高島 健

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.