[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-04] Oxygen dominant ring current during large geomagnetic storms at solar maximum observed by the Arase satellite

*Naritoshi Kitamura¹, Kazuhiro Yamamoto¹, Shoichiro Yokota², Satoshi Kasahara³, Atsuki Shinbori¹, Yusuke Ebihara⁴, Kunihiro Keika³, Yoshizumi Miyoshi¹, Lynn M Kistler⁵, Tomoaki Hori¹, Chae-Woo Jun¹, Akimasa Ieda¹, Ayako Matsuoka⁶, Mariko Teramoto⁷, Iku Shinohara⁸ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.Department of Earth and Space Science, Graduate School of Science, Osaka University, 3.Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, 4.Research Institute for Sustainable Humanosphere, Kyoto University, 5.Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, 6.Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, 7.Department of Space Systems Engineering, Kyushu Institute of Technology, 8.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)

Keywords:geomagnetic storm, ring current, Arase satellite

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.