Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

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 $L \sim 10$. 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

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Other Link： https://link.springer.com/article/10.1007/s11214-022-00885-4/fulltext.html

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2022GL098105

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2022GL098105

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2021JA030008

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Geophysical Research: Space Physics  

Substorm activity, due to either substorms or substorm-associated disturbances, shows two preferred recurrent intervals for periodic events. While the longer recurrent interval of ∼3 hr has been extensively studied, albeit still without a conclusive answer on its source mechanism, the source of the shorter 1-hr recurrent interval has not been addressed at all. Here, we provide statistics on 102 events, each event being a sequence of substorm-like dipolarizations with recurrent intervals of between 30 min and 2 hr, using Kp, Dst, AE, and solar wind speed, and relate them to statistics of other response modes, such as steady magnetospheric convections, sawtooth events, and isolated substorms. It becomes apparent that the 1-hr events are a nonstorm phenomenon, which clearly distinguishes them from sawtooth events, which occur during larger Dst and Kp values. While there are statistical similarities to isolated substorms in terms of solar wind driving and Kp, our events also encompass pseudo-breakups and smaller substorm activations. Overall, the range of AE values for the 102 events is very broad, suggesting small to large auroral activities during the events. Furthermore, we provide a comparison to published global MHD simulations with kinetic corrections, which suggests that the source mechanism of these events could be internally driven, quasi-periodic reconnection in the magnetotail. Finally, we advocate the treatment of this global phenomenon as a separate response mode of the magnetosphere-ionosphere system.

DOI： 10.1029/2021JA030064

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2021JA029677

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JA029677

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Earth, Planets and Space  

DOI： 10.1186/s40623-021-01569-z

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2021JA029786

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JA029786

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2020JA029073

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA029073

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2021JA029168

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2020JA028790

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020JA028790

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Geophysical Research: Space Physics  

Dayside cusp aurorae are created from particles precipitating into the cusp, and ionospheric convection is driven by solar wind electric fields. In this study, we coordinated the observations obtained from the all-sky camera on Svalbard, the Super Dual Auroral Radar Network, SuperMAG magnetometer data, and far ultraviolet imagers on board the Defense Meteorological Satellite Program satellites for the event January 4, 2014 to examine the morphology of aurorae and the patterns of ionospheric convection for radial interplanetary magnetic field (IMF). During the event, a poleward-moving auroral form and antisunward ionospheric convection were observed when the IMF turned into almost purely radial. Moreover, both types of antisunward and sunward convection were simultaneously observed near the footprint of the cusp at different times during the radial IMF period. The antisunward convection and sunward convection are typically an indicator of the dayside reconnection for the southward IMF and the lobe reconnection for the northward IMF, respectively. All those observations support the concept of low-latitude dayside and high-latitude lobe reconnection for the radial IMF. This study further shows that the coexistence of the two types of reconnection for radial IMF, resulting in an interplay of repetitive antisunward and sunward convection.

DOI： 10.1029/2019JA027664

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2020GL091384

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020GL091384

Language：English   Publishing type：Research paper (scientific journal)  

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Earth, Planets and Space  

Prompt penetration electric fields were detected during the intense geomagnetic storm on 22 June, 2015 by the HF Doppler sounders at middle and low latitudes; Prague (Czech Republic), Iitate, Oarai, Onna (Japan), and Zhongli (Republic of China). The storm was initiated by the storm sudden commencement (SC) at 1833 UT, immediately followed by the main phase with the minimum SYM-H of − 139 nT at 2017 UT. The convection electric field was found to be up to 5.1 mV/m at low latitudes, which is enough to cause ionospheric disturbances such as the enhancement of the total electron content, equatorial ionization anomaly and so on. The electric fields of the SC and main phase are westward in the night (0330 MLT), while eastward in the evening (1930 MLT). The main phase electric field intensified the eastward equatorial electrojets (EEJ) at Huancayo in South America on the dayside (1330 MLT) and the westward EEJ on the nightside at Guam in the western Pacific (0330 MLT) and Tirunelveli in India (00 MLT). The eastward direction of the evening-time electric field agrees with a feature of the evening anomaly of the penetration electric field, while the intensity is suppressed because of the sunlit condition in the evening in the summer solstice. The sudden northward turning of the interplanetary magnetic field (IMF) caused a transition from the main phase to the recovery phase, when the EEJ turned into the counterelectrojet (CEJ). It was found that the HF Doppler sounders detected an impulsive eastward overshielding electric field at 2005 UT on the nightside during the northward IMF-driven CEJ with the duration of 17 min and intensity of 8.0 mV/m at Iitate (0500 MLT) and 11.8 mV/m at Prague (2130 MLT). The impulsive overshielding electric field was accompanied by the anti-sunward plasma flows at middle latitude (50°–60° GML) equatorward of the dusk sunward convection as observed with SuperDARN, which signifies intensification of the Region-2 field-aligned currents at the flow reversal. Substorm signatures are identified with intensified westward auroral electrojet and magnetic dipolarization as detected by the geosynchronous orbit satellite, ETS-VIII. Consequently, the stormtime substorm may provide strong overshielding electric fields to the middle latitude ionosphere on the nightside.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-020-01196-0

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1186/s40623-020-01182-6

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Other Link： http://link.springer.com/article/10.1186/s40623-020-01182-6/fulltext.html

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>
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 (<italic>M</italic>) up to 3–4 amu at <italic>L</italic> = 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 <italic>M</italic>. 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 <italic>M</italic> 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.

DOI： 10.1186/s40623-020-01235-w

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Other Link： http://link.springer.com/article/10.1186/s40623-020-01235-w/fulltext.html

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2019JA027310

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We have statistically studied the average structure and properties of the plasma sheet in the Earth's magnetotail at radial distances between R similar to 8 and 32 RE on the basis of ion, electron, and magnetic field data from the Geotail spacecraft. Here we discuss the transition between the inner plasma sheet and the outer plasma sheet near the equatorial plane. The ion and electron number densities, pressures, and energy fluxes at high energies, as well as the magnetic field, generally decrease with increasing radial distance from the Earth. The characteristics of the flux changes are reflected in the radial pressure gradients. If the transition between the inner and outer plasma sheet is determined by the radial pressure gradient change, the ion and electron transitions are located, on average, at R similar to 11 to 13 RE and R similar to 11 to 17 RE, respectively, at the midnight meridian. It is possible that the ion transition is located earthward of the electron transition. Furthermore, we have estimated the electric field, the electric and diamagnetic drift velocities, and the.. parameter (the square root of the ratio of the minimum curvature radius of the magnetic field line to the maximum gyroradius), which differ between the ion inner and outer

DOI： 10.1029/2019JA027561

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CiNii Research

Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2019JA027686

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Language：English   Publishing type：Research paper (scientific journal)  

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/s40623-019-0990-1

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/s40623-019-1053-3

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Journal of Geophysical Research: Space Physics  

Two wave packets of second harmonic poloidal Pc 4 waves with a wave frequency of ~7 mHz were detected by Van Allen Probe A at a radial distance of ~5.8 RE and magnetic local time of 13 hr near the magnetic equator, where plasmaspheric refilling was in progress. Proton butterfly distributions with energy dispersions were also measured at the same time; the proton fluxes at 10–30 keV oscillated with the same frequency as the Pc 4 waves. Using the ion sounding technique, we find that the Pc 4 waves propagated eastward with an azimuthal wave number (m number) of ~220 and ~260 for each wave packet, respectively. Such eastward propagating high-m (m > 100) waves were seldom reported in previous studies. The condition of drift-bounce resonance is well satisfied for the estimated m numbers in both events. Proton phase space density was also examined to understand the wave excitation mechanism. We obtained temporal variations of the energy and radial gradient of the proton phase space density and find that temporal intensification of the radial gradient can generate the two wave packets. The cold electron density around the spacecraft apogee was >100 cm−3 in the present events, and hence the eigenfrequency of the Pc 4 waves became lower. This causes the increase of the m number which satisfies the resonance condition of drift-bounce resonance for 10–30 keV protons and meets the condition for destabilization due to gyrokinetic effect.

DOI： 10.1029/2019JA027158

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union ({AGU})  

DOI： 10.1029/2019GL084379

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2019ja027312

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2019JA027312

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2019JA026682

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2019SW002168

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CiNii Research

Language：English   Publishing type：Research paper (bulletin of university, research institution)  

DOI： 10.20637/JAXA-RR-18-005E/0004

Language：English   Publishing type：Research paper (bulletin of university, research institution)  

DOI： 10.20637/JAXA-RR-18-005E/0005

Language：English   Publishing type：Research paper (scientific journal)  

© 2019, The Author(s). With the advent of the Heliophysics/Geospace System Observatory (H/GSO), a complement of multi-spacecraft missions and ground-based observatories to study the space environment, data retrieval, analysis, and visualization of space physics data can be daunting. The Space Physics Environment Data Analysis System (SPEDAS), a grass-roots software development platform (www.spedas.org), is now officially supported by NASA Heliophysics as part of its data environment infrastructure. It serves more than a dozen space missions and ground observatories and can integrate the full complement of past and upcoming space physics missions with minimal resources, following clear, simple, and well-proven guidelines. Free, modular and configurable to the needs of individual missions, it works in both command-line (ideal for experienced users) and Graphical User Interface (GUI) mode (reducing the learning curve for first-time users). Both options have “crib-sheets,” user-command sequences in ASCII format that can facilitate record-and-repeat actions, especially for complex operations and plotting. Crib-sheets enhance scientific interactions, as users can move rapidly and accurately from exchanges of technical information on data processing to efficient discussions regarding data interpretation and science. SPEDAS can readily query and ingest all International Solar Terrestrial Physics (ISTP)-compatible products from the Space Physics Data Facility (SPDF), enabling access to a vast collection of historic and current mission data. The planned incorporation of Heliophysics Application Programmer’s Interface (HAPI) standards will facilitate data ingestion from distributed datasets that adhere to these standards. Although SPEDAS is currently Interactive Data Language (IDL)-based (and interfaces to Java-based tools such as Autoplot), efforts are under-way to expand it further to work with python (first as an interface tool and potentially even receiving an under-the-hood replacement). We review the SPEDAS development history, goals, and current implementation. We explain its “modes of use” with examples geared for users and outline its technical implementation and requirements with software developers in mind. We also describe SPEDAS personnel and software management, interfaces with other organizations, resources and support structure available to the community, and future development plans.

DOI： 10.1007/s11214-018-0576-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

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

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PubMed

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Berlin Heidelberg  

© 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

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Language：Japanese   Publishing type：Research paper (scientific journal)  

©2018. American Geophysical Union. All Rights Reserved. We investigate the spatial distributions and composition of contributing energies, which we term an energy range that makes the dominant contribution to energy density, in the inner magnetosphere during the main phase of magnetic storms. We analyze data from the medium-energy particle experiments-ion mass analyzer (MEP-i) on board the Arase satellite during six magnetic storms in year 2017 with the SYM-H minimum smaller than −50 nT. The results show that the inner part (L ≤ 5) is dominated by relatively low-energy ions adiabatically transported from the plasma sheet by enhanced convection. The contributing energies are higher for O+ than for H+ at higher L shells (L > 5), particularly during the storms driven by coronal mass ejections. The results provide in situ evidence of the contribution from mass-dependent/selective acceleration processes associated with substorm activity to the buildup of the outer part the ring current.

DOI： 10.1029/2018GL080047

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Language：English   Publishing type：Research paper (scientific journal)  

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

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2018JA025865

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.2322/tastj.16.687

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union ({AGU})  

DOI： 10.1029/2018GL080122

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2018GL079103

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The Arase spacecraft is capable of observing ultralow frequency waves in the inner magnetosphere at intermediate magnetic latitudes, a region sparsely covered by previous spacecraft missions. We report a series of impulsively excited fundamental toroidal mode standing Alfven waves in the midnight sector observed by Arase outside the plasmasphere at magnetic latitudes 13-24 degrees. The wave onsets are concurrent with Pi2 onsets detected by the Van Allen Probe B spacecraft at the magnetic equator in the duskside plasmasphere and by ground magnetometers at low latitudes. The duration of each toroidal wave packet is similar to 20 min, which is much longer than that of the corresponding Pi2 wave packet. The toroidal waves cannot be the source of high-latitude Pi2 waves because they were not detected on the ground near the magnetic field footprint of Arase. Overall, the toroidal wave event lasted more than 2 hr and allowed us to use the wave frequency to estimate the plasma mass density at L = 6.1-8.3. The mass density (in amu/cm(3)) is higher than the electron density (cm(-3)) by a factor of similar to 6, which implies that 17-33% of the ions were O+.Plain Language Summary Magnetospheric substorms begin with a sudden change of the configuration of the magnetotail, which excites magnetohydrodynamic waves with periods of the order of 100 s. The Arase satellite detects these waves at distances less than similar to 6 Earth radii and magnetic latitudes as high as similar to 40 degrees, a region sparsely covered by previous spacecraft missions. On 24 May 2017, Arase, located off the equator, observed substorm-associated resonant oscillation of magnetic field lines (standing waves) continuously for 2 hr, whereas a Van Allen Probes spacecraft, located on the equator, observed short-lived oscillations with different frequencies. This multipoint observation indicates that different wave modes are excited from a common impulsive source located in the near-Earth magnetotail. The waves at Arase allow us for the first time to continuously monitor the plasma mass density in the nightside magnetosphere during substorms.

DOI： 10.1029/2018GL078731

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CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union ({AGU})  

DOI： 10.1029/2018GL078825

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union ({AGU})  

DOI： 10.1029/2018GL078961

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union ({AGU})  

We present observational evidence of drift resonance between westward propagating odd mode standing ultralow frequency waves and energetic protons. Compressional similar to 13 mHz (Pc4 band) waves and proton flux oscillations at >50 keV were detected at similar to 03 hr magnetic local time by the Arase satellite on 15 April 2017. The azimuthal wave number (m number) is estimated to be similar to-50 from ground observations, while the theory of drift resonance gives m similar to-49 for odd mode waves and similar to 110-keV protons, providing evidence that the drift resonance indeed took place in this event. We also found a steep earthward gradient of proton phase space density, which can quantitatively explain the wave excitation. The observed waves show typical features of giant pulsations (Pgs), regarding local time, m number, and flux oscillations. This study, therefore, has great implications to the field line mode structure and excitation mechanism of Pgs.

DOI： 10.1029/2018GL078293

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CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Nature  

Substorm onset has originally been defined as a longitudinally extended sudden auroral brightening (Akasofu initial brightening: AIB) followed a few minutes later by an auroral poleward expansion in ground-based all-sky images (ASIs). In contrast, such clearly marked two-stage development has not been evident in satellite-based global images (GIs). Instead, substorm onsets have been identified as localized sudden brightenings that expand immediately poleward. To resolve these differences, optical substorm onset signatures in GIs and ASIs are compared in this study for a substorm that occurred on December 7, 1999. For this substorm, the Polar satellite ultraviolet global imager was operated with a fixed-filter (170 nm) mode, enabling a higher time resolution (37 s) than usual to resolve the possible two-stage development. These data were compared with 20-s resolution green-line (557.7 nm) ASIs at Muonio in Finland. The ASIs revealed the AIB at 2124:50 UT and the subsequent poleward expansion at 2127:50 UT, whereas the GIs revealed only an onset brightening that started at 2127:49 UT. Thus, the onset in the GIs was delayed relative to the AIB and in fact agreed with the poleward expansion in the ASIs. The fact that the AIB was not evident in the GIs may be attributed to the limited spatial resolution of GIs for thin auroral arc brightenings. The implications of these results for the definition of substorm onset are discussed herein.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

A poloidal Pc4 wave and proton flux oscillations are observed in the inner magnetosphere on the dayside near the magnetic equator by the Van Allen Probes spacecraft on 2 March 2014. The flux oscillations are observed in the energy range of 67.0 to 268.8 keV with the same frequency of the poloidal Pc4 wave. We find pitch angle and energy dispersion in the phase difference between the poloidal magnetic field and the proton flux oscillations, which are features of drift-bounce resonance. We estimate the resonance energy to be ~120 keV for pitch angle (α) of 30° or 150° and 170–180 keV for α = 50° or 130°. To examine the direction of energy flow between protons and the wave, we calculate the sign of the gradient of proton phase space density (df/dW) on both the inbound and outbound legs of the orbit. We find the gradient to be outward on both legs, which means that energy is transferred from the protons to the wave. During the poloidal Pc4 wave event, the Dst* index shows a measurable increase of ~6.7 nT. We estimate the total energy loss of the ring current from the recovery of the Dst* index and from the variation of proton flux by the drift-bounce resonance. We suggest that energy transfer from the ring current protons to the poloidal Pc4 wave via the drift-bounce resonance contributes to up to ~85% of the increase of the Dst* index.

DOI： 10.1029/2017JA025087

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.23919/URSI-AT-RASC.2018.8471561

Scopus

Other Link： http://orcid.org/0000-0002-8160-3553

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Nature  

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 format, archived on a data server, and made available to the science community. Magnetic field observation by the MGF will significantly improve our knowledge of the growth and decay of radiation belts and ring currents, as well as the dynamics of geospace storms.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Nature  

Understanding of underlying mechanisms of drastic variations of the near-Earth space (geospace) is one of the current focuses of the magnetospheric physics. The science target of the geospace research project Exploration of energization and Radiation in Geospace (ERG) is to understand the geospace variations with a focus on the relativistic electron acceleration and loss processes. In order to achieve the goal, the ERG project consists of the three parts: the Arase (ERG) satellite, ground-based observations, and theory/modeling/integrated studies. The role of theory/modeling/integrated studies part is to promote relevant theoretical and simulation studies as well as integrated data analysis to combine different kinds of observations and modeling. Here we provide technical reports on simulation and empirical models related to the ERG project together with their roles in the integrated studies of dynamic geospace variations. The simulation and empirical models covered include the radial diffusion model of the radiation belt electrons, GEMSIS-RB and RBW models, CIMI model with global MHD simulation REPPU, GEMSIS-RC model, plasmasphere thermosphere model, self-consistent wave–particle interaction simulations (electron hybrid code and ion hybrid code), the ionospheric electric potential (GEMSIS-POT) model, and SuperDARN electric field models with data assimilation. ERG (Arase) science center tools to support integrated studies with various kinds of data are also briefly introduced.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGEROPEN  

Understanding of underlying mechanisms of drastic variations of the near-Earth space (geospace) is one of the current focuses of the magnetospheric physics. The science target of the geospace research project Exploration of energization and Radiation in Geospace (ERG) is to understand the geospace variations with a focus on the relativistic electron acceleration and loss processes. In order to achieve the goal, the ERG project consists of the three parts: the Arase (ERG) satellite, ground-based observations, and theory/modeling/integrated studies. The role of theory/modeling/integrated studies part is to promote relevant theoretical and simulation studies as well as integrated data analysis to combine different kinds of observations and modeling. Here we provide technical reports on simulation and empirical models related to the ERG project together with their roles in the integrated studies of dynamic geospace variations. The simulation and empirical models covered include the radial diffusion model of the radiation belt electrons, GEMSIS-RB and RBW models, CIMI model with global MHD simulation REPPU, GEMSIS-RC model, plasmasphere thermosphere model, self-consistent wave-particle interaction simulations (electron hybrid code and ion hybrid code), the ionospheric electric potential (GEMSIS-POT) model, and SuperDARN electric field models with data assimilation. ERG (Arase) science center tools to support integrated studies with various kinds of data are also briefly introduced.

DOI： 10.1186/s40623-018-0785-9

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We examine the spatiotemporal variations of the energy density and the energy spectral evolution of energetic ions in the inner magnetosphere during the main phase of the 17 March 2015 storm, using data from the RBSPICE and EMFISIS instruments onboard Van Allen Probes. The storm developed in response to two southward IMF intervals separated by about 3 h. In contrast to two steps seen in the Dst/SYM-H index, the ring current ion population evolved in three steps: the first subphase was apparently caused by the earlier southward IMF, and the subsequent subphases occurred during the later southward IMF period. Ion energy ranges that contribute to the ring current differed between the three subphases. We suggest that the spectral evolution resulted from the penetration of different plasma sheet populations. The ring current buildup during the first subphase was caused by the penetration of a relatively low-energy population that had existed in the plasma sheet during a prolonged prestorm northward IMF interval. The deeper penetration of the lower-energy population was responsible for the second subphase. The third subphase, where the storm was unexpectedly intensified to a Dst/SYM-H level of &lt
−200 nT, was caused by the penetration of a hot, dense plasma sheet population. We attribute the hot, dense population to the entry of hot, dense solar wind into the plasma sheet and/or ion heating/acceleration in the near-Earth plasma sheet associated with magnetotail activity such as reconnection and dipolarization.

DOI： 10.1002/2017JA024462

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Scopus

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

Long-lasting second-harmonic poloidal standing Alfvén waves (P2 waves) were observed by the twin Van Allen Probes (Radiation Belt Storm Probes, or RBSP) spacecraft in the noon sector of the plasmasphere, when the spacecraft were close to the magnetic equator and had a small azimuthal separation. Oscillations of proton fluxes at the wave frequency (∼10 mHz) were also observed in the energy (W) range 50–300 keV. Using the unique RBSP orbital configuration, we determined the phase delay of magnetic field perturbations between the spacecraft with a 2nπ ambiguity. We then used finite gyroradius effects seen in the proton flux oscillations to remove the ambiguity and found that the waves were propagating westward with an azimuthal wave number (m) of ∼−200. The phase of the proton flux oscillations relative to the radial component of the wave magnetic field progresses with W, crossing 0 (northward moving protons) or 180° (southward moving protons) at W ∼ 120 keV. This feature is explained by drift-bounce resonance (mωd ∼ ωb) of ∼120 keV protons with the waves, where ωd and ωb are the proton drift and bounce frequencies. At lower energies, the proton phase space density (FH+) exhibits a bump-on-tail structure with ∂FH+/∂W &gt
0 occurring in the 1–10 keV energy range. This FH+ is unstable and can excite P2 waves through bounce resonance (ω ∼ ωb), where ω is the wave frequency.

DOI： 10.1002/2017JA024869

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Berlin Heidelberg  

Substorm onset has originally been defined as a longitudinally extended sudden auroral brightening (Akasofu initial brightening: AIB) followed a few minutes later by an auroral poleward expansion in ground-based all-sky images (ASIs). In contrast, such clearly marked two-stage development has not been evident in satellite-based global images (GIs). Instead, substorm onsets have been identified as localized sudden brightenings that expand immediately poleward. To resolve these differences, optical substorm onset signatures in GIs and ASIs are compared in this study for a substorm that occurred on December 7, 1999. For this substorm, the Polar satellite ultraviolet global imager was operated with a fixed-filter (170 nm) mode, enabling a higher time resolution (37 s) than usual to resolve the possible two-stage development. These data were compared with 20-s resolution green-line (557.7 nm) ASIs at Muonio in Finland. The ASIs revealed the AIB at 2124:50 UT and the subsequent poleward expansion at 2127:50 UT, whereas the GIs revealed only an onset brightening that started at 2127:49 UT. Thus, the onset in the GIs was delayed relative to the AIB and in fact agreed with the poleward expansion in the ASIs. The fact that the AIB was not evident in the GIs may be attributed to the limited spatial resolution of GIs for thin auroral arc brightenings. The implications of these results for the definition of substorm onset are discussed herein.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-018-0843-3

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Scopus

PubMed

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

The plasmas (electrons and ions) in the inner magnetosphere have wide energy ranges from electron volts to mega-electron volts (MeV). These plasmas rotate around the Earth longitudinally due to the gradient and curvature of the geomagnetic field and by the co-rotation motion with timescales from several tens of hours to less than 10 min. They interact with plasma waves at frequencies of mHz to kHz mainly in the equatorial plane of the magnetosphere, obtain energies up to MeV, and are lost into the ionosphere. In order to provide the global distribution and quantitative evaluation of the dynamical variation of these plasmas and waves in the inner magnetosphere, the PWING project (study of dynamical variation of particles and waves in the inner magnetosphere using ground-based network observations, http://www.isee.nagoya-u.ac.jp/dimr/PWING/) has been carried out since April 2016. This paper describes the stations and instrumentation of the PWING project. We operate all-sky airglow/aurora imagers, 64-Hz sampling induction magnetometers, 40-kHz sampling loop antennas, and 64-Hz sampling riometers at eight stations at subauroral latitudes (similar to 60 degrees geomagnetic latitude) in the northern hemisphere, as well as 100-Hz sampling EMCCD cameras at three stations. These stations are distributed longitudinally in Canada, Iceland, Finland, Russia, and Alaska to obtain the longitudinal distribution of plasmas and waves in the inner magnetosphere. This PWING longitudinal network has been developed as a part of the ERG (Arase)-ground coordinated observation network. The ERG (Arase) satellite was launched on December 20, 2016, and has been in full operation since March 2017. We will combine these ground network observations with the ERG (Arase) satellite and global modeling studies. These comprehensive datasets will contribute to the investigation of dynamical variation of particles and waves in the inner magnetosphere, which is one of the most important research topics in recent space physics, and the outcome of our research will improve safe and secure use of geospace around the Earth.

DOI： 10.1186/s40623-017-0745-9

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

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

DOI： 10.1002/2017JA024342

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Scopus

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The ionospheric Alfven resonator (IAR) is found in a dynamic power spectrum of the geomagnetic field variations as spectral resonance structures in the frequency range of 0.1-10Hz. To date, observations of IAR at low latitude have only been made by a few studies employing a single ground observatory and are not sufficient enough to reveal its general characteristics. We analyze data measured with an induction magnetometer installed at Muroto, Japan (24.40 degrees geomagnetic latitude) for the period from 28 December 2013 to 13 August 2016. A statistical analysis reveals that the occurrence probability of the low-latitude IAR is (1) dominant during nighttime with a gradual increase from dusk to midnight and a broad maximum at 00-05 LT followed by a sudden decrease at dawn, (2) slightly higher during May through September, and (3) independent of the Kp index for Kp &lt;= 5. We also find that (4) the frequency separation between the harmonics (f) of the low-latitude IAR is 0.1-0.5Hz with a peak at 0.2-0.275Hz. These observational results are compared with the model calculation results along with previous studies about possible energy sources. It is found that the global thunderstorm activity is a likely source for IAR at Muroto; that is, it produces electromagnetic disturbances that are converted into the Alfven waves and that are then trapped in the ionospheric cavity bounded by the conductive E layer and a steep gradient of Alfven velocity above the F-2 layer to form the resonant Alfven waves.

DOI： 10.1002/2017JA024204

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SpringerOpen  

The Geotail satellite has been operating for almost two solar cycles (~23 years) since its launch in July 1992. The satellite carries the energetic particle and ion composition (EPIC) instrument that measures the energetic ion flux (9.4–212 keV/e) and enables the investigation of long-term variations of the ion composition in the plasma sheet for solar cycles 22–24. From the statistical analysis of the EPIC data, we find that (1) the plasma ion mass (M) is approximately 1.1 amu during the solar minimum, whereas it increases to 1.5–2.7 amu during the solar maximum
(2) the increases in M seem to have two components: a raising of the baseline levels (~1.5 amu) and a large transient enhancement (~1.8–2.7 amu)
(3) the baseline level change of M correlates well with the Mg II index, which is a good proxy for the solar extreme ultraviolet (EUV) or far ultraviolet (FUV) irradiance
and (4) the large transient enhancement of M is caused by strong magnetic storms. We also study the long-term variations of substorm occurrences in 1992–2015 that are evaluated with the number of Pi2 pulsations detected at the Kakioka observatory. The results suggest no clear correlation between the substorm occurrence and the Mg II index. Instead, when the substorms are classified into externally triggered events and non-triggered events, the number of the non-triggered events and the Mg II index are negatively correlated. We interpret these results that the increase in the solar EUV/FUV radiation enhances the supply of ionospheric ions (He+ and O+ ions) into the plasma sheet to increase M, and the large M may suppress spontaneous plasma instabilities initiating substorms and decrease the number of the non-triggered substorms. The present analysis using the unprecedentedly long-term dataset covering ~23 years provides additional observational evidence that heavy ions work to prevent the occurrence of substorms.

DOI： 10.1186/s40562-015-0033-0

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigate the magnetic field dipolarization in the inner magnetosphere and its associated ion flux variations, using the magnetic field and energetic ion flux data acquired by the Van Allen Probes. From a study of 74 events that appeared at L = 4.5-6.6 between 1 October 2012 and 31 October 2013, we reveal the following characteristics of the dipolarization in the inner magnetosphere: (1) its time scale is approximately 5 min; (2) it is accompanied by strong magnetic fluctuations that have a dominant frequency close to the O+ gyrofrequency; (3) ion fluxes at 20-50 keV are simultaneously enhanced with larger magnitudes for O+ than for H+; (4) after a few minutes of the dipolarization, the flux enhancement at 0.1-5 keV appears with a clear energy-dispersion signature only for O+; and (5) the energy-dispersed O+ flux enhancement appears in directions parallel or antiparallel to the magnetic field. From these characteristics, we discuss possible mechanisms that can provide selective acceleration to O+ ions at &gt; 20 keV. We conclude that O+ ions at L = 5.4-6.6 undergo nonadiabatic local acceleration caused by oscillating electric field associated with the magnetic fluctuations and/or adiabatic convective transport from the plasma sheet to the inner magnetosphere by the impulsive electric field. At L = 4.5-5.4, however, only the former acceleration is plausible. We also conclude that the field-aligned energy-dispersed O+ ions at 0.1-5 keV originate from the ionosphere and are extracted nearly simultaneously to the onset of the dipolarization.

DOI： 10.1002/2016JA022549

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We examine enhancements of energetic (&gt;50 keV) oxygen ions observed by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on board the Van Allen Probes spacecraft in the inner magnetosphere (L similar to 6) at 22-23 h magnetic local time (MLT) during an injection event of the 6 June 2013 storm. Simultaneous observations by two Van Allen Probes spacecraft located close together (similar to 0.5 R-E) indicate that particle injections occurred in the premidnight sector (&lt;similar to 24 h MLT). We also examine the evolution of the proton and oxygen energy spectra at L similar to 6 during the injection event. The spectral slope did not significantly change during the storm. The oxygen phase space density (PSD) was shifted toward higher PSD in a wide range of the first adiabatic invariant. The spectral evolution manifests the characteristics of adiabatic acceleration and density increase of oxygen ions. Warm (0.1-10 keV) oxygen measured by the Helium, Oxygen, Proton, and Electron (HOPE) instrument was enhanced prior to the storm mostly in magnetic field-aligned directions. The most reasonable scenario of this event is that warm oxygen ions that preexisted in the inner magnetosphere were picked up and adiabatically transported and accelerated by spatially localized, temporarily impulsive electric fields.

DOI： 10.1002/2016JA022384

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Scopus

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

This study addresses the ion composition in the magnetosphere before the satellite era. We estimate the plasma ion mass for 1956-1975 from the period of low-latitude Pi2 pulsations found in digital geomagnetic field data that are created from analog magnetograms at Kakioka. The period of investigation covers most of solar cycle 19 and the whole solar cycle 20. To consider long-term variation, the moving average of the estimated plasma ion mass is calculated with a 1 year time window. We find that 1 year moving average of the plasma ion mass changed by a factor of similar to 2 during one solar cycle (i.e., between similar to 1.1 amu and similar to 2.4 amu for solar cycle 19 and between similar to 1.1 amu and similar to 2.0 amu for solar cycle 20). The correlation coefficient between the 1 year moving average of the plasma ion mass and that of the F-10.7 index is 0.86. This result supports the idea that in long-term variation, solar radiation increases the density and the temperature of O+ ions in the ionosphere, leads to the outflow of O+ ions, and contributes to the enhancement of the plasma ion mass in the nightside magnetosphere. The digital data created from analog magnetograms provide an important clue to know the space environment in old days and are advantageous for studies of the space weather and space climate.

DOI： 10.1002/2016JA022510

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report subpacket structures found in electromagnetic ion cyclotron (EMIC) rising tone emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes. We investigate three typical cases in detail. The first case shows a continuous single rising tone with four obvious subpackets, and the second case is characterized by a patchy emission with multiple subpackets triggered in a broadband frequency. The third case looks like a smooth rising tone without any obvious subpacket in the fast Fourier transform spectrum, while its amplitude contains small peaks with increasing frequencies. The degree of polarization of each subpacket is generally higher than 0.8 with a left-handed polarization, and the wave direction of the subpackets is typically field aligned. We show that the time evolution of the observed frequency and amplitude can be reproduced consistently by nonlinear growth theory. We also compare the observed time span of each subpacket structure with the theoretical trapping time for second-order cyclotron resonance. They are consistent, indicating that an individual subpacket is generated through a nonlinear wave growth process which excites an element in accordance with the theoretically predicted optimum amplitude.

DOI： 10.1002/2014JA020764

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CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDP SCIENCES S A  

The aim of this study is to provide a complete scope of a magnetic sudden impulse (SI) event along its way through interplanetary space and the magnetosphere until its arrival to the ground. In our case, we chose the event of 19th November 2007 because of the availability of enough well-located spacecraft at that moment for our purpose. We have used a 16 spacecraft data set. We calculated the mass flux variation and the change in magnetic field components across the discontinuity. Thus, we identified the solar wind discontinuity as a shock. We also calculated the orientation of the solar wind shock front. Then, we examined the effects of the shock front propagation in detail. With this large data set, we obtained a global view of the travelling wave front and identified the effects of the compressional wave front. Thus, we determined in detail the shock front passing through the different parts of the magnetosphere. We described the compressional effects in the bow shock, the magnetosheath, and the magnetopause and we depicted the propagation inside the inner magnetosphere. Moreover, we used an extensive data set from magnetic observatories on the ground and so we studied the global distribution of the SI waveform. Finally, the comparison of the observational facts with those derived from the theoretical model showed a good consistency. On the basis of the waveforms and polarizations of this SI, we determined the location in latitude where ionospheric currents (ICs) changed their sense. And also, we related polarization at ground to polarization measured by GOES spacecraft.

DOI： 10.1051/swsc/2015016

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Recent results by the Van Allen Probes mission showed that the occurrence of energetic ion injections inside geosynchronous orbit could be very frequent throughout the main phase of a geomagnetic storm. Understanding, therefore, the formation and evolution of energetic particle injections is critical in order to quantify their effect in the inner magnetosphere. We present a case study of a substorm event that occurred during a weak storm (Dst similar to-40nT) on 14 July 2013. Van Allen Probe B, inside geosynchronous orbit, observed two energetic proton injections within 10min, with different dipolarization signatures and duration. The first one is a dispersionless, short-timescale injection pulse accompanied by a sharp dipolarization signature, while the second one is a dispersed, longer-timescale injection pulse accompanied by a gradual dipolarization signature. We combined ground magnetometer data from various stations and in situ particle and magnetic field data from multiple satellites in the inner magnetosphere and near-Earth plasma sheet to determine the spatial extent of these injections, their temporal evolution, and their effects in the inner magnetosphere. Our results indicate that there are different spatial and temporal scales at which injections can occur in the inner magnetosphere and depict the necessity of multipoint observations of both particle and magnetic field data in order to determine these scales.

DOI： 10.1002/2014JA020872

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

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.

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We study the formation process of an oxygen torus during the 12-15 November 2012 magnetic storm, using the magnetic field and plasma wave data obtained by Van Allen Probes. We estimate the local plasma mass density ((L)) and the local electron number density (n(eL)) from the resonant frequencies of standing Alfven waves and the upper hybrid resonance band. The average ion mass (M) can be calculated by M approximate to (L)/n(eL) under the assumption of quasi-neutrality of plasma. During the storm recovery phase, both Probe A and Probe B observe the oxygen torus at L = 3.0-4.0 and L = 3.7-4.5, respectively, on the morning side. The oxygen torus has M = 4.5-8 amu and extends around the plasmapause that is identified at L approximate to 3.2-3.9. We find that during the initial phase, M is 4-7 amu throughout the plasma trough and remains at approximate to 1 amu in the plasmasphere, implying that ionospheric O+ ions are supplied into the inner magnetosphere already in the initial phase of the magnetic storm. Numerical calculation under a decrease of the convection electric field reveals that some of thermal O+ ions distributed throughout the plasma trough are trapped within the expanded plasmasphere, whereas some of them drift around the plasmapause on the dawnside. This creates the oxygen torus spreading near the plasmapause, which is consistent with the Van Allen Probes observations. We conclude that the oxygen torus identified in this study favors the formation scenario of supplying O+ in the inner magnetosphere during the initial phase and subsequent drift during the recovery phase.

DOI： 10.1002/2014JA020593

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigate the responses of different ion species (H+, He+, He++, and O+) to fast plasma flows and local dipolarization in the plasma sheet in terms of energy density. We use energetic (9-210keV) ion composition measurements made by the Geotail satellite at r=10 similar to 31 R-E. The results are summarized as follows: (1) whereas the O+-to-H+ ratio decreases with earthward flow velocity, it increases with tailward flow velocity with steeper V-x dependence for perpendicular flows than for parallel flows; (2) for fast earthward flows, the energy density of each ion species increases without any clear preference for heavy ions; (3) for fast tailward flows, the ion energy density initially increases, then it decreases to below the preceding levels except for O+; (4) the O+-to-H+ ratio does not increase through local dipolarization irrespective of dipolarization amplitude, background B-z, X distance, and V-x; (5) in general, the H+ and He++ ions behave similarly. Result (1) can be attributed to radial transport in the presence of the earthward gradient of the background O+-to-H+ ratio. Results (2) and (4) suggest that ion energization at local dipolarization is not mass dependent in the energy range of our interest because the ions are not magnetized irrespective of species. Result (3) can be attributed to the thinning of the plasma sheet and the preferable field-aligned escape of the H+ ions on the tailward side of the reconnection site. Result (5) suggests that the solar wind is the primary source of the high-energy H+ ions.

DOI： 10.1002/2014JA020517

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

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

DOI： 10.1186/s40623-014-0155-1

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The Inter-university Upper atmosphere Global Observation NETwork (IUGONET) project focuses on handling ground-based observational data of the upper atmosphere. To this end, the project members have been developing a data analysis software package which is based on Interactive Data Language (IDL). Filling the spatial gaps in observational data requires the use of numerical models. In this paper, we discuss an IDL software package for global ionospheric conductivity by integration of 3rd party numerical models. The model can be used to create further derived models. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cpc.2014.08.011

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Information and Knowledge  

In 2008, the International Council for Science (ICSU) established the World Data System (WDS) to ensure the long-term stewardship and provision of quality-assessed data and data services to the international science community and other stakeholders. One of the objectives of the WDS is to support and make progress of interdisciplinary and cross-over studies data analyses covering a variety of research fields. In 2010, the NICT Science Cloud, which is one of the projects of the National Institute of Information and Communications Technology (NICT), has initiated a sub-project to realize the WDS's concept of the interdisciplinary data activity. In this paper, we demonstrate potential capability of our Web application for interdisciplinary data analysis, which is now operated on the NICT Science Cloud. In the initial stage of the system development, we constructed a basic concept and an initial design of the Web application, taking into account our long-term experiences in data-oriented activities under the WDC (World Data Center) system of the ICSU, which was an ICSU's data program operated from 1957 to 2008, preceded the WDS. Based on our design, we have implemented the Web application for data plots on the basis of the HTML5 and the Ajax technologies. Data plots can be previewed on the Web application with higher usability and better handleability than those of traditional data-plot tools developed for multiple-datasets. We finally discuss feasibility and potential usefulness of the Web application for future interdisciplinary data–analysis activities of the WDS.

DOI： 10.2964/jsik_2014_030

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We study magnetic fluctuations embedded in dipolarizations in the inner magnetosphere (a geocentric distance of &lt;= 6.6 R-E) and their associated ion flux changes, using the Engineering Test Satellite VIII and Active Magnetospheric Particle Tracer Explorers/CCE satellites. We select seven events of dipolarization that occur during the main phase of magnetic storms having a minimum value of the Dst index less than -40 nT. It is found that (1) all of the dipolarization events are accompanied by strong magnetic fluctuations with the major frequency close to the local O+ gyrofrequency; (2) the magnetic fluctuations appear with significant amplitude in the component nearly parallel to the local magnetic field; (3) the strong flux enhancement is seen in the energy range of 1-10 keV only for O+ ions. In terms of frequency and dominant components of the magnetic fluctuations, they are considered to be excited by the drift-driven electromagnetic ion cyclotron (EMIC) instability that is recently identified with the linear theory. We perform particle tracing for H+ and O+ ions in the electromagnetic fields modeled by the linear dispersion relation of the drift-driven EMIC instability. Results show that the O+ ions are accelerated to the energy range of 0.5-5 keV and undergo a significant modification of the spectral shape, while the H+ ions have no clear change of spectral shape, being consistent with the observations. We therefore suggest that the electromagnetic fluctuations associated with the dipolarizations can accelerate O+ ions locally and nonadiabatically in the inner magnetosphere. This selective acceleration of O+ ions may play a role in enhancing the O+ energy density in the storm time ring current.

DOI： 10.1002/2014JA019806

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We report observations of electromagnetic ion cyclotron (EMIC) triggered emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes outside the plasmasphere. Although these phenomena have recently received much attention because of the possibility of strong interaction with particles, only a few events of EMIC triggered emissions have been reported near the equatorial plasmapause. We performed a survey of the THEMIS probe data and found various types of emissions mainly on the dayside at radial distances of 6-10 R-E. We study three distinctive events in detail. The first is a typical event with an obvious rising tone emission in the afternoon sector. The emissions in the second event are simultaneously excited in different frequency bands separated by the cyclotron frequency of helium ions. In the third event, which occurred near local noon, rising tone emissions were excited in an extended region near the equator where the field-aligned B gradient was much reduced because of compression of the magnetosphere by the solar wind. We compare these events with the nonlinear wave growth theory developed by Omura et al. (2010). In all events, it is found that the observed relationship between the amplitudes and frequencies of the emissions are in good agreement with the theory.

DOI： 10.1002/2013JA019146

Web of Science

Scopus

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Information and Knowledge  

After the Internet is widely used, new concept and information technology haves shown up; semantic web and linked open data (LOD). These technologies enable information on the Internet machine readable. In many science fields, it is pointed out that the semantic web will play an important role for the interdisciplinary research works. However, there have been few ideas to be ever proposed as a methodology or roadmap to the interdisciplinary science using semantic web. Herein we present a concept of professional knowledge and academic knowledge following collective knowledge proposed as a Web 2.0. Based on the concept, we design an application for interdisciplinary science.

DOI： 10.2964/jsik_2014_007

CiNii Research

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

To investigate the physical mechanism responsible for substorm triggering, we performed a superposed-epoch analysis using plasma and magnetic-field data from THEMIS probes. Substorm onset timing was determined based on auroral breakups detected by all-sky imagers at the THEMIS ground-based observatories. We found earthward flows associated with north-south auroral streamers during the substorm growth phase. At around X = -12 Earth radii (R-E), the northward magnetic field and its elevation angle decreased markedly approximately 4 min before substorm onset. Moreover, a northward magnetic-field increase associated with pre-onset earthward flows was found at around X = -17 R-E. This variation indicates that local dipolarization occurs. Interestingly, in the region earthwards of X = -18 R-E, earthward flows in the central plasma sheet (CPS) reduced significantly approximately 3 min before substorm onset, which was followed by a weakening of dawn-/duskward plasma-sheet boundary-layer flows (subject to a 1 min time lag). Subsequently, approximately 1 min before substorm onset, earthward flows in the CPS were enhanced again and at the onset, tailward flows started at around X = -20 R-E. Following substorm onset, an increase in the northward magnetic field caused by dipolarization was found in the near-Earth region. Synthesizing these results, we confirm our previous results based on GEOTAIL data, which implied that significant variations start earlier than both current disruption and magnetic reconnection, at approximately 4 min before substorm onset roughly halfway between the two regions of interest; i.e. in the catapult current sheet.

DOI： 10.5194/angeo-32-99-2014

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：CODATA  

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

DOI： 10.2481/dsj.IFPDA-07

Scopus

CiNii Research

Language：English   Publishing type：Part of collection (book)   Publisher：American Geophysical Union  

We investigated the spatial and temporal properties of the 9-210 keV/e ion composition in the near-Earth plasma sheet (at geocentric distance, r, of 8-15 RE), using energetic ion flux data acquired by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument onboard the Geotail spacecraft. We analyzed data covering 8.3 years to find the MLT-r distribution of the H+, He+, and O+ energy densities (as well as the He+/H+ and O+/H+ energy density ratios) as a function of the SYM-H index. We obtained the following results: (1) The energy density increases as SYM-H decreases
(2) The energy density change depends on ion mass: the largest change occurs in O+ and the smallest change occurs in H+
(3) The energy density shows a dawn-dusk asymmetry when SYM-H &lt
- 50 nT, being larger on the duskside than on the dawnside
and (4) The He+/H+ and O+/H+ energy density ratios in the plasma sheet are similar to those in the ring current. From these results we conclude that ionospheric He+ and O+ ions are transported to the plasma sheet, accelerated there by the dawn-to-dusk electric field in a mass-dependent manner (heavier ions gain more energy than lighter ions), and injected into the ring current region. Both the ion flux from the ionosphere and the energy gain in the plasma sheet become large when the geomagnetic disturbance becomes intense.

DOI： 10.1029/155GM08

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Saturation of the auroral electrojet (AE) index during storm times is a phenomenon that has been known but not fully understood. To address this puzzle, here we correlate the (provisional) AE index with net field-aligned current (Net-dB) index, which is a data product derived with magnetic field measurements from the Iridium satellite constellation (Anderson et al., 2010), with an assumption that AE is largely measuring convection driven by the field-aligned currents represented by the Net-dB index. The Net-dB index has a time resolution of similar to 45 mm and is currently available from February 18, 1999, to May 31, 2008. It is found that, for the entire data period, there is a good linear correlation (r=0.74) between the AE index, when averaging over the Net-dB index time grids, and the Net-dB index, suggesting that statistically similar to 55% of the ionospheric Hall currents correlate with large scale convection as reflected in the field-aligned currents. It is also found that the correlation decreases during storm times, with a clear decreasing trend toward a more negative Sym-H. For large storms (Sym-H &lt; - 100 nT), the correlation becomes weak (r &lt; 0.4). Two correlation peaks are identified: a major peak (r=0.73) occurring at Sym-H &gt; 30 nT and a secondary peak (r=0.66) at Sym-H similar to -50 to -30 nT. A further study shows that the variations of the AE-Net-dB correlation are associated with the location of the field-aligned currents into and out of the ionosphere relative to the ground magnetometer stations, as expected often but not demonstrated in the past. (C) 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2012.09.017

Web of Science

J-GLOBAL

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Three rare occasions are introduced, where the excitation of vertical acoustic resonance between the ground and the ionosphere, and the resultant generation of a field-aligned current, just after earthquakes are observationally confirmed. In the case of two inland earthquakes, barometric observations very close to the epicenters (i.e., only 30 km apart) were available, and they showed a sharp spectral peak which appeared within one hour after the origin time and lasted a few hours. The observed periods of the spectral peaks around 260 seconds are close to the period of the theoretically-expected fundamental mode of the resonance. On the other hand, magnetic observations on the ground showed a dominant period at 220-230 seconds which corresponds to the first overtone among theoretically-expected major resonance peaks. In the third case, i.e., during the 2010 Chile earthquake, a long-period magnetic oscillation in the east-west direction, which has two major resonance periods at 265 and 190-195 seconds, was observed on the night-side magnetic dip equator in Peru, where the distance is more than 2600 km from the epicenter, under a very quiet geomagnetic condition. The oscillation was interpreted as the effect of field-aligned current generated through a dynamo process in the ionosphere over the epicenter caused by the resonance.

DOI： 10.5047/eps.2013.02.002

Web of Science

J-GLOBAL

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Geomagnetic field data with high time resolution (typically 1 s) have recently become more commonly acquired by ground stations. Such high time resolution data enable identifying Pi2 pulsations which have periods of 40-150 s and irregular (damped) waveforms. It is well-known that pulsations of this type are clearly observed at mid-and low-latitude ground stations on the nightside at substorm onset. Therefore, with 1-s data from multiple stations distributed in longitude around the Earth's circumference, substorm onset can be regularly monitored. In the present study we propose a new substorm index, the Wp index (Wave and planetary), which reflects Pi2 wave power at low-latitude, using geomagnetic field data from 11 ground stations. We compare the Wp index with the AE and ASY indices as well as the electron flux and magnetic field data at geosynchronous altitudes for 11 March 2010. We find that significant enhancements of the Wp index mostly coincide with those of the other data. Thus the Wp index can be considered a good indicator of substorm onset. The Wp index, other geomagnetic indices, and geosynchronous satellite data are plotted in a stack for quick and easy search of substorm onset. The stack plots and digital data of the Wp index are available at the Web site (http://s-cubed.info) for public use. These products would be useful to investigate and understand space weather events, because substorms cause injection of intense fluxes of energetic electrons into the inner magnetosphere and potentially have deleterious impacts on satellites by inducing surface charging. Citation: Nose, M., et al. (2012), Wp index: A new substorm index derived from high-resolution geomagnetic field data at low latitude, Space Weather, 10, S08002, doi:10.1029/2012SW000785.

DOI： 10.1029/2012SW000785

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

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

Other Link： http://orcid.org/0000-0002-8160-3553

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Our previous study showed that the energy release associated with substorm expansion onsets is the most significant midway between the magnetic reconnection and initial dipolarization regions (-12 &gt
X &gt
-18 R E in the premidnight sector) in the magnetotail. In the present paper, we have statistically studied the substorm-associated energy balance and transport in the magnetotail, focusing on the midway region as well as the near-Earth initial dipolarization region (X &gt
∼-12 RE). We find that a large amount of energy is released in the midway region, associated with onsets, but only a part of this energy is transported to the near-Earth initial dipolarization region mainly in the form of the thermal flux and the wave Poynting flux. It is possible that the energy carried by fast earthward flows and waves from the reconnection region is not sufficient for the thermal energy increase and the outward transported energy in the initial dipolarization region, although the magnetic flux may be sufficiently carried. A considerably large amount of the magnetic energy comes from the lobes in the form of the Poynting flux also in the initial dipolarization region. © 2012. American Geophysical Union. All Rights Reserved.

DOI： 10.1029/2012JA017925

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

Using energetic (9-212 keV/e) ion flux data obtained by the Geotail spacecraft, Ono et al. (2009) statistically examined changes in the energy density of H+ and O+ ions in the near-Earth plasma sheet during substorm-associated dipolarization. They found that ions are nonadiabatically accelerated by the electric field induced by the magnetic field fluctuations whose frequencies are close to their gyrofrequencies. The present paper revisits this result and finds it still holds. Copyright 2012 by the American Geophysical Union.

DOI： 10.1029/2012JA017518

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The present study statistically examines the characteristics of energetic ions in the plasma sheet using the Geotail/Energetic Particle and Ion Composition data. An emphasis is placed on the O+ ions, and the characteristics of the H+ ions are used as references. The following is a summary of the results. (1) The average O+ energy is lower during solar maximum and higher during solar minimum. A similar tendency is also found for the average H+ energy, but only for geomagnetically active times; (2) The O+-to-H+ ratios of number and energy densities are several times higher during solar maximum than during solar minimum; (3) The average H+ and O+ energies and the O+-to-H+ ratios of number and energy densities all increase with geomagnetic activity. The differences among different solar phases not only persist but also increase with increasing geomagnetic activity; (4) Whereas the average H+ energy increases toward Earth, the average O+ energy decreases toward Earth. The average energy increases toward dusk for both the H+ and O+ ions; (5) The O+-to-H+ ratios of number and energy densities increase toward Earth during all solar phases, but most clearly during solar maximum. These results suggest that the solar illumination enhances the ionospheric outflow more effectively with increasing geomagnetic activity and that a significant portion of the O+ ions is transported directly from the ionosphere to the near-Earth region rather than through the distant tail.

DOI： 10.1029/2011JA016532

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We statistically studied the spatial characteristics of Pi2 pulsations using magnetic field observations in the magnetosphere at the equatorial-orbiting Active Magnetospheric Particle Tracer Explorers (AMPTE)/Charge Composition Explorer (CCE) and the polar-orbiting Dynamics Explorer 1 (DE 1) satellites and on the ground at the low-latitude station Kakioka (KAK, L = 1.23). We defined Pi2 pulsations from wavelet analysis of KAK data covering August 1984 to January 1989 and obtained 849 nightside (20:00-04:00 LT) events. For each KAK Pi2 event, we evaluated the coherence between the ground H component and the magnetospheric radial (B(perpendicular to R)), azimuthal (B(perpendicular to A)), and compressional (B(//)) components at the frequency of the ground Pi2. High-coherence ground-satellite Pi2 events were found most often in B(//), with many of them evident at both spacecraft when CCE was located near the equator (magnetic latitude &lt;30 degrees) and inside of the plasmapause estimated from an empirical formula while DE 1 was located at high latitude (&gt;30 degrees) and outside of the estimated plasmapause. In these simultaneous CCE and DE 1 events, the B(//) amplitude was larger at CCE; in addition, the H-B(//) cross phase was similar to 0 degrees at CCE but was similar to 180 degrees at DE 1. We show detailed analysis of one such event as well as two other events that occurred when CCE was outside of the estimated plasmapause and exhibited different coherence and cross-phase properties at this spacecraft. Overall, the two-satellite observations provide additional evidence of plasmaspheric virtual resonance, which was previously suggested from studies using single satellites.

DOI： 10.1029/2010JA016199

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Using data from the high-energy neutral atom (HENA) imager onboard the IMAGE satellite, we examined the relation between the SYM-H index and the ring current energy during a storm main phase. The energy range of the energetic neutral atom (ENA) flux data used here is 16-120 keV for hydrogen and &lt;180 keV for oxygen. From the data for the period 2000-2002, we selected 24 storm main phase events during which the IMAGE satellite was located at a geomagnetic latitude of &gt;= 45 degrees and a geocentric distance of &gt;= 6 R-E. According to the Dessler-Parker-Sckopke (DPS) equation, the ring current energy is expected to increase as the SYM-H index decreases. When the ENA energy flux is superimposed as a function of the SYM-H index for all 24 events, their overall correlation is negative; that is, the relation between the ENA energy flux and the SYM-H index is generally consistent with the DPS equation. However, an analysis of individual events showed only 10 events (42%) in which the ENA energy flux was negatively correlated with the SYM-H index (negative correlation events). There were 10 events showing no clear correlation between the ENA energy flux and the SYM-H index (no correlation events) and 4 events which contradicted the DPS equation (positive correlation events). In the superimposed plot, we noted that a smooth curve can be drawn for an upper limit of the data distribution, and data from the no correlation or positive correlation events create downward branches in the distribution. These observational results are not explained by the conventional DPS equation but by the "generalized" DPS equation, which includes a term representing energy stored in the stretched magnetic field. We can reasonably presume that the stretched magnetic field prevents energetic particles from being injected into the ring current. From the generalized DPS equation, we conclude that the total (kinetic and magnetic) energy stored in the stretched field and ring current loss mechanisms are important for understanding the relation between the ground magnetic field variation and ring current energy variation.

DOI： 10.1029/2010JA015799

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

DOI： 10.1029/2010JA016389

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2010JA016389

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We examine the temporal evolution of spatially averaged energy spectra of ring current ions (27-120 keV for H+, 79-264 keV for O+) during the recovery phase of three magnetic storms. The energy spectra are derived from energetic neutral atom (ENA) imaging provided by the High Energy Neutral Atom (HENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite
ENAs generated within HENA lines of sight passing by close to the Earth are excluded in order to distinguish high-altitude emissions from low-altitude emissions. We confirm, by performing a simple model of ENA production, that the retrieved spectra are a good indication of those at 3.5 &lt
L &lt
5. The spectral evolution during the early and rapid recovery phase of the examined intense and weak storms shows that higher-energy H + were lost more than lower-energy H+. The loss rate of ∼100 keV H+ is higher than or comparable to that of ∼100 keV O+. During the early and rapid recovery of the examined moderate storm, the O+ loss rate was higher for lower energies. These results suggest that charge exchange makes a small contribution to the ring current rapid recovery. We conclude that ion precipitation through pitch angle scattering (probably due to interactions with electromagnetic ion cyclotron waves) plays a significant role in ion loss during the early and rapid recovery phase. For the late and slow recovery phase, on the other hand, the spectral evolution is consistent with that expected from charge exchange. The retrieved phase space density of &lt
60 keV H+ decreased down to around the prestorm level when Dst did not yet recover to the prestorm value, while the phase space density of &gt
60 keV H+ was higher than the prestorm level. It is likely that the contribution from &gt
60 keV H+ to the ring current intensity (and H+ pressure) increases as a storm recovers. We suggest that the ring current intensity is dominated by higher-energy (&gt
60 keV) H+ during the late recovery phase. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA015628

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Michel Menvielle, Toshihiko Iyemori, Aurélie Marchaudon, Masahito Nosé, 2011, &#039;Geomagnetic Indices&#039;, &lt;i&gt;Geomagnetic Observations and Models&lt;/i&gt;, pp. 183-228

DOI： 10.1007/978-90-481-9858-0_8

Web of Science

Other Link： http://orcid.org/0000-0002-2789-3588

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

Using the magnetic field and plasma wave data obtained by the Combined Release and Radiation Effects Satellite (CRRES), we search for enhancements of O+ ion density in the deep inner magnetosphere known as "the oxygen torus". We examine 4 events on the dayside in which toroidal standing Alfvén waves appear clearly. From the frequency of the toroidal waves, the magnetospheric local mass density (ρL) is estimated by solving the MHD wave equation for realistic models of the magnetic field and the field line mass distribution. We also estimate the local electron number density (neL) from the plasma wave spectrograms by identifying narrow-band emission at the upper-hybrid resonance frequency. Assuming the quasi-neutral condition of plasma, we infer the local average ion mass (M L) by ρL/neL. It is found that M L is approximately 3 amu in the plasma trough, while it shows an enhancement of &gt
7 amu at L ∼ 4.5-6.5 that is close to the plasmapause at L ∼ 3.5-6.0. This indicates an existence of the oxygen torus in the vicinity of the plasmapause. The oxygen torus is found preferentially during the storm recovery phase. We interpret that these features of the oxygen torus (i.e., close relations with the plasmapause and the storm recovery phase) reflect its generation mechanism
that is, the ionospheric temperature is enhanced by heat conduction from high altitudes in the limited L range where the plasmasphere, because of its inflation during the recovery phase, encounters the ring current, and then the ionosphere has a larger scale height and supplies O+ ions to the inner magnetosphere, resulting in the oxygen torus. We also discuss the contribution of the oxygen torus to the formation of the O+-rich ring current. It is proposed that the O+-rich ring current is formed in a recurrent process, in which the oxygen torus, the plasmasphere, and the ring current interact with each other. Copyright © 2011 by the American Geophysical Union.

DOI： 10.1029/2011JA016651

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

The superstorm on 20-21 November 2003 was the largest geomagnetic storm in solar cycle 23 as measured by Dst, which attained a minimum value of -422 nT. We have simulated this storm to understand how particles originating from the solar wind and ionosphere get access to the magnetosphere and how the subsequent transport and energization processes contribute to the buildup of the ring current. The global electromagnetic configuration and the solar wind H + distribution are specified by the Lyon-Fedder-Mobarry (LFM) magnetohydrodynamics model. The outflow of H+ and O+ ions from the ionosphere are also considered. Their trajectories in the magnetosphere are followed by a test-particle code. The particle distributions at the inner plasma sheet established by the LFM model and test-particle calculations are then used as boundary conditions for a ring current model. Our simulations reproduce the rapid decrease of Dst during the storm main phase and the fast initial phase of recovery. Shielding in the inner magnetosphere is established at early main phase. This shielding field lasts several hours and then breaks down at late main phase. At the peak of the storm, strong penetration of ions earthward to L shell of 1.5 is revealed in the simulation. It is surprising that O+ is significant but not the dominant species in the ring current in our calculation for this major storm. It is very likely that substorm effects are not well represented in the models and O+ energization is underestimated. Ring current simulation with O+ energy density at the boundary set comparable to Geotail observations produces excellent agreement with the observed symH. As expected in superstorms, ring current O+ is the dominant species over H+ during the main to midrecovery phase of the storm. © Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JA015720

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Sarah J. Reay, Donald C. Herzog, Sobhana Alex, Evgeny P. Kharin, Susan McLean, Masahito Nosé, Natalia A. Sergeyeva, 2011, &#039;Magnetic Observatory Data and Metadata: Types and Availability&#039;, &lt;i&gt;Geomagnetic Observations and Models&lt;/i&gt;, pp. 149-181

DOI： 10.1007/978-90-481-9858-0_7

Web of Science

Other Link： http://orcid.org/0000-0002-2789-3588

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS GESELLSCHAFT MBH  

The Auroral Electrojet (AE) indices, which are composed of four indices (AU, AL, AE, and AO), are calculated from the geomagnetic field data obtained at 12 geomagnetic observatories that are located in geomagnetic latitude (GMLAT) of 61.7 degrees-70 degrees. The indices have been widely used to study magnetic activity in the auroral zone. In the present study, we examine magnetic local time (MLT) dependence of geomagnetic field variations contributing to the AU and AL indices. We use 1-min geomagnetic field data obtained in 2003. It is found that both AU and AL indices have two ranges of MLT (AU: 15:00-22:00 MLT, similar to 06:00 MLT; and AL: similar to 02:00 MLT, 09:00-12:00 MLT) contributing to the index during quiet periods and one MLT range (AU: 15:00-20:00 MLT, and AL: 00:00-06:00 MLT) during disturbed periods. These results are interpreted in terms of various ionospheric current systems, such as, S(q)(p), S(q), and DP2.

DOI： 10.5194/angeo-29-673-2011

Web of Science

J-GLOBAL

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We examine the temporal variations of energy spectra of energetic neutral atoms (ENAs) detected by the High Energy Neutral Atom imager (HENA) onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite during three substorms on 21 October 2001 and three substorms on 19 March 2002; the substorms occurred during the storm main phase. The ENA energy used in the present study ranges from 10 to 198 keV for hydrogen and from 29 to 222 keV for oxygen. We use ENA data obtained from two independent areas of a HENA image, for which HENA lines of sight pass through the inner magnetosphere (similar to-6 RE &lt; X &lt; similar to-3 R-E around the magnetic equator) and the outer magnetosphere (X &lt; similar to-6 R-E around the magnetic equator). The analyses of the selected ENA data yield the following results: (1) The oxygen ENA flux displays 20-30 min bursts during all substorms, while the hydrogen ENA flux did not increase or less significantly increased than the oxygen flux. (2) The temporal evolution of energy spectra is mass dependent for all examined substorms. (3) For two of the substorms, the oxygen flux ratio between before and after a substorm increases with increasing energy, indicating the hardening of an O+ energy spectrum. (4) The flux ratio for the inner image area is comparable to or higher than that in the outer area. The results confirm that nonadiabatic acceleration with regard to the first adiabatic invariant did occur in the near-Earth magnetotail (X &gt; -8 R-E). Although the influence of the nonadiabatic acceleration appears in the inner magnetosphere (similar to-6 R-E &lt; X &lt; similar to-3 R-E) as well as the outer magnetosphere (similar to-8 R-E &lt; X &lt; similar to-6 R-E), it is not clear from the present results whether O+ energization in the inner magnetosphere is due to nonadiabatic acceleration in the inner magnetosphere or adiabatic transport of O+ nonadiabatically accelerated in the outer magnetosphere. It is likely, for at least the two substorms, that the nonadiabatic acceleration makes a more significant contribution to O+ energization than increase of the O+ density in the plasma sheet.

DOI： 10.1029/2010JA015889

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

A new generation mechanism of a butterfly pitch angle distribution (PAD) of energetic ions is demonstrated by a test particle simulation in the stretched magnetic field. The Polar satellite detected some events of the butterfly PADs of the energetic protons (greater than or similar to 80 keV) in the outer ring current region around midnight near the equatorial plane. They were observed at relatively disturbed time in the inner magnetosphere and the ring current appeared to be developed; that is, magnitude of the magnetic field at the Polar satellite was highly depressed and the adiabaticity of the protons were expected to be violated. To reproduce the butterfly PAD, we perform a test particle simulation in which the first adiabatic invariant mu can be changed. When the radius of the field line becomes almost comparable to the Larmor radius of a proton, it suffers significant scattering of pitch angle (a) due to change of mu. This mu-scattering process causes reform of the PADs. Owing to cumulative mu-scattering, the flux of the protons ends to have a peak at alpha similar to 40 degrees (or 140 degrees) and collapses at alpha similar to 0 degrees (or 180 degrees) and 90 degrees, which is a butterfly PAD. The reproduced PAD resembles the butterfly PADs observed by the Polar satellite. We suggest that the nonadiabatic effect is important for not only the generation mechanisms of the butterfly PAD but also the development of the storm-time ring current.

DOI： 10.1029/2010JA015281

Web of Science

Scopus

Language：English   Publishing type：Part of collection (book)   Publisher：World Scientific Publishing Co.  

It has been expected that the atmospheric perturbation causes the acoustic resonance between the lower atmosphereand the thermosphere. The main resonance frequencies theoretically reported are around 3.7, 4.4-4.6 and 5.1-5.3mHz. We calculated mean power spectral densities and the occurrence rates of spectral peaks for the pressure perturbations and geomagnetic variations and found that the local maxima appeared in both data sets around the resonance frequencies. These maximaappear most clearly in August to October for pressure and in every season for geomagnetic fields. These results confirm the existence of the acoustic resonance coupling between the lower atmosphere and the ionosphere and suggest that the geomagnetic oscillations are frequently caused not only through magnetospheric plasma processes but also by lower atmospheric disturbances.

DOI： 10.1142/9789812838209_0010

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We have studied plasma (ion) pressure changes that occurred in association with the dipolarization in the near-Earth plasma sheet around substorm onsets. Using Geotail data, we have performed a superposed epoch analysis in addition to detailed examinations of two individual cases with special emphasis on the contribution of high-energy particles to the plasma pressure. It is found that, unlike previously reported results, the plasma pressure does increase in association with the initial dipolarization at X &gt
∼-12 RE and -2 &lt
Y &lt
6 RE, with the increase largely due to high-energy particles. Outside the initial dipolarization region, particularly tailward and duskward of this region, the plasma pressure begins to decrease owing to the magnetic reconnection before onset or before the dipolarization region reaches there. At later times, the plasma pressure tends to increase there, related to the expanding dipolarization region, but the contribution of high-energy particles is not very large. These observations suggest the following. The rarefaction wave scenario proposed in the current disruption model is questionable. The radial and azimuthal pressure gradients may strengthen between the initial dipolarization and outside regions, possibly resulting in stronger braking of fast earthward flows and changes in field-aligned currents. The characteristics of the dipolarization may differ between the initial dipolarization and tailward regions, which would be possibly reflected in the auroral features. Furthermore, we have examined the specific entropy and the ion β. The specific entropy increases in the plasma sheet in the dipolarization region as well as in the midtail region in conjunction with substorm onsets, suggesting from the ideal MHD point of view that the substorm processes are nonadiabatic. The ion β is found to peak at the magnetic equator in the initial dipolarization region around dipolarization onsets. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2010JA015608

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

There are two schools of thought regarding the excitation mechanism of low-latitude Pi2 pulsations. One is the cavity mode resonance, which is established by fast mode waves that are emitted at substorm onset and form the standing oscillation within the plasmasphere. The other is the bursty bulk flow (BBF)-driven process, in which periodical BBFs traveling from the magnetotail cause pressure pulses in the inner magnetosphere, generating Pi2 pulsations. This study intends to examine which of these two excitation mechanisms is more plausible. A working assumption is that in the case of the cavity mode resonance the wave period of Pi2 pulsations depends on the size and the plasma mass density of the plasmasphere, while in the case of the BBF-driven process the wave period is controlled by the BBF period and is independent of the plasmaspheric parameters. We investigated long-term variation in wave period of Pi2 pulsations observed at the Kakioka observatory for the period from March 1983 to October 2009. Multiple correlation analysis revealed that the Pi2 period is negatively correlated with the ∑Kp index and positively correlated with ion mass in the near-Earth plasma sheet or the F10.7 index. The ∑Kp index is considered as a proxy for the size of the plasmasphere, and the plasma sheet ion mass or the F10.7 index can be considered as a proxy for the mass density of the plasmasphere, indicating that the Pi2 period is proportional to both the size and mass density of the plasmasphere. This result strongly supports the plasmaspheric cavity mode resonance as an excitation mechanism of low-latitude Pi2 pulsations. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2009JA015205

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

Multipoint observations of a dayside Pc4 pulsation event provide evidence of fast mode waves trapped in the plasmasphere (plasmaspheric cavity mode or virtual resonance). Time History of Events and Macroscale Interactions during Substorms (THEMIS)-A, the primary source of data for the present study, was moving outward near noon and detected poloidal oscillations, characterized by the azimuthal electric field component Ey and the radial and compressional magnetic field components Bx and Bz. The structure of the plasmasphere was constructed from the mass density radial profile estimated from the frequency of toroidal standing Alfvn waves observed at this spacecraft. The outer edge of the plasmapause (the maximum of the equatorial Alfvn velocity VAeq) was located at L ∼ 7, and the minimum of VAeq was located at L ∼ 4, forming a potential well structure required for mode trapping. Relative to the ground magnetic pulsations observed in the H component at a low-latitude station (L = 1.5), the E y component exhibited a broad amplitude maximum around L ∼ 3.5 and maintained a nearly constant phase from L = 2 to L = 5. In contrast, the Bz component exhibited an amplitude minimum and switched its phase by 180° at L = 3.8. This radial mode structure is consistent with theoretical models of mode trapping. Also, the Ey and Bz components oscillated 90 out of phase, as is expected for radially standing waves. Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2010JA015956

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We studied magnetic field dipolarization and associated ion acceleration in the deep inner magnetosphere, using magnetic field data obtained by the magnetometer on board the Mission Demonstration Satellite 1 (MDS-1) and the energetic neutral atom (ENA) flux data obtained by the high-energy neutral analyzer imager on board the Imager for Magnetopause-to-Aurora Global Exploration satellite. Because the MDS-1 satellite has a geosynchronous transfer orbit, we could survey magnetic field variations at L = 3.0-6.5. Analyzing data in the period from February to July 2002, we found that (1) dipolarization can be detected over a wide range of L (i.e., L = 3.5-6.5, which is far inside the geosynchronous altitude)
(2) when the MDS-1 satellite was located close to auroral breakup longitude, the occurrence probability of dipolarization was about 50% just inside the geosynchronous altitude and about 16% at L = 3.5-5.0, suggesting that dipolarization in the deep inner magnetosphere is not unusual
(3) magnetic storms were developing whenever dipolarization was found at L = 3.5-5.0
(4) dipolarization was accompanied by magnetic field fluctuations having a characteristic timescale of 3-5 s, which is comparable to the local gyroperiod of O+ ions
and (5) after dipolarization, the oxygen ENA flux in the nightside ring current region was predominantly enhanced by a factor of 2-5 and stayed at an enhanced level for more than 1 h, while clear enhancement was scarcely seen in the hydrogen ENA flux. From these results, we conjectured a scenario for the generation of an O+-rich ring current, in which preexisting thermal O+ ions in the outer plasmasphere (i.e., an oxygen torus known from satellite observations) experience local and nonadiabatic acceleration by magnetic field fluctuations that accompany dipolarization in the deep inner magnetosphere (L = 3.5-5.0). Copyright 2010 by the American Geophysical Union.

DOI： 10.1029/2010JA015321

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We examined long-term variations of ion composition in the plasma sheet, using energetic (9.4-212.1 keV/e) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. EPIC/STICS observations are available from 17 October 1992 for more than 16 years, covering the declining phase of solar cycle 22, all of solar cycle 23, and the early phase of solar cycle 24. This unprecedented long-term data set revealed that (1) the He+/H+ and O+/H+ flux ratios in the plasma sheet were dependent on the F10.7 index; (2) the F10.7 index dependence is stronger for O+/H+ than He+/H+; (3) the O+/H+ flux ratio is also weakly correlated with the Sigma Kp index; and (4) the He2+/H+ flux ratio in the plasma sheet appeared to show no long-term trend. From these results, we derived empirical equations related to plasma sheet ion composition and the F10.7 index and estimated that the average plasma ion mass changes from similar to 1.1 amu during solar minimum to similar to 2.8 amu during solar maximum. In such a case, the Alfven velocity during solar maximum decreases to similar to 60% of the solar minimum value. Thus, physical processes in the plasma sheet are considered to be much different between solar minimum and solar maximum. We also compared long-term variation of the plasma sheet ion composition with that of the substorm occurrence rate, which is evaluated by the number of Pi2 pulsations. No correlation or negative correlation was found between them. This result contradicts the O+ triggering substorm model, in which heavy ions in the plasma sheet increase the growth rate of the linear ion tearing mode and play an important role in localization and initiation of substorms. In contrast, O+ ions in the plasma sheet may prevent occurrence of substorms.

DOI： 10.1029/2009JA014203

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigate variations of ion flux over the ionosphere and in the plasma sheet when storm time substorms are initiated, using simultaneous observations of neutral atoms in the energy range of up to a few keV measured by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration ( IMAGE) satellite, outflowing ion flux of &lt;1 keV measured by the ion electrostatic analyzer (IESA) on board the Fast Auroral SnapshoT (FAST) satellite, and energetic (9-210 keV/e) ion flux measured by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite. We examined three storm intervals during which the IMAGE or FAST satellite was in a suitable location to observe ionospheric ion outflow and the Geotail satellite was in the plasma sheet on the nightside. The neutral atom flux observed by IMAGE/LENA in the first interval and outflowing ion flux observed by FAST/IESA in the second and third intervals indicate that storm time substorms can cause increases of low-energy ion flux over the ionosphere by a factor of 3-50 with time delay of less than several minutes. In the plasma sheet, the flux ratio of O+/H+ is rapidly enhanced at the storm time substorms and then increased gradually or stayed at a constant level in a time scale of similar to 1 h, suggesting a mass-dependent acceleration of ions at local dipolarization and a subsequent additional supply of O+ ions to the plasma sheet which have been extracted from the ionosphere at the substorms. These coordinated observations revealed that substorms have both an immediate effect and a delayed effect (i.e., two-step effect) on the ion composition in the plasma sheet.

DOI： 10.1029/2009JA014048

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We statistically examine changes in the composition of two different ion species, proton and oxygen ions, in the near-Earth plasma sheet (X = -16 R(E) similar to -6 R(E)) during substorm-associated dipolarization. We use 10 years of energetic (9-212 keV/e) ion data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particles and ion composition (EPIC) instrument on board the Geotail spacecraft. The results are as follows: (1) Although the percentage increase in the energy density of O(+) ions before and after a dipolarization exceeds that of H(+) ions in the low-energy range (9-36 keV/e), this property is not evident in the high-energy range (56-212 keV/e); (2) the energy spectrum of H(+) and that of O(+) become harder after dipolarization in almost all events; and (3) in some events the energy spectrum of O+ becomes harder than that of H(+) as reported by previous studies, and, importantly, in other events, the spectrum of H(+) becomes harder than that of O(+). In order to investigate what mechanism causes these observational results, we focus on magnetic field fluctuations during dipolarization. It is found that the increase of the spectrum slope is positively correlated with the power of waves whose frequencies are close to the gyrofrequency of H(+) or O(+), respectively (the correlation coefficient is 0.48 for H(+) and 0.68 for O(+)). In conclusion, ions are nonadiabatically accelerated by the electric field induced by the magnetic field fluctuations whose frequencies are close to their gyrofrequencies.

DOI： 10.1029/2008JA013918

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigate variations of ion flux over the ionosphere and in the plasma sheet when storm time substorms are initiated, using simultaneous observations of neutral atoms in the energy range of up to a few keV measured by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration ( IMAGE) satellite, outflowing ion flux of &lt;1 keV measured by the ion electrostatic analyzer (IESA) on board the Fast Auroral SnapshoT (FAST) satellite, and energetic (9-210 keV/e) ion flux measured by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite. We examined three storm intervals during which the IMAGE or FAST satellite was in a suitable location to observe ionospheric ion outflow and the Geotail satellite was in the plasma sheet on the nightside. The neutral atom flux observed by IMAGE/LENA in the first interval and outflowing ion flux observed by FAST/IESA in the second and third intervals indicate that storm time substorms can cause increases of low-energy ion flux over the ionosphere by a factor of 3-50 with time delay of less than several minutes. In the plasma sheet, the flux ratio of O+/H+ is rapidly enhanced at the storm time substorms and then increased gradually or stayed at a constant level in a time scale of similar to 1 h, suggesting a mass-dependent acceleration of ions at local dipolarization and a subsequent additional supply of O+ ions to the plasma sheet which have been extracted from the ionosphere at the substorms. These coordinated observations revealed that substorms have both an immediate effect and a delayed effect (i.e., two-step effect) on the ion composition in the plasma sheet.

DOI： 10.1029/2009JA014048

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Ubiquity Press Ltd  

Ultra-low frequency acoustic waves called "acoustic gravity waves" or "infrasounds" are theoretically expected to resonate between the ground and the thermosphere. This resonance is a very important phenomenon causing the coupling of the solid Earth, neutral atmosphere, and ionospheric plasma. This acoustic resonance, however, has not been confirmed by direct observations. In this study, atmospheric perturbations on the ground and ionospheric disturbances were observed and compared with each other to confirm the existence of resonance. Atmospheric perturbations were observed with a barometer, and ionospheric disturbances were observed using the HF Doppler method. An end point of resonance is in the ionosphere, where conductivity is high and the dynamo effect occurs. Thus, geomagnetic observation is also useful, so the geomagnetic data were compared with other data. Power spectral density was calculated and averaged for each month. Peaks appeared at the theoretically expected resonance frequencies in the pressure and HF Doppler data. The frequencies of the peaks varied with the seasons. This is probably because the vertical temperature profile of the atmosphere varies with the seasons, as does the reflection height of infrasounds. These results indicate that acoustic resonance occurs frequently.

DOI： 10.2481/dsj.8.S68

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We have obtained a state-of-the-art picture of substorm-associated evolution of the near-Earth magnetotail and the inner magnetosphere for understanding the substorm triggering mechanism. We performed superposed epoch analysis of Geotail, Polar, and GOES data with 2-min resolution, utilizing a total of 3787 substorms for each of which auroral breakup was determined from Polar UVI or IMAGE FUV auroral imager data. The decrease of the north-south magnetic field associated with plasmoids and the initial total pressure decrease suggest that the magnetic reconnection first occurs in the premidnight tail, on average, at X ∼ -16 to -20 RE at least 2 min before auroral onset. The magnetic reconnection site is located near the tailward edge of a region of considerably taillike magnetic field lines and intense cross-tail current, which extends from X ∼ -5 to -20 RE in the premidnight sector. Then the plasmoid substantially evolves tailward of X ∼ -20 R E immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X ∼ -7 to -10 RE 2 min before onset. The dipolarization region then expands tailward as well as in the dawn-dusk directions and earthward. We find that the total pressure generally enhances in association with the dipolarization, with the contribution of high-energy particles. Also, energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2008JA013225

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：COPERNICUS PUBLICATIONS  

We investigated the temporal and spatial development of the near-Earth magnetotail during substorms based on multi-dimensional superposed-epoch analysis of Geotail data. The start time of the auroral break-up (t=0) of each substorm was determined from auroral data obtained by the Polar and IMAGE spacecraft. The key parameters derived from the plasma, magnetic-field, and electric-field data from Geotail were sorted by their meridional X(GSM)-Z(proxy) coordinates.
The results show that the Poynting flux toward the plasma-sheet center starts at least 10 min before the substorm onset, and is further enhanced at X similar to-12 R(E) (Earth radii) around 4 min before the onset. Simultaneously, large-amplitude fluctuations occurred, and earthward flows in the central plasma sheet between X similar to-11 R(E) and X similar to-19 R(E) and a duskward flow around X=-10 R(E) were enhanced. The total pressure starts to decrease around X=-16 R(E) about 4 min before the onset of the substorm. After the substorm onset, a notable dipolarization is observed and tailward flows commence, characterised by southward magnetic fields in the form of a plasmoid.
We confirm various observable-parameter variations based on or predicted by the relevant substorm models; however, none of these can explain our results perfectly. Therefore, we propose a catapult (slingshot) current-sheet relaxation model, in which an earthward convective flow produced by catapult current-sheet relaxation and a converted duskward flow near the Earth are enhanced through flow braking around 4 min before the substorm onset. These flows induce a ballooning instability or other instabilities, causing the observed current disruption. The formation of the magnetic neutral line is a natural consequence of the present model, because the relaxation of a highly stretched catapult current-sheet produces a very thin current at its tailward edge being surrounded by intense earthward and tailward magnetic fields which were formerly the off-equatorial lobe magnetic fields. This location is the boundary between a highly stressed catapult current sheet and a Harris-type current sheet characterized by little stress. In addition, the flows induced around the boundary toward the current-sheet center may enhance the formation of the magnetic neutral line and the efficiency of magnetic reconnection. After magnetic reconnection is induced, it plays a significant role in driving the substorm.

DOI： 10.5194/angeo-27-1035-2009

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigate in detail the time history of substorm disturbances in the magnetotail observed during a major tail conjunction of Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites on 29 January 2008, 0700-0900 UT. During this interval, all THEMIS satellites were closely aligned along the tail axis near midnight and were bracketed in local time by GOES 11 and 12. The radial distance covered ranges from the geosynchronous altitude to similar to 30 R-E in the tail. This interval consists of three activations detected by the THEMIS satellites with good ground all-sky-camera observations of auroral activity. The first activation is a small substorm with spatially limited disturbance in the tail. The onset arc was equatorward of an undisturbed arc. The second activation is a moderate size substorm with the onset arc also being equatorward of an undisturbed arc. The third activation is an intensification of the substorm with its onset indicated by the second activation. The active auroral arc for this intensification was near the poleward boundary of the auroral oval. Analysis of these observations indicates that the first activation is a small substorm initiated in the near-Earth plasma sheet and does not involve magnetic reconnection of open magnetic field lines. Magnetic reconnection on closed field lines can be ruled out for this substorm because it cannot generate the observed high-speed plasma flow. The second and third activations are part of a moderate size substorm initiated also in the near-Earth plasma sheet, with a subsequent substorm intensification involving activity initiated tailward of similar to 30 R-E. Overall, the time history of substorm activity for these two substorms is consistent with the near-Earth initiation model.

DOI： 10.1029/2008JA013424

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Here we present the first evidence for particle injection in the dawn-side equatorial plane caused by a sudden enhancement of the solar wind dynamic pressure (SEP(dyn))The observational results demonstrate that the SEP(dyn) caused two effects: (1) instantaneous particle energization in the dayside, and (2) particle injection in the dawn side at approximately 7 min later. The former effect has been suggested to be energization of the preexisting particles by the sudden compression of the magnetosphere. We argue that the latter effect was due to acceleration of relatively hot ions in the dawn-side plasma sheet by a bipolar electric field caused by the SEP(dyn). (C) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2008.07.019

Web of Science

J-GLOBAL

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The dawn-dusk locations of reconnection in the near-earth magnetotail at the time of isolated auroral breakup are studied to clarify whether breakup is always accompanied by reconnection. The near-earth reconnection is identified by tailward plasma flows faster than 200 km/s with southward magnetic field. We first identified 66 breakups in the Polar ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail in situ observations of the plasma sheet between 25 and 31 RE down the tail. It was found that the dawn-dusk (Y) locations of relatively fast (&gt;= 400 km/s) tailward flows were associated with breakup magnetic local time (MLT) by a regression line of YAGSM = -5.7 x (MLT + 0.6) R(E) with a correlation coefficient of 0.8. Most tailward flows were observed within 5 R(E) of the modeled Y locations, where tailward flows occurred in 88% of the 26 cases of breakups between 22 and 0 MLT. It is thus inferred that in most cases, breakup is accompanied by tailward flow near the breakup MLT with its dawn-dusk dimension similar to 10 R(E). There were only two events without tailward flows in the region where flows have been expected. These two events were an earthward flow event and a traveling compression region event, which are not inconsistent with the initiation of the near-earth reconnection. Auroral breakup is thus likely to always be accompanied by near-earth reconnection near breakup MLT. It is also inferred that reconnection and breakup occur simultaneously within a few minutes, assuming a time delay between reconnection onset and the arrival of tailward flows at satellite locations.

DOI： 10.1029/2008JA013127

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] We have statistically analyzed Pi2 pulsations, using the 6-second magnetic field data acquired by the polar orbiting DE-1 satellite ( a perigee of about 500 km altitude and an apogee of about 3.6 RE altitude). From February 1983 to January 1991, 746 events were found in the H component of the geomagnetic field data from Kakioka or Hermanus, which were located near midnight at substorm onsets determined with the AL index. Out of these 746 events, 91 events had high coherence (&gt; 0.6) between DE-1 and ground stations. We examined the spatial distributions of the occurrence probability and the L dependence of the power ratio to the ground and phase difference. The following observational results are obtained: ( 1) There are many high- coherence events in the compressional component at high latitude as well as at low latitude on the nightside; ( 2) In the compressional component, the power ratio of DE-1 to ground stations has a weak peak around L similar to 5 on the nigthside and flankside; and ( 3) the phase difference between DE-1 and ground stations is changed by similar to 180 degrees around L similar to 5 on the nightside and flankside. Some high- coherence events were found in the compressional component in the polar region. We concluded Pi2 pulsations might be excited by plasmaspheric virtual resonance (PVR) mode, in which compressional waves are produced inside the plasmasphere and oscillate the ambient magnetic field at high latitude.

DOI： 10.1029/2007JA012740

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration ( IMAGE) spacecraft can observe energetic neutral atoms (ENA) of 10 eV to a few keV generated by upflowing ions through charge exchange with the Earth's exosphere. Using IMAGE/LENA data, we statistically analyzed behaviors of the ion outflow in the main and recovery phases of the magnetic storms from June 2000 to December 2001. Results show that during the main phase, most of ENA emissions from the Earth's direction are accompanied by the solar wind dynamic pressure (P-dy) enhancements. For the recovery phase, there are no such tendencies. Instead, the ENA flux shows large values at the beginning of the recovery phase, and then decreases with the storm recovery. These results suggest that the dominant mechanism responsible for the ion outflow during the magnetic storms can be totally different between the two phases.

DOI： 10.1029/2007GL029877

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] A typical Pi3 pulsation is examined by magnetic field measurements from multiple satellites and ground stations. Low-latitude ground observations with a wide longitudinal span indicate that the amplitude of the Pi3 pulsation peaks on the dayside and is gradually decreasing toward the nightside. This effect and the fact that the wave phase on the dayside leads that on the nightside, imply that the source of the Pi3 lies on the dayside. Variations in solar wind dynamic pressure observed by the GEOTAIL satellite (just outside of the magnetosphere) are highly correlated with these ground magnetic field variations. We argue in this case that the global Pi3 pulsation is directly driven by impulsive variations in the solar wind dynamic pressure. The Pi3 pulsation observed along the latitudinal magnetometer chain at 0930LT shows significant equatorial enhancement and additional observations along a latitudinal chain at 1630LT show that the phase of the Pi3 pulsation at high latitudes lags behind that at low latitudes. The low-altitude polar orbiting satellite Oersted also observed this pulsation in the dayside inner magnetosphere. The B-parallel to (northward) component at Oersted is strictly out of phase with the X component observed at the dip equator below the spacecraft path, which indicates that the Pi3 pulsation at the dip equator is caused by oscillation of an ionospheric current. We propose that the Pi3 pulsations at different latitudes are generated by different mechanisms.

DOI： 10.1029/2006JA011675

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

In this contribution, we extend a series of previous works focused on an investigation of signal's polarization attributes using the continuous wavelet transform, where we proposed a method to map instantaneous polarization attributes of multicomponent signals in the wavelet domain and explicitly relate these attributes with the wavelet transform coefficients of the analyzed signal. In this work, we applied our polarization method to an examination of characteristics of Pi2 pulsations. We have shown some merits of the use of the continuous wavelet transform for the Pi2 pulsations' analysis. First, we used our polarization method for the geomagnetic field data from the MSR, KAK, GUA, SMA, BLM and LAQ observatories and showed some correlations between the polarization parameters of pulsation and the station's position (nightside or dayside). Secondly, we considered the signal's north components of a pair of stations and demonstrated a time-frequency variations of the phase difference between two stations during the pulsation.

DOI： 10.1186/BF03352035

Web of Science

J-GLOBAL

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

The present study addresses the contribution of charge exchange loss of the ring current ions to the decay of the storm time ring current, based on measurements of energetic neutral atoms (ENAs) made by the High Energy Neutral Atom (HENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. The energy range of ENA fluxes examined in the present study is from 27 to 119 keV for hydrogen and from 79 to 264 keV for oxygen. This is the first statistical estimate of the charge exchange contribution made with dedicated measurements of both neutral hydrogen and oxygen. We estimate charge exchange energy losses of the ring current ions, employing two independent methods. The decay rate of the ring current is derived from the temporal variations of the Dst index. Our estimates show that the rate of the charge exchange energy losses is comparable to the ring current decay rate for the intervals of the slow decay, while the loss rate is much smaller than the decay rate in the rapid decay phase in particular for the early stage of a storm recovery. The logarithm of the energy loss rate is well correlated with the simultaneous Dst index, increasing as Dst decreases. Utilizing the high correlations, we simulate a Dst recovery due to charge exchange loss alone. Pressure-corrected Dst (Dst0) recoveries derived from measured Dst cannot be reproduced by charge exchange loss alone. We conclude that the charge exchange loss is not primarily responsible for the rapid decay of the storm time ring current. Copyright 2006 by the American Geophysical Union.

DOI： 10.1029/2006JA011789

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Using a two dimensional image of the Earth's plasmasphere taken by the Extreme Ultraviolet Imager (EUV) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft, in-situ electron density measurements from the IMAGE Radio Plasma Imager (RPI) instrument, measurements of magnetospheric mass density inferred from field line resonant frequencies measured by magnetometers on the Earth's surface, and a model for the density field aligned variation, we construct a computer model for the magnetospheric density on 29 August 2000 at 1519 UT. The purposes of this study are to demonstrate how a density model can be constructed using multiple data sources, to document this particular model, which is being used in studies of ultra low frequency Pi-2 oscillations and plasmaspheric cavity modes, to describe some of the problems involved with EUV density inversion, and to demonstrate some features of the plasmaspheric density, particularly in the region of the dusk plasmatrough and plume. (c) 2005 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jastp.2005.11.009

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J-GLOBAL

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The Earth's inner magnetosphere (inside 10 Re) is a region where particle energy increases to the relativistic energy range. This region is very important as a laboratory where high-energy particle acceleration can be directly measured in a dipolar field configuration, as well as for human activities in space including space weather prediction. Despite abundant in situ satellite measurements, this region has been "missing" because of several difficulties arising from the measurements, such as high-energy particle contamination of low-energy particle measurement, protection against the possible incidence of radiation belt particles on the satellite, and the difficulties of measuring three-dimensional particles over a broad energy range, from a few electron volts to more than 10 MeV. In this paper, we address important scientific topics and propose a possible configuration of small satellites termed Energization and Radiation in Geospace (ERG), which would provide new insights into the dynamics of the inner magnetosphere and strongly contribute to the International Living With a Star project. (C) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2005.05.089

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE LTD  

Shock accelerated nonthermal particles are thought to contribute to modify the shock structure. Here we present two such cases at two strong interplanetary shocks in 1994 and 2003, and try to see how the nonlinear feature depend on the shock parameters, such as Mach number and shock angle. (c) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2006.03.012

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

We statistically investigated longitudinal dependence of characteristics of low-latitude Pi2 pulsations to find the longitudinal structure of the plasmaspheric cavity mode. We used the geomagnetic field data from two ground stations, Kakioka (27.2 degrees geomagnetic latitude, 208.5 degrees geomagnetic longitude) and Hermanus (-33.9 degrees geomagnetic latitude, 82.2 degrees geomagnetic longitude), and auroral image data acquired by the ultraviolet imager onboard the Polar satellite for the period of December 4, 1996 to March 3, 1997. Our findings include the following: (1) Pi2 amplitude is the largest around the magnetic local time of the auroral breakup site and decreases away from it; (2) when a nightside Pi2 pulsation has large amplitude, a dayside Pi2 pulsation can be observed with a similar waveform; (3) Pi2 pulsations generally have no clear phase differences (mean phase difference of 3.3 degrees) between Kakioka and Hermanus, except for some events; and (4) the phase difference is independent on Delta MLT (difference of magnetic local time between a station and the auroral breakup). These observations suggest that the plasmaspheric cavity mode can be excited globally with a very small value of the azimuthal wave number (m approximate to 0).

DOI： 10.1186/BF03351981

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J-GLOBAL

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

A long period Pc5 pulsation was observed at Phimai in Thailand, shortly after the origin time of the Sumatra earthquake on December 26, 2004. The localized nature and the period of oscillations suggest that the long period magnetic pulsation was generated by dynamo action in the lower ionosphere, set up by an atmospheric pressure pulse which propagated vertically as an acoustic wave when the ocean floor suddenly moved vertically. It is speculated that a Pc3 type pulsation observed at Tong Hai in China, 10 degrees north of Phimai in latitude, was the result of magnetic field line resonance with a magnetosonic wave generated from the electric and magnetic fields of the dynamo current caused by the Earthquake.

DOI： 10.1029/2005GL024083

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Two coronal mass ejections associated with the X17 and X10 solar flares reached the Earth's environment at very high speeds on 29 and 30 October 2003, respectively, causing very intense geomagnetic storms (Dst similar to -400 nT). The present study focused on the main phase of the 30 October storm during which the Geotail spacecraft was within the near-Earth magnetotail at X similar to -8 R-E. A number of extremely intense substorms occurred during this period. In one of them, the intensity of the westward auroral electrojet exceeded 3000 nT, which was one of the largest magnitudes ever observed. The energetic particle observations from the low-altitude, polar-orbiting NOAA satellites indicate that the auroral oval shifted equatorward to magnetic latitudes much lower than usual, as low as 50 degrees. Throughout the interval, the magnetic field in the near-Earth magnetotail, and possibly the plasma density, was much larger than usual, indicating a considerable degree of energy accumulation in the lobe region and compression of the plasma sheet and very intense cross-tail currents. The dense plasma may be responsible for the intense auroral electrojet and the intense ring current. Very large, rapid dipolarizations occurred in relation to the intense substorms. High-energy particle fluxes were an order of magnitude higher than usual, and their increases took place immediately after the dipolarizations. Fast tailward flows with large southward magnetic fields as well as fluxes of energetic heavy ions (oxygen) were also observed, suggesting that the magnetic reconnection took place in the near-Earth magnetotail, associated with the very intense substorms. This location is much closer to the Earth than usual, probably as close to the Earth as ever reported. These magnetotail and auroral observations as well as other results reported previously suggest that the entire magnetosphere was considerably distorted during the storm.

DOI： 10.1029/2005JA011070

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] We statistically and quantitatively examine the outflow of energetic ions from the magnetosphere during magnetic storms. We also evaluate the contribution of the outflow to the decay of the ring current. We use energetic ion ( 9 - 210 keV) data obtained by the energetic particle and ion composition ( EPIC) instrument and magnetic field data obtained by the magnetic field measurements (MGF) system, both on board the Geotail spacecraft. The outflowing energy flux, that is, the energy flux lost by ring current ions flowing through the magnetopause, is defined as the energy flux normal to the magnetopause and is calculated based on measurements made adjacent to the earthward side of the low-latitude boundary layer. Our statistics show that the outflowing energy flux is about 10(5) - 10(8) keV/(cm(2) s) during both the main phase and the recovery phase. It is higher on the afternoonside than on the morningside. It is better correlated with the square root of the dynamic pressure of the solar wind than the electric field of the solar wind, which is a proxy for the strength of the convection electric field. The contribution of the outflow to the rapid decay of the ring current is estimated to be at least 23% and could be much higher than 23% for the 23 September 2001 storm, based on an underestimated leakage area which is determined from magnetic field measurements. We suggest that the drift governing the ion outflow mainly is the del B drift which has a radial component that arises from a day-night gradient of the magnetic field in the magnetosphere caused by the solar wind compression. We conclude that the ion outflow contributes significantly to the rapid decay of the ring current, even in the case of a sudden northward turning of the interplanetary magnetic field which causes a sudden decrease in the convection electric field.

DOI： 10.1029/2004JA010970

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] We studied dynamics of O+ ions during the superstorm that occurred on 29 - 31 October 2003, using energetic (9 - 210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Since the low-energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near-Earth plasma sheet of X-GSM similar to - 6 R-E to - 8.5 R-E, we found that the H+ energy density showed no distinctive differences between the superstorm and quiet intervals ( 1 - 10 keV cm(-3)), while the O+ energy density increased from 0.05 - 3 keV cm(-3) during the quiet intervals to similar to 100 keV cm(-3) during the superstorm. The O+/ H+ energy density ratio reached 10 - 20 near the storm maximum, which is the largest ratio in the near-Earth plasma sheet ever observed by Geotail, indicating more than 90% of O+ in the total energy density. We argued that such extreme increase of the O+/ H+ energy density ratio during the October 2003 superstorm was due to mass-dependent acceleration of ions by storm-time substorms as well as an additional supply of O+ ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near-Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had the O+/ H+ energy density ratio as large as 10 - 20 for the October 2003 superstorm.

DOI： 10.1029/2004JA010930

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The present study observationally addresses the role of the magnetospheric substorm in the storm-time ring current intensification. The intensity of energetic neutral atom (ENA) emission, which is measured by the high-energy neutral atom (HENA) imager onboard the IMAGE satellite, is carefully used as a guide for inferring the change of the ring current intensity. First, a storm event of August 2000 is examined in detail with a focus on a substorm that occurred at the start of the storm recovery phase (as defined by Sym-H). During the expansion phase of this substorm, the Sym-H index recovered ( increased) as the geosynchronous magnetic field dipolarized. At the same time the low-energy (27-60 keV) hydrogen, high-energy (60-119 keV) hydrogen, and total oxygen(&lt;160 keV for this event) ENA intensities increased, suggesting that the ring current intensified. The apparent recovery of Sym-H can therefore be attributed to the reduction of the tail current rather than the decay of the ring current. The substorm-related change of the ENA intensity is examined statistically by conducting a superposed epoch analysis, for which the onset of geosynchronous dipolarization is adopted as a reference time. The result reveals that the ENA intensity tends to decrease before substorm onsets and to increase after onsets, and so does the Sym-H index although its pre-onset decrease is less clear than the post-onset increase. It is therefore suggested that in the course of substorms, the change of the ring current intensity is opposite to what is expected from the change of the Sym-H index. The decay and intensification of the ring current can be attributed to substorm-related changes of the near-Earth magnetic field and convection. Another important result is that the response of the ENA intensity to substorms strongly depends on species and energy range. The variation of the low-energy hydrogen ENA intensity is not clearly organized by the substorm onset, and its relative change is less than 10%. The high-energy hydrogen intensity decreases by about 20% during the growth phase and then recovers to the initial level leaving no significant net increase. In contrast, the increase in the oxygen ENA intensity during the expansion phase overcompensates for the preceding reduction, resulting in a net increase of 20%. The net enhancement of the oxygen ENA intensity suggests the importance of non-adiabatic acceleration associated with near-Earth dipolarization. The timescale of dipolarization is comparable to the oxygen gyroperiod, and therefore the oxygen ions may be accelerated preferably by the associated inductive electric field. It is inferred that the substorm-related energization of the oxygen ions makes an important contribution to the storm-time ring current intensification.

DOI： 10.1029/2004JA010954

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Scopus

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

We calculated the energy density of H+, He+, and O+ ions in the plasma sheet during the development of a magnetic storm, using energetic (9-210 keV) particle flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. We found that the energy density ratio of O+/H+ stayed at similar to 0.1 before storms, but increased as storms developed, reaching 0.5-1.0 at a peak of storms. The energy density ratio of He+/H+ was rather constant at 0.01-0.02. These results are comparable with those in the outer ring current reported by the previous studies. This implies that the ions of ionospheric origin (O+ and He+) are transported to the ring current through the plasma sheet.

DOI： 10.1016/S0964-2749(05)80031-9

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

We investigated upstream events observed by the ion composition system (ICS) sensor of the energetic particles and ion composition (EPIC) instrument on board the Geotail spacecraft. We examined how occurrence probability and spatial distribution of upstream events depend on the geomagnetic activity. The results showed that the upstream events were observed more frequently in the dawn side during intense geomagnetic activity in particular. We also analyzed carbon-nitrogen-oxygen ions during the upstream events. From the above results we discuss origin of the upstream energetic ions.

DOI： 10.1016/S0964-2749(05)80045-9

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We have studied magnetic pulsations in the 5- to 20-mHz (Pc4) band observed on 7 January 1997 from ∼1400 UT to ∼1600 UT at ground stations located on the nightside. At low latitudes (L &lt
2) the spectral content of the pulsations did not differ much from the Pi2 pulsations that were observed adjacent to the above interval. However, during the Pc4 event the auroral luminosity measured by the Ultra Violet Imager instrument on the Polar spacecraft was steadily declining toward a very low level. Also, the Geotail spacecraft, located in the plasmasheet at a geocentric distance of ∼30 RE did not detect strong plasma flows. Looking for clues of possible dayside sources for the pulsations, we find that the interplanetary magnetic field measured by the IMP-8 spacecraft made a small (&lt
30°) angle with the Sun-Earth axis. This condition is favorable for generation of ultra-low-frequency (ULF) waves in the region upstream of the bow shock, and it is known that the upstream waves are a major source of Pc4 pulsations observed on the dayside. Pointing to a common upstream energy source, strong low-latitude Pc4 pulsations were observed on the dayside during the period of the nightside Pc4 pulsations. However, the spectrum of the upstream magnetic field oscillations at IMP-8 was characterized by broadband power below 20 mHz instead of a strong peak at the frequency of the observed ground Pc4 pulsations. Plasmaspheric cavity mode resonance may have played a role in filtering the Pc4 pulsations from the broadband input to the magnetosphere. Copyright 2005 by the American Geophysical Union.

DOI： 10.1029/2005JA011093

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

In the present study we aim to derive empirical equations relating source plasma of the ring current to the solar wind. We used the energy spectra at energies of 9-135 keV obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. The plasma parameters (i.e., number density and temperature) of H+, O+, and He were estimated by fitting the K-distribution function to the energy spectra in the region of a geocentric distance of 8.5-10.5 RE and magnetic local time of 2200-0200 hour. The results showed that the H+ number density in the plasma sheet correlated with the solar wind density, while the O+ and He+ number density had no correlation with the solar wind parameters. Thus the origin of H+ ions in the plasma sheet is thought to be the solar wind. O+ and He+ ions in the plasma sheet are expected to have different origin from the solar wind. It was also found that the temperature of H+, O+ and He+ has a good correlation with the solar wind velocity and that gradients of the derived empirical equations can be ordered by ion mass. This implies that ions are accelerated in a mass-dependent way.

DOI： 10.1016/S0964-2749(05)80009-5

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Part of collection (book)   Publisher：Blackwell Publishing Ltd  

We investigated the spatial and temporal properties of the 9-210 keV/e ion composition in the near-Earth plasma sheet (at geocentric distance, r, of 8-15 RE), using energetic ion flux data acquired by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument onboard the Geotail spacecraft. We analyzed data covering 8.3 years to find the MLT-r distribution of the H+, He+, and O+ energy densities (as well as the He+/H+ and 0+/H+energy density ratios) as a function of the SYM-H index. We obtained the following results: (1) The energy density increases as SYM-H decreases
(2) The energy density change depends on ion mass: the largest change occurs in 0+, and the smallest change occurs in H+ (3) The energy density shows a dawn-dusk asymmetry when SYM-H &lt
- 5 0 nT, being larger on the duskside than on the dawnside
and (4) The He+/H+and 0+/H+ energy density ratios in the plasma sheet are similar to those in the ring current. From these results we conclude that ionospheric He+ and O+ ions are transported to the plasma sheet, accelerated there by the dawn-to-dusk electric field in a mass-dependent manner (heavier ions gain more energy than lighter ions), and injected into the ring current region. Both the ion flux from the ionosphere and the energy gain in the plasma sheet become large when the geomagnetic disturbance becomes intense.

DOI： 10.1029/155GM08

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] In order to identify acceleration sites of energetic ions upstream of the Earth's bow shock and in the dayside magnetosheath, we investigated energetic ion events observed by the Energetic Particles and Ion Composition ( EPIC) instrument on board the Geotail spacecraft from 1995 to 2001. Energetic ion data obtained by the Ion Composition System ( ICS) sensor were used to select " events,'' that is, intervals when the 77 - 107 keV ion flux increased by more than two orders of magnitude within 10 min, and examine their spatial distribution and its geomagnetic activity dependence. Heavy ions with energy range of 9 - 210 keV/ e detected by the Suprathermal Ion Composition Spectrometer ( STICS) sensor were chosen to calculate the percentage of low- charge- state heavy ions ( P-LCS = ( O+ + N+)/( O+ + N+ + O6+ + O7+)) in each event, and the events were classified into three groups: high- charge- state ( HCS) events ( P-LCS less than or equal to 25%), intermediate ( IM) events ( 25% &lt; P-LCS &lt; 75%), and low- charge- state ( LCS) events ( P-LCS greater than or equal to 75%). The HCS and LCS events occurred at almost the same rate in geomagnetically quiet conditions ( SYM- H greater than or equal to 0 nT), while the LCS events were observed more frequently than the HCS events in geomagnetically disturbed conditions ( SYM- H less than or equal to - 30 nT). The percentage of the IM and LCS events summed was higher in disturbed conditions than in quiet conditions. The HCS and LCS events were frequently observed in the dawnside upstream region and in the duskside magnetosheath, respectively. The IM events occurred both in the dawnside upstream region and in the magnetosheath around noon. We conclude that energetic ion events including ions accelerated in the magnetosphere are dominant. Ions accelerated in the magnetosphere leak out from the duskside magnetopause, in particular when geomagnetic activity is high, and then most likely mix with ions accelerated at the bow shock on the way to the dawnside upstream region. The acceleration site of the HCS events is also discussed.

DOI： 10.1029/2003JA009953

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] Orsted is a low-altitude polar orbiting satellite, which is advantageous for investigating the spatial structure of low-frequency events like Pi2 pulsations. Using 1-s vector magnetic field data from April 1999 to May 2002 obtained from Orsted and Kakioka magnetic observatories, we found that the field aligned component (Bparallel to) of Pi2 pulsations at the satellite height is highly correlated with the H component on the ground, but the other two components, i.e., eastward component (B-E) and radial component (B-R), do not show a clear correlation with the ground observations. The observed results for nightside events provide evidence that nightside Pi2s at low latitude are generated from cavity resonance. Two cases observed by Orsted when it was located on the dawnside or duskside also show cavity resonance properties. However, when Orsted was on the dayside, the oscillations observed by the satellite are out of phase with that observed on the ground, suggesting that the dayside Pi2s are more likely related to some dayside ionospheric current systems rather than that caused by a global cavity resonance mode. The amplitude variation recorded by Orsted shows a peak in the equatorial region, which, for the first time, gives clear observational support for earlier model calculations. We also estimated the screening effect of the ionosphere on MHD waves and suggest that when the cavity resonance mode is valid for generation of Pi2 pulsations at low latitudes, the screening effect is negligible; that is, the compressional waves are seen directly as Pi2 pulsations at the ground.

DOI： 10.1029/2004JA010576

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] The present study aims to investigate how and where ions of ionospheric origin are accelerated to the ring current energy (a few tens to a few hundreds of keV) and how they are supplied to the ring current. We examined the plasma sheet ion composition during magnetic storm development, using energetic (9-210 keV) ion flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on board the Geotail spacecraft. We selected two magnetic storms, that is, the 16-17 May 2000 storm and the 25 December 1998 storm, for which the energy density ratios of O+/H+ and He+/H+ in the plasma sheet were calculated from the EPIC/STICS data. These magnetic storms had a minimum of the SYM-H index (the 1-min Dst index) less than -50 nT and a duration of the main phase shorter than 6 hours. We obtained the following results: (1) Both the O+/H+ and He+/H+ energy density ratios were anticorrelated with the SYM-H index (\r\ = 0.73-0.88); (2) The O+ /H+ energy density ratio was rather constant at similar to0.1 before storms, but reached 0.3-1.0 at the storm maximum; and (3) The He+ /H+ energy density ratio increased from 0.01-0.02 before storms to 0.04-0.1 at the storm maximum. These ion composition changes are comparable to those in the ring current, which have been reported by previous studies, indicating that ions of ionospheric origin are possibly convected to the ring current via the plasma sheet. A close inspection of ion energy spectra revealed that the observed ion composition changes can be attributed to the mass-dependent acceleration of ions by the dawn-to-dusk electric field in the current sheet and the additional transport of ionospheric ions into the plasma sheet.

DOI： 10.1029/2002JA009660

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The radial mode structure of Pi2 pulsations in the inner magnetosphere (L&lt;7) and its relation to the plasmapause are studied using data acquired by the Combined Release and Radiation Effects Satellite (CRRES) between August 1990 and September 1991. Low-latitude Pi2 pulsations detected on the ground at Kakioka (L=1.25) are used as the reference signal to determine the relative amplitude and phase of the electric field oscillations detected at CRRES. The plasmapause is identified using electron density inferred from the plasma wave spectra observed on CRRES. Pi2 events at CRRES are defined to be 10-min intervals of high coherence between oscillations in the Kakioka horizontal northward magnetic field (H) and CRRES dusk-to-dawn electric field (E-&phi;) components within the Pi2 band (6-25 mHz). The E-&phi; component represents the poloidal oscillation of the geomagnetic field lines for satellite local times near midnight. Fifty-five high-coherence E-&phi;-H Pi2 events occurred when both CRRES and Kakioka were within 3 hours of magnetic midnight. For these events CRRES was on L shells ranging from 2 to 6.5 and was either in the plasmasphere or in the close vicinity of the plasmapause, providing evidence for the plasmaspheric origin of low-latitude Pi2 pulsations. The amplitude of E-&phi; varied significantly but there is an indication of a maximum near L=4. The phase of E-&phi; (relative to Kakioka H) remained near -90&DEG; at all distances. These properties are consistent with the radial structure of the fundamental cavity mode oscillations confined in the plasmasphere. For some events observed at L&gt;3.5 it was also possible to determine the amplitude and phase of the compressional component B-z at CRRES. In contrast to E-phi, the phase of B-z (relative to H) was clustered both at similar to180 and similar to0 for events occurring near the plasmapause. This observation still is consistent with the cavity mode according to a numerical simulation using a dipole magnetic field and a realistic plasmapause plasma density structure, which indicates that the node of B-z is located near the plasmapause. Depending on the satellite position relative to the node, the phase can be either -180degrees or 0. A negative correlation is found between the Pi2 frequency and the distance of the plasmapause, which is additional support for the cavity mode origin of low-latitude Pi2 pulsations.

DOI： 10.1029/2002JA009761

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

One-second resolution geomagnetic data from 5 stations located at low-latitudes (i.e., L = 1.2 similar to 1.6) were used to examine the local time dependence of the dominant frequency of Pi2 pulsations. We analyzed 183 Pi2 events simultaneously recorded at the 5 stations and discussed their possible generation mechanisms. The averaged dominant frequency of the H (horizontal) component is higher on the dawn side than that on the dusk side and shows a peak value in the post-midnight at around 03 LT, which confirms the LT dependence previously suggested from single station data (i.e., non-simultaneous observation). However, some of the events have no LT dependence. For the events which show the LT dependence at low-latitudes, we infer that the cavity resonance mode is a plausible generation mechanism, but we do not rule out the possibility of the plasmaspheric surface wave mode which has also been suggested. For the events having a common frequency between the eastern and western stations, we suggest the cavity resonance mode to be the mechanism. The averaged dominant frequency of the D component does not show any clear LT dependence, and only about 20% of Pi2s have identical frequency for both the H and D components, therefore we suggest that the H and D oscillations of Pi2s are generated from different mechanisms. We also found that the frequency of different parts of a Pi2 pulsation, i.e., the dominant frequency for leading part and trailing part of the pulsation, is different, and the frequency of the trailing part is lower than that of the leading part.

DOI： 10.1186/BF03352467

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J-GLOBAL

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing Ltd  

We investigated a Pi2 pulsation that occurred at 0538 UT on 20 September 1995, using data from ground stations and the ETS-VI and EXOS-D satellites. Since ground stations at L = 1.45 - 12.6 and the two satellites were located at 7-10 hours of magnetic local time (MLT), we could investigate characteristics of the morning side Pi2 pulsation in detail. We also examined geomagnetic field data from equatorial and low-latitude {L ≤ 1.5) stations at 0200 MLT and 1500 MLT. Our findings include the following: (1) Pi2 pulsations on the morning side were observed over a wide range of L (L &lt
6.1) with almost identical period (T - 70 s) and waveforms
(2) the ETS-VI satellite located above the geomagnetic equator at L = 6.3 observed a Pi2 pulsation that had nearly the same period and waveforms as the ground Pi2 pulsation
(3) the Pi2 pulsation observed by ETS-VI appeared in the compressional and radial components
(4) phase lag between the compressional and radial components was ∼180° (5) the ground-to-satellite phase lag was ∼180° (∼0°) for the X component and the compressional (radial) component
(6) the EXOS-D observation placed the plasmapause location at L = 6.8, across which ground Pi2 pulsations changed their characteristics
and (7) no phase delay was found between low-latitude Pi2 pulsations observed around 0700 MLT, 0200 MLT, and 1500 MLT. From these results we concluded that the morning side Pi2 pulsation was caused by the plasmaspheric cavity mode resonance and that its longitudinal structure was rather uniform. Copyright 2003 by the American Geophysical Union.

DOI： 10.1029/2002JA009747

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] The present study seeks to systematically examine the fast plasma flow in the plasma sheet at geomagnetically quiet time. The study uses plasma measurements made by the Geotail satellite in the midnight sector at x &gt; -50 R-E during quiet intervals with a total duration of 446 hours. Comparison with the results of previous studies suggests that the occurrence frequency of the perpendicular flow velocity V-Xperpendicular to depends more clearly on the radial distance than on geomagnetic activity. Two extreme events were selected for detailed studies; they occurred on 23 November and 10-11 December 1994. In the 23 November event, Geotail was located similar to37 R-E from Earth and observed a fast tailward flow (V-Xperpendicular to similar to -1250 km s(-1)) with a strongly southward magnetic field (B-Z similar to -8.9 nT). The signature indicates that a near-Earth neutral line was formed earthward of the satellite and the reconnection reached the lobe magnetic field. On the ground, however, only weak (&lt; 100 nT) magnetic disturbances were observed at high (&SIM;75&DEG;) latitudes but not at auroral zone stations. The result strongly suggests that lobe reconnection is not sufficient for the global development of a substorm. The 10-11 December 1994 event is very similar to the 23 November 1994 event except that Geotail observed a fast (V-X&BOTTOM; &SIM; +1500 km s(-1)) earthward flow rather than a tailward flow, along with the dipolarization of the local magnetic field. It is asserted that the near-Earth substorm process, that is, tail current disruption, controls the development of a substorm. The fast plasma flow may set a favorable condition for this process to proceed, and therefore the substorm may tend to develop following generation of the fast flow, but the result of the present study indicates that near-Earth reconnection does not necessarily trigger the global substorm.

DOI： 10.1029/2001JA000116

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

We carried out a statistical analysis of Pit pulsations using the geomagnetic field data obtained at three ground stations. A local time dependence of the dominant frequency of Pit was found on the nightside. The frequency of mid-latitude Pit pulsations is lower on the dusk side than that on the dawn side. This tendency is attributed to the shape of the plasmasphere which bulges out to the dusk side. It was confirmed that the Pit frequency depends also on the geomagnetic activity measured with Kp index. During the disturbed periods, Pit pulsations have higher frequency than that in the quiet periods. This dependence is interpreted to be caused by the size of the plasmapause which is smaller under the disturbed conditions than that under the quiet conditions. The dominant frequency of Pit pulsations at lower latitudes has a peak in post-midnight, and a Kp dependence similar to that at mid-latitudes is also observed. However, the result for low-latitude Pit's is different from that for mid-latitude Pit. We consider that the dominant mechanism of mid-latitude Pit is the plasmaspheric surface wave. In order to examine the idea that the surface wave on the plasmapause is the dominant mechanism of Pit pulsations at mid-latitudes, we estimated the resonance frequency of the surface wave on the plasmapause using a plasmaspheric model which includes the effect of the plasmaspheric bulge. The estimated frequency of the surface wave is higher on the dawn side than that on the dusk side, which is essentially consistent with the observational results. The predicted frequency under quiet conditions (Kp less than or equal to 3) is nearly equal to the observed Pit frequency at mid-latitudes. These results suggest that the dominant frequency of Pit pulsations at mid-latitudes depends on the structure of the plasmapause.

DOI： 10.1186/BF03351730

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J-GLOBAL

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2000ja000400

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The Dst index has been conventionally used as a measure of the storm intensity, which ideally assumes that the associated ground magnetic disturbance is caused by the ring current. The present study examines the contribution of the tail current to Dst, focusing on the occurrence of geosynchronous dipolarization close to the Dst minimum, in other words, the start of the storm recovery phase. The Sym-H (referred to as Dst((Sy-H)) hereafter) index rather than the conventional Dst index is used because of its higher time resolution (1 min). For the June 1998 storm event, dipolarization started at two GOES satellites and the Geotail satellite in the near-Earth tail when Dst((Sym-H)) reached its minimum. This result indicates that the source current was located outside of geosynchronous orbit, and therefore the recovery of Dst((Sym-H)) can be attributed to the reduction of the tail current rather than the decay of the ring current. A statistical study based on 59 storm events (79 GOES events) confirms the tendency for geosynchronous magnetic field to dipolarize at the Dst((Sym-H)) minimum. It is therefore highly likely that the Dst((Sym-H)) minimum is misidentified as the start of the ring current (storm) decay at a time when the ring current may actually be intensifying owing to substorm-associated injection, From the magnitude of the Dst((Sym-H)) recovery during the interval of geosynchronous dipolarization, the contribution of the tail current to Dst((Sym-H)) at the Dst((Sym-H)) minimum is estimated to be 20-25%. However, the contribution of the tail current may be even larger because the tail current may not return to preintensification levels and may continue to contribute to Dst(sym-fn after dipolarization. The trigger of dipolarization (substorm) and the subsequent recovery of Dst((Sym-H)) tend to take place in the course of the reduction of the southward interplanetary magnetic field (IMF) B-Z. It is therefore suggested that the ring current (storm) recovers after the substorm since the magnetospheric convection weakens because of weaker southward IMF B-Z.

DOI： 10.1029/2000JA000400

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We investigate the ion composition of the near-Earth plasma sheet in storm and quiet intervals, using energetic (9-210 keV) particle flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. In 1998 four magnetic storms (minimum Dst &lt; -50 nT) occurred when Geotail was located in the near-Earth plasma sheet (X greater than or equal to -10 R-E) For each of the storms, we have selected a col responding quiet interval from time periods when Geotail revisited the near-Earth plasma. sheet under the condition Dst &gt; -20 nT, The energy density of the H+, He+, and O+ ions was computed from the EPIC/STICS data for these storm and quiet-time events. We obtained. the following results: (1) The energy density is higher during storms than during quiet times for all ion species (H+, He+, and O+); (2) the He+/H+ energy density ratio during storms is 0.01-0.02, while that, during quiet times is similar to0.01; and (3) the O+/H+ energy density ratio is significantly larger during storms (0.2-0.6) than during quiet times (0.05-0.1). To explain these results we suggested a current sheet acceleration mechanism in which ions are energized by the dawn-to-dusk convection electric field in a mass-dependent way in the course of interaction with the current sheet.

DOI： 10.1029/2000JA000376

DOI： 10.1029/2000ja000376

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The first composition observation of energetic neutral atom (ENA) emissions during the main phase of a geomagnetic storm is reported here. The measurements were made by the Energetic Particles and Ion Composition (EPIC) instrument on the Geotail on October 22, 1999 when the ring current was strengthening. For this geomagnetic storm, while the Dst index was monotonically decreasing during the main phase, ENA fluxes showed two major enhancements with distinctly different temporal profiles. For the first enhancement, the increases in ENA fluxes were rapid and the subsequent evolution was species dependent. Both the integral ENA (E &gt;similar to 12 keV) and neutral hydrogen (E &gt; 67 keV) fluxes remained relatively unchanged after the increase while the neutral oxygen (E &gt; 203 keV) fluxes decreased rapidly. For the second enhancement, the increases in ENA fluxes of all species were rather gradual and the subsequent decrease in the neutral oxygen flux was also slow. The slow decrease may be related to the continual AE enhancement during that time. For total energies above 200 keV, the intensity of ENA oxygen was the highest, followed by hydrogen, and then by helium. This ordering of ENA intensity for these species is similar to the previous ENA composition results obtained for the recovery phase of another geomagnetic storm.

DOI： 10.1029/2000GL012612

DOI： 10.1029/2000gl012612

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Characteristics of the lobe plasma convection enhancements around Pi2 substorm onset have been clarified with Geotail data. We identified 92 lobe substorm onset events within 5 R-E from an estimated neutral sheet from X = -10 to -35 R-E, and we found 27 convection enhancement events. These events were categorized into two types depending on whether the magnetic field tilts northward or southward in coincidence with intensity. decrease. The former type was identified in 12 of the 27 and found to be generally at shorter distances from the Earth than the latter, The frequencies of the lobe observations in the above Z width are small (less than or equal to 11%); that is, the plasma sheet usually occurs. In this situation, 80%: of the 27 are identified only within 2.5 R-E from the estimated neutral sheet. This indicates that the convection enhancement events occur. in association with the lobe appearance in the vicinity of the neutral sheet. This is also recognized in the X-Y distribution of the lon-latitude lobe observation; the relatively high frequencies of greater than or equal to4% which have a dawn-dusk asymmetry roughly correspond to the event location. Examination of the convection speed shows that the peak lies in 0.04V(A) to 0.1V(A) (V-A is the local Alfven velocity) and that the duration is 1 - 3 min for most cases. Identification probability of the convection enhancement events nas also examined. Both types of events have very similar occurrence probability in Y = 0-10 R-E, which is consistent with an interpretation that both are created by a common process, i,e,, lobe reconnection in the near-Earth neutral line (NENL). For the downside of this Y region, however, the southward field tilting type more often appears than he other, implying that the convection enhancements occur in an asymmetric manner with respect to the NENL.

DOI： 10.1029/2000JA900097

DOI： 10.1029/2000ja900097

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

It I-las been reported by previous studies that the energetic particle flux of ions of ionospheric origin like O+ ions is more enhanced than that of Hf ions in the near-Earth tail (X similar to-6 to -16 R-E) during substorms. To explain this strong O+ flux enhancement, some studies have surmised that thermal O+ ions in the plasma sheet boundary layer or the lobe are strongly accelerated at the magnetic reconnection region (X similar to-20 to -30 R-E), and are subsequently transported into the near-Earth plasma sheet; with earthward plasma flows. However, other studies have supposed that the strong O+ flux enhancement is caused by local magnetic field reconfiguration (local dipolarization). In the present study, we used Geotail/EPIC measurements of energetic (60 keV to 3.6 MeV) ion flux to test the above two scenarios. We investigated ion composition in the plasma sheet while earthward plasma flows and/or dipolarization signatures were observed. In terms of energy density ratio of oxygen ions to protons, the observational results can be summarized as follows: (1) earthward plasma flows without dipolarization signatures did not accompany large increases of the ratio in most cases; (2) when earthward plasma flows appeared with dipolarization signatures, they accompanied increases of the ratio; and (3) most of dipolarization events were associated with large increases of the ratio. These results suggest that the strong increase in the energetic oxygen constituent in the near-Earth plasma sheet is due to acceleration of ions during dipolarization, consistent with the latter scenario.

DOI： 10.1029/2000JA000129

DOI： 10.1029/2000ja000129

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Measurements from the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft were used to investigate dynamics of O+ ions of ionospheric origin at energies of 9 keV to 210 keV in the near-Earth plasma, sheet during the substorm expansion phase. Substorm signatures were clearly observed on the ground at 1850 UT on May 17, 1995. In the expansion phase of this substorm, Geotail stayed in the plasma sheet at X similar to-10.5 R-E and observed a local dipolarization signature accompanied by strong disturbances of the magnetic field. From the energetic ion flux data of EPIC/STICS, we obtained the following results: (1) energetic flux enhancement was more pronounced for O+ than for H+; (2) the flux was enhanced almost simultaneously with local dipolarization; (3) the enhancement factor of O+ ions (EO+), which represents the enhancement of the O+ flux ratio (after and before substorm onset) relative to the H+ flux ratio, was Bs large as 1.31; and (4) thermal energy increased from 8.9 keV to 42.8 keV for O+ ions and from 9.4 keV to 15.9 keV for H+ ions. We also performed statistical analysis for 35 events of local dipolarization found in the near-Earth region (X similar to-6 to -16 R-E). We found that EO+ is larger than unity in all ranges of radial distance and that the average value of EO+ is 1.37. These results suggest that O+ ions are commonly more energized than H+ ions during the substorm expansion phase. To interpret these observational results, we propose a mechanism in which ions are accelerated in a non-adiabatic way during substorm-associated field reconfiguration.

DOI： 10.1029/1999JA000318

DOI： 10.1029/1999ja000318

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We have investigated a near-Earth dipolarization event in the midnight sector using simultaneous observations of Polar and Geotail. We have found evidence for near-Earth dipolarization to be a non-MHD process: dipolarization occurring without significant plasma flow or with tailward flow and during dawnward electric field different from that inferred based on the frozen-in condition. These observations are inconsistent with the idea that dipolarization is an MHD process of magnetic flux pileup from braking of sunward plasma flow. Possible variances of the flow braking scenario are considered but none is satisfactory in accounting for the observed features. On the other hand, these findings are quite consistent with the expectations from the current disruption scenario.

DOI： 10.1029/1999GL003620

DOI： 10.1029/1999gl003620

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

We investigated statistical characteristics of dayside Pi 2 pulsations observed at Mineyama (25.5 degrees geomagnetic latitude) from November 1994 through June 1996, using an algorithm to detect Pi 2 pulsations which was introduced in Part I of the accompanying paper. We obtained the following results. (1) The ratio of the number of Pi 2 pulsations on the dayside (06-18 MLT) to that on the nightside (18-06 MLT) was about 31%. (2) The polarization of the dayside Pi 2 pulsations changed from right-handed before local noon to left-handed after local noon. (3) Wave power of dayside Pi 2 pulsations in the H-component has a peak around local noon and dips on both dawn and dusk sides. (4) Frequency of the fundamental wave is ranging from 9 mHz to 30 mHz with dominant frequency of 17-24 mHz, and frequency ratios of the first three harmonics are 1 : (1.7 +/- 0.5) : (2.3 +/- 0.7). We found that the magnetospheric cavity model can explain most of these observational results.

DOI： 10.1186/BF03352206

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J-GLOBAL

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

This paper reports on the enhancement of the lobe plasma velocity towards the neutral sheet. Analyzing the Geotail plasma and magnetic field data in the lobes earthward of X = -45 R-E, we identified 30 events of several-minute magnetic field increase-and-decrease during which the field tilts only southward; that is, the B-Z variation is unipolar. For most of these events, we found that the lobe plasma velocity towards the neutral sheet is enhanced and peaked around the most southward tilting of the field. The average of the peak magnitude is about 130 km/s in a plane perpendicular to the magnetic field. For each case, this magnitude is roughly 1 - 10 % of the lobe Alfven speed. We interpret the velocity enhancement having these characteristics as sig-natures of the transition from plasma sheet reconnection to lobe reconnection at the near-Earth neutral line.

DOI： 10.1029/1998GL900132

DOI： 10.1029/1998gl900132

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The timing of the near-Earth neutral line (NENL) formation relative to substorm onset signatures has been a central debate in substorm research for some time. Several recent studies have suggested that the direct identification of the reconnection site, at least in its initial stage, is difficult because it may be a very limited region spatially. Our approach to this timing problem is the examination of the midtail lobe traveling compression regions (TCRs) relative to onset during substorm events. Fifty-nine midtail TCRs in the IMP 8 magnetotail observations are analyzed. These midtail TCRs differ from the distant tail TCRs in that they lack the first half of the north and south tilting of the field during the compression. The flat B-Z phase preceding the south tilting interval in these midtail compression regions has been interpreted as being due to the close proximity to the growing plasmoid bulge just prior to plasmoid ejection down the tail. The results of our analysis show that these TCRs start a few minutes before or simultaneously (accuracy of 1 min) with Pi2 onset at low-latitude or midlatitude ground stations and the particle injection at the geosynchronous orbit. Before the plasmoid bulge formation starts, the NENL must start to form in the plasma sheet; that is, the formation of NENL must initially occur on closed field lines to form the plasmoids which compress the lobes to cause the TCRs. Our results support this view and allow us to conclude that NENL forms before ground and geosynchronous substorm onset signatures, i.e., in the late growth phase. The implications of these results for magnetospheric substorm models are discussed.

DOI： 10.1029/98JA02617

DOI： 10.1029/98ja02617

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Using data from the polar orbiting Dynamic Explorer (DE) -1 and -2 satellites and a ground-based station, we investigated electron precipitation phenomena accompanying Pc 5 pulsations. DE-2 observed oscillatory disturbances in the magnetic and electric fields in the upper ionosphere at the geomagnetic footprint of the high altitude region in which transverse Pc 5 pulsations were detected by DE-1. DE-2 observed electrons precipitating into the ionosphere with energies of several keV to several tens of keV, These electrons were accelerated in the direction of the ambient magnetic field. When Pc 5 pulsations in the H-component and periodic variations of cosmic radio noise absorption (CNA pulsations) were observed at Syowa Station, DE-2 which was in geomagnetic conjunction with Syowa Station also observed oscillatory disturbances in the magnetic and electric fields. These oscillatory disturbances are caused by small-scale field-aligned currents each with. width of 0.5 degrees-1.4 degrees invariant latitude. This suggests that Pc 5 pulsations have a small-scale resonance structure in the radial direction. The resonance structure has a small scale comparable to the ion acoustic gyroradius, then kinetic Alfven waves having electric fields parallel to the ambient magnetic field can arise. The parallel electric field generates a field-aligned potential drop of about 3-5 kV. Electrons accelerated by these kinetic Alfven waves would cause CNA pulsations, the phase of which leads that of the B-component of the Pc 5 pulsations by 90 degrees in the southern hemisphere. This is consistent with the observations at Syowa Station.

DOI： 10.1029/98JA01187

DOI： 10.1029/98ja01187

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER-VERLAG BERLIN  

Tohru Araki, Toshihiko Iyemori, Masahito Nose, Takaaki Wada, Yoshifumi Futaana, Genta Ueno, Toyohisa Kamei, Akiniro Saito, 1998, &#039;Application of Discovery Science to Solar-Terrestrial Physics&#039;, &lt;i&gt;Discovey Science&lt;/i&gt;, pp. 451-452

DOI： 10.1007/3-540-49292-5_66

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Wavelet analysis is suitable for investigating waves, such as Pi 2 pulsations, which are limited in both time and frequency. We have developed an algorithm to detect Pi 2 pulsations by wavelet analysis. We tested the algorithm and found that the results of Pi 2 detection are consistent with those obtained by visual inspection. The algorithm is applied in a project which aims at the nowcasting of substorm onsets. In this project we use real-rime geomagnetic field data. with a sampling rate of 1 second, obtained at mid- and low-latitude stations (Mineyama in Japan, the York SAMNET station in the U.K., and Boulder in the U.S.). These stations are each separated by about 120 degrees in longitude, so at least one station is on the nightside at all times. We plan to analyze the real-time data at each station using the Pi 2 detection algorithm, and to exchange the detection results among these stations via the Internet. Therefore we can obtain information about substorm onsets in real-time, even if we are on the dayside. We have constructed a system to detect Pi 2 pulsations automatically at Mineyama observatory. The detection results for the period of February to August 1996 showed that the rate of successful detection of Pi 2 pulsations was 83.4% for the nightside (18-06MLT) and 26.5% for the dayside (06-18MLT). The detection results near local midnight (20-02MLT) give the rate of successful detection of 93.2%.

DOI： 10.1186/BF03352169

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

To study the ring current structure in the inner magnetosphere, we have statistically examined the magnetic field data acquired by ETS-VI (the Engineering Test Satellite-VI). During a magnetic storm, the Dst index shows a rapid recovery of its amplitude for about 9 hours on average after the main phase and a subsequent long-lasting slow recovery. We have investigated this "two-step recovery" of the Dst index by obtaining magnetic field vectors and calculating the current structure in the inner magnetosphere for each magnetic storm phase determined by the Dst index. From this study, following results are obtained: (1) Throughout the storm-time, disturbed magnetic fields exhibit clear day-night asymmetry with strong peak in the nightside. (2) During the main phase, southward perturbed field components have a relative bump in the nightside region between similar to 2000 and similar to 0400 MLT and between similar to 4.0 and 6.4 R-E (geocentric distances in Earth radii). (3) The initial rapid recovery of the Dst index is considerably influenced by the nightside currents flowing between similar to 1800 and similar to 0600 MLT and between 5.6 and 7.2 R-E. These currents are thought to be mainly composed of the particles that escape the magnetosphere on the duskside flank, which are simulated in particle tracing in a realistic magnetosphere.

DOI： 10.1186/BF03352179

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

The magnetic field data from the Engineering Test Satellite-VI (ETS-VI) have been analyzed to investigate the occurrence distributions of pulsations in Pc 3-5 frequency ranges in the magnetosphere. The observation of ETS-VI covered the invariant latitude (ILAT) range of 64.5 degrees-69 degrees ILAT near the geomagnetic equator (-10 degrees-20 degrees magnetic latitude) at all magnetic local time (MLT). Magnetic pulsations were selected by the Fast Fourier Transform method and checked by visual scanning if they have continuous waveforms. From the occurrence distributions of pulsations, we have found distinctive features in the following pulsations: (1) azimuthal Pc 5 pulsation; (2) azimuthal Pc 3 pulsation; (3) radial Pc 4 pulsation on the dayside; (4) azimuthal Pc 4 pulsations on the nightside, In respect of the first three types of pulsations (i.e., the azimuthal Pc 5 pulsation, the azimuthal Pc 3 pulsation, and the radial Pc 4 pulsation on the dayside), the results presented in this study confirm the previous results obtained by other satellite observations. The azimuthal Pc 4 pulsations on the nightside were observed in continuous waveforms lasting for about 10 minutes. Although the azimuthal Pc 4 pulsations on the nightside start at almost the same time as substorm onsets, they are different from Pi 2 pulsations in the magnetosphere. They are observed frequently in the MLT range of 23-04MLT with an occurrence peak at 01-02MLT. We suggest that the azimuthal Pc 4 pulsations on the nightside are excited through coupling to the fast mode Alfven waves which were launched at substorm onset.

DOI： 10.1186/BF03352087

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J-GLOBAL

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

The magnetic field experiment on board ETS-VI (the Engineering Test Satellite-VI) and initial results from the experiment are presented. ETS-VI was launched on August 28, 1994 and has been placed in a near equatorial orbit with a perigee of 2.3Re, an apogee of 7.1Re, an inclination of 13.4 degrees, and an orbital period of 14.4 hours. The spacecraft is three-axis stabilized. The magnetic field experiment on board the satellite consists of a triaxial fluxgate magnetometer. The magnetometer has a sampling interval of 3 s, and except near perigee, it is operated in the mode having a +/-256 nT dynamic range and a 0.125 nT resolution. We have examined the magnetometer data obtained at radial distances from 5.0 to 7.1Re, magnetic latitudes from -10 degrees to 25 degrees, and local times from 14 MLT through midnight to 04 MLT, to study magnetic field variations associated with substorms. Substorm-associated field variations are easily seen at larger radial distances and at local times later than 19 MLT. Using 92 substorm events that occurred in the 21-01 MLT sector, we have constructed the average field configuration during the growth phase. The magnetic field becomes highly taillike and its intensity increases at high latitudes (&gt;10 degrees), while the field intensity decreases at lower latitudes (&lt;10 degrees). The field configuration suggests that the inner edge of the growth phase current system is located near the synchronous altitude at the end of the growth phase.

DOI： 10.5636/jgg.48.741

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CiNii Research

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Language：English   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Earth, Planets and Space  

The magnetic field experiment on board ETS-VI (the Engineering Test Satellite-VI) and initial results from the experiment are presented. ETS-VI was launched on August 28, 1994 and has been placed in a near equatorial orbit with a perigee of 2.3Re, an apogee of 7.1 Re, an inclination of 13.4°, and an orbital period of 14.4 hours. The spacecraft is three-axis stabilized. The magnetic field experiment on board the satellite consists of a triaxial fluxgate magnetometer. The magnetometer has a sampling interval of 3 s, and except near perigee, it is operated in the mode having a ±256 nT dynamic range and a 0.125 nT resolution. We have examined the magnetometer data obtained at radial distances from 5.0 to 7.1Re, magnetic latitudes from -10° to 25°, and local times from 14 MLT through midnight to 04 MLT, to study magnetic field variations associated with substorms. Substorm-associated field variations are easily seen at larger radial distances and at local times later than 19 MLT. Using 92 substorm events that occurred in the 21-01 MLT sector, we have constructed the average field configuration during the growth phase. The magnetic field becomes highly taillike and its intensity increases at high latitudes (>10°), while the field intensity decreases at lower latitudes (<10°). The field configuration suggests that the inner edge of the growth phase current system is located near the synchronous altitude at the end of the growth phase.

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Using the magnetic field data obtained by the DE-1 polar orbiting satellite, statistical characteristics of transverse Pc5 pulsations in the inner magnetosphere are examined. The occurrence distribution is found to have a strong dawn/dusk asymmetry. The occurrence is most frequent in the region around 72 degrees invariant latitude (ILAT) between 0800 and 1000MLT. The distribution shifts to lower ILAT both in the early morning and in the afternoon sectors. Our results are generally consistent with the occurrence distributions obtained by previous studies except for appreciable differences in the local time of the peak occurrence. The dependence of the transverse Pc5 occurrence on solar wind velocity is also investigated. Results show that the higher the solar wind velocity, the more frequent the Pc5 occurrence. This fact suggests that the energy source of these Pc5's is in the Kelvin-Helmholtz instability (KHI) on the magnetopause. To explain the strong dawn/dusk asymmetry, the dependence of the Pc5 occurrence on the angle between the solar wind velocity and the IMP in the ecliptic plane is investigated. It is found that this angle controls the magnetic local time of Pc5 appearance when the solar wind velocity is small. This dependence may be explained by the low threshold of KHI due to an influence of a quasi-parallel bow shock.

DOI： 10.1029/95GL01794

DOI： 10.1029/95gl01794

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J-GLOBAL

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J-GLOBAL

Language：Japanese   Publishing type：Lecture material (seminar, tutorial, course, lecture, etc.)  

Language：English   Publishing type：Lecture material (seminar, tutorial, course, lecture, etc.)  

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J-GLOBAL

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

Language：English   Publishing type：Research paper, summary (international conference)  

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

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Language：English   Publishing type：Book review, literature introduction, etc.   Publisher：Blackwell Publishing Ltd  

We investigate variations of ion flux over the ionosphere and in the plasma sheet when storm time substorms are initiated, using simultaneous observations of neutral atoms in the energy range of up to a few keV measured by the low-energy neutral atom (LENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite, outflowing ion flux of &lt
1 keV measured by the ion electrostatic analyzer (IESA) on board the Fast Auroral SnapshoT (FAST) satellite, and energetic (9-210 keV/e) ion flux measured by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite. We examined three storm intervals during which the IMAGE or FAST satellite was in a suitable location to observe ionospheric ion outflow and the Geotail satellite was in the plasma sheet on the nightside. The neutral atom flux observed by IMAGE/LENA in the first interval and outflowing ion flux observed by FAST/IESA in the second and third intervals indicate that storm time substorms can cause increases of lowenergy ion flux over the ionosphere by a factor of 3-50 with time delay of less than several minutes. In the plasma sheet, the flux ratio of O+/H+ is rapidly enhanced at the storm time substorms and then increased gradually or stayed at a constant level in a time scale of ∼1 h, suggesting a mass-dependent acceleration of ions at local dipolarization and a subsequent additional supply of O+ ions to the plasma sheet which have been extracted from the ionosphere at the substorms. These coordinated observations revealed that substorms have both an immediate effect and a delayed effect (i.e., two-step effect) on the ion composition in the plasma sheet. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2009JA014048

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

We investigate the ion composition of the near-Earth plasma sheet in storm and quiet intervals, using energetic (9-210 keV) particle flux data obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. In 1998 four magnetic storms (minimum Dst &lt; -50 nT) occurred when Geotail was located in the near-Earth plasma sheet (X greater than or equal to -10 R-E) For each of the storms, we have selected a col responding quiet interval from time periods when Geotail revisited the near-Earth plasma. sheet under the condition Dst &gt; -20 nT, The energy density of the H+, He+, and O+ ions was computed from the EPIC/STICS data for these storm and quiet-time events. We obtained. the following results: (1) The energy density is higher during storms than during quiet times for all ion species (H+, He+, and O+); (2) the He+/H+ energy density ratio during storms is 0.01-0.02, while that, during quiet times is similar to0.01; and (3) the O+/H+ energy density ratio is significantly larger during storms (0.2-0.6) than during quiet times (0.05-0.1). To explain these results we suggested a current sheet acceleration mechanism in which ions are energized by the dawn-to-dusk convection electric field in a mass-dependent way in the course of interaction with the current sheet.

DOI： 10.1029/2000JA000376

Web of Science

Language：English   Publisher：AMER GEOPHYSICAL UNION  

Measurements from the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft were used to investigate dynamics of O+ ions of ionospheric origin at energies of 9 keV to 210 keV in the near-Earth plasma, sheet during the substorm expansion phase. Substorm signatures were clearly observed on the ground at 1850 UT on May 17, 1995. In the expansion phase of this substorm, Geotail stayed in the plasma sheet at X similar to-10.5 R-E and observed a local dipolarization signature accompanied by strong disturbances of the magnetic field. From the energetic ion flux data of EPIC/STICS, we obtained the following results: (1) energetic flux enhancement was more pronounced for O+ than for H+; (2) the flux was enhanced almost simultaneously with local dipolarization; (3) the enhancement factor of O+ ions (EO+), which represents the enhancement of the O+ flux ratio (after and before substorm onset) relative to the H+ flux ratio, was Bs large as 1.31; and (4) thermal energy increased from 8.9 keV to 42.8 keV for O+ ions and from 9.4 keV to 15.9 keV for H+ ions. We also performed statistical analysis for 35 events of local dipolarization found in the near-Earth region (X similar to-6 to -16 R-E). We found that EO+ is larger than unity in all ranges of radial distance and that the average value of EO+ is 1.37. These results suggest that O+ ions are commonly more energized than H+ ions during the substorm expansion phase. To interpret these observational results, we propose a mechanism in which ions are accelerated in a non-adiabatic way during substorm-associated field reconfiguration.

DOI： 10.1029/1999JA000318

Web of Science

Language：Japanese  

CiNii Books

Other Link： http://hdl.handle.net/2433/80381

Language：English   Publisher：TERRA SCIENTIFIC PUBL CO  

Wavelet analysis is suitable for investigating waves, such as Pi 2 pulsations, which are limited in both time and frequency. We have developed an algorithm to detect Pi 2 pulsations by wavelet analysis. We tested the algorithm and found that the results of Pi 2 detection are consistent with those obtained by visual inspection. The algorithm is applied in a project which aims at the nowcasting of substorm onsets. In this project we use real-rime geomagnetic field data. with a sampling rate of 1 second, obtained at mid- and low-latitude stations (Mineyama in Japan, the York SAMNET station in the U.K., and Boulder in the U.S.). These stations are each separated by about 120 degrees in longitude, so at least one station is on the nightside at all times. We plan to analyze the real-time data at each station using the Pi 2 detection algorithm, and to exchange the detection results among these stations via the Internet. Therefore we can obtain information about substorm onsets in real-time, even if we are on the dayside. We have constructed a system to detect Pi 2 pulsations automatically at Mineyama observatory. The detection results for the period of February to August 1996 showed that the rate of successful detection of Pi 2 pulsations was 83.4% for the nightside (18-06MLT) and 26.5% for the dayside (06-18MLT). The detection results near local midnight (20-02MLT) give the rate of successful detection of 93.2%.

DOI： 10.5636/eps.50.773

Web of Science