[P-EM12] Dynamics of the Inner Magnetospheric System

[PEM12-04] Multisatellite observations of field-aligned low-energy O⁺ ion flux enhancements in the inner magnetosphere: September 22, 2018, Event

*能勢 正仁¹、松岡 彩子²、三好 由純¹、浅村 和史³、寺本 万里子⁴、篠原 育³、平原 聖文¹、Kletzing C. A.⁵、Smith C. W.⁶、MacDowall R. J.⁷、Spence H. E.⁶、Reeves G. D.⁸ (1.名古屋大学 宇宙地球環境研究所、2.京都大学 理学研究科、3.宇宙航空研究開発機構 宇宙科学研究本部、4.九州工業大学 工学研究院、5.Department of Physics and Astronomy, University of Iowa、6.Institute for the Study of Earth, Oceans, and Space, University of New Hampshire、7.Solar System Exploration Division, Goddard Space Flight Center、8.Space Sciences and Applications Group, Los Alamos National Laboratory)

キーワード：沿磁力線方向低エネルギーO+イオンフラックス増加、内部磁気圏、warm plasma cloak、酸素イオントーラス

Recent studies employing the Arase and Van Allen Probes satellites [Chaston et al., 2015; Kistler et al., 2016; Nosé et al., 2016, 2018; Gkioulidou et al., 2019] have shown that unidirectional/bidirectional energy-dispersed O⁺ flux appears a few minutes after substorms in the inner magnetosphere and lasts for ~10 min with a decrease in its energy from ~5 keV to 10–100 eV. Nosé et al. [2016] found that the unidirectional energy-dispersed O⁺ flux is observed in 80% of the total events and that its direction is parallel (antiparallel) to the magnetic field when the satellites are located below (above) the geomagnetic equator. This strongly implies that these O⁺ ions are extracted from the ionosphere at the onset of substorms and flow along the magnetic field toward the geomagnetic equator. Low-energy O⁺ ions may be scattered near the geomagnetic equator and remain there, although the scattering mechanism is yet unknown. They may contribute to the O⁺ content of the inner magnetospheric plasma such as the warm plasma cloak and the oxygen torus, and the resultant increase in the O⁺ density may provide a precondition for the O⁺-rich ring current.
In the present study, we examine the low-energy O⁺ ion flux variations simultaneously observed by multiple satellites, Arase, Van Allen Probe A and B satellite, on September 22, 2018. The O⁺ fluxes are enhanced after a substorm onset at 05:24 UT, at which three satellites are located in the nightside inner magnetosphere (Arase at MLT=0.3 hr, L=6.2, GMLAT=−9.6°; Probe A at MLT=0.7 hr, L=5.5, GMLAT=14.7°; Probe B at MLT=0.0 hr, L=5.3, GMLAT=10.6°). Arase observes O⁺ flux enhancements only in the parallel direction to the magnetic field in the energy range from a few keV to 200 eV. Probe A and B, however, identify O⁺ flux enhancements in both parallel and antiparallel directions at 1 keV to 10 eV. The antiparallel fluxes appear earlier than the parallel fluxes. Multiband flux enhancements are detected only by Probe A. We perform numerical calculation of O⁺ ion trajectories to reproduce the observed E-t spectorgrams at three satellites. In the presentation, we will show results of data analysis and numerical simulation in more detail, and discuss the contribution of the low-energy O⁺ ion flux enhancements to the O⁺ content of the inner magnetospheric plasma.