, are used to identify the occurrence of EMSTIDs. Thermospheric wind measurements made with the Fabry-Perot interferometer observations of OI 630 nm from Shigaraki, Japan, and Darwin, Australia, are used along with ionosonde observations over Yamagawa, Japan, and Darwin, Australia, to study the thermospheric and ionospheric characteristics. These are the first results from such multi-instrument observations simultaneously made from geomagnetic conjugate locations. Our results show that the amplitudes of the EMSTIDs are often different between the hemispheres. Thermospheric meridional winds appear to control the EMSTID amplitudes in the respective hemisphere. However, EMSTIDs are generated only when there is significant sporadic E activity with foEs often reaching greater than 6 MHz and (foEs − fbEs) reaching above 5 MHz at least for a short duration occurred. Existence of strong sporadic E activity on one of the hemispheres is found to be sufficient enough for generation of EMSTIDs in the conjugate F regions. These results conclusively indicate the importance of sporadic E layers in the generation of EMSTIDs. Further, it shows the significance of interhemispheric coupling between the E and F region ionospheres in the formation of EMSTIDs while their amplitudes in the respective hemispheres appear to have control of thermospheric neutral winds in the same hemisphere.

DOI： 10.1029/2018JA025261

Web of Science

Scopus

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 Exploration of energization and Radiation in Geospace (ERG) project explores the acceleration, transport, and loss of relativistic electrons in the radiation belts and the dynamics for geospace storms. This project consists of three research teams for satellite observation, ground-based network observation, and integrated data analysis/simulation. This synergetic approach is essential for obtaining a comprehensive understanding of the relativistic electron generation/loss processes of the radiation belts as well as geospace storms through cross-energy/cross-regional couplings, in which different plasma/particle populations and regions are strongly coupled with each other. This paper gives an overview of the ERG project and presents the initial results from the ERG (Arase) satellite.[Figure not available: see fulltext.]

DOI： 10.1186/s40623-018-0862-0

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40645-018-0189-2

Web of Science

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 report the first observations of ~100-s periodic oscillations of intensity in low-frequency (LF) standard radio waves over Japan at 05:51–05:56 UT, which was about 4 min and 42 s after mainshock onset of “the 2011 off the Pacific coast of Tohoku earthquake” on 11 March 2011 (Mw 9.0). The LF radio wave propagation paths used in this study were JJY (Japan, 60 kHz)-Rikubetsu (RKB, Japan), and BPC (China, 68.5 kHz)-RKB, where the minimum distances between the epicenter and the LF propagation paths were 413.6 and 561.5 km, respectively. The observed modulations in intensity and phase were about 0.1 dB and 0.5°, respectively. Based on a numerical simulation of the neutral atmosphere and the wave-hop method, the occurrence time of the ~100-s periodic oscillations was in good agreement with the total propagation time of the Rayleigh wave that spread concentrically from the epicenter to the LF propagation paths and acoustic waves propagating vertically from the Earth's surface to the bottom of the ionosphere. Variations in synthesized electric field intensity of ground waves and sky waves, calculated by the wave hop method up to 10 hops, reproduced similar ~100-s periodic oscillations that were caused by the difference in arrival time of the acoustic waves excited by the Rayleigh waves at each hop point. The amplitude of the D region electron density variations at 70 km altitude during the ~100-s periodic oscillation was estimated to be about 1% compared to the background electron density, based on the numerical simulation.

DOI： 10.1029/2018JA025289

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017JA025021

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017GL076486

Web of Science

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：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.73.1.0_766

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017JA024602

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-017-0745-9

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-017-0742-z

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-017-0729-9

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jastp.2017.08.030

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.5194/angeo-35-1131-2017

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-017-0698-z

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017JA024146

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017JA023919

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We analyzed the horizontal phase velocity of gravity waves and medium-scale traveling ionospheric disturbances (MSTIDs) by using the three-dimensional fast Fourier transform method developed by Matsuda et al. (2014) for 557.7 nm (altitude: 90–100 km) and 630.0 nm (altitude: 200–300 km) airglow images obtained at Shigaraki MU Observatory (34.8°N, 136.1°E, dip angle: 49°) over ∼16 years from 16 March 1999 to 20 February 2015. The analysis of 557.7 nm airglow images shows clear seasonal variation of the propagation direction of gravity waves in the mesopause region. In spring, summer, fall, and winter, the peak directions are northeastward, northeastward, northwestward, and southwestward, respectively. The difference in east-west propagation direction between summer and winter is probably caused by the wind filtering effect due to the zonal mesospheric jet. Comparison with tropospheric reanalysis data shows that the difference in north-south propagation direction between summer and winter is caused by differences in the latitudinal location of wave sources due to convective activity in the troposphere relative to Shigaraki. The analysis of 630.0 nm airglow images shows that t

DOI： 10.0002/2017JA023919

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016JA023774

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017JA024048

Web of Science

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：Journal of Physics: Conference Series  

© Published under licence by IOP Publishing Ltd. The ERG (Exploration of energization and Radiation in Geospace) is Japanese geospace exploration project. The project focuses on relativistic electron acceleration mechanism of the outer belt and dynamics of space storms in the context of the cross-energy coupling via wave-particle interactions. The project consists of the satellite observation team, the ground-based network observation team, and integrated-data analysis/simulation team. The satellite was launched on December 20 2016 and has been nicknamed, &quot;Arase&quot;. This paper describes overview of the project and future plan for observations.

DOI： 10.1088/1742-6596/869/1/012095

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017JA024025

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017JA024211

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2017GL072956

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016JA023754

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/s11141-017-9764-4

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-017-0642-2

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-017-0643-1

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016JA023417

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016JA023853

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016GL071168

Web of Science

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：2017 32nd General Assembly and Scientific Symposium of the International Union of Radio Science, URSI GASS 2017  

We investigated a post-midnight Field-Aligned Irregularities (FAIs) event observed with the Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S, 100.3°E, dip lat. 10.4°S) in Indonesia on the night of 9 July 2010, using a comprehensive dataset of both neutral and plasma parameters. We compared FAI echoes collocated with 630 nm airglow depletion detected by an all-sky imager. The thermospheric neutral winds and temperatures obtained by a Fabry-Perot interferometer at Kototabang and the altitudes of F-layer (h'F) observed with ionosondes at Kototabang, Chiang Mai, and Chumphon were also examined. We found that the 3-m scale post-midnight FAIs occurred within plasma bubbles. The convergence of the equatorward neutral winds happened in this particular event related to midnight temperature maximum (MTM) and that the equatorward winds in both northern and southern hemispheres could be responsible for the growth of plasma bubbles around midnight. The uplift of F-layer at low latitudes could increase the growth rate of the Rayleigh-Taylor instability. Eastward electric currents driven by the equatorward winds could also contribute to the generation of the irregularities at post-midnight.

DOI： 10.23919/URSIGASS.2017.8105106

Web of Science

Scopus

その他リンク： http://orcid.org/0000-0002-4957-764X

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.icarus.2016.08.004

Web of Science

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  

Ionospheric weather maps using the total electron content (TEC) monitored by ground-based Global Navigation Satellite Systems (GNSS) receivers over South American continent, TECMAP, have been operationally produced by Instituto Nacional de Pesquisas Espaciais's Space Weather Study and Monitoring Program (Estudo e Monitoramento Brasileiro de Clima Especial) since 2013. In order to cover the whole continent, four GNSS receiver networks, (Rede Brasileiro de Monitoramento Contínuo) RBMC/Brazilian Institute for Geography and Statistics, Low-latitude Ionospheric Sensor Network, International GNSS Service, and Red Argentina de Monitoreo Satelital Continuo, in total ~140 sites, have been used. TECMAPs with a time resolution of 10 min are produced in 12 h time delay. Spatial resolution of the map is rather low, varying between 50 and 500 km depending on the density of the observation points. Large day-to-day variabilities of the equatorial ionization anomaly have been observed. Spatial gradient of TEC from the anomaly trough (total electron content unit, 1 TECU = 1016 el m−2 (TECU) <10) to the crest region (TECU > 80) causes a large ionospheric range delay in the GNSS positioning system. Ionospheric plasma bubbles, their seeding and development, could be monitored. This plasma density (spatial and temporal) variability causes not only the GNSS-based positioning error but also radio wave scintillations. Monitoring of these phenomena by TEC mapping becomes an important issue for space weather concern for high-technology positioning system and telecommunication.

DOI： 10.1002/2016SW001474

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016JA022665

Web of Science

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 scenario is presented for the cause of magnetospheric relativistic electron decreases (REDs) and potential effects in the atmosphere and on climate. High-density solar wind heliospheric plasmasheet (HPS) events impinge onto the magnetosphere, compressing it along with remnant noon-sector outer-zone magnetospheric ~10-100 keV protons. The betatron accelerated protons generate coherent electromagnetic ion cyclotron (EMIC) waves through a temperature anisotropy (T⊥/T|| > 1) instability. The waves in turn interact with relativistic electrons and cause the rapid loss of these particles to a small region of the atmosphere. A peak total energy deposition of ~3 × 1020 ergs is derived for the precipitating electrons. Maximum energy deposition and creation of electron-ion pairs at 30-50 km and at < 30 km altitude are quantified. We focus the readers' attention on the relevance of this present work to two climate change mechanisms. Wilcox et al. (1973) noted a correlation between solar wind heliospheric current sheet (HCS) crossings and high atmospheric vorticity centers at 300 mb altitude. Tinsley et al. () has constructed a global circuit model which depends on particle precipitation into the atmosphere. Other possible scenarios potentially affecting weather/climate change are also discussed.

DOI： 10.1002/2016JA022499

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016JA022957

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.polar.2016.03.005

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2016RS006035

Web of Science

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  

To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8 s) images of proton-induced N2+ emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. These results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

DOI： 10.1002/2016GL070008

Web of Science

Scopus

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 this work we present direct ground-based observational evidence for the merging of individual equatorial plasma bubbles (EPBs) obtained through the imaging of OI 630.0 nm airglow. Three potential mechanisms have been identified: (1) One of the EPBs tilts and reaches location of the adjacent growing EPB finally merging with it. (2) Some of the branches of an EPB arising from secondary instabilities reach out to adjacent EPB and merge with it. (3) The eastward zonal drift of the EPB on the eastern side slows down while the adjacent EPB on the western side drifts relatively faster and catches up. In one of the cases, a branch of an EPB was observed to get interchanged with another EPB as a result of merging and consequent pinching off from the parent EPB.

DOI： 10.1002/2016JA022861

Web of Science

Scopus

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 formation of equatorial plasma bubbles (EPBs) associated with spread F irregularities are fairly common phenomenon in the postsunset equatorial ionosphere. These bubbles grow as a result of eastward polarization electric field resulting in upward E × B drift over the dip equator. As they grow they are also mapped to low latitudes along magnetic field lines. The EPBs are often observed as airglow depletions in the images of OI 630 nm emission. On occasions the growth of the features over the dip equator is observed as poleward extensions of the depletions in all-sky images obtained from low latitudes. Herein, we present interesting observations of decrease in the latitudinal extent of the EPBs corresponding to a reduction in their apex altitudes over the dip equator. Such observations indicate that these bubbles not only grow but also shrink on occasions. These are the first observations of shrinking EPBs. The observations discussed in this work are based on all-sky airglow imaging observations of OI 630.0 nm emission made from Panhala (11.1°N dip latitude). In addition, ionosonde observations made from dip equatorial site Tirunelveli (1.1°N dip latitude) are used to understand the phenomenon better. The analysis indicates that the speed of shrinking occurring in the topside is different from the bottomside vertical drifts. When the EPBs shrink, they might decay before sunrise hours.

DOI： 10.1002/2016JA022633

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA022264

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-016-0471-8

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA022123

Web of Science

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  

Before the onset of a geomagnetic storm on 22 January 2012 (Ap = 24), an enhancement in solar wind number density from 10/cm3 to 22/cm3 during 0440–0510 UT under northward interplanetary magnetic field (IMF Bz) condition is shown to have enhanced the high-latitude ionospheric convection and also caused variations in the geomagnetic field globally. Conspicuous changes in ΔX are observed not only at longitudinally separated low-latitude stations over Indian (prenoon), South American (midnight), Japanese (afternoon), Pacific (afternoon) and African (morning) sectors but also at latitudinally separated stations located over high and middle latitudes. The latitudinal variation of the amplitude of the ΔX during 0440–0510 UT is shown to be consistent with the characteristics of prompt penetration electric field disturbances. Most importantly, the density pulse event caused enhancements in the equatorial electrojet strength and the peak height of the F layer (hmF2) over the Indian dip equatorial sector. Further, the concomitant enhancements in electrojet current and F layer movement over the dip equator observed during this space weather event suggest a common driver of prompt electric field disturbance at this time. Such simultaneous variations are found to be absent during magnetically quiet days. In absence of significant change in solar wind velocity and magnetospheric substorm activity, these observations point toward perceptible prompt electric field disturbance over the dip equator driven by the overcompression of the magnetosphere by solar wind density enhancement.

DOI： 10.1002/2016JA022475

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA022129

Web of Science

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  

Three meter field-aligned irregularities (3 m FAIs) associated with medium-scale traveling ionospheric disturbances (MSTIDs) that occurred on 5 February 2008 were observed by using the Chung-Li 52 MHz coherent scatter radar. Interferometry measurements show that the plasma structures responsible for the 3 m FAI echoes are in a clumpy shape with a horizontal dimension of about 10-78 km in a height range of 220-300 km. In order to investigate the dynamic behaviors of the plasma irregularities at different scales in the bottomside of F region, the VHF radar echo structures from the 3 m FAIs combined with the 630 nm airglow images provided by the Yonaguni all-sky imager are compared and analyzed. The results show that the radar echoes were located at the west edge of the depletion zones of the 630 nm airglow image of the MSTIDs. The bulk echo structures of the 3 m FAIs drifted eastward at a mean trace velocity of about 30 m/s that is in general agreement with the zonal trace velocity of the MSTIDs shown in the 630 nm airglow images. These results suggest that the observed F region 3 m FAIs for the present case can be regarded as the targets that are frozen in the local region of the MSTIDs. In addition, the radar-observed 3 m FAI echo intensity and spectral width bear high correlations to the percentage variations of the 630 nm emission intensity. These results seem to suggest that through the nonlinear turbulence cascade process, the MSTID-associated 3 m FAIs are very likely generated from the kilometer-scale plasma irregularities with large amplitude excited by the gradient drift instability.

DOI： 10.1002/2016JA022511

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-016-0392-6

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015GL067432

Web of Science

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  

Long-lasting (> 20 h) electromagnetic ion cyclotron (EMIC) Pc1-Pc2 waves were observed by the Athabasca (L =∼ 4.6) induction magnetometer and Canadian Array for Realtime Investigations of Magnetic Activity (L =∼ 4-6) fluxgate magnetometers on 5 April 2007. These waves showed a systematic frequency change with local time, the minimum frequency near dusk, and the maximum frequency near dawn. Assuming the plasmapause as a potential source region of the waves, we estimated the plasmapause location from localized proton enhancement (LPE) events observed at NOAA-Polar Orbiting Environmental Satellites and METOP-2 satellites. We found that the longitudinal frequency variation of EMIC waves has a clear correlation with the estimated plasmapause location and that the waves are in the frequency band between the equatorial helium and oxygen gyrofrequencies at the estimated plasmapause. With our analysis results we suggest that the LPE events are caused by wave-particle interaction with the helium band EMIC waves generated near the plasmapause.

DOI： 10.1002/2015GL067536

Web of Science

Scopus

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 statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosynchronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to identify the geosynchronous EMIC waves, we use high time resolution magnetic field data acquired from GOES 10, 11, and 12 over a 2 year period from 2007 and 2008 and examine the local time distribution of EMIC wave events. Unlike previous studies, which reported high EMIC wave occurrence in the postnoon sector with a peak around 1500-1600 magnetic local time (MLT) during magnetically disturbed times (i.e., storm and/or substorm), we observed that quiet time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100-1200 MLT. To investigate whether the quiet time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure variations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and interplanetary magnetic field Bz) and geomagnetic indices (AE and SYM-H). From the superposed epoch analysis we found that solar wind dynamic pressure variation is a more important parameter than AE and SYM-H for quiet time EMIC wave occurrence.

DOI： 10.1002/2015JA021968

Web of Science

Scopus

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 found rising tone emissions with a dispersion of ∼1 Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over ∼30 min from 0250 UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7°N, 246.7°E, magnetic latitude 61.7°N). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, ∼10% variations in intensity, and fine structures of 3° in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of ∼1 Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time.

DOI： 10.1002/2015JA021681

Web of Science

Scopus

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 zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to >550 km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13°N and 25.98°S magnetic dip latitudes indicating their altitude development to ~1670 km at apex. In contrast, at few degrees east in the premidnight sector (Thailand-Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM-A and SWARM-C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field Bz.

DOI： 10.1002/2015JA021932

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA021816

Web of Science

記述言語：英語   掲載種別：論文集(書籍)内論文   出版者・発行元：Auroral Dynamics and Space Weather  

Isolated proton aurora (IPA) in the subauroral ionosphere is created by energetic proton precipitation through wave-particle interactions with electromagnetic ion cyclotron (EMIC) waves in the conjugate inner magnetosphere. In this study, spatial distribution and occurrence probability of IPAs were statistically investigated as a proxy for regions of EMIC wave occurrence using ground-based imaging data during 2006-2012 at Athabasca, Canada. The 7-year average of the IPA occurrence probability over the total observation interval was estimated to be 0.83%, and a factor of 5 change was found between maximum and minimum years. Local time (between 16 and 06 MLT) distribution shows double peaks at premidnight and at dusk. The occurrence probability increases with Kp, and the MLT location tends to shift duskward. The statistical distribution of IPA size shows a clear peak at a spatial size of 10,000 km2, and latitudinal and longitudinal lengths have peaks at 56 and 340 km, respectively, at the ionospheric altitude. The equatorial projections of IPA source locations and two-dimensional (2D) structures are estimated by magnetic field tracing. These spatial structures are essential to quantitatively estimate the loss rate of energetic particles, contributing to space weather studies.

DOI： 10.1002/9781118978719.ch5

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA021381

Web of Science

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 present a study of concentric gravity waves (CGWs) event from the coordinated observation between Ionosphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping (IMAP)/Visible and near-Infrared Spectral Imager (VISI), all-sky camera at Rikubetsu, Multi-functional Transport Satellite (MTSAT), Tropical Rainfall Measuring Mission, and MF radar at Wakkanai combined with Modern-Era Retrospective Analysis for Research and Application data. IMAP/VISI is the first space-based imager that capable of imaging the airglow in the mesosphere and lower thermosphere region in the nadir-looking direction. Therefore, it has a unique ability to observe a great extend of CGWs propagation. Arc-like shaped, part of CGWs pattern was observed around the mesopause (∼95 km) in the O2 762 nm airglow emission obtained by IMAP/VISI at 1204 UT on 18 October 2012. Similar patterns were also observed by the all-sky imager at Rikubetsu (43.5N, 143.8E) in OI 557.7 nm and OH band airglow emissions from ∼1100 to 1200 UT. Horizontal wavelengths of the observed small-scale gravity waves are ∼50 km (OH band and OI 557.7 nm) and ∼67 km (O2 762 nm). The source is suggested to be a deep convective activity over Honshu Island which likely was an enhanced convective activity related to a typhoon in the south of Japan. The data showed that the CGWs could propagate up to ∼1400-1500 km horizontally from the source to the mesopause but not farther away. Using atmospheric temperature profiles obtained by Thermospheric Ionosphere Mesosphere Energetics Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry, we conclude that this long-distance propagation of the waves could be caused by thermal duct in the middle atmosphere. The arc-like shaped instead of full circle pattern points out that the wind filtering effect is significant for the particular direction of wave propagation.

DOI： 10.1002/2015JA021424

Web of Science

Scopus

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  

Common volume measurements by the Resolute Bay Incoherent Scatter Radar-North (RISR-N) and Optical Mesosphere and Thermosphere Imagers (OMTI) have been used to clarify the electrodynamic structure of a Sun-aligned arc in the polar cap. The plasma parameters of the dusk-to-dawn drifting arc and surrounding ionosphere are extracted using the volumetric imaging capabilities of RISR-N. Multipoint line-of-sight RISR-N measurements of the plasma drift are inverted to construct a time sequence of the electric field and field-aligned current system of the arc. Evidence of dramatic electrodynamic and plasma structuring of the polar cap ionosphere due to the arc is described. One notable feature of the arc is a meridionally extended plasma density depletion on its leading edge, located partially within a downward field-aligned current region. The depletion is determined to be a by-product of enhanced chemical recombination operating on a time scale of 15 min. A similarly shaped electric field structure of over 100 mV/m and line-of-sight ion temperatures nearing 3000 K were collocated with the depletion.

DOI： 10.1002/2015JA021790

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA021347

Web of Science

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  

Magnetotail processes and structures related to substorm growth phase/onset auroral arcs remain poorly understood mostly due to the lack of adequate observations. In this study we make a comparison between ground-based optical measurements of the premidnight growth phase/onset arcs at subauroral latitudes and magnetically conjugate measurements made by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) at ~780 km in altitude and by the Van Allen Probe B (RBSP-B) spacecraft crossing L values of ~5.0-5.6 in the premidnight inner tail region. The conjugate observations offer a unique opportunity to examine the detailed features of the arc location relative to large-scale Birkeland currents and of the magnetospheric counterpart. Our main findings include (1) at the early stage of the growth phase the quiet auroral arc emerged ~4.3 equatorward of the boundary between the downward Region 2 (R2) and upward Region 1 (R1) currents; (2) shortly before the auroral breakup (poleward auroral expansion) the latitudinal separation between the arc and the R1/R2 demarcation narrowed to ~1.0; (3) RBSP-B observed a magnetic field signature of a local upward field-aligned current (FAC) connecting the arc with the near-Earth tail when the spacecraft footprint was very close to the arc; and (4) the upward FAC signature was located on the tailward side of a local plasma pressure increase confined near L ~5.2-5.4. These findings strongly suggest that the premidnight arc is connected to highly localized pressure gradients embedded in the near-tail R2 source region via the local upward FAC.

DOI： 10.1002/2015JA021676

Web of Science

Scopus

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

Transionospheric radio signals in the high-latitude polar cap are susceptible to degradation when encountering sharp electron density gradients associated with discrete plasma structures, or patches. Multi-instrument measurements of polar cap patches are examined during a geomagnetic storm interval on 22 January 2012. For the first time, we monitor the transportation of patches with high spatial and temporal resolution across the polar cap for 1-2 h using a combination of GPS total electron content (TEC), all-sky airglow imagers (ASIs), and Super Dual Auroral Radar Network (SuperDARN) HF radar backscatter. Simultaneous measurements from these data sets allow for continuous tracking of patch location, horizontal extent, and velocity despite adverse observational conditions for the primary technique (e.g., sunlit regions in the ASI data). Spatial collocation between patch-like features in relatively coarse but global GPS TEC measurements and those mapped by high-resolution ASI data was very good, indicating that GPS TEC can be applied to track patches continuously as they are transported across the polar cap. In contrast to previous observations of cigar-shaped patches formed under weakly disturbed conditions, the relatively narrow dawn-dusk extent of patches in the present interval (500-800 km) suggests association with a longitudinally confined plasma source region, such as storm-enhanced density (SED) plume. SuperDARN observations show that the backscatter power enhancements corresponded to the optical patches, and for the first time we demonstrate that the motion of the optical patches was consistent with background plasma convection velocities.

DOI： 10.1002/2015RS005672

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA021000

Web of Science

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  

Equatorial ionosphere plasma bubbles over the South American continent were successfully observed by mapping the total electron content (TECMAP) using data provided by ground-based GNSS receiver networks. The TECMAP could cover almost all of the continent within ~4000. km distance in longitude and latitude, monitoring TEC variability continuously with a time resolution of 10. min. Simultaneous observations of OI 630. nm all-sky image at Cachoeira Paulista (22.7°S, 45.0°W) and Cariri (7.4°S, 36.5°W) were used to compare the bubble structures. The spatial resolution of the TECMAP varied from 50. km to 1000. km, depending on the density of the observation sites. On the other hand, optical imaging has a spatial resolution better than 15. km, depicting the fine structure of the bubbles but covering a limited area (~1600. km diameter). TECMAP has an advantage in its spatial coverage and the continuous monitoring (day and night) form. The initial phase of plasma depletion in the post-sunset equatorial ionization anomaly (PS-EIA) trough region, followed by development of plasma bubbles in the crest region, could be monitored in a progressive way over the magnetic equator. In December 2013 to January 2014, periodically spaced bubble structures were frequently observed. The longitudinal spacing between the bubbles was around 600-800. km depending on the day. The periodic form of plasma bubbles may suggest a seeding process related to the solar terminator passage in the ionosphere.

DOI： 10.1016/j.jastp.2015.06.003

Web of Science

Scopus

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  

Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earth's outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large-amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p-p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ∼5-20 cm-3. Waves were also observed by ground-based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA-POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30-80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field-aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC-induced loss processes for this event.

DOI： 10.1002/2015JA021227

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2015JA021096

Web of Science

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  

Coordinated digisonde and OI 630.0 nm airglow observations from Thumba (TVM), an Indian dip equatorial station, in conjunction with magnetic and geosynchronous particle flux measurements, reveal three different types of electric field disturbances in the equatorial ionosphere-thermosphere system (ITS) occurring in succession over a period of 6 h on a single night (22-23 January,2012; Ap = 24). These include (1) westward electric field perturbations owing to a pseudo-breakup and a substorm event, each lasting for about 30 min; (2) eastward electric field perturbations continuing for about an hour, owing to the southward excursion of Z component of interplanetary magnetic field (Bz); and (3) DP2-type fluctuating (period 40 min) electric field perturbation sustaining for about 4 h. The pseudo-breakup and the fully grown substorm events are found to be longitudinally localized and different in terms of response in the westward auroral electrojet index (AL) as well as geosynchronous electron/proton injections. The polarity of the prompt penetration of interplanetary electric field that affects the equatorial ionosphere is observed to be eastward during 2100-2200 IST (Indian Standard Time) which is observationally sparse but consistent with modeling studies. Interestingly, on the same night, DP2-type electric field fluctuations with 40 min periodicity and occasional eastward polarity (akin to daytime) are also found to affect the equatorial ITS for about 4 h (2200-0200 IST). The case study, thus, brings out different processes that constitute a long duration prompt penetration event which, otherwise, would have been categorized as a single event. Key Points Different types of electric field disturbances on a given night These include substorm, prompt penetration, and DP2-type events Impact on equatorial vertical drift and 630.0 nm airglow intensity

DOI： 10.1002/2014JA020759

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2014JA020946

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-015-0202-6

Web of Science

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  

This is the first paper that reports simultaneous observations of zonal drift of plasma bubbles and the thermospheric neutral winds at geomagnetically conjugate points in both hemispheres. The plasma bubbles were observed in the 630 nm nighttime airglow images taken by using highly sensitive all-sky airglow imagers at Kototabang, Indonesia (geomagnetic latitude (MLAT): 10.0S), and Chiang Mai, Thailand (MLAT: 8.9N), which are nearly geomagnetically conjugate stations, for 7 h from 13 to 20 UT (from 20 to 03 LT) on 5 April 2011. The bubbles continuously propagated eastward with velocities of 100-125 m/s. The 630 nm images at Chiang Mai and those mapped to the conjugate point of Kototabang fit very well, which indicates that the observed plasma bubbles were geomagnetically connected. The eastward thermospheric neutral winds measured by two Fabry-Perot interferometers were 70-130 m/s at Kototabang and 50-90 m/s at Chiang Mai. We compared the observed plasma bubble drift velocity with the velocity calculated from the observed neutral winds and the model conductivity, to investigate the F region dynamo contribution to the bubble drift velocity. The estimated drift velocities were 60-90% of the observed velocities of the plasma bubbles, suggesting that most of the plasma bubble velocity can be explained by the F region dynamo effect.

DOI： 10.1002/2014JA020398

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Exploration of energization and Radiation in Geospace project Science Center (ERG-SC) has developed the science data file archive and integrated data analysis tool for the ground network observation data of the ERG project. We have organized data design consortiums to develop and elaborate the standard metadata and data variable sets for each type of the ERG-related ground data being archived as Common Data Format (CDF) files. The integrated data analysis software for the project has also been developed on the basis of the Space Physics Environment Data Analysis Software (SPEDAS) which works in concert with the CDF data file repository. The software tools, which are provided as plug-in libraries for SPEDAS, are made available to the international science community so that scientists and students are ready to proceed to integrated studies combining the ground data with other satellite and simulation data seamlessly. The integrated data analysis software can make scientific activities more productive and help the ERG project achieve the scientific goals.

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-014-0178-7

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40645-014-0031-4

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2014JA020375

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-014-0160-4

Web of Science

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  

Thanks to the work of a number of scientists who made it known that severe space weather can cause extensive social and economic disruptions in the modern high-technology society. It is therefore important to understand what determines the severity of space weather and whether it can be predicted. We present results obtained from the analysis of coronal mass ejections (CMEs), solar energetic particle (SEP) events, interplanetary magnetic field (IMF), CME-magnetosphere coupling, and geomagnetic storms associated with the major space weather events since 1998 by combining data from the ACE and GOES satellites with geomagnetic parameters and the Carrington event of 1859, the Quebec event of 1989, and an event in 1958. The results seem to indicate that (1) it is the impulsive energy mainly due to the impulsive velocity and orientation of IMF Bz at the leading edge of the CMEs (or CME front) that determine the severity of space weather. (2) CMEs having high impulsive velocity (sudden nonfluctuating increase by over 275 km s-1 over the background) caused severe space weather (SvSW) in the heliosphere (failure of the solar wind ion mode of Solar Wind Electron Proton Alpha Monitor in ACE) probably by suddenly accelerating the high-energy particles in the SEPs ahead directly or through the shocks. (3) The impact of such CMEs which also show the IMF Bz southward from the leading edge caused SvSW at the Earth including extreme geomagnetic storms of mean DstMP <-250 nT during main phases, and the known electric power outages happened during some of these SvSW events. (4) The higher the impulsive velocity, the more severe the space weather, like faster weather fronts and tsunami fronts causing more severe damage through impulsive action. (5) The CMEs having IMF Bz northward at the leading edge do not seem to cause SvSW on Earth, although, later when the IMF Bz turns southward, they can lead to super geomagnetic storms of intensity (Dstmin) less than even -400 nT. Key Points Impulsive energy at CME front causes severe space weather (SvSW) in heliosphere Impulsive energy and IMF Bz southward at the CME front causes SvSW at the Earth Northward IMF Bz at the CME front causes normal space weather at the Earth

DOI： 10.1002/2014JA020151

Web of Science

Scopus

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 nighttime medium-scale traveling ionospheric disturbance (MSTID) features observed over Yonaguni (24.5°N, 123.0°E; 19.3°N dip latitude), Japan are studied using all-sky imaging of OI 630.0 nm airglow emission. The uniqueness of these observations is that the area observed by the imager covers the transition region between low to middle latitudes in the ionosphere. Typical low-latitude limit of midlatitude-type nighttime MSTIDs possessing phase front alignments along the northwest to the southeast occurs in this region. These MSTID features are rarely sighted at dip latitudes below 15°. We selected 2 year period for analysis in which 1 year corresponded to the solar minimum conditions and another year to the solar maximum conditions. The MSTIDs were observed to extend to farther lower latitudes during the solar minimum conditions than during the solar maximum periods. Their observed range of wavelengths, phase velocities, phase front alignment, and propagation directions are similar to those observed at typical midlatitude sites. However, on many occasions the phase fronts of the observed MSTIDs did not extend over the whole field of view of the imager indicating that some process inhibits their extension to further lower latitudes. Detailed investigation suggests that the poleward propagating enhancement of airglow intensity, probably associated with the midnight pressure bulge, causes the MSTID features to disappear when they reach lower latitudes later in the night. When the MSTIDs reach lower latitudes well before midnight, they are found to be inhibited by the equatorial ionization anomaly crest region.

DOI： 10.1002/2014JA020368

Web of Science

Scopus

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  

The global distribution of traveling planetary wave (PW) activity in the mesopause region is estimated for the first time from ground-based airglow measurements. Monthly and total mean climatologies of PW power are determined from rotational temperatures measured at 19 sites from 78° N to 76° S which contribute to the Network for the Detection of Mesospheric Change (NDMC). Wave power is expressed as the standard deviation of nocturnal mean temperature around the seasonal temperature variation. The results from 20° N confirm the SABER traveling PW proxy by Offermann et al. (2009, J. Geophys. Res. 114, D06110) at two altitudes. Most sites between 69° S and 69° N show total mean traveling PW activity of about 6. K, and only some high latitude sites have considerably higher activity levels. At the two tropical sites, there is practically no seasonal variation of PW activity. At 70% of the midlatitude sites, the seasonal variation is moderate for most of the year, but it is quite appreciable at all high latitude sites. Results about traveling PW activity at 87. km and 95. km available from several sites signal similar behavior at both altitudes. The total mean climatological results here obtained have further been used to separate the traveling PW contribution from the superposition of wave types contained in OH rotational temperature fluctuations measured by the SCIAMACHY instrument on Envisat. A narrow equatorial wave activity maximum is probably caused by gravity waves, while a tendency towards greater activity at higher northern latitudes may be due to stationary planetary waves. © 2014 Elsevier Ltd.

DOI： 10.1016/j.jastp.2014.07.002

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

オーロラは,磁気圏でのプラズマ変動を,地球大気に投影した形で2次元的に可視化し,その面的な把握を可能にする.近年のオーロラカメラの高感度化や多点化,米国THEMIS衛星や日本のれいめい衛星により,オーロラを通して磁気圏のさまざまな変動の様子が明らかになってきた.本節ではその中で,オーロラサブストーム,パルセィティングオーロラや波動と粒子の相互作用,共回転オーロラやブラックオーロラなどに代表されるプラズマの微細空間構造に関して,近年の主だった研究成果を紹介する.

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1186/s40623-014-0140-8

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2014JA020050

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1093/astrogeo/atu215

Web of Science

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：日本気象学会  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/2014JA020161

Web of Science

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: Atmospheres  

We have developed a new analysis method for obtaining the power spectrum in the horizontal phase velocity domain from airglow intensity image data to study atmospheric gravity waves. This method can deal with extensive amounts of imaging data obtained on different years and at various observation sites without bias caused by different event extraction criteria for the person processing the data. The new method was applied to sodium airglow data obtained in 2011 at Syowa Station (69°S, 40°E), Antarctica. The results were compared with those obtained from a conventional event analysis in which the phase fronts were traced manually in order to estimate horizontal characteristics, such as wavelengths, phase velocities, and wave periods. The horizontal phase velocity of each wave event in the airglow images corresponded closely to a peak in the spectrum. The statistical results of spectral analysis showed an eastward offset of the horizontal phase velocity distribution. This could be interpreted as the existence of wave sources around the stratospheric eastward jet. Similar zonal anisotropy was also seen in the horizontal phase velocity distribution of the gravity waves by the event analysis. Both methods produce similar statistical results about directionality of atmospheric gravity waves. Galactic contamination of the spectrum was examined by calculating the apparent velocity of the stars and found to be limited for phase speeds lower than 30 m/s. In conclusion, our new method is suitable for deriving the horizontal phase velocity characteristics of atmospheric gravity waves from an extensive amount of imaging data.

DOI： 10.1002/2014JD021543

Web of Science

Scopus

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

The total electron content (TEC) in the equatorial and low-latitude ionosphere over Brazil was monitored in two dimensions by using 2011 data from the ground-based global navigation satellite system (GNSS) receiver network operated by the Brazilian Institute for Geography and Statistics. It was possible to monitor the spatial and temporal variations in TEC over Brazil continuously during both day and night with a temporal interval of 10 min and a spatial resolution of about 400 km. The daytime equatorial ionization anomaly (EIA) and post-sunset plasma enhancement (PS-EIA) were monitored over an area corresponding to a longitudinal extension of 4000 km in South America. Considerable day-to-day variation was observed in EIA and PS-EIA. A large latitudinal and longitudinal gradient of TEC indicated a significant ionospheric range error in application of the GNSS positioning system. Large-scale plasma bubbles after sunset were also mapped over a wide range. Depletions with longitudinally separated by more than 800 km were observed. They were extended by more than 2000 km along the magnetic field lines and drifted eastward. It is expected that 2-dimensional TEC mapping can serve as a useful tool for diagnosing ionospheric weather, such as temporal and spatial variation in the equatorial plasma trough and crest, and particularly for monitoring the dynamics of plasma bubbles. © 2013 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2014.01.032

Web of Science

Scopus

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  

We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all-sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and Defense Meteorological Satellite Program (DMSP)-17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low-latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open-closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet. © 2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2014GL060301

Web of Science

Scopus

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  

High-latitude ionospheric variations at times near auroral substorms exhibit large temporal variations in both vertical and horizontal extents. Statistical analysis was made of data from the European Incoherent Scatter UHF radar at Tromsø, Norway, and International Monitor for Auroral Geomagnetic Effects magnetometer for finding common features in electron density, ion and electron temperatures and relating these to currents and associated heating. This paper particularly focused on the height dependencies. Results show clear evidences of large electric field with corresponding frictional heating and Pedersen currents located just outside the front of the poleward expanding aurora, which typically appeared at the eastside of westward traveling surge. At the beginning of the substorm recovery phase, the ionospheric density had a large peak in the E region and a smaller peak in the F region. This structure was named as C form in this paper based on its shape in the altitude-time plot. The lower altitude density maximum is associated with hard auroral electron precipitation probably during pulsating aurora. We attribute the upper F region density maximum to local ionization by lower energy particle precipitation and/or long-lived plasma that is convected horizontally into the overhead measurement volume from the dayside hemisphere. Key Points Ionosphere near poleward moving aurora is studied using EISCAT data Joule energy is dissipated just outside the poleward expanding aurora Ionospheric density at early recovery phase has a double peak in height ©2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013JA019704

Web of Science

Scopus

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 investigated Pi2 pulsations in the nightside ionosphere that began at 14:15 UT (2315 LT) on 11 July 2010, and they were observed with high-temporal (8 s) resolution by beam 4 of the Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar. These pulsations were simultaneously observed in both the ground/sea scatter echoes reflected from the F region height and in ionospheric echoes from field-aligned irregularities in the sporadic Es region. They had the same period of 110 s and approximately no phase lag. From the radar observations and the International Geomagnetic Reference Field model, the amplitude of the eastward (EEW) component of the electric field of the Pi2 pulsations in the ionosphere was estimated ~8.0 mV/m in the F region and ~2.0 mV/m in the E region. Corresponding Pi2 pulsations appeared dominantly in the horizontal northward magnetic field component (H) at nearby ground stations, Moshiri (MSR), St. Paratunka (PTK), and Stecolny (STC), with amplitudes ranging from 6 nT (MSR) to 10 nT (STC). At the dominant frequency of 8.8 mHz, the coherences between H and EEW were high (>0.9), the cross phases of EEW relative to H were -56° and -45°, and the amplitude ratios were 2.7 × 105 m/s and 8.4 × 105 m/s, in the E and F regions, respectively. Based on a comparison of these results with theoretical predictions, we suggest that the concept of a pure cavity mode is not sufficient to explain the combined observations for midlatitude Pi2 waves and that the contribution of an Alfvén waves must be taken in account. Key Points Midlatitude SuperDARN radar observed Pi2 pulsations in the E and F regions The electric field of Pi2 pulsations in the ionosphere are estimated The conception of pure cavity mode is not sufficient ©2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2012JA018585

Web of Science

Scopus

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  

During a 2 h interval from 2240 to 2440 UT on 12 November 2012, regions of increased 630.0 nm airglow emissions were simultaneously detected by dual all-sky imagers in the polar cap, one at Longyearbyen, Norway (78.1°N, 15.5°E) and the other at Resolute Bay, Canada (74.7°N, 265.1°E). The Resolute Bay incoherent scatter radar observed clear enhancements of the F region electron density up to 1012 m-3 within these airglow structures which indicates that these are optical manifestations of polar cap patches propagating across the polar cap. During this interval of simultaneous airglow imaging, the nightside/dawnside (dayside/duskside) half of the patches was captured by the imager at Longyearbyen (Resolute Bay). This unique situation enabled us to estimate the dawn-dusk extent of the patches to be around 1500 km, which was at least 60-70% of the width of the antisunward plasma stream seen in the Super Dual Auroral Radar Network convection maps. In contrast to the large extent in the dawn-dusk direction, the noon-midnight thickness of each patch was less than 500 km. These observations demonstrate that there exists a class of patches showing cigar-shaped structures. Such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation. Key Points Image the entire shape of polar cap patches by combining dual airglow imagers Shape of the patches was much more elongated in the dawn-dusk direction Patches could be produced in a wide range of local time nearly simultaneously © 2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013GL058748

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.5194/npg-21-347-2014

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IEEJ Transactions on Power and Energy  

The electromagnetic radiations from lightning discharges have been intensively studied for a long time in different frequency ranges. Recent observations of electromagnetic radiations from lightning in the ELF (extremely low frequency) frequency range so-called ELF transients are recognized as a powerful tool to obtain one of the most important properties of lightning discharges; the charge moment changes (Qds). In this paper we demonstrate the spatio-temporal distributions of lightning discharges together with their charge moment change (CMC) around Japan by using our newly developed domestic ELF observation network. This is the first time to obtain such type of distribution by using only ELF observations in the spatial scale of Japan (a few thousands km). We found that the obtained lightning source distributions both over the Pacific Ocean and the Sea of Japan are originated from the thunderstorm active regions confirmed by other measurements such as WWLLN. Statistical properties of the charge moment changes indicate that both number and CMC of positive CGs are superior to those of negative CGs. Moreover considerably large CMC with both polarities are identified for the CGs over the Pacific Ocean as well as those with positive polarity over the Sea of Japan. © 2013 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejpes.133.994

Scopus

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：国立極地研究所  

電離圏ダイナミクスに大きく寄与する熱圏風についての理解を深めるため,イオノゾンデの磁気共役点観測により推定された南北方向の熱圏風(推定熱圏風)と,ファブリ・ペロー干渉計(FPI)観測により直接測定された南北方向の熱圏風との相関を調べた.推定熱圏風は,磁気共役点における熱圏風は等しい(赤道横断風モードが卓越する)という仮定のもとで導出される.2 つの手法で観測された夜間の熱圏風を比較するのは本研究が初めてである.2010 年のチェンマイ(タイ)とコトタバン(インドネシア)のイオノゾンデとFPI データを比較した結果,両者はおおむね良い相関を示したが,相関が悪い日もあった.相関が悪い事例は,赤道から収束・発散する成分を無視できず,赤道横断風モードが卓越するという仮定が成立しないと解釈されるものである.また,2 つの手法で求めた熱圏風の相関を季節別に調べると,2-4 月に両者の相関が高い一方,5-7 月に両者の相関が低いことがわかった.

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：Geophysical Research Letters  

We present the first reported gravity wave patterns in the mesopause region caused by a typhoon in the troposphere. On 10 December 2002, concentric rings of gravity waves in OH airglow were observed simultaneously by all-sky imagers in the Optical Mesosphere and Thermosphere Imager system in Japan, located at Rikubetsu (43.5°N, 143.8°E), Shigaraki (34.9°N, 136.1°E), and Sata (31.0°N, 130.7°E). The airglow structures, which were well defined and formed a coherent wave pattern expanding concentrically, were identified over 8 h (2135-2947 LT). We estimate the horizontal wavelength, horizontal phase speed, and wave period as 34.5 km, 50.2 m s-1, and 11.5 min, respectively. Infrared cloud images from the Geostationary Meteorological Satellite show that the center of the rings estimated from the airglow data corresponds to a spiral band of Typhoon Pongsona (T0226). This unique event provides new insight into coupling between the lower and upper atmosphere. ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013GL058087

Web of Science

Scopus

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

To comprehend ionospheric-thermospheric coupling, one must understand the thermospheric wind system. However, measuring the thermospheric wind using a Fabry-Perot interferometer (FPI) is not an easy task. Because of this difficulty, some researchers have estimated meridional wind velocities using data obtained from a pair of ionosonde stations near the geomagnetic conjugate points, under the assumption that the meridional wind is the same at the two ionosonde stations (transequatorial mode wind). In this paper, we construct the first comparison of the estimated meridional wind velocities with meridional wind observed with FPIs. We analyzed data from the ionosondes and FPIs installed at Chiang Mai, Thailand, and Kototabang, Indonesia, from 2010. We found that the estimated and observed wind velocities were generally in good agreement on most nights, although on some nights, the wind velocities were different. The assumption that the meridional wind is equal anywhere between the two ionosonde stations would not be suitable for the days when the winds were not in good agreement. We also investigated the seasonal dependence of the correlation between the estimated and observed meridional winds. They were in good agreement from February to April and were not in good agreement from May to July. © 2013 National Institute of Polar Research.

Scopus

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  

DOI： 10.1016/j.jastp.2013.08.017

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jastp.2013.03.024

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.jastp.2013.03.020

Web of Science

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  

Upper mesosphere airglow emissions and temperature observed at Rikubetsu (43.5°N,143.8°E) and mesospheric winds observed at Wakkanai (45.4°N,141.8°E), Japan, from January to December 2005 were used to analyze periodic oscillations of 2-16 days. During the January to March period and after September, both winds and airglow demonstrated clear 8-, 10- and 16-day oscillations. Downward phase progressions observed in the oscillations indicate that these are a signature of Rossby mode planetary waves. The 16-day wave was more evident in the zonal wind than the meridional. The 10-day wave was observed in January and March, on the basis of only a few cycles superposed on the 16-day wave. Airglow OI 557.7nm, O2 and OH(6,2) band emissions and O2 rotational temperature also showed significant amplitude of oscillation induced by the wave passages. For the 10-day wave, OI557.7nm showed an amplitude of oscillation equals to or more than 50% of the mean intensity level, O2 ~45% and OH ~25%. Large amplitudes of oscillation of the airglow during the passage of planetary waves suggest the possible vertical transport of atomic oxygen in addition to the density and temperature variations intrinsic to the wave events. © 2013 Elsevier Ltd.

DOI： 10.1016/j.jastp.2013.03.014

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1002/jgra.50279

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

CiNii Books

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：Geophysical Research Letters  

We present observational evidence of gravity wave ducting and show, for the first time, that the ducting occurs over large horizontal distances in the mesopause region. An optical network of four all-sky imagers in Japan identified two-dimensional patterns of small-scale gravity wave bands in OH airglow images on 13 June 2004. The wave signatures clearly showed northward propagation of more than 1800 km over a wide range of latitudes. The horizontal wavelength, horizontal phase speed, and wave period were estimated from the airglow data as 33.4 km, 42.8 m s^-1, and 13.4 min, respectively. The wave structure lasted for the whole 5-h airglow observation period. Simultaneous MF radar wind data and TIMED/SABER measurements suggested that the wave was trapped and ducted at the airglow height. The ducting likely contributed to the remarkable coherence of the wave as it propagated northward. Key Points An airglow imaging network in Japan provides gravity wave ducting signatures Gravity wave bands appear over a horizontal extent of more than 1800 km Simultaneous winds and temperature are consistent with gravity wave ducting © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012GL054605

Web of Science

Scopus

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 report ground and satellite observations of unique low-latitude red auroras that appear at the initial phase of geomagnetic storms. For two events on 21 October 2001, and 6 April 2000, the low-latitude red auroras appeared at ∼45° MLAT (L ∼ 2) ∼ 1.5 h after the storm sudden commencement in the postmidnight sector in Japan. Comprehensive satellite data were available for the former event. The energetic neutral atom images obtained by the Imager for Magnetopause-to-Aurora Global Exploration satellite show rapid enhancement of ring current hydrogen and oxygen fluxes at radial distances of ∼ 2-8 RE after the storm sudden commencement and associated with several storm-time substorms. The hydrogen ring-current enhancement occurred particularly in the postmidnight sector where the red aurora was observed. The timing of oxygen flux enhancement associated with a storm-time substorm coincided with the red aurora appearance. This rapid and significant enhancement of energetic neutral atom flux was also confirmed by energetic ion data obtained by the NOAA/POES-16 satellite. Extreme ultraviolet plasmaspheric images obtained by Magnetopause-to-Aurora Global Exploration indicate that the plasmapause was located at L = 2.3-2.5 in the postmidnight sector during the event, indicating that a spatial overlap occurs between the plasmasphere and the enhanced ring current ions at L ∼ 2. Based on these observations, we suggest that large energization of high-energy ring-current ions in the postmidnight inner magnetosphere caused the spatial overlap of these ring-current ions with the low-energy plasmaspheric plasmas at L ∼ 2, producing the low-latitude red auroras at the very beginning of the storms. ©2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA018001

Web of Science

Scopus

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 pitch-angle distributions and spectral shapes of auroral electrons simultaneously observed during three conjunction events by the FAST satellite at altitudes of 2500-3500 km and by the THEMIS satellite in the plasma sheet at 7-13 RE. All three events were on the lower latitude of the auroral oval. Conjunction event 1 occurred at ~1914:45 UT on 10 April 2008. Electron spectra at energies of 0.1-3 keV are correlated well between the two satellites, while the precipitating electrons above 3 keV are missing at FAST. Event 2 occurred at 1255:26 UT on 26 April 2008. Electron spectra above 3 keV are correlated well between the two satellites. An additional broad spectral peak at energies of 0.1-0.5 keV was observed by FAST. Event 3 occurred at 0058:04 UT on 25 December 2008. Precipitating electrons of 0.5-5 keV obtained by FAST are correlated well with those of THEMIS-C, while a monoenergetic peak at 0.1-0.2 keV was observed only by FAST. For the three conjunction events, we conclude that high-energy precipitating auroral electrons observed by FAST directly come from the equatorial plasma sheet, while low-energy precipitating electrons may come from middle altitudes as a result of acceleration by static potential differences. For missing high-energy (>3 keV) electrons of event 1, we speculate that the pitch-angle scattering by waves occurs only at a limited energy range. ©2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017805

Web of Science

Scopus

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

Two-dimensional structures of medium-scale traveling ionospheric disturbances (MSTIDs) over Europe have been revealed, for the first time, by using maps of the total electron content (TEC) obtained from more than 800 GPS receivers of the European GPS receiver networks. From statistical analysis of the TEC maps obtained 2008, we have found that the observed MSTIDs can be categorized into two groups: daytime MSTID and nighttime MSTID. The daytime MSTID frequently occurs in winter. Its maximum occurrence rate in monthly and hourly bin exceeds 70% at lower latitudes over Europe, whereas it is approximately 45% at higher latitudes. Since most of the daytime MSTIDs propagate southward, we speculate that they could be caused by atmospheric gravity waves in the thermosphere. The nighttime MSTIDs also frequently occur in winter but most of them propagate southwestward, in a direction consistent with the theory that polarization electric fields play an important role in generating the nighttime MSTIDs. The nighttime MSTID occurrence rate shows distinct latitudinal difference: The maximum of the occurrence rate in monthly and hourly bin is approximately 50% at lower latitudes in Europe, whereas the nighttime MSTID was rarely observed at higher latitudes. We have performed model calculations of the plasma density perturbations caused by a gravity wave and an oscillating electric field to reproduce the daytime and nighttime MSTIDs, respectively. We find that TEC perturbations caused by gravity waves do not show dip angle dependencies, while those caused by the oscillating electric field have a larger amplitude at lower latitudes. These dip angle dependencies of the TEC perturbation amplitude could contribute to the latitudinal variation of the MSTID occurrence rate. Comparing with previous studies, we discuss the longitudinal difference of the nighttime MSTID occurrence rate, along with the E- and F-region coupling processes. The seasonal variation, of the nighttime MSTID occurrence rate in Europe, is not consistent with the theory that the longitudinal and seasonal variations of the nighttime MSTID occurrence could be attributed to those of the Es layer occurrence. © 2013 Author(s).

DOI： 10.5194/angeo-31-163-2013

Web of Science

Scopus

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

A new stereoscopic measurement technique is developed to obtain an all-sky altitude map of aurora using two ground-based digital single-lens reflex (DSLR) cameras. Two identical full-color all-sky cameras were set with an 8 km separation across the Chatanika area in Alaska (Poker Flat Research Range and Aurora Borealis Lodge) to find localized emission height with the maximum correlation of the apparent patterns in the localized pixels applying a method of the geographical coordinate transform. It is found that a typical ray structure of discrete aurora shows the broad altitude distribution above 100 km, while a typical patchy structure of pulsating aurora shows the narrow altitude distribution of less than 100 km. Because of its portability and low cost of the DSLR camera systems, the new technique may open a unique opportunity not only for scientists but also for night-sky photographers to complementarily attend the aurora science to potentially form a dense observation network. © Author(s) 2013.

DOI： 10.5194/angeo-31-1543-2013

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.15094/00009711

Scopus

その他リンク： http://orcid.org/0000-0001-8392-6455

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.5047/eps.2012.08.001

Web of Science

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  

DOI： 10.1016/j.jastp.2012.10.002

Web of Science

Scopus

担当区分：筆頭著者   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 have developed new Fabry-Perot interferometers (FPIs) that are designed to measure thermospheric winds and temperatures as well as mesospheric winds through the airglow/aurora emissions at wavelengths of 630.0 nm and 557.7 nm, respectively. One FPI (FP01), possessing a large aperture etalon (diameter: 116 mm), was installed at the EISCAT Tromsø site in 2009. The other FPIs, using 70-mm diameter etalons, were installed in Thailand, Indonesia, and Australia in 2010-2011 (FP02-FP04) by the Solar-Terrestrial Environment Laboratory, and in Peru (Nazca and Jicamarca) and Alaska (Poker Flat) by Clemson University. The FPIs with 70-mm etalons are low-cost compact instruments, suitable for multipoint network observations. All of these FPIs use low-noise cooled-CCD detectors with 1024 × 1024 pixels combined with a 4-stage thermoelectric cooling system that can cool the CCD temperature down to - 80°C. The large incident angle (maximum: 1.3°-1.4°) to the etalon achieved by the use of multiple orders increases the throughput of the FPIs. The airglow and aurora observations at Tromsø by FP01 show wind velocities with typical random errors ranging from 2 to 13 m s-1 and from 4 to 27 m s-1 for mesosphere (557.7 nm) and thermosphere (630.0 nm) measurements, respectively. The 630.0-nm airglow observations at Shigaraki, Japan, by FP02-FP04 and by the American FPI instruments give thermospheric wind velocities with typical random errors that vary from 2 m s-1 to more than 50 m s-1 depending on airglow intensity. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

DOI： 10.5047/eps.2012.05.004

Web of Science

Scopus

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 report multipoint observations of daytime tweek atmospherics during the solar eclipse of 22 July 2009. Sixteen and sixty-three tweek atmospherics were observed at Moshiri and Kagoshima, Japan, where the magnitudes of the solar eclipse were 0.458 and 0.966, respectively. This was the first observation of tweek atmospherics during a low-magnitude eclipse (0.458). The average and standard deviation of the reflection height were 94.9 13.7km at Moshiri and 87.2 12.9km at Kagoshima. The reflection height at Moshiri was almost the same as that for normal nighttime conditions in July (96.7 12.6km) in spite of the low magnitude of the eclipse. The reflection height at Kagoshima seems be divided into two parts: propagation across the total solar eclipse path and propagation in the partial solar eclipse path. During the eclipse, we also observed the phase variation in the LF transmitter signals. The average change in the phase delay of the LF signals was 109 for the paths that crossed the eclipse path and 27 for the paths that did not cross the eclipse path. Assuming a normal daytime height for LF waves of 65km, a ray tracing analysis indicates that the variations in phase correspond to a height increase of 5-6km for the paths across the eclipse and 1-2km for partial eclipse paths. The wide range of estimated tweek reflection heights at Kagoshima also suggests a difference in electron density in the lower ionosphere between total and partial solar eclipses. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA018151

Web of Science

Scopus

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  

[1] We report on nighttime medium-scale traveling ionospheric disturbances (MSTIDs) observed at Kototabang, Indonesia (geographic longitude: 100.3°E; geographic latitude: 0.2°S; and geomagnetic latitude: 10.6°S) during a 7-year period from October 2002 to October 2009. MSTIDs were observed in 630-nm nighttime airglow images by using a highly sensitive all-sky airglow imager at Kototabang. The averages and standard deviations of horizontal phase velocity, period, and horizontal wavelength of MSTIDs observed during the 7 years were 320 ±170 m/s, 42 ±11 min, and 790 ± 440 km, respectively. The occurrence rate of the observed MSTIDs decreased with decreasing solar activity. The average horizontal wavelength of MSTIDs increased with decreasing solar activity. Southward MSTIDs were dominant throughout the 7 years of observations. These facts are consistent with the hypothesis that the observed MSTIDs are caused by gravity waves in the thermosphere. Moreover, we compared the propagation directions of the observed MSTIDs with the locations of tropospheric convection activity for the events where gravity waves producing the observed MSTIDs could have existed in the lower atmosphere. Strong tropospheric convection was found within ±30 degrees from the source directions of MSTIDs in 81% of the MSTID events. In such events, gravity waves were possibly generated from deep convection in the troposphere and directly propagated into the thermosphere. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017758

Web of Science

Scopus

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 report for the first time the rapid oscillating motion of nighttime medium-scale traveling ionospheric disturbances (MSTIDs) based on airglow imaging observations at Troms (magnetic latitude: 67.1N), Norway on 8 December, 2009. The MSTIDs appeared in 630-nm airglow images at 1530 UT as wave-like structures south of Troms with a horizontal wavelength of ∼200km and a phase surface of north to south. They moved eastward with velocities of 30-60m/s. The velocity was faster in the poleward-side of the MSTIDs, forming a northeast-southwest phase surface at later times. This phase surface direction is opposite to that of midlatitude MSTIDs. The MSTIDs show sudden oscillations and phase jump in the east-west direction with a timescale of ∼10min at 1730 UT. The oscillations were associated with an auroral brightening observed at the poleward edge of the images and small magnetic field perturbations observed by ground magnetometers. The Doppler measurement of the 630-nm airglow by a Fabry-Perot interferometer at Troms showed a stable southeastward thermospheric wind with a velocity of ∼150m/s. These observations indicate that the MSTID oscillations were linked to auroral electric field in the ionosphere, implying that the observed MSTIDs are ionospheric plasma structures. We suggest that the observed MSTIDs were created by atmospheric gravity waves at the beginning, left as fossil plasma structures even after the gravity wave packet dissipated in the thermosphere, moved eastward according to the background electric field driven by the F-region dynamo, and oscillated associated with the auroral electric field. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017928

Web of Science

Scopus

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  

The passage of an interplanetary (IP) shock was detected by Wind, ACE, Geotail, and THEMIS-B in the solar wind on 24 November 2008. From the propagation time of the IP shock at the spacecraft, it is expected that the IP shock front is aligned with the Parker spiral and strikes the postnoon dayside magnetopause first. Using multipoint observations of the sudden commencement (SC) at THEMIS probes, GOES 11, and ETS in the dayside magnetosphere, we confirmed that the magnetospheric response to the IP shock starts earlier in the postnoon sector than in the prenoon sector. We found that the estimated normal direction of the SC front is nearly aligned with the estimated IP shock normal. We also found that the SC front normal speed is much slower than the fast mode speed and is about 22-56% of the IP shock speed traveling in the solar wind. Thus, we suggest that the major field changes of the SC in the dayside magnetosphere are not due to the magnetic flux carried by hydromagnetic waves but to the increased solar wind dynamic pressure behind the shock front sweeping the magnetopause. The SC event appears as a step-like increase in the H component at the low-latitude Bohyun station and a negative-then-positive variation in the H component at the high-latitude Chokurdakh (CHD) station in the morning sector. During the negative perturbation at CHD, the SuperDARN Hokkaido radar detected a downward motion in the ionosphere, implying westward electric field enhancement. Using the THEMIS electric field data, it is confirmed that the westward electric field corresponds to the inward plasma motion in the dayside magnetosphere due to the magnetospheric compression.

DOI： 10.1029/2012JA017871

Web of Science

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  

Interaction with EMIC (electromagnetic ion cyclotron) waves is thought to be a key component contributing to the very rapid loss of both ring current and radiation belt particles into the atmosphere. Estimated loss rates are heavily dependent on the assumed spatial distribution of the EMIC wave. Statistical maps of the spatial distribution have been produced using in-situ satellite data. However, with limited satellite data it is impossible to deduce the true spatial distribution. In this study, we present ground-based observations using all-sky imager and search coil magnetometer networks, which provide the large-scale distribution and motion of the EMIC wave-particle interaction regions. We observed several spots of isolated proton auroras simultaneously with Pc1/EMIC waves at subauroral latitudes during the expansion phase of a storm-time substorm on 9 March 2008. The isolated auroras were distributed over similar to 4-hours MLT preceding midnight. The POES-17 satellite confirmed enhancements of 30-keV proton precipitations over the isolated auroras. The equatorward motion of the auroras and frequency drift of the wave were consistent with the plasmasphere eroding due to a polar cap potential enhancement modeled by a numerical simulation. We also found that relativistic electron precipitation was not always associated with the isolated aurora, depending strongly on the plasma density profile near the plasmapause. This study shows that the specific distribution of ring current proton precipitation can be visualized through the ground network observations. By combining with upcoming inner-magnetosphere satellite missions, these remote-sensing observations are very important for quantitative understanding of the particle loss in the inner magnetosphere.

DOI： 10.1029/2012JA018180

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2012JA017961

Web of Science

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 high correlation between a pulsating auroral patch and grouped chorus waves was observed on 17 April 2006 at Syowa Station in Antarctica. The spatial distribution of aurora-chorus correlation coefficients is evaluated in order to determine the source region. A pulsating patch at the highest-correlation pixel shows a one-to-one correspondence with the intensity variation of the grouped chorus waves, consisting of successive rising-tone elements with a duration and spacing of 2-3s and 20-30s, respectively. The generation region of the chorus waves is estimated from the latitude and longitude dependence of the equatorial electron gyrofrequencies using the IGRF geomagnetic field model. The extent of the estimated latitude and longitude is consistent with the spatial distribution of the high-correlation aurora-chorus region. The time difference between the chorus waves and the scattered electrons is also evaluated to discuss the validity of the source region. It shows that electrons reached the ionosphere sooner than the associated chorus waves by ∼1 s, consistent with the theoretical value for conjugate pulsating aurora generated at the equator. These results support the hypothesis that pulsating aurora is caused by pitch angle scattering of high-energy electrons by whistler mode chorus waves, via a cyclotron resonance at the equator. These results are the first ground-based observations of high correlations between a spatially extended aurora and chorus waves. © (c)2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2011JA017478

Web of Science

Scopus

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  

We investigated the relationship between post-midnight F-region field aligned irregularities (FAIs) and F-layer altitude by analyzing data of a 30.8-MHz radar installed 5at Kototabang, Indonesia (0.2 degrees S, 100.3 degrees E; geomagnetic latitude 10.4 degrees S) and an ionosonde installed at Chumphon, Thailand (10.7 degrees N, 99.4 degrees E; geomagnetic latitude 3.3 degrees N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of the F-layer rose at Chumphon a half hour before the post-midnight FAIs appeared at Kototabang. The Doppler velocity of the E-region FAIs observed simultaneously by the 30.8-MHz radar was downward, indicating that the F-layer uplift was not caused by the electric field. We also investigated seasonal variations of the post-midnight FAI occurrence and the F-layer altitude. Both the post-midnight FAIs and the uplift of the F-layer were frequently seen around midnight between May and August. The seasonal variation of the midnight F-layer uplift around the geomagnetic equator coincided with that of the post-midnight FAI occurrence at Kototabang. These results suggest that the uplift of the F-layer would play an important role in the generation of post-midnight FAIs. We evaluated the linear growth rate of the Rayleigh-Taylor instability based on the altitude of the F-layer observed at Chumphon. The result shows that the uplift of the F-layer can enhance the growth rate because gravity-driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the uplifted F-layer.

DOI： 10.1029/2012JA017692

Web of Science

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  

We report simultaneous observations of an equatorial plasma bubble and a Medium-Scale Traveling Ionospheric Disturbance (MSTID) in 630-nm airglow images taken with an all-sky airglow imager at Shigaraki (34.9°N, 136.1°E; dip angle of the geomagnetic field ̃49°), Japan. Clear depletion of the 630-nm airglow intensity was observed as the equatorial plasma bubble propagated eastward, whereas the MSTID, which had a wavefront aligned from northwest to southeast, propagated southwestward. This result indicates that MSTIDs do not propagate at the same velocity as the ambient plasma, which is clearly shown by the eastward motion of the plasma bubbles. We found that the airglow depletion caused by the plasma bubble disappeared when the plasma bubble encountered the MSTID. The plasma depletion could be filled with ambient rich plasma that moved into the plasma-depleted region by E - B drift associated with the MSTID, indicating that MSTIDs are accompanied by electric field perturbations. © 2012. American Geophysical Union.

DOI： 10.1029/2012GL052286

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2011JA017423

Web of Science

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  

Ionospheric plasma density structures in the dayside F-region of the polar cap are commonly occurring events, but adequate measurements of their formation and evolution have been sparse. With the advent of the advanced modular incoherent scatter radar RISR-N (Resolute Bay Incoherent Scatter Radar) it is now possible for the first time to study the temporal evolution of the plasma properties in the polar cap region in three dimensions, with a spatial resolution of tens of kilometers, from which the plasma rest frame can be experimentally established. We demonstrate the strength of the diagnostic with observations from an event of enhanced plasma density observed over Resolute Bay in December 2009. A colocated all-sky imager showed faint 630.0 and 557.7 nm emission corresponding to the plasma enhancements, and the structures could be traced back to a formation region in the open/closed field line boundary. This new plasma imaging technique will provide important information on the mechanisms controlling the structuring in the high latitude ionosphere. Copyright © 2012 by the American Geophysical Union.

DOI： 10.1029/2012GL050895

Web of Science

Scopus

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  

Based on conjugate ground and THEMIS satellite observations, we show electron spectra and wave characteristics near the magnetic equatorial plane during a pulsating aurora event on the high latitude side of the auroral oval. The pulsating aurora was observed by a 30-Hz sampled all-sky imager (ASI) at Gillam (56.4 degrees N, 265.4 degrees E), Canada, at similar to 0840-0910 UT on 8 January 2008. The auroral intensity pulsation at the possible THEMIS D (THD) footprints had frequency peaks at similar to 0.1-0.2 Hz. The footprint of THD was in the poleward part of the proton aurora observed by a meridian-scanning photometer. After auroral pulsation began at similar to 0842 UT, both THD and THEMIS E which was near THD in the mid-tail at 11.6-11.8 R-E, observed enhanced field-aligned electron fluxes at energies of 1-10 keV. However, the amplitudes of whistler mode waves and electrostatic cyclotron harmonics (ECH) waves observed by THD with the highest sampling rate of 8 kHz were not significant, showing a marked contrast to the recent report of clear correlation between whistler mode waves and auroral pulsations observed at 5-9 R-E. We suggest that the observed field-aligned electrons, which are probably caused by Fermi-type acceleration associated with earthward plasma flow in the mid-tail plasma sheet, are modulated by some wave processes to cause pulsating auroras.

DOI： 10.1029/2011JA017066

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2011JA017241

Web of Science

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

Joint observations of the all-sky camera at Resolute Bay (Nunavut, Canada) and the Polar Dual Auroral Radar Network (PolarDARN) HF radars at Rankin Inlet and Inuvik (Canada) are considered to establish radar signatures of poleward moving polar cap arcs "detaching" from the auroral oval. Common features of the events considered are enhanced power or echo occurrence in the wake of the arcs and enhanced spectral width of these echoes. When the arcs were oriented along some of the radar beams, velocity reversals at the arc location were observed with the directions of the arc-associated flows corresponding to a converging electric field. For the event of 9 December 2007, two arcs were poleward progressing almost along the central beams of the Inuvik radar at the speed close to the E × B drift of the bulk of the F-region plasma as inferred from HF Doppler velocities and from independent measurements by the Resolute Bay ionosonde. In global-scale convection maps inferred from all Super Dual Auroral Radar Network (SuperDARN) radar measurements, the polar cap arcs were often seen close to the reversal line of additional mesoscale convection cells located poleward of the normal cells related to the auroral oval. © Author(s) 2012.

DOI： 10.5194/angeo-30-441-2012

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：Geophysical Monograph Series  

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space storms. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through crossenergy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012GM001304

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：Geophysical Monograph Series  

Understanding the physical connections of the coupled magnetosphere- ionosphere system will result in a more complete explanation of the aurora and will further the goal of being able to interpret the global auroral distributions as a dynamic map of the magnetosphere. Significant advances have been made in recent years toward this goal. In this chapter, we briefly review, while focusing on recent observations, selected auroral phenomena that are driven by magnetospheric processes. These include expansion of substorm aurora, plasma sheet waves and auroral modulations, ballooning instability and auroral beads, dipolarization/plasma injections and the auroral bulge, poleward boundary intensifications and auroral streamers, vortical flows and auroral spirals, and plasma flows prior to auroral onset. In addition, other auroral phenomena, such as auroral arcs, diffuse auroras, auroral asymmetry/conjugacy in the conjugate hemispheres, and auroral magnetospheric currents, are highlighted here but expanded in other chapters of this monograph for in-depth reviews. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012GM001231

Web of Science

Scopus

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

A VHF backscatter radar with operating frequency 30.8 MHz has been operated at Kototabang (0.20°S, 100.32°E; dip latitude 10.4°S), Indonesia since February 2006. The F-region field-aligned irregularities (FAIs) observed by this radar from February 2006 to December 2010 during a solar minimum period have been analyzed and found that FAIs appeared frequently at the post-midnight sector between May and August every year from 2006 to 2010. Five-beam measurements by the radar revealed zonal propagation of the F-region FAIs. The present paper reports, for the first time, statistics of the zonal propagation velocity of the post-midnight FAIs. Between May and August, 46% (14%) of the post-midnight FAIs propagated westward (eastward), and zonal propagation was not discernible for 40% of the post-midnight FAIs. Average velocity was approximately 50 m s-1 westward. The post-midnight FAIs were likely associated with either plasma bubbles or medium-scale traveling ionospheric disturbances (MSTIDs).

Scopus

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  

Using ionospheric total electron content (TEC) measured by Global Positioning System (GPS) receivers of the Canadian High Arctic Ionospheric Network (CHAIN) we provide clear evidence for a systematic and propagating temporary TEC enhancement produced by compression of the magnetosphere due to a sudden increase in solar wind dynamic pressure. The magnetospheric compression is evident in THEMIS/GOES satellite data. Application of a GPS triangulation technique revealed that the TEC changes propagated with a speed of 3-6 km/s in the antisunward direction near noon and ∼8 km/s in the sunward direction in the pre-noon lower latitude sector. Characteristics of these TEC changes along with riometer absorption measurements seems to indicate that the TEC change is due to electron density enhancement in the F region and is possibly due to particle precipitation associated with sudden magnetospheric compression. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2011GL050041

Web of Science

Scopus

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  

Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2011GL049856

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2011JA016954

Web of Science

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 investigated, for the first time, long-term (1976-2010) variations in reflection heights of tweek atmospherics based on very low frequency (VLF) observations at Kagoshima, Japan. The results revealed the effects of the solar cycle on the nighttime lower ionosphere at low to middle latitudes. The tweek reflection heights on geomagnetically quiet days were analyzed every month over three solar cycles by using an automated spectral fitting procedure to estimate the cutoff frequency. The average and standard deviation of the reflection height were 95.9 km and ±3.1 km, respectively. Typical periods of time variation for the reflection height were 13.3, 3.2, 1.3, 1.0, 0.6, and 0.5 years. The variations in tweek reflection heights did not show simple anticorrelation with solar activity. The correlation coefficient between tweek reflection height and sunspot number was 0.03 throughout the three solar cycles. Hilbert-Huang transform analysis successfully indicated the presence of 0.5-1.5 year and ∼10 year variations as intrinsic mode functions (IMF). The decomposed IMF with the ∼10 year variation had a positive correlation with sunspot numbers and a negative correlation with galactic cosmic rays (GCRs). We hypothesize that these variations in tweek reflection heights could be caused by coupling of several ionization effects at the D and lower E regions, effects such as geocorona, GCRs, particle precipitation, and variations in neutral density in the lower thermosphere. Among these processes, the geocorona and particle precipitation could show negative correlation, while the GCRs and neutral density could show positive correlation with solar activities. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016800

Web of Science

Scopus

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：一般社団法人電子情報通信学会  

ジオスペースERG探査計画の紹介.

その他リンク： http://repository.kulib.kyoto-u.ac.jp/dspace/handle/2433/193849

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  

An empirical model of the quiet daily geomagnetic field variation has been constructed based on geomagnetic data obtained from 21 stations along the 210 Magnetic Meridian of the Circum-pan Pacific Magnetometer Network (CPMN) from 1996 to 2007. Using the least squares fitting method for geomagnetically quiet days (Kp &lt;= 2+), the quiet daily geomagnetic field variation at each station was described as a function of solar activity SA, day of year DOY, lunar age LA, and local time LT. After interpolation in latitude, the model can describe solar-activity dependence and seasonal dependence of solar quiet daily variations (S) and lunar quiet daily variations (L). We performed a spherical harmonic analysis (SHA) on these S and L variations to examine average characteristics of the equivalent external current systems. We found three particularly noteworthy results. First, the total current intensity of the S current system is largely controlled by solar activity while its focus position is not significantly affected by solar activity. Second, we found that seasonal variations of the S current intensity exhibit north-south asymmetry; the current intensity of the northern vortex shows a prominent annual variation while the southern vortex shows a clear semi-annual variation as well as annual variation. Thirdly, we found that the total intensity of the L current system changes depending on solar activity and season; seasonal variations of the L current intensity show an enhancement during the December solstice, independent of the level of solar activity.

DOI： 10.1029/2011JA016487

Web of Science

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  

It has been suggested that dispersive Alfven waves (DAWs) are capable of accelerating electrons via Landau resonance, and the interference of DAWs plays an essential role to create flickering auroral patterns. Here we show evidence that the leading front of a typical interference pattern is more energetic than the trailing part, based on ground-based high-speed imaging observations at wavelengths of 670.5 nm and 844.6 nm, which are sensitive to relatively hard and soft electrons, respectively. The fine spatial resolution of 9.5 deg field-of-view at magnetic zenith and the 100 Hz sampling rate of electron multiplying charge-coupled device (EMCCD) enabled us to resolve the spatiotemporal variation of the flickering aurora. It is found that there is only 10 ms time delay with 0.5 km spatial shift on average in the obtained flickering patterns at two wavelengths. The time delay and spatial shift can be comprehensively explained by the traveling inhomogeneous interference pattern of DAWs, probably associated with the Landau damping and/or time-of-flight effect, which is only detectable using the highest resolved temporal and spatial observations of flickering aurora. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2011GL048317

Web of Science

Scopus

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 report an event in which a polar cap patch was detected with an all-sky imager (ASI) at Resolute Bay, Canada (74.73° N, 265.07° E; AACGM latitude 82.9°), on the nightside. The patch stopped its antisunward motion associated with a northward turning of interplanetary magnetic field and stayed within the field of view of the ASI for more than 1 h. When the patch stagnated, its luminosity decreased gradually, which allows us to investigate how the patch plasma decayed in a quantitative manner. The decay of the patch can be quantitatively explained by the loss through recombinations of O+ with ambient N2 and O2 molecules, if we assume the altitude of the optical patch to be around 295 km. The derived altitude of the patch around 295 km is much higher than the nominal value at 235 km obtained from the MSIS-E90 and IRI-2007 models, indicating that climatological models such as IRI are not suitable for describing the actual density profile of patches. This is probably because the loss process was much faster in the lower-altitude part of the patch; thus, the peak altitude of the patch increased as it traveled across the polar cap because of rapid recombination at the bottomside of the F region. This suggests that we should employ higher emission altitude when we investigate optical patches transported deep into the nightside polar cap. Such information is important when we compare the optical data with other instruments such as coherent radars and GPS scintillation measurements by mapping the all-sky image on the geographic coordinate system with an assumption of the patch emission altitude. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016297

Web of Science

Scopus

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 conducted a ground-based optical measurement pointing at local magnetic zenith with a narrow field of view of 9.3 by 9.3, using a high-speed electron multiplier charge-coupled device camera at Poker Flat Research Range. We show evidence that auroral folds were periodically formed in a breakup arc and the luminosity is exponentially increased for about 10 s before an auroral breakup onset. The evolution of turbulent microstructures and the formation of folds may be interpreted by the nonlinear evolution of inertial Alfvén wave (IAW) turbulence in the thin current sheet. On the basis of these optical observations, we discuss a possible role of the ionosphere for modulating the triggering process of the onset via the self-organization of the IAW turbulence associated with Kelvin-Helmholtz and tearing instabilities of the thin current sheet. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA016334

Web of Science

Scopus

記述言語：日本語   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：Geophysical Research Letters  

On the night of May 16, 2007, a satellite limb imager of FORMOSAT-2/ISUAL observed wave-like structures of the 630-nm airglow simultaneously with an all-sky imager deployed at Darwin in Australia. The height of the airglow layer was estimated as 220 km, and the structures were aligned in the northeast-southwest orientation with a wavelength of ∼300 km and propagated toward the northwest with a phase velocity of ∼100 m s^-1, showing typical characteristics of the nighttime medium-scale traveling ionospheric disturbances (MSTIDs). We conclude that ISUAL for the first time succeeded in observing the airglow layer altitude and airglow structures modulated by MSTID from space. Such a satellite limb airglow imaging could be a new tool to characterize ionospheric irregularities on a global level. Copyright © 2011 by the American Geophysical Union.

DOI： 10.1029/2010GL046268

Web of Science

Scopus

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  

Periodic black auroral patches were observed in the Canadian postmidnight sector by ground-based all-sky cameras during an auroral substorm. The aurora consisted of several black patches of similar to 50 km (similar to 1 degrees in longitude) scales that appeared along the equatorward edges of the proton auroras over a 3 h local time segment from Gillam to Sanikiluaq ground stations 15 min after the auroral intensification occurred in Alaska. To the best of our knowledge, this type of black auroras has never been reported before, although the surrounding luminous auroras evolve into the well-known torch structures later. In situ observations by the THEMIS satellites, three of which were close to being magnetically conjugate in the plasma sheet, as well as ground-based proton auroral observations, show that a dipolarization front and associated dawnward ion flows propagated eastward at poleward of the black patches coinciding with their appearance. The flow motions of auroras observed poleward of the black patches are possibly generated by decelerations of dawnward ion flow because of the consistency of both durations and positions. We speculate, based on manifestations of high-pressure regions in dipolarized flux tubes, the pressure-driven interchange instability formed a beading shape (periodic black patches) of field-aligned currents. The black auroral patches developed into torch structures after the flow disappearance, indicating the dawnward flows might stabilize and limit the instability within a narrow L shell where the pressure gradient is directed Earthward and flows cannot enter. We conclude that the periodic black patches arise as a consequence of a relaxation process of enhanced plasma pressure near the inner boundary of the plasma sheet during a substorm.

DOI： 10.1029/2010JA015957

Web of Science

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  

Induction magnetometer data from the mid-latitude station Moshiri (geomagnetic latitude 35.6°) has been examined in search of a transient ULF response to the regional lightning activity. For many events, besides the main impulse produced by the lightning discharge, a secondary impulse delayed about 1 sec was observed. These secondary echo-impulses are probably caused by the partial reflection of wave energy of the initial lightning pulse from the upper boundary of the ionospheric Alfven resonator in the topside ionosphere. The modeling with artificial signals has shown that a multi-band spectral resonant structure (SRS) can be formed owing to the occurrence of paired pulses in analyzed time series. The statistical superposed epoch method indeed has revealed a dominance of two-pulse structure in the magnetic field background during the periods of the SRS occurrence. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).

DOI： 10.5047/eps.2010.12.009

Web of Science

Scopus

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

From nocturnal variations of the airglow O2 (0-1) and OH Meinel (6-2) band emission intensity and the rotational temperature, gravity waves and the damping effect in the MLT region were investigated. The data set was obtained from photometer measurements at Rikubetsu (43.5° N, 143.8° E), Japan, from March 2004 to August 2005. The ratio of the amplitude of oscillation and their phase difference between the two emissions were calculated when simultaneous periodic variations were observed. The ratio showed a linear correlation with the phase difference. The vertical wavelength and damping rate were estimated by using a model calculation carried out by previous works. The results show that the wave damping is significant when the vertical wavelength is shorter than 30-40 km. Krassovsky's parameter η, which represents a ratio between the emission intensity and temperature oscillations, was also calculated. The results show that the η also depends on the damping effect. © Author(s) 2011.

DOI： 10.5194/angeo-29-631-2011

Web of Science

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2010JA015906

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 have investigated Pc1 geomagnetic pulsations observed by induction magnetometers at three low-latitude stations (Paratunka (PTK), Moshiri (MSR), and Sata (STA), L=1.2-2.1). A detailed polarization analysis shows that polarization parameters (angle of polarization ellipse orientation, ψ, and polarization sense, ε) of individual Pc1 bands depend on frequency at all three stations. The dependence of ψ on frequency was seen in ∼70% of the 93 Pc1 events observed at MSR (L=1.5) from 14 July 2007 to 13 July 2009. The maxima of seasonal and diurnal variations of the occurrence rate were in winter and during the nighttime, as reported previously, indicating that the transmission of observed Pc1 pulsations to lower latitudes is controlled by the density of the F layer plasma. These facts suggest that spatially distributed Pc1 waves with varying frequencies depending on longitude or latitude at high latitudes propagated in the ionospheric duct to cause the frequency dependence of polarization parameters at low latitudes. We also suggest that the Pc1 pearl structure with a repetition period of ∼5-30 s observed at low latitudes is a beat of high-latitude waves with slightly different frequencies. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA015684

Scopus

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_21

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2010JA015848

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2010JD014674

Web of Science

記述言語：日本語   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：AMER GEOPHYSICAL UNION  

Broadband electrons (BBEs) exhibit remarkable electron flux enhancements over a broad energy range (0.03-30 keV) near the equatorward edge of the auroral oval during geomagnetic storms. Here, we report a BBE event observed by the Fast Auroral Snapshot (FAST) satellite at 1355-1359 UT, similar to 61 degrees-66 degrees invariant latitudes, similar to 0600 magnetic local time (MLT), and similar to 3800 km altitude during a storm on 25 July 2004. The Double Star (DS) TC1 satellite was located near the magnetic equator at L = 5.7, close to the same local time as FAST. We investigate the acceleration process of BBEs from the inner magnetosphere to near the ionosphere by comparing electron data obtained by FAST and DS TC1. We also investigate both plasma and field variations in the inner magnetosphere associated with substorm onset using DS TC1 data to examine the relationship between the BBEs and the storm time substorm. Ground geomagnetic field data show a positive H-bay at similar to 1349 UT at similar to 0600 MLT, indicating that a storm time substorm started just before the appearance of the BBEs. At similar to 1350 UT, a tailward ion flow was observed by DS TC1. Then, DS TC1 observed a local dipolarization and a drastic ion density enhancement at similar to 1351 UT, indicating that particle heating associated with the substorm was occurring in the inner magnetosphere. From similar to 1352 UT, electron fluxes were isotropically enhanced at energies above similar to 0.5 keV as observed by DS TC1. On the other hand, the pitch angle distribution of BBEs at the FAST altitude showed field-aligned lower-energy electrons below similar to 0.5 keV and isotropic higher-energy electrons above similar to 0.5 keV. From these data, it was inferred that the BBEs might consist of two energy components due to the acceleration or heating of electrons at different altitudes in association with the storm time substorm.

DOI： 10.1029/2009JA014907

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

CiNii Research

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2009JA014273

Web of Science

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1029/2009JA014515

Web of Science