To elucidate the relationship between electric field and electron-density variations in the high-latitude ionosphere associated with Pc5 ultralow-frequency (ULF) waves from subauroral to high latitudes, we analyzed the global navigation satellite system (GNSS)-total electron content (TEC), ionospheric plasma flow observed by the Super Dual Auroral Radar Network (SuperDARN), and electron density in the inner magnetosphere measured by the Arase satellite. On 23 November 2022, the SuperDARN Prince George (PGR) radar in the dusk sector detected meridional plasma flow oscillations with periods and amplitudes of 5 min and 10–60 m/s, respectively. The plasma flow oscillations began at approximately 01:10 UT and persisted until 03:30 UT over a magnetic latitude range of 65°–72°. The amplitude increased as the magnetic latitude increased. The electron density profile observed by the Arase satellite did not exhibit a sharp gradient during the inner magnetosphere. This indicates that the plasmasphere extended beyond the apogee of the Arase satellite (6.1 Re, where Re is Earth’s radius) under quiet geomagnetic conditions. A detailed comparison between SuperDARN radar and GNSS-TEC data revealed that meridional plasma flow oscillations occurred in the mid-latitude trough and the auroral oval (increased TEC region). Additionally, the equatorward boundary of the auroral oval was located between magnetic latitudes of 72° and 74°. The 15-min detrended TEC measured over the Fort Simpson radar, inside the field-of-view of the PGR radar, showed oscillations similar to the ionospheric plasma flow variations. Through a spectral analysis of the detrended TEC and meridional plasma flow oscillations, we identified a phase difference of ∼135° (∼1.9 min) between them. This phase difference indicates that the upward and downward motion of the ionosphere, driven by an external electric field resulting from Alfvén waves propagating from the dusk-side magnetosphere, was responsible for the observed GNSS-TEC perturbation.

Abstract The geomagnetic superstorm in May 2024 represents the most extreme space weather event over the past two decades, offering a unique opportunity to investigate radiation belt electron dynamics under exceptionally strong solar wind driving conditions. Observations from the Arase satellite show that relativistic electron fluxes dropped by several orders of magnitude during the storm main phase. Although magnetopause shadowing and wave‐driven losses are established mechanisms, the response of radiation belt electrons to superstorm‐level solar wind driving remains poorly constrained. Using the Versatile Electron Radiation Belt model, we present the first physics‐based simulation of the May 2024 electron dropout. Utilizing multi‐satellite and ground‐based observations, we show that the decades‐long, widely adopted Kp‐driven radial diffusion parameterization fails to represent radial transport under superstorm conditions, substantially misrepresenting both the timing and magnitude of enhanced diffusion relative to observed ultra‐low‐frequency wave activity. Accurate modeling of the observed extreme dropout requires precise specification of the onset of enhanced radial diffusion, which should coincide with the nearly simultaneous storm‐time magnetopause compression. These coupled processes drive the dramatic depletion, followed by scattering from whistler‐mode plasma waves.

Super geomagnetic storms are characterized by extreme intensification of the ring current in near-Earth space. The origin of the ions that carry the ring current is key to understanding its development. In situ measurements of ring current ions by the Arase satellite demonstrate an unprecedented dominance of heavy ions originating from the Earth during the May 2024 super geomagnetic storm, despite the high solar wind density. The solar wind, another expected source of ions, contributes little to the energy density of the ring current. This observational evidence highlights the critical role of ion supply processes from the Earth and transport in the magnetosphere in developing the ring current for the super geomagnetic storm. Furthermore, the super-intense ring current penetrated close to the Earth, strongly deforming the local geomagnetic field and driving unusual outward transport of electrons, which led to the loss of radiation belt electrons from the near-Earth region.

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.

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.

Abstract We investigated the spatial distributions and pitch‐angle distributions of 30–300 eV H + and O + ion fluxes using data obtained by the low‐energy particle experiments‐ion mass analyzer (LEPi) onboard the Arase satellite. A statistical analysis over 3.7 years reveals that the H + ion flux is enhanced at L > 4 from the premidnight sector through dawn to noon, with the peak location shifting inward during geomagnetic disturbances. The pitch‐angle distribution of H + ions varies with both L and geomagnetic activity, which can be interpreted as the influence of transport processes and the ring current effect. In contrast, the O + ion flux exhibits a pronounced enhancement in a confined region at L = 3–5 and 19–9 magnetic local time, with a strong dawn‐dusk asymmetry. The O + flux increases with geomagnetic activity at L = 3–5, while remaining nearly unchanged at L = 6. Its pitch‐angle distribution is consistently bidirectional and field‐aligned, with little dependence on geomagnetic conditions. These distinct spatial and pitch‐angle characteristics indicate that the observed H + ions correspond to the inner part of the warm plasma cloak, whereas the observed O + ions represent the high‐energy tail of the oxygen torus. The results further suggest that low‐energy O + ions are supplied directly from the nightside ionosphere along geomagnetic field lines, rather than being transported inward from the outer magnetosphere. Our findings demonstrate that the warm plasma cloak and the oxygen torus constitute likely independent plasma populations in the inner magnetosphere.

During the May 2024 super storm, we identified sub-second microburst-type relativistic electron precipitation events at McIlwain's in low-Earth orbit using the Gamma-ray Burst Monitor (CGBM) onboard the CALorimetric Electron Telescope on the International Space Station. The high time resolution of CGBM (1/8 s) enables the detection of sub-MeV electron microbursts that are unresolved by most currently available radiation belt monitors, which typically lack sub-second cadence. Simultaneous plasma wave observations from the Arase satellite reveal enhanced whistler-mode chorus activity at Roederer's -4. These observations indicate that chorus-driven wave-particle interactions can operate at unusually low L-shells near the earthward-displaced inner edge of the outer radiation belt during extreme geomagnetic storms. Quantifying whether such chorus activity also contributes to local acceleration or slot filling requires future evaluation of wave properties, plasma conditions, diffusion coefficients, and acceleration and loss timescales.

Abstract The irregularities of ionospheric plasma bubbles (EPBs) sequentially generated over wide longitudes are expected to share similar lifetime under normal conditions. In this study, we report a special case where the EPB irregularity lifetime over wide longitudes showed a quasi‐wavelike undulation pattern with wavelength of ∼3,000 km, that is, the lifetime of EPB irregularities sequentially generated at different longitudes experienced several gradually decreasing/increasing cycles. Irregularities with longer (shorter) lifetime corresponded to the EPBs with larger (smaller) development in latitude/altitude. Further observations revealed that smaller‐scale (hundred‐kilometer) and larger‐scale (thousand‐kilometer) waves may coexist at the bottomside of ionosphere around sunset. The former acted as the seeding source for EPB generation, whereas the latter superimposed on PRE and modulated EPB development in altitude over different longitudes, leading to the wavelike undulation morphology in irregularity lifetime ultimately. This study provides implications on how the complex wave structures influence EPB morphology over wide longitudes.

The May 2024 storm exhibited a minimum Dst of -406 nT and was recorded as one of the largest geomagnetic storms in recent decades, causing various geomagnetic phenomena, such as the penetration of enhanced ring current particles into the deep inner magnetosphere, the acceleration and loss of radiation belt electrons, the generation of various plasma waves, etc. Electromagnetic ion cyclotron (EMIC) waves, also known as Pc1 geomagnetic pulsations on the ground, play an important role in the loss processes of energetic ring current protons and relativistic electrons in the radiation belts through pitch-angle scattering. In this study, we present observations of EMIC wave activity during the great geomagnetic storm of May 2024 obtained by the PWING ground-based observation network and the POES and Arase satellites. Using data obtained from the Arase satellite, we found that unusual high-frequency EMIC waves with frequencies above 5 Hz were predominantly observed at L* < 3 during the main and early recovery phases. POES observations also revealed significant 30–80 keV proton precipitation events by EMIC wave–particle interaction at L* 2 during the main phase. PWING ground-based observations exhibit special types of Pc1 pulsations at subauroral and low-latitude stations with frequencies higher than those of typical Pc1 waves. We also found that the variation in Pc1 wave frequency shows a relationship with the plasmapause locations and the inner boundary of 33–78 keV proton fluxes. We investigate the evolution of ring current protons and their contribution to the generation of EMIC waves during the storm. From these observational facts, we suggest that the energetic ring current protons penetrate the deep inner magnetosphere (L* < 3) during the main and early recovery phase of the severe storms and drive EMIC waves in the near-Earth regions. These waves can contribute to the loss of enhanced ring current protons and relativistic electrons in the deep inner magnetosphere. These observations provide new insights into the generation processes and evolution of EMIC waves and the inner magnetospheric dynamics during intense geomagnetic storms.

In this study, we present simultaneous multi-point observations of Pi2 magnetic pulsations studied, for the first time, through joined measurements of multiple missions: the CSES-01 and Swarm (A and C), in the topside ionosphere, Van Allen Probe (or Radiation Belt Storm Probes, RBSP) (A and B) and Arase in the magnetosphere. We focused on the compressional component of the satellites and the horizontal component of magnetic field from Kakioka (KAK) ground station in Japan. The Pi2 event occurred from 12:40 to 12:56 UT on January 12, 2019, where CSES-01, RBSP-A, and KAK were on the night side; Swarm-A/C, were on the day side while RBSP-B and Arase were in the dusk sector. We observed 90-degree phase delay between RBSP-A-Bz and RBSP-A-Ey which can be interpreted as a radially trapped fast mode for the compressional oscillation. Both the wavelet transforms and the Hilbert-Huang transform (HHT) were applied for signal analysis, revealing wave-like structures and strong coherence among all data sets and confirming the Pi2 pulsation nature. The compressional component in the topside ionosphere and in the magnetosphere seem very similar with the horizontal component of KAK station. During 12:43-12:45 UT, CSES-01 and Swarm-A/C exhibited an in-phase variation while both were in the Southern Hemisphere. However, as CSES-01 transitioned to the Northern Hemisphere between 12:47 and 12:56 UT, the corresponding signals became out of phase. During the selected Pi2 event, RBSP-B was located very close to Arase in the dusk sector and detected compressional oscillations with a waveform nearly identical to that observed by RBSP-A, suggesting that the observed Pi2 exhibited cavity resonance characteristics. To understand their propagation mechanism, we conduct further analysis of the duskside Pi2 pulsations in this event. We found that the penetration/propagation speed of the low-frequency Pi2 pulsations is high (|m|~ 0.3) and much larger than the average Alfven speed in the plasmasphere, while the high-frequency Pi2 pulsations have a finite m number (m ~ -1.7) and their phase speed is comparable to the average Alfven speed. We suggest that the nightside Pi2 pulsations propagate sunward through a waveguide-like mode, consistent with the high-frequency Pi2 signatures detected on the duskside in the magnetosphere.

Field-line curvature scattering (FLCS) is believed to be the primary mechanism forming electron isotropy boundaries (IB) and can rapidly scatter relativistic electrons from the outer radiation belt. However, its direct and quantitative impact on controlling outer belt electron lifetimes has never been directly assessed. Using simultaneous observations of IBs from low-altitude satellites and in situ electron fluxes from equatorial satellites, we report IBs intruding into the outer belt (reaching L similar to 4.5), closely synchronized with sharp flux radial gradients near IBs, caused by significant electron loss outside IBs during a 4-day storm recovery period. By combining observations with simulations, we provide the first direct and quantitative evidence that FLCS-induced electron loss outside the IB dominantly controls the outer belt electron lifetimes. Our findings reveal that this simple yet fundamental physical process, which has been historically neglected in global radiation belt models, can explain the outer electron belt configuration.

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

Chorus waves play a significant role in the dynamic evolution of energetic electrons in the inner magnetosphere. Therefore, understanding the spatial and temporal dynamics of these electrons requires global distributions of chorus waves, which in turn necessitates combining data products from multiple satellite missions to achieve sufficient spatial coverage. In this study, we use 11 years of data from both the Van Allen Probes and the Arase satellite to create a global model of the magnetic intensity of chorus waves. The agreement between the two satellite missions was assessed using observations during close conjunctions. The statistical model is based on data with good spatial coverage up to 40° magnetic latitude, across all magnetic local times (MLT), and at high L-shells, resulting in a model with excellent spatial continuity. The model is generated for both Upper-Band Chorus (UBC; 0.5 fce

The all-sky meteor radars are primarily designed for observations of mesospheric neutral wind. Recently, the capability of all-sky radars in investigating ionospheric irregularities of the Bragg scales has been developed. However, limited by the relatively small range gate span of the all-sky radar, this capability is mainly constrained to the observations of irregularities at lower altitudes, e.g., E-region irregularities. The F-region irregularities at higher altitudes up to hundreds of kilometers, e.g., equatorial plasma bubbles (EPBs), are usually considered beyond the detection range of all-sky radars. In this paper, we try to extend the capability of a conventional interferometric all-sky meteor radar located at Sanya (18.4°N, 109.7°E) for investigating the spatial features of EPBs. Based on the arriving angles of irregularity backscatter echoes obtained by the radar interferometry technique, and according to the magnetic sensitivity of the EPB field-aligned irregularities, the true ranges of the irregularity structures could be determined to further reveal the spatial features of EPB structures. The results are confirmed by the collocated narrow-beam very high frequency (VHF) radar and the Low lAtitude long Range Ionospheric raDar (LARID). It is revealed that the all-sky radar could be employed to investigate EPB irregularities in a larger zonal region than narrow-beam VHF radars, which could be up to ∼2000 km. The observations could well cover the blind area of the LARID field-of-view, and thus could be employed to continuously trace EPB occurrences and evolutions over thousands of kilometers by combining with LARID in future studies.

Abstract. Studies of high-latitude plasma turbulence in Earth's upper atmosphere fundamentally focus on the differential response of electrons and ions to strong external electric fields generated during geomagnetic storms. Because ions in the E region are heavy and highly collisional, they remain largely tied to the neutral gas, whereas magnetized electrons undergo rapid E×B-drift. Microscopic polarization electric fields are generated when the relative drift velocity between these streaming electrons and the background ions exceeds the local ion-acoustic speed, triggering two-stream plasma instabilities, producing Farley Buneman waves. We propose a new theory that explicitly considers thousands, or millions, of such waves being excited inside a limited volume of space around aurorae, subject to the renormalization group. The resulting theory constitutes an effective field-theory for Farley-Buneman turbulence in the Martin-Siggia-Rose formalism. At the core of this theory is a statistical description of Farley-Buneman waves, where we allow each individual wave to produce a polarization electric field. We treat the sum total of these "micro-fields" that occur inside a turbulent volume as a stochastic variable, or simply noise. That noise, now a thermodynamic property, becomes the basis for anomalous diffusion, and an effective diffusion tensor, and we recover the expression for Bohm diffusion. In support of this theory, we present a large statistical analysis of how auroral electrojet turbulence responds to magnetospheric driving, revealing a clear tendency for the observed number density of turbulent waves to scale linearly with driving power, matching the predictions made by our field theory's overdamped equations of motion. Crucially, the effective field theory offers closed-form calculations of macroscopic transport relations that are uniquely suitable for sub-grid parameterization in space weather modeling, mimicking the success of stochastic parameterization in hydrodynamic Earth-system models. The derivation of these parameterized equations demonstrates how Bohm diffusion arises from a statistical-mechanical treatment of turbulent volumes, at the expense of an explicit treatment of the turbulent cascade. The equations should be investigated further, and in future, they may model the evolution of momentum and energy in numerical treatments of global magnetohydrodynamic circulation, below the scale-sizes normally considered accessible to fast, predictive models.

Plasmaspheric hiss is a whistler-mode emission in the Earth’s plasmasphere and is a major contributor to the pitch-angle scattering and loss of radiation belt electrons. Previous statistical studies based on single-satellite observations have limited a systematic understanding of plasmaspheric hiss waves. In this study, we present a statistical analysis of plasmaspheric hiss using combined observations from the Van Allen Probes and the Arase spacecraft during 2012-2024. The use of two missions improves spatial coverage and enables a more comprehensive characterization of the hiss intensity distribution within magnetic latitudes up to 45°. The results show that hiss intensity is enhanced on the dayside and peaks at L ≈ 3-4. Based on these results, we develop an empirical regression model that parameterizes the dependence of the root-mean-square hiss magnetic field intensity on L-shell, magnetic local time (MLT) and magnetic latitude (MLAT). The influence of geomagnetic activity is further parameterized using polynomial fits to the Kp index. The model is applicable for L ≤ 6.5, Kp ≤ 6, all MLTs, and MLAT up to 45°, providing a practical representation of plasmaspheric hiss for radiation belt modeling applications.

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. In the auroral ionosphere, plasma turbulence acts as an important dissipation mechanism for magnetospheric energy and the primary cause of radio wave scintillation. Characterizing auroral plasma turbulence across its full spatial extent has historically been limited by the narrow bandwidths of individual instruments. Our investigation approaches the problem of obtaining accurate, scale-dependent information using the physics of the Farley-Buneman (FB) instability, a modified two-stream plasma instability. In this study, we construct a composite spatial powerspectrum of plasma turbulence in the auroral electrojets spanning roughly four orders of magnitude in scale (from ~100 km down to ~20 m). This is achieved by combining a recent Monte-Carlo-based method of spatial clustering of very-high-frequency (VHF) radar echoes, with phase screen information derived from global navigation satellite system (GNSS) signals, using ground-based instrumentation in Canada. Through multi-instrument conjunctions with the European Swarm and Japanese Arase missions, we observe that the clustering of electrojet turbulence matches the structuring of field-aligned currents, and correlates with magnetospheric electron fluxes. Statistical analysis of the composite spectra, as well as a very large database of radar clustering spectra only, reveals a consistently steep decay of spectral power in the auroral electrojets, with the most probable spectral index being near −8/3. The observations suggest a continuous, scale-invariant cascade that frequently preserves the spatial signature of its magnetospheric drivers, where we outline a way for Alfvén waves to structure the turbulent E-region. Furthermore, we demonstrate that the plasma structures guilty of causing GPS scintillations (~270 meters in size) were moving at the ion acoustic speed, implying that those structures were, in fact, FB waves, and we thereby establish an observational basis for low-frequency electrojet turbulence. The method that we present, the composite radar-GNSS spectra, will on both counts offer useful empirical constraints for future efforts seeking to simulate the "sub-grid" turbulence that complicates the magnetosphere-ionosphere coupling around aurorae.

Abstract Magnetospheric chorus waves often accompany a gap in intensity near half of the electron cyclotron frequency, , whose generation process is still debated. One mechanism involving a nonlinear damping process proposed by Omura et al. (2009, https://doi.org/10.1029/2009ja014206 ) predicts a characteristic spectral signature where the lower cutoff of upper band waves, , follows local . Here, we statistically analyze spectral features of banded chorus waves from the Van Allen Probes and Arase spacecraft. Observationally, is almost one‐to‐one correlated with up to magnetic latitude for the banded chorus events in the midnight‐to‐dawn sector, but becomes nearly independent of for the events found on the dayside. By contrast, the upper cutoff of lower band waves, , depends only weakly on magnetic latitude and remains at approximately (where is the equatorial ) near the equatorial region, meaning that the power gap should have been already formed there. These results indicate that while the good correlation between and lends strong support for the nonlinear damping mechanism as a power gap enhancer during poleward propagation of chorus waves, its formation near the equatorial source region and/or on the dayside likely needs more concrete explanation.