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.

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.

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 similar to 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 degrees 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.

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.

Using recent in-situ Arase satellite observations during Solar Cycle 25, between March 2017 and December 2023, we perform a comprehensive statistical study of the inner magnetospheric ion (H + , He + , and O + ) distributions as a function of geomagnetic conditions and solar activity. The analysis employed the intercalibration method to combine two ion datasets (LEPi and MEPi) onboard the Arase satellite, covering an energy range of 0.03–187 keV/q. In this study, we found distinct ion populations on energy: (1) plasmaspheric population (E < 30 eV) at L < 5; (2) suprathermal population at energies of several tens of eV to several keV at L > 5 for H + and L < 5 for He + and O + ; (3) ring current population (E = 1 keV – several tens of keV) with the ion nose structures; and (4) high-energy ring current particles (E > 30 keV) with symmetric MLT distributions. Ion fluxes and partial number densities exhibited significant enhancements with increasing Kp levels and solar activity. The O + fraction increased dramatically during periods of elevated Kp and enhanced solar activity, becoming comparable to the H + contribution within the plasmasphere. From our observations, we suggest that ion behavior in the inner magnetosphere is strongly influenced by geomagnetic disturbance levels and solar activity. This ion behavior can provide the favorable conditions for understanding the major driver of various magnetospheric plasma waves.

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 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.

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 ∼ 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 ∼ 2.5 . Combined with mass density estimates, the inferred ion composition consistently indicates heavy-ion dominance, with O ^+ fractions exceeding 90

Earth’s main magnetic field forms the magnetosphere, a barrier against solar wind plasmas. A southward magnetic field carried by the solar wind makes the magnetosphere smaller, which is termed “magnetopause erosion.” This process leads to equatorward extension of the polar region, where solar wind plasmas directly intrude into the magnetosphere. Although previous studies have suggested that magnetopause reconnection causes magnetopause erosion, the erosion mechanism during super geomagnetic storms remains controversial owing to a lack of in situ measurements. Here, we report a magnetopause crossing by the Arase satellite at 4.96 R E during the May 2024 super geomagnetic storm, which is the innermost magnetopause ever detected by magnetospheric spacecraft. An unexpected decrease in the magnetic field intensity extended from the inner magnetosphere to the magnetopause, indicating that magnetospheric currents reduced Earth’s main magnetic field on a global scale. Consequently, the solar wind pushed the weakened magnetosphere further toward the Earth, resulting in the severe shrinkage of the magnetosphere. This shrinkage led to a significant equatorward penetration of solar energetic particles on the dayside, suggesting that the magnetospheric currents can increase the risk of aircraft exposure to solar energetic particles at mid-latitudes.

Electromagnetic ion cyclotron (EMIC) waves are believed to cause the loss of relativistic electrons from the outer radiation belt into the atmosphere due to pitch angle scattering. However, it is still unclear whether all EMIC waves can scatter relativistic electrons or which conditions are favorable for pitch angle scattering by EMIC waves. In this study, we performed a 2‐year data analysis of EMIC waves and EMIC wave‐driven electron precipitation (EP), from 1 November 2016–31 October 2018. Electromagnetic ion cyclotron waves were observed using a ground‐based magnetometer installed at Athabasca (ATH, 4.3), Canada. Electron precipitation events were identified from very low‐frequency radio waves propagated from the transmitters at North Dakota (NDK, 3.0) and Seattle (NLK, 2.9) stations in USA to the receiver installed at ATH. The magnetic local time dependence of EMIC waves showed higher occurrence rates in the dawn sector. In contrast, EMIC waves accompanied by EP were localized in the dusk sector and were likely to occur during geomagnetic substorms. We found that EMIC waves accompanied by EP were associated with the main phase of geomagnetic storms and occurred inside the plasmapause. These results suggest that the EMIC waves that cause EP occur in the overlap region between the ring current and dense cold plasma during the main phase of geomagnetic storms. This is consistent with previous studies describing that the electron resonant energy with EMIC waves is lower in regions with high plasma density.

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 the enhanced ring current particles into the deep inner magnetosphere, the acceleration and loss of radiation belt electrons, and 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 of those particles. 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. We found that the Arase satellite detected various EMIC waves, which differed depending on the storm phase. High-frequency EMIC waves with frequencies above 5 Hz were predominantly observed at L < 3 during the main and early recovery phase, suggesting that energetic particles had penetrated regions close to the Earth. POES observations revealed significant 30–80 keV proton precipitation events at L ~ 2 during the main phase in support of the ring current penetration to low L-shells. PWING ground-based observations exhibit special types of Pc1 pulsations at sub-auroral and low-latitudinal stations with frequencies higher than those of typical Pc1 waves. The variation in Pc1 wave frequency shows a relationship with the plasmapause locations and inner boundary of 33–78 keV proton fluxes. We investigate the evolution of the ring current protons and their contribution to the generation of EMIC waves during the storm. The observations provide new insights into the generation processes and evolution of EMIC waves and the inner magnetospheric dynamics during intense geomagnetic storms.

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.

We have analyzed Electrostatic Electron Cyclotron Harmonic (ECH) waves observed using interferometry observation mode performed by the Arase satellite to estimate low-energy electron temperatures. Interferometry can be used to calculate velocities, but the Arase satellite can only perform interferometry observations in a one-dimensional direction. We proposed a method to estimate the wave vector of the observed ECH waves from the observed electric fields and calculated the phase velocity for each frequency. We determined the particle parameters from the particle detector and the upper hybrid resonance and estimated the unknown low-energy electron temperature from the agreement between the observed ECH dispersion relation and the theoretical dispersion curves. We performed our analysis for six events and found that the low-energy electron temperature in the observed region is on the order of 1 eV.

Recent simulation studies using the RAM-SCB model showed that proton precipitation contributes significantly to the total energy flux deposited into the subauroral ionosphere thereby affecting the magnetosphere-ionosphere coupling. In this study, we use the BATS-R-US + RAM-SCB model to understand the evolution of ElectroMagnetic Ion Cyclotron (EMIC) waves in the inner magnetosphere, their correspondence to the proton precipitation into the subauroral ionosphere, and to assess the performance of the model in reproducing the EMIC wave-particle interactions. During the 27 May 2017 storm, Arase and RBSP-A satellites observed typical signatures of EMIC waves in the inner magnetosphere. Within this interval, Defense Meteorological Satellite Program (DMSP) and National Oceanic and Atmospheric Administration (NOAA)/MetOp satellites observed significant proton precipitation in the dusk-midnight sector. Simulation results show that H- and He-band EMIC waves are excited within regions of strong temperature anisotropy near the plasmapause. The simulated growth rates of EMIC waves show a similar trend to that of the EMIC wave power observed by the Arase and RBSP-A satellites, suggesting that the model can reproduce the EMIC wave activity qualitatively. The simulated H-band waves in the dusk sector are stronger than He-band waves possibly due to the presence of excess protons in the boundary conditions obtained from the BATS-R-US code. The precipitating proton fluxes reproduced by the simulation with EMIC waves are found to agree reasonably well with the DMSP and NOAA/MetOp satellite observations. It is suggested that EMIC wave scattering of ring current ions can account for proton precipitation observed by the DMSP and MetOp satellites during the 27 May 2017 storm. During geomagnetic storms, plasma waves are generated in the Earth's magnetosphere. Among these waves, ElectroMagnetic Ion Cyclotron (EMIC) waves can scatter protons from the ring current, causing them to precipitate into the subauroral ionosphere. Such precipitation not only affects the midlatitude ionosphere but also impacts the dynamics of the magnetosphere. Understanding the origin of magnetospheric plasma waves and how they interact with the magnetospheric populations, along with their subsequent impact on the ionosphere, is crucial for predicting space weather accurately. In our study, we combined ground and satellite observations with simulations using the BATS-R-US + RAM-SCB to investigate EMIC wave-particle interactions in the inner magnetosphere and the resulting proton precipitation during the 27 May 2017 storm. We found that EMIC waves were excited in the dusk-midnight sector during the storm's main phase, within the regions of strong temperature anisotropy. The simulations reproduced the proton precipitation observed in the dusk-midnight sector by the Defense Meteorological Satellite Program /National Oceanic and Atmospheric Administration MetOP satellites fairly well. The model qualitatively captured the growth of the EMIC waves during the storm and showed that the EMIC waves, by scattering the ring current, were responsible for the proton precipitation into the dusk-midnight sector during the storm. ElectroMagnetic Ion Cyclotron (EMIC) wave activity and proton precipitation were observed simultaneously in the dusk-midnight sector during the 27 May 2017 storm The BATS-R-US + RAM-SCB model can capture the EMIC wave growth during the storm qualitatively The EMIC wave scattering of ring current ions can account for the proton precipitation in the dusk-midnight sector during the storm

Abstract The ring current is an important component of the Earth's near‐space environment, as its variations are the direct driver of geomagnetic storms that can disrupt power grids, satellite communications, and navigation systems, thereby impacting a wide range of technological and human activities. Oxygen ions (O+) are one of the major components of the ring current and play a significant role in both the enhancement and depletion of the ring current during geomagnetic storms. Although a standard statistical study can provide average global distributions of ring current ions, it can't offer insight into the short‐term dynamic variations of the global distribution. Therefore, we employed the Artificial Neural Network technique to construct a global ring current O+ ion model based on the Van Allen Probes observations. Through optimization of the combination of input geomagnetic indices and their respective time history lengths, the model can well reproduce the spatiotemporal variation of the oxygen ion flux distributions and demonstrates remarkable accuracy and minimal errors. Additionally, the model effectively reconstructs the temporal variation of ring current O+ ions for non‐training set data. Furthermore, the model provides a comprehensive and dynamic representation of global ring current O+ ion distribution. It accurately captures the dynamics of O+ ions during a geomagnetic storm with the oxygen ion fluxes enhancement and decay, and reveals distinct characteristics for different energy levels, such as injection from the plasma sheet, outflow from the ionosphere, and magnetic local time asymmetry.

&lt;p&gt;Geomagnetic storms are the main component of space weather. Enhancement of the ring current is a typical feature of the geomagnetic storm and a global decrease in the &lt;em&gt;H&lt;/em&gt; component of the geomagnetic field is observed during the main phase of the geomagnetic storm. &amp;#160;The ring current represents a diamagnetic current driven by the plasma pressure in the inner magnetosphere. The plasma pressure is mainly dominated by protons in an energy range of a few to a few hundred keVs during quiet times. The O&lt;sup&gt;+&lt;/sup&gt; contribution is also important, and sometimes dominates more than H&lt;sup&gt;+&lt;/sup&gt; during intense geomagnetic storms. However, electron contribution to the ring current is not studied well. Recently, we showed that the electron pressure also contributes to the depression of ground magnetic field during the November 2017 CIR-driven storm by comparing &lt;strong&gt;R&lt;/strong&gt;ing current&amp;#160;&lt;strong&gt;A&lt;/strong&gt;tmosphere interactions&amp;#160;&lt;strong&gt;M&lt;/strong&gt;odel with&amp;#160;&lt;strong&gt;S&lt;/strong&gt;elf&amp;#160;&lt;strong&gt;C&lt;/strong&gt;onsistent magnetic field (RAM-SCB) simulation, Arase in-situ plasma/particle data, and ground-based magnetometer data [Kumar et al., 2021]. Arase satellite observed 26 geomagnetic storms driven by Corotating Interaction Regions (CIR) during 2017-2021. In this study, we examine statistically the spatial and temporal distribution of ions (H&lt;sup&gt;+&lt;/sup&gt;, He&lt;sup&gt;+&lt;/sup&gt;, O&lt;sup&gt;+&lt;/sup&gt;) and electrons pressure as a function of magnetic local time, L shell and wide range of energies during prestorm, main phase, early recovery and late recovery phase for 26 CIR storms using in situ plasma/particle data obtained by Arase. The results indicate that the electrons (20-50 keV) contribution to the ring current pressure is non-negligible.&lt;/p&gt;