[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-04] あらせ衛星の観測による太陽活動極大期の酸素イオンが支配的なリングカレントの研究

*北村 成寿¹、山本 和弘¹、横田 勝一郎²、笠原 慧³、新堀 淳樹¹、海老原 祐輔⁴、桂華 邦裕³、三好 由純¹、Kistler Lynn⁵、堀 智昭¹、Jun Chae-Woo¹、家田 章正¹、松岡 彩子⁶、寺本 万里子⁷、篠原 育⁸ (1.名古屋大学 宇宙地球環境研究所、2.大阪大学 大学院理学研究科 宇宙地球科学専攻、3.東京大学 大学院理学系研究科 地球惑星物理学専攻、4.京都大学 生存圏研究所、5.ニューハンプシャー大学 地球海洋宇宙研究所、6.京都大学 大学院理学研究科 地磁気世界資料解析センター、7.九州工業大学 宇宙システム工学科、8.宇宙航空研究開発機構 宇宙科学研究所)

キーワード：磁気嵐、環電流、あらせ衛星

The reduction in the horizontal component of the geomagnetic field due to the ring current is an essential feature of geomagnetic storms. The composition of ring current ions is critically important for growth and decay of geomagnetic storms, since the supply, acceleration, pitch angle scattering, and loss processes of the ions depend on ion species. Past satellite missions have yielded a limited amount of the composition measurement for ring current ions during high solar activity, because the level of the solar maximum in the last solar cycle was lower than that of other recent solar cycles. The Arase satellite has observed ring current ions (H⁺, He⁺⁺, He⁺, O⁺⁺, O⁺, and molecular ions in the energy per charge range of 9.6-184 keV/q) with the Medium-Energy Particle experiments-Ion mass analyzer (MEP-i) during geomagnetic storms in 2023 and 2024, when the solar activity level was much higher than the previous solar cycle. In this presentation, we show in-situ observational results of the ring current dominated by oxygen ions. We identified that O⁺ can easily become dominant, which is consistent with expectations on the basis of studies with data obtained by past satellite missions. During some of geomagnetic storms, enhancements of O⁺⁺ (>10% of the total energy density), which is difficult to discuss in detail due to low counts during geomagnetically quiet periods, were also identified in the inner part of the ring current (mainly L < 3). The decrease in the energy density ratio between O⁺⁺ and O⁺ toward the outer regions seems inconsistent with a hypothesis of transport from the outer region. Shifting the focus from the spatial distribution to the energy spectrum of ions, the energy per charge of O⁺⁺ tended to be smaller than that of O⁺ in the region of O⁺⁺ energy density enhancements. The enhancement limited in the inner part and smaller energy per charge of O⁺⁺ are consistent with a scenario in which O⁺⁺ is generated locally from energetic O⁺ without changing energy. If O⁺⁺ is generated preferentially near the Earth, this enhancement of O⁺⁺ is probably a unique phenomenon to large magnetic storms, when energetic O⁺ can be transported close to the Earth. Although O⁺⁺ has not been considered in modeling studies of the ring current, the importance of O⁺⁺ should be investigated in the future, especially for super geomagnetic storms.