Kazushi Asamura

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.

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 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 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.