Kazushi Asamura

Earth's main magnetic field forms the magnetosphere, shielding the planet from solar wind plasma. Southward interplanetary magnetic fields erode the magnetosphere, a process known as magnetopause erosion, allowing solar wind penetration close to the Earth. While magnetic reconnection is considered a key driver, the mechanism during super geomagnetic storms remains unclear due to limited in situ observations. Here, we report a magnetopause crossing by Arase at 4.96 R _E during the May 2024 superstorm, one of the innermost magnetopause crossings ever detected by magnetospheric spacecraft. We observe an anomalous and global reduction in magnetic field intensity extending from the inner magnetosphere to the magnetopause, suggesting global weakening by magnetospheric currents. This weakened magnetic field is responsible for the enhanced magnetopause erosion.

During geomagnetic storms, energetic ions are injected from the Earth's magnetotail along the plasma sheet boundary layer (PSBL). Owing to its off‐equatorial inclined orbit, the Arase satellite is in a prime position to observe these injections in the PSBL on the nightside at the apogee. The injections are observed as energy and pitch angle dispersed features in the Arase LEP‐i and MEP‐i data. While these injections have often been observed in H ⁺ , reports of O ⁺ dispersions are rare. During the main phase of a geomagnetic storm on 16 July 2017 we identify frequent strong earthward injections of O ⁺ . The distance to the injection location varied from about 10 to 40 Earth radii. Some successive injections showed an increasing trend in distance, indicating that the ion acceleration region moved down the magnetotail over time. The earthward flux of these ions was comparable to the flux of nightside auroral outflow during this time, indicating that these injections may be a significant contributor to the hot O ⁺ population in the storm‐time near‐earth plasma sheet. Additionally, we present statistics derived from 180 individual O ⁺ injections during storms in 2017–2020, confirming that the injection source is mainly located in 10–40 R _E with a median at 20 R _E .

At subauroral latitudes, discrete auroras, such as strong thermal emission velocity enhancement and stable auroral red arcs, have been extensively studied. On the other hand, diffuse emissions at subauroral latitudes have not been studied well. In this paper, we report eight events of conjugate observations of faint diffuse 557.7‐nm emissions with intensities of 300–1,000 rayleighs without discrete structures equatorward of the auroral oval, using the Arase satellite and high‐sensitivity all‐sky airglow imagers in North America. Ground‐magnetosphere conjugate observations of such diffuse emissions have not been previously reported. Diffuse 557.7‐nm emissions were observed during both geomagnetically disturbed and quiet conditions. Preference for magnetic local time was not found in these eight events. Arase observed plasma‐sheet electrons with energies from several hundred eV to beyond 10 keV in the source magnetosphere, which is located inside the plasmapause. Electromagnetic waves, such as plasmaspheric hiss and lower‐band chorus waves, were also observed there by the Arase satellite. For five events, the resonance energies estimated from the observed wave frequencies ranged from 30 eV to 30 keV, suitable for generating the 557.7‐nm emission, though the pitch‐angle diffusion coefficient estimated for one event was not so large. We discuss the possibility that plasma‐sheet electrons in the plasmasphere precipitated into the ionosphere through wave‐particle interaction, forming faint diffuse 557.7‐nm emission equatorward of the auroral oval.

Electron and proton precipitation into the nightside ionosphere is a key process in magnetosphere–ionosphere coupling. Although auroral electron precipitation at –10 keV is traditionally considered the main driver of auroral emissions, recent studies show that keV electrons and protons can significantly contribute to ionospheric energy deposition, especially during storms. While energetic electron precipitation has been studied extensively, the role of energetic proton precipitation remains poorly quantified. Here, we use coordinated ELFIN and DMSP nightside observations during two substorm events, resolving electron and proton precipitation spectra from eV to ∼MeV. We show that keV proton precipitation, consistent with curvature scattering, can have an impact on E‐region ionization comparable to that of energetic electrons. These analyses are further supported by magnetospheric observations from the Arase and THEMIS spacecraft and ground‐based incoherent scatter radar measurements from PFISR.

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.

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.

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.

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.

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

This study investigates field‐aligned low‐energy oxygen (FALEO) ions observed by the Arase satellite during 2020–2023. Using 4 years of data from the low‐energy particle experiment ion mass analyzer and magnetic field investigation instruments, we identified 90 energy‐dispersed FALEO events and our analysis reveals several key findings: (a) FALEO ions exhibit field‐aligned motion, appearing parallel to the magnetic field below the equator and anti‐parallel above it (b) the stop energy of nightside FALEO ions increases with L ‐value but decreases at low L ‐value during the high K _p values, (c) unidirectional FALEO events occur primarily between 18 magnetic local time (MLT) and 08 MLT, peaking near 00 MLT, (d) bidirectional events are found between 22 and 10 MLT, with a broad maximum around 06 MLT, (e) for the first time, using Arase bidirectional FALEO events are observed on the dawnside magnetosphere within 06–10 MLT, and (f) most of the dawnside FALEO events are associated with geomagnetic storms, indicating a strong link to enhanced geomagnetic activity. These findings suggest the existence of a previously unrecognized dawnside source of FALEO ions in addition to the well‐established nightside source. The presence of such events on the dawnside may impact dayside reconnection processes and affect the global magnetospheric convection.