篠原 育

Abstract Ground‐based very low frequency transmitters emit signals that primarily propagate within the Earth–ionosphere waveguide, and some of their energy can propagate into the magnetosphere. Ionospheric observations from the DEMETER satellite reveal a distinct concentric rings pattern of the wave power distribution of the North West Cape transmitter on the transverse (longitude–L‐shell) plane. Using data from the Van Allen Probes and Arase/energization and Radiation in Geospace satellites, we find that the concentric rings pattern is still visible but becomes less distinct. The pattern shifts outward, and becomes more elliptic as the wave propagates from the southern ionosphere to the Northern Hemisphere. To investigate the cause of this evolution, we performed ray tracing simulations under three conditions: ducted propagation, non‐ducted propagation with vertical initial wave normal angles and non‐ducted propagation with spread initial wave normal angles. The results show that non‐ducted propagation with spread wave normal angles best explains the observed evolution of wave power distribution pattern during the propagation.

Abstract Pulsating aurorae are prominent auroral emissions in the polar regions, typically occurring in the morning hours during the recovery phase of auroral substorms. These aurorae usually consist of round‐shaped patches of emission, with luminosity that pulsates at intervals ranging from less than a second to several tens of seconds. Here, we present, for the first time, a unique case of a pulsating aurora that expanded radially outward in all the directions and repeatedly formed a ring‐shaped structure. The speed of expansion, which was at least several tens of kilometers per second at ionospheric altitudes, cannot be attributed to the horizontal convective motion of plasma in the ionosphere. In the magnetosphere, corresponding to the expanding ring‐shaped aurora, the Arase satellite detected successive enhancements of natural electromagnetic waves known as a “chorus.” These chorus waves scatter energetic magnetospheric electrons into the ionosphere, resulting in pulsating diffuse aurorae. Notably, the satellite observed systematic delay in the timing of chorus detections, which suggests that a similar circularly expanding feature existed in space. These simultaneous observations of expanding features in both the ionosphere and the magnetosphere demonstrate that the temporal evolution of the shape of a pulsating aurora manifests the spatiotemporal evolution of the source of plasma waves in space.

Abstract During the super geomagnetic storm of 10–11 May 2024, an extreme enhancement in plasma mass density was observed in the deep inner magnetosphere near $$L \sim 2.5$$ . Multi-point ground magnetometer observations revealed that this enhancement extended across widely separated longitudinal sectors—from New Zealand through Europe to eastern North America—during the storm main phase and early recovery phase. The maximum density, approximately 35,000 amu/cm $$^{3}$$ , was detected near $$L = 2.1$$ in the New Zealand longitude sector during the storm main phase. To investigate the origin of this anomalous mass loading and the associated highly O $$^{+}$$ -rich plasma state, we employ an integrated analysis combining multi-point ground magnetometer measurements, Arase satellite observations, DMSP satellite data, and total electron content (TEC) distributions derived from global GNSS networks. Ground-based magnetometer observations provide spatially distributed field line resonance (FLR) signatures that enable estimation of equatorial plasma mass density based on assumed field-aligned density profiles. Arase in situ measurements of plasma wave spectra, magnetic fields, and energetic particle fluxes enable estimation of local plasma density and characterization of ion and electron energy distributions. DMSP-F17 observations supply complementary ionospheric parameters including electron temperature, while GNSS TEC maps reveal large-scale ionospheric electron depletion and its regional evolution. This coordinated multi-dataset approach enables systematic characterization of the unique inner magnetospheric plasma state during this extreme event. Plasmaspheric electron densities derived from Arase plasma wave measurements indicate in situ electron densities of approximately 1,500 cm $$^{-3}$$ at $$L \sim 2.5$$ . Combined with mass density estimates, the inferred ion composition consistently indicates heavy-ion dominance, with O $$^{+}$$ fractions exceeding 90% in some regions. The coexistence of cold and warm plasma populations observed by Arase near the plasmapause, together with elevated ionospheric electron temperatures detected by DMSP-F17 and significant TEC depletion, suggests that cold plasmaspheric electrons were heated through Coulomb collisions with storm-time injected warm ions. This process likely led to enhanced heating of ionospheric electrons and subsequent heavy-ion upflow along affected flux tubes. These results indicate that superstorm-level magnetospheric convection can produce rapid plasma mass loading at unusually low L -shells during the storm main phase, leading to the formation of an O $$^{+}$$ -rich plasmasphere, in contrast to the conventional recovery-phase refilling scenario. The findings highlight the critical role of ionospheric outflow in regulating inner magnetospheric plasma mass density under superstorm conditions.

Abstract Inverted‐V ion structures in energy‐time spectrograms are typically associated with quasi‐static potential structures and have generally been observed as unidirectional signatures in previous studies. Based on observations from the Arase satellite, we report an event featuring counter‐streaming inverted‐V ion structures that occurred on 16 February 2021. The inverted‐V ions parallel and anti‐parallel to the magnetic field are observed with a time difference of ∼5‐min, likely because they originate from the quasi‐static structures in the southern and northern hemispheres, which may have slightly different spatial locations along the satellite trajectory. This spatial difference between the two structures is also suggested by a time difference in the electron flux depletion observed in the parallel and anti‐parallel directions. Auroral images from multiple satellites further support the existence of quasi‐static structures in both the northern and southern hemispheres. In addition, the parallel inverted‐V ions exhibit a wider pitch angle distribution than that of the anti‐parallel ions, possibly due to pitch angle scattering of about 5° as they crossed the magnetic equator from the southern hemisphere. These results contribute to a better understanding of the spatial configuration and dynamics of auroral acceleration processes.

Abstract On 23 November 2022, during exceptionally quiet geomagnetic conditions, a distinct ultra‐low frequency (ULF) wave in the Pc5 band (∼2.4 mHz) was observed in the duskside sub‐auroral region by two Canadian SuperDARN (Super Dual Auroral Radar Network) radars. The wave exhibited a periodic Doppler velocity signature resembling a “caterpillar,” characterized by anti‐sunward propagation and an azimuthal wave number of ∼12. Ground‐based magnetometer data revealed latitudinal variations in wave amplitude and phase consistent with field line resonance, peaking at ∼66° magnetic latitude. The Arase satellite, whose ionospheric footprint traversed the region of the caterpillar ULF wave, detected toroidal oscillations in electric and magnetic fields at magnetically conjugate locations in the inner magnetosphere, with high coherence and phase consistency with ground‐based observations. Arase observed the electric field variation preceding the magnetic field variation by ∼45°, which is unusual for a pure standing mode and suggests a departure from a simple standing‐wave interpretation. Instead, the observations can be interpreted as mixed standing and propagating characteristics along the field line. Simultaneously, the clear anti‐sunward azimuthal phase propagation and sustained wave activity suggest a driving mechanism by a large‐scale surface perturbation at the magnetospheric flank. These properties are consistent with excitation by the Kelvin‐Helmholtz instability. A minor enhancement in solar wind dynamic pressure was observed, potentially contributing to the wave onset. This study highlights SuperDARN's capability to detect ULF waves during quiet intervals and demonstrates the utility of multi‐instrumental conjugate observations in revealing the spatiotemporal variations of ULF waves and their generation mechanisms.

Abstract The afternoon detached auroral arc is an important phenomenon in the subauroral region, reflecting coupling processes between the Earth's magnetosphere and ionosphere. Previous studies have not identified fine‐scale structures in such arcs, leaving the dynamics underlying their formation poorly understood. Here we report an afternoon detached auroral arc event on 13 September 2017 during the recovery phase of a storm. For the first time, the sawtooth‐like undulations were observed along the equatorward boundary of the afternoon detached arc in the Lyman‐Birge‐Hopfield Long (LBHL) wavelength band of Defense Meteorological Satellite Program/Special Sensor Ultraviolet Spectrographic Imager (DMSP/SSUSI). This auroral structure is accompanied by >10 keV ion precipitation and by tens to hundreds of eV electron precipitation at higher latitudes. Detailed analyses based on coordinated observations from the Arase satellite indicate that the structure is associated with a plasmaspheric plume, with surface waves occurring along its boundary. Joint observations from ground‐based magnetometer stations indicate that magnetic pulsations in the Pc1‐2 band were also distinctly detected. We suggest that surface waves perturb the cold plasma density within the plume, thereby modulating Electromagnetic Ion Cyclotron (EMIC) waves. The modulated EMIC waves resonate with energetic ions, producing precipitation that contributes to the formation of the sawtooth‐like undulations in afternoon detached auroral arc.

Abstract Analyzing the dynamics of trapped electron fluxes in the Earth's outer radiation belt is a complex task, due to the presence of insufficiently known parameters and the long runtimes of multi‐dimensional radiation belt codes, preventing a thorough examination of dependencies on all parameters. Here, we present an approximate eigenfunction modeling of whistler‐mode wave‐driven electron pitch‐angle diffusion, slightly generalized compared to previous work. This new model can approximately describe, in an easy, flexible, and fast way, both the asymptotic electron pitch‐angle distribution (PAD) at all pitch angles and its temporal evolution toward this final state, in both weak and strong diffusion regimes, in the presence of a finite, time‐varying electron source. In this model, wave‐driven pitch‐angle diffusion is assumed to prevail over energy diffusion and radial diffusion, limiting its applicability to the plasmasphere or intervals of smooth decay of the electron flux outside the plasmasphere, during moderately active periods. We propose a new method, based on this model, for estimating the energy spectrum and temporal variation of the electron source. We investigate the dynamics of the electron flux measured by the Van Allen Probes and Arase spacecraft during two events in 2018 and 2022 in the outer radiation belt. We demonstrate that the new model can reproduce the evolution of the measured electron flux and of its PAD, provided that the magnitude of diffusion rates is normalized to the observed decay timescale in the 300–600 keV range and that a finite electron source term is included below 300 keV.

Abstract The May 2024 geomagnetic superstorm provided the opportunity to explore how strong wave‐particle interactions affect energetic electron precipitation under intense driving. Using coordinated measurements from a balloon‐borne Timepix‐based X‐ray detector, ground‐based riometers and magnetometers, and Arase satellite observations, we identified quasi‐periodic bursts of energetic electron precipitation coincident with Pc5 ultra low frequency (ULF) wave oscillations. Arase satellite data revealed energy‐dispersed trapped energetic electron flux modulations in the “seed” energy range, indicating that trapped electron flux was likely modulated by ULF waves. This letter reveals that these flux enhancements surpassed the Kennel‐Petschek (K‐P) limit, creating intense chorus waves and driving periodic electron precipitation. Drift‐dispersion analysis traced these modulations back to a source in the post‐noon magnetospheric sector, matching balloon and ground‐based measurements. Here, we propose a novel indirect ULF wave‐driven mechanism for modulated energetic electron precipitation, whereby periodic modulations of “seed” electron fluxes enhance electron losses.

Abstract Electron conics are a distinct type of electron distribution observed in Earth’s magnetosphere, characterized by enhanced fluxes of upgoing electrons at several-keV energies, particularly in the auroral acceleration region. This study analyzes high-altitude (27,000–32,000 km) observations made by the Arase satellite to investigate the characteristics of electron conics after passing through the heating region, employing the high angular resolution of the low-energy particle experiments—electron analyzer (LEPe) onboard the satellite. We analyzed eight electron conic events between 2017 and 2022 to estimate their source altitudes using mirror ratios and potential differences and by comparing pre- and post-heating data to investigate heating properties. Our results show that the source region of conics has an upper boundary at 9,000–14,000 km, with the peak flux originating from a central altitude of 3,000–7,000 km. This region spatially coincides with the source of auroral kilometric radiation (AKR): the central altitude of the source of conics corresponds to the lower boundary of the AKR source, suggesting that a longer residence time of particles within the AKR source region leads to stronger heating. The comparison of pre- and post-heating populations demonstrated that upgoing conic electrons exhibit higher temperatures and lower densities. The number flux remains conserved, indicating the energization of a magnetospheric population, whereas the energy flux is enhanced by up to a factor of four, significantly higher than that reported in previous studies. A test particle simulation, using observed plasma parameters and incorporating stochastic perpendicular heating, reproduces the main features of observed conics in terms of both energy and pitch angle. Our simulation shows that electron conics evolve into narrow, field-aligned beams at higher altitudes, suggesting that some of the anti-Earthward-flowing beams observed in the magnetotail may actually be unresolved conics. These findings contribute to the understanding of energy transport between the auroral acceleration region and the magnetotail and show the importance of high-angular-resolution instrumentation. Graphical Abstract

Abstract Understanding how the properties of Pc1 waves change during their propagation from the magnetospheric source regions to the middle or low-latitude ionosphere have not yet been clearly revealed by observations. In this study, we present the first quantitative comparison of Pc1 wave power attenuation both along the geomagnetic field lines and in the ionospheric wave ducts, using simultaneous observations from the Arase satellite and dynamical variation of Particles and Waves in the INner magnetosphere using Ground-based network observations (PWING) ground magnetometers. One of our key findings is that the polarization sense of the waves changed from left-handed polarization (LHP) at the satellite to right-handed polarization (RHP) on the ground, providing observational evidence of polarization transformation from space to the ionosphere. By examining polarization angles, we confirm that the Pc1 waves observed at multiple ground stations originated from the same magnetospheric source as the EMIC waves detected by the Arase. Importantly, we quantify the wave power attenuation factor along the magnetic field line to be only 0.37 dB/1000 km, which is nearly an order of magnitude smaller than that in the ionospheric wave duct (4.7–8.2 dB/1000 km). This result establishes a previously unreported minimum Pc1 wave attenuation rate in the magnetosphere, highlighting that the wave energy loss occurs more rapidly in the ionospheric duct than in space. These findings provide new insights into Pc1 wave transmission mechanisms and emphasize the importance of combined space- and ground-based observations for characterizing wave propagation processes across geospace. Graphical Abstract

Abstract During the Mother's Day Storm, the most intense storm of the last 20 years, with a peak Dst of less than −400 nT, the Macau Science Satellite‐1 observed the penetration of relativistic electrons of energies greater than 1 MeV into the inner radiation belt at Low Earth Orbit (LEO). The arrival of the MeV electrons was observed to occur instantaneously following the Dst minimum, with their continuous enhancement in the South Atlantic Anomaly over 7 days in the recovery phase reaching L 1.5. The so‐called impenetrable barrier, previously estimated to be located at L 2.8 during the Van Allen Probes' era, has been significantly violated. A combined analysis of observations with Arase data at mid‐latitude reveals the evolution of electron spectrum and pitch angle distribution for the first time, including zebra stripe patterns, an increase in electron flux near the loss cone, and a decrease in electron flux at higher pitch angles. These new results suggest that MeV electrons might undergo several steps to reach the inner radiation belt at LEO during this storm, which includes radial transport, radial diffusion, local acceleration and pitch angle scattering.

Abstract While low‐altitude observations indicate that field line curvature (FLC) scattering produces isotropic precipitation of energetic protons, it remains unclear whether this mechanism also leads to a fully isotropic pitch angle distribution in the magnetosphere. In this study, we distinguished the low‐latitude boundaries of the loss cone filling and the isotropic distribution of energetic protons in an energy range of 10–180 keV using middle‐altitude (3–5 ) off‐equatorial observations made by the Arase satellite. The isotropic distribution boundary (IDB) is defined by the ratio of proton fluxes at pitch angles of 0°–45° to 45°–90° for the northern hemisphere. The latitude of the IDB exhibits energy dependence, such that higher energy protons become isotropic at lower latitudes, implying isotropization of protons by FLC. Around the IDB, the downward loss cone was filled, while the corresponding upward loss cone was empty due to atmospheric loss. The low‐latitude boundary of the loss cone filling tended to be located at 0.1°–0.4° lower latitude than the IDB. Numerical simulations using the centrifugal impulse model for FLC scattering reproduced this latitude difference in the low‐latitude boundaries of the loss cone filling and the isotropic distribution. These results suggest that effective pitch angle scattering occurs only near the loss cone on the field line with a larger equatorial curvature radius, resulting in loss cone filling, whereas it occurs more broadly across the pitch angles on the field line with a smaller equatorial curvature radius, leading to the isotropic distribution.

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, ] we follow the lengthy argument to a logical conclusion, leading to an alternative description of electrodynamics in the cusp region.

Abstract. In the polar middle and upper atmosphere, nitric oxide (NO) is produced in large amounts by both solar EUV and X-ray radiation and energetic particle precipitation, and its chemical loss is driven by photodissociation. As a result, polar atmospheric NO has a clear seasonal variability and a solar cycle dependency which have been measured by satellite-based instruments. On shorter timescales, NO response to magnetospheric electron precipitation has been shown to take place on a day-to-day basis. Despite recent studies using observations and simulations, it remains challenging to understand NO daily distribution in the mesosphere–lower thermosphere during geomagnetic storms and to separate contributions of electron forcing and atmospheric chemistry and dynamics. This is due to the uncertainties existing in the available electron flux observations, differences in representation of NO chemistry in models, and differences between NO observations from satellite instruments. In this paper, we use mesospheric–lower-thermospheric NO column density data measured with a millimeter-wave spectroscopic radiometer at the Syowa station in Antarctica. In the period 2012–2017, we study both the long-term and short-term variability of NO. Comparisons are made with results from the Whole Atmosphere Community Climate Model to understand the shortcomings of current electron forcing in models and how the representation of the NO variability can be improved in simulations. We find that, qualitatively, the simulated year-to-year and day-to-day variability of NO is in agreement with the observations. On the other hand, there is up to a factor of 2 underestimation of the NO column density in wintertime. Also, the model captures only 27 % of the range of observed daily NO values. The observed day-to-day variability has a good correlation with three different geomagnetic indices, indicating the importance of electron forcing in atmospheric NO production. Using electron flux measurements from the Arase satellite, we demonstrate their potential in atmospheric research. Our results call for improved representation of electron forcing in simulations to capture the observed day-to-day variability.

Abstract 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.

Abstract The Earth's outer radiation belt is populated by relativistic ( 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 ‐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 ‐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.

Abstract 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.

Abstract Ducted propagation of whistler‐mode waves has attracted attention as a process that explains how whistler‐mode waves propagate to high latitudes, resulting in the loss of relativistic electrons to the atmosphere and changes in the upper atmosphere due to electron precipitation. However, few studies have compared the observed density ducts and wave propagation characteristics to theoretical predictions in detail, particularly for low‐density ducts. We present four patterns of ducting modes as electron density increases or decreases, as observed by the Arase satellite. (a) Lower‐band (LB) waves propagating along a high‐density duct with small wave normal angles (WNAs), (b) LB waves propagating along a low‐density duct with a wide distribution of WNAs up to above the Gendrin Angle, (c) LB waves propagating along a low‐density duct with WNAs around the Gendrin Angle, and (d) upper‐band waves propagating along a low‐density duct with small WNAs. We derived the WNAs for these cases, and their characteristics were consistent with the ducting theory. Based on this theory, we calculated the frequency range in which the waves were likely to be trapped in the ducts. We compared this frequency range with the power spectra of the recorded whistler‐mode waves and found consistency between the theory and observations. Furthermore, it is suggested that the WNAs for cases (b) and (c) have azimuthal distributions based on a comparison of the WNA analysis of the simple simulated waveforms and the observed data.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract We report a statistical result of electrons inside the loss cone with energies of 67 eV–88 keV using electron measurements obtained in situ by the Arase satellite in the inner magnetosphere around the magnetic equator for 60 months. Loss cone electrons are found with a high occurrence probability from the nightside to the dawnside at approximately L = 6. For 641 eV–88 keV electrons, the high‐occurrence region shifts toward later magnetic local times (MLTs) with increasing loss cone electron energy. The spatial distribution of the occurrence probability around MLT = 22–3 at L = 5–6 is consistent with the calculated average resonance energy distribution of whistler mode chorus waves near the magnetic equator. These results suggest that pitch angle scattering driven by chorus waves plays the main role in electron precipitation in this region.

Abstract Strong Thermal Emission Velocity Enhancement (STEVE) is a latitudinally narrow, purple‐band emission observed at subauroral latitudes. Stable Auroral Red (SAR) arcs characterized by major red emission, and red/green arcs with both red and green emissions also occur at subauroral latitudes. Characteristics of magnetospheric source plasma and electromagnetic fields of these three types of arcs have not been fully understood because of the limited conjugate observations between magnetosphere and the ground. In this study, we report 11 conjugate observations (2 STEVEs, 7 SAR arcs, and 2 red/green arcs), using all‐sky images obtained at seven ground stations over more than four years from January 2017 to April 2021 and magnetospheric satellites (Arase and Van Allen Probes). We found that, in the inner magnetosphere, the source region of STEVEs and red/green arcs were located outside the plasmasphere, and that of the SAR arc was in the region of spatial overlap between the plasmasphere and ring current region. Electromagnetic waves at frequencies below 1 Hz were observed for STEVEs and red/green arcs. SuperDARN radar data showed a strong westward plasma flow in the ionosphere, especially during STEVE events, whereas the plasma flows associated with SAR arcs and red/green arcs were generally weaker and variable. The STEVE and SAR arc can appear simultaneously at slightly different latitudes and STEVEs and red/green arcs can transform into SAR arcs. These first comprehensive ground‐satellite measurements of three types of subauroral‐latitude auroras increase our understanding on similarlity, differences, and coupling of these auroras in the ionosphere and the magnetosphere.

Abstract We investigate the dynamics of relativistic electrons in the Earth's outer radiation belt by analyzing the interplay of several key physical processes: electron losses due to pitch angle scattering from electromagnetic ion cyclotron (EMIC) waves and chorus waves, and electron flux increases from chorus wave‐driven acceleration of 100–300 keV seed electrons injected from the plasma sheet. We examine a weak geomagnetic storm on 17 April 2021, using observations from various spacecraft, including GOES, Van Allen Probes, ERG/ARASE, MMS, ELFIN, and POES. Despite strong EMIC‐ and chorus wave‐driven electron precipitation in the outer radiation belt, trapped 0.1–1.5 MeV electron fluxes actually increased. We use theoretical estimates of electron quasi‐linear diffusion rates by chorus and EMIC waves, based on statistics of their wave power distribution, to examine the role of those waves in the observed relativistic electron flux variations. We find that a significant supply of 100–300 keV electrons by plasma sheet injections together with chorus wave‐driven acceleration can overcome the rate of chorus and EMIC wave‐driven electron losses through pitch angle scattering toward the loss cone, explaining the observed net increase in electron fluxes. Our study emphasizes the importance of simultaneously taking into account resonant wave‐particle interactions and modeled local energy gradients of electron phase space density following injections, to accurately forecast the dynamical evolution of trapped electron fluxes.

Abstract Microbursts are impulsive injections of energetic (few keV to >MeV) electrons into the atmosphere, primarily caused by nonlinear scattering driven by whistler mode chorus waves. While the relative importance of microburst precipitation as a loss process has not been fully quantified, many studies have shown microbursts may play a significant role in the loss of outer radiation belt electrons. We present a multi‐platform statistical analysis of chorus and energetic electron precipitation in an attempt to constrain the azimuthal spatial extent (MLT) of the microburst precipitation region and determine how this extent varies with geomagnetic activity. Statistical upper bounds of this azimuthal extent are determined with observations of general energetic electron precipitation that can include direct microburst detections, while statistical lower bounds determination requires direct microburst detections. The resulting distributions of both upper and lower bounds azimuthal extent suggest that microbursts may frequently constitute an important source of electron loss from the outer radiation belt. We find that 36% of upper bound events in the dawn sector span more than 5 hr in MLT. This azimuthal extent increases with geomagnetic activity, particularly in the dawn and noon MLT sectors.

The plasmasphere within Earth’s magnetosphere plays a crucial role in space physics, with its electron density distribution being pivotal and strongly influenced by solar activity. Very Low Frequency (VLF) waves, including whistlers, provide valuable insights into this distribution, making the study of their propagation through the plasmasphere essential for predicting space weather impacts on various technologies. In this study, we evaluate the performance of different deep learning model sizes for lightning whistler detection using the YOLO (You Only Look Once) architecture. To achieve this, we transformed the entirety of raw data from the Arase (ERG) Satellite for August 2017 into 2736 images, which were then used to train the models. Our approach involves exposing the models to spectrogram diagrams—visual representations of the frequency content of signals—derived from the Arase Satellite’s WFC (WaveForm Capture) subsystem, with a focus on analyzing whistler-mode plasma waves. We experimented with various model sizes, adjusting epochs, and conducted performance analysis using a partial set of labeled data. The testing phase confirmed the effectiveness of the models, with YOLOv5n emerging as the optimal choice due to its compact size (3.7 MB) and impressive detection speed, making it suitable for resource-constrained applications. Despite challenges such as image quality and the detection of smaller whistlers, YOLOv5n demonstrated commendable accuracy in identifying scenarios with simple shapes, thereby contributing to a deeper understanding of whistlers’ impact on Earth’s magnetosphere and fulfilling the core objectives of this study.

Abstract 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, , that describes electron flux dependence on equatorial pitch‐angle, , . 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.

Abstract We analyzed time‐of‐flight (TOF) data from the Arase satellite to investigate temporal variations of the molecular ion group (O₂⁺, NO⁺, and N₂⁺) at 19.2 keV/q in the inner magnetosphere for 6 years from the solar declining to rising phase. The molecular ions counts were estimated by subtracting the background contamination of oxygen counts. While the number of clear molecular ion events was small, the estimated counts exhibited good correlation with the solar wind dynamic pressure and SYM‐H index. Long‐term variations of the molecular ions differed from those of counts of the O⁺ and N⁺ group. Additionally, we discuss the importance of the solar wind dynamic pressure in causing variations of molecular ions in the inner magnetosphere.

Abstract The Arase satellite observed the precipitation of monoenergetic electrons accelerated from a very high altitude above 32,000 km altitude on 16 September 2017. The event was selected in the period when the high‐angular resolution channel of the electron detector looked at pitch angles within ∼5° from the ambient magnetic field direction, and thereby was the first to examine the detailed distribution of electron flux near the energy‐dependent loss cone at such high altitudes. The potential energy below the satellite estimated from the observed energy‐dependence of the loss cone was consistent with the energy of the upgoing ion beams, indicating that ionospheric ions were accelerated by a lower‐altitude acceleration region. The accelerated electrons inside the loss cone carried a significant net field‐aligned current (FAC) density corresponding to ionospheric‐altitude FAC of up to ∼3μA/m². Based on the anisotropy of the accelerated electrons, we estimated the height of the upper boundary of the acceleration region to be >∼2 R_E above the satellite. The height distribution of the acceleration region below the satellite, estimated from the frequency of auroral kilometric radiation, was ∼4,000–13,000 km altitude, suggesting that the very‐high‐altitude acceleration region was separated from the lower acceleration region. Additionally, we observed time domain structure (TDS) electric fields on a subsecond time scale with a thin FAC indicated by magnetic deflections. Such a TDS may be generated by the formation of double layers in the magnetotail, and its potential drop could significantly contribute (∼40%–60%) to the parallel energization of precipitating auroral electrons.

Abstract We report an Arase‐all sky imager (ASI) conjugate event in which the pulsating aurora (PsA) has a one‐to‐one correspondence with chorus bursts. Wavelet analysis displayed three peaks at ∼0.3 Hz, 4 Hz, and >10 Hz, corresponding to the main pulsation, internal modulation, and fast modulation, respectively. These correspond to the old terms of ∼5–15 s pulsations, chorus risers/elements and subelements/subpackets, respectively. Electron “microbursts” correspond to the 4‐Hz peak. The internal and fast modulations are further verified by the analysis based on fast Fourier transform analyses. Moreover, the spatial distributions of the Fourier spectral amplitude show that the internal and fast modulations are well‐structured within auroral patches. The above results indicate a paradigm shift away from quasilinear theory which implicitly assumes diffuse wave generation. The three time‐scale modulations are consistent with coherent chorus which has been theoretically argued to lead to pitch angle transport three orders of magnitude faster.