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

At night in Earth's polar regions, energetic aurorae frequently penetrate into the atmosphere, with the peculiar effect of driving turbulent electrojet currents in the bottomside ionosphere. During the day, however, Earth's plasma environment becomes highly conductive, owing to the constant flux of extreme ultraviolet radiation emitted from the Sun. The high-conductivity plasma in the dayside ionosphere can effectively short out plasma turbulence around aurorae, and so electrojet turbulence is thought rare in the dayside high-latitude ionosphere. In this paper we show observations to the contrary. During the onset of the 23 April 2023 geomagnetic storm, we observed prolific small-scale plasma turbulence in the dayside E region on closed magnetic field lines just equatorward of the cusp. Using data from two orbiting satellites, we infer the locations of the cusp and the distributed presence of diffuse aurorae, through observations of electron fluxes and wave-particle interactions near the magnetospheric equator, on nearby magnetic field lines. The resulting diffuse aurorae pass electric fields and produce unstable gradients in the plasma density. The number and intensity of the falling charges momentarily overwhelm the capacity of the lower ionosphere to extinguish the strong electric fields that follow from this action, spurring the growth of transient, turbulent electrojets, or Hall currents. In the 23 April 2023 case study, we establish a correlation between observations of chorus-wave activity near the magnetospheric equator and observations of turbulent electrojets in the ionosphere on closed magnetic field lines, from which we infer a causal chain where magnetospheric plasma waves ultimately drive small-scale turbulence in the ionosphere. We show how the predictions are brought to fruition in similar supporting events. Finally, we briefly discuss the implications that this discovery bears for the electrodynamics of the dayside ionosphere. In the following paper [M. F. Ivarsen, Phys. Rev. E 112, 045203 (2025)10.1103/3bzj-bsf8] we follow the lengthy argument to a logical conclusion, leading to an alternative description of electrodynamics in the cusp region.

DOI： 10.1103/r6bv-pzlq

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その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/r6bv-pzlq/fulltext

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

Using Arase satellite observations, this study provides a comprehensive statistical analysis of ions (H⁺, He⁺, O⁺) and electron contributions to the total ring current pressure during storms with two different drivers. The results demonstrate the effect of different solar wind drivers on the composition, energy distribution, and spatial characteristics of the ring current. Using 32 CIR- and 30 Interplanetary Coronal Mass Ejection (ICME)-driven storms, we characterize the ring current pressure evolution during the prestorm, main, early-recovery, and late-recovery storm phases as a function of magnetic local time and L-shell. In CIR-driven storms, H⁺ ions are the dominant (∼70%) contributor to the total ring current pressure during main/early recovery phases and increasing to ∼80% during late recovery. In contrast, the O⁺ pressure (E = 20–50 keV) response is significantly stronger in ICME-driven storms contributing ∼40% to the overall pressure during the main/early recovery phases and even dominate (∼53%) in certain MLT sectors. Additionally, ICME-driven storms tend to have peak pressure at lower L-shells (L ≈ 3–4), while CIR-driven storms show pressure peaks at slightly higher L-shells (L ≈ 4–5). Interestingly, electron pressure also plays a notable role in specific MLT sectors, contributing ∼18% (03–09 MLT) during the main phase of CIR-driven storms and ∼11% (21–03 MLT) during ICME-driven storms. The results highlight that the storm time electron pressure plays a crucial role in the ring current buildup. Another noteworthy feature of this study is that Arase's fine-energy resolution and broad coverage enable a detailed investigation of energy-dependent ring current dynamics.

DOI： 10.1029/2025JA034182

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

The number of resident space objects (RSOs), including debris and inactive satellites, is rapidly increasing, posing a growing threat to operational spacecraft. Additionally, many of these RSOs are too small to be reliably tracked by current radars. A novel solution to this problem is to use the disturbances in the plasma environment to track these objects. Over the last decade, theoretical, numerical, laboratory, and observational efforts have studied the generation and propagation of these disturbances. This study investigates these disturbances statistically using 5 months of Very Low Frequency (VLF) Electric (E) field data collected by the Arase satellite. This represents the first large-scale statistical observational study of RSO-generated waves. Specifically, we use E field power between 1 and 20 kHz from the Onboard Frequency Analyzer, which is part of the Plasma Wave Experiment (PWE) onboard the Arase satellite. We analyze this dataset by comparing the power between an “experiment” population of measurements taken where Arase was in a situation where RSO-generated waves would be observed, and a “control” population where Arase was in a situation where no RSO-generated waves would be observed. For robust comparisons, the two populations should have similar distributions of latitude, longitude, altitude, and time. However, rigidly enforcing this matching results in very small sample sizes. We therefore use three different approaches to this balancing. Our strongest results have a p-value of 1.3e-5, indicating that there is a less than 1 in 77,000 chance that the experiment and control populations are drawn from the same parent population. We take this as strong statistical evidence of RSO-generated waves. These encouraging results bolster the feasibility of using these waves as part of a future detection, identification, and tracking system.

DOI： 10.1016/j.asr.2025.07.013

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

The Earth's outer radiation belt is populated by relativistic ((Formula presented.) keV) electrons, which are typically confined by the strong dipole magnetic field but can precipitate into the atmosphere through scattering by electromagnetic waves. In contrast, the magnetotail primarily contains electrons with energies below 200 keV, which are predominantly scattered and precipitated due to magnetic field-line curvature scattering (FLCS). In this study, we demonstrate that FLCS can also scatter and precipitate relativistic electrons from the outer radiation belt. Using coordinated observations from the ERG/Arase satellite and low-altitude ELFIN CubeSats in the outer radiation belt, we compare electron fluxes across different (Formula presented.) -shells and energy ranges. Our analysis reveals that the outer edge of the radiation belt exhibits isotropic electron populations above a minimum energy that increases with proximity to Earth. Such isotropization energy dependence on distance, or (Formula presented.) -shell, agrees with that observed simultaneously at the ELFIN satellite, at low-Earth orbit, where it has been known as the electron isotropy boundary (IBe). This agreement between low-altitude and near-equatorial observations during satellite conjunctions suggests that the IBe pattern may extend to the outskirts of the traditional outer radiation belt. From that distance, the associated FLCS may facilitate precipitation of relativistic electrons up to several MeV. Therefore, FLCS—known to shape the IBe pattern —plays a key role in radiation belt dynamics.

DOI： 10.1029/2025JA034184

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

Substorm energetic electron injections serve as a significant energy source for chorus wave generation, markedly altering the distribution of energetic electrons. Using the Arase satellite data, we present direct evidence for the nonlinear evolution of chorus waves following a substorm injection. The substorm injection causes the enhancement of energetic electron fluxes (∼20–200 keV) during which chorus waves appear as clear and intense rising-tone elements. Linear theoretical analysis shows that anisotropic energetic electrons provide free energy for the generation of seed chorus waves and the enhancement of energetic electrons increases the linear growth rate. Furthermore, nonlinear theoretical analysis shows that the increase in energetic electrons reduces the threshold amplitude, which is conducive to the chorus wave entering the nonlinear growth stage. These results indicate that nonlinear growth plays a significant role in the amplification and spectral evolution of chorus waves through a decrease in the threshold amplitudes.

DOI： 10.1029/2025JA033931

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記述言語：日本語   出版者・発行元：京都大学生存圏研究所電波科学計算機実験共同利用・共同研究専門委員会  

DOI： 10.14989/kdk_report_2024_31

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

This study demonstrates the influence of electromagnetic ion cyclotron (EMIC) rising-tone emissions on relativistic electrons in the inner magnetosphere using data obtained from the Van Allen Probes and Arase satellites. We find that the intense EMIC rising-tone emissions occur during the increase in the solar wind pressures, creating favorable conditions for triggering nonlinear wave growth. The strong flux drop-out of relativistic electrons in the parallel directions of the magnetic field, with energies of 0.2–4 MeV, was associated with the wave activity. We calculated the nonlinear triggering conditions and the minimum resonant energy of relativistic electrons interaction with EMIC waves, based on our observations. We conclude that EMIC rising-tone emissions contribute not only to the rapid loss of MeV electrons through EMIC wave-particle interactions while extending the resonance energy to a few MeV by broadening bandwidth via nonlinear wave growth but also to interactions with sub-MeV electrons through the nonresonant effect.

DOI： 10.1029/2024GL113855

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

Whistler-mode chorus waves play important roles in the development of energetic electron populations in the Earth's inner magnetosphere. We have statistically analyzed rapid changes in the electron flux associated with chorus waves using data from the Arase satellite. The Arase satellite observations obtained from 23 March 2017 to 12 October 2018 show that the rapid changes are concentrated near the magnetic equator from nightside to dawnside. We compared the energy and pitch angle range of the rapid changes in the electron flux with the region bounded by the resonance energy curve of whistler mode waves which are calculated from properties of the observed chorus waves in 46 events. This comparison shows that, for most of the events, the energy and pitch angle range of the rapid changes in the electron flux can be explained by the first-order cyclotron resonance with the observed chorus waves. We also found that the timescale for the change in the electron pitch angle distribution ranges from several seconds to a few tens of seconds. This timescale is much faster than that expected by quasi-linear diffusion theory, suggesting that nonlinear wave-particle interactions play important roles in the deformation of the electron pitch angle distributions.

DOI： 10.1029/2024JA033684

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

During the May 2024 storm, the minimum Dst index was approximately −412 nT, marking the largest geomagnetic storm of the past decade. This event caused the inner edge of the ring current to penetrate deeply into the inner magnetosphere during the main phase of the storm. We present observations of high-frequency electromagnetic ion cyclotron (HF EMIC) wave activity during this intense geomagnetic storm using data from the Arase satellite. Arase observations showed that HF EMIC waves with frequencies of 5–36 Hz at L ∼ 2, occurred mainly during the main and early-recovery phases. The minimum resonance energy of energetic protons and relativistic electrons associated with HF EMIC waves suggests their potential to cause the loss of relativistic electrons in the low L-shell region. Our observations provide new insights into the generation of EMIC waves and the dynamics of energetic particles at low L-shells in the inner magnetosphere.

DOI： 10.1029/2024GL112489

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

Electrostatic Cyclotron Harmonic (ECH) waves have been considered a potential cause of pitch angle scattering of electrons in the energy range from a few hundred eV to tens of keV. Theoretical studies have suggested that scattering by ECH waves is enhanced at lower pitch angles near the loss cone. Due to the insufficient angular resolution of particle detectors, it has been a great challenge to reveal ECH-driven scattering based on electron measurements. This study reports on variations in electron pitch angle distributions associated with ECH wave activity observed by the Arase satellite. The variation is characterized by a decrease in fluxes near the loss cone, and energy and pitch angle dependence of the flux decrease is consistent with the region of enhanced pitch angle scattering rates predicted by the quasi-linear diffusion theory. This study provides direct evidence for energy-pitch angle dependence of pitch angle scattering driven by ECH waves.

DOI： 10.1029/2024GL113188

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出版者・発行元：2025 Ursi Asia Pacific Radio Science Meeting AP Rasc 2025  

Space debris is a rapidly growing hazard for space-based assets, with even millimeter sized debris capable of inflicting serious or mission-ending damage. While debris larger than ~10 cm can be tracked with radar, and collision avoidance maneuvers taken as precaution, no such recourse is yet available against the smaller debris. To answer this threat, the Intelligence Advanced Research Projects Activity (IARPA) Space Debris Identification and Tracking (SINTRA) program aims to develop a next-generation capability to detect, track, and characterize small space debris (<10 cm). In this paper, we present work done as a part of the SINTRA effort in which plasma waves, potentially generated by resident space objects (RSOs) are detected using the Japan Aerospace Exploration Agency's (JAXA's) Arase satellite. The effort seeks to verify that plasma waves observed in the vicinity of Arase are generated by RSOs in close proximity. This work also investigates the potential to determine properties of the RSO by evaluating properties of the plasma waves observes by Arase. The work presented covers the first step in this process including detecting plasma waves and verifying statistically significant confidence of their association with RSOs.

DOI： 10.46620/URSIAPRASC25/SVMB7532

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

In this study, we investigated terrestrial-origin O⁺ ions below 1 keV around the Moon using data from the Kaguya satellite between December 2007 and June 2009. These terrestrial-origin low-energy O⁺ ions were identified based on three parameters: the periodicity of O⁺ ion count enhancement corresponding to Kaguya’s 2-h orbital period, the count ratio of O⁺ ions to Na⁺ and Al⁺ ions, and the direction of ion bulk velocity in the Sun–Earth direction. We identified three intervals that included such O⁺ ions: 14:30–20:30 UT on June 19, 2008, 19:00 UT on July 16, 2008 to 03:00 UT on July 17, 2008, and 14:00–24:00 UT on June 7, 2009. These intervals were found in the dawn sector, the dusk sector, and the midnight to dawn sector within the magnetotail, respectively. We examined the relation between geomagnetic storm conditions and increases in terrestrial-origin O⁺ ion counts and found that all three intervals occurred during the late recovery phase of moderate/weak magnetic storms. Since moderately/weakly disturbed conditions (Dst = –40 nT to –20 nT) account for approximately 21% of the total time between 1957 and 2016, we suggest that low-energy O⁺ ions from the Earth have a non-negligible impact on the ion composition and the ion mass density in the lunar plasma environment.

DOI： 10.1186/s40623-024-02107-3

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

Near-equatorial measurements of energetic electron fluxes, in combination with numerical simulation, are widely used for monitoring of the radiation belt dynamics. However, the long orbital periods of near-equatorial spacecraft constrain the cadence of observations to once per several hours or greater, that is, much longer than the mesoscale injections and rapid local acceleration and losses of energetic electrons of interest. An alternative approach for radiation belt monitoring is to use measurements of low-altitude spacecraft, which cover, once per hour or faster, the latitudinal range of the entire radiation belt within a few minutes. Such an approach requires, however, a procedure for mapping the flux from low equatorial pitch angles (near the loss cone) as measured at low altitude, to high equatorial pitch angles (far from the loss cone), as necessitated by equatorial flux models. Here we do this using the high energy resolution ELFIN measurements of energetic electrons. Combining those with GPS measurements we develop a model for the electron anisotropy coefficient, (Formula presented.), that describes electron flux (Formula presented.) dependence on equatorial pitch-angle, (Formula presented.), (Formula presented.). We then validate this model by comparing its equatorial predictions from ELFIN with in-situ near-equatorial measurements from Arase (ERG) in the outer radiation belt.

DOI： 10.1029/2024JA033155

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

We present the first direct evidence of an in situ excitation of drift-compressional waves driven by drift resonance with ring current protons in the magnetosphere. Compressional Pc4–5 waves with frequencies of 4–12 mHz were observed by the Arase satellite near the magnetic equator at L ∼ 6 in the evening sector on 19 November 2018. Estimated azimuthal wave numbers (m) ranged from −100 to −130. The observed frequency was consistent with that calculated using the drift-compressional mode theory, whereas the plasma anisotropy was too small to excite the drift-mirror mode. We discovered that the energy source of the wave was a drift resonance instability, which was generated by the negative radial gradient in a proton phase space density at 20–25 keV. This proton distribution is attributed to a temporal variation of the electric field, which formed the observed multiple-nose structures of ring current protons.

DOI： 10.1029/2023GL107707

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

Internally driven Pc4-5 waves are excited in the plasmaspheric drainage plume (PDP) and near the plasmapause. The excitation of ultralow frequency (ULF) waves was investigated by using the magnetosphere-ionosphere coupled model between Geospace Environment Modeling System for Integrated Studies-Ring Current (GEMSIS-RC) and GEMSIS-POTential solver (GEMSIS-POT). In order to investigate the effects of cold plasma on the wave excitation, the simulation code to describe the dynamics of cold plasma was included in the model. The model can reproduce the shrink of the plasmasphere on the nightside and the formation of the PDP on the dayside. First, fundamental Pc5 waves are excited through the drift resonance on the dayside. The waves are caused by positive energy gradient of ion phase space density (PSD) at 50–130 keV. Second harmonic waves (drift-bounce resonance) are generated outside the plasmapause. These two types of ULF waves are also seen in the case of constant density. Unlike the case of constant density, localized eastward propagating Pc4 waves (drift resonance) are seen on the dawnside associated with the azimuthal density gradient. The azimuthal wave number of Pc4 waves is about 70 and anti-earthward gradient of PSD about 10 keV contributes to wave growth. We also detect fundamental Pc4-5 waves near the plasmapause on the nightside in the drift resonance with 100–150 keV ions. Simulation results suggest that the plasmapause has an effect to sustain the excitation of Pc4-5 ULF wave through the drift resonance.

DOI： 10.1029/2023JA031638

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