[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-02] Spatial and pitch angle distributions of low-energy (<300 eV) O⁺ plasma in the inner magnetosphere

*Masahito Nose¹, Kazushi Asamura², Yoshizumi Miyoshi³, Trunali Shah⁴, Ayako Matsuoka⁵, Mariko Teramoto⁶, Atsushi Kumamoto⁷, Fuminori Tsuchiya⁷, Yoshiya Kasahara⁸, Atsuki Shinbori³, Iku Shinohara² (1.School of Data Science, Nagoya City University, 2.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3.Institute for Space-Earth Environmental Research, Nagoya University, 4.Indian Institute of Geomagnetism, 5.Graduate School of Science, Kyoto University, 6.Department of Space Systems Engineering, Kyushu Institute of Technology, 7.Graduate School of Science, Tohoku University, 8.Emerging Media Initiative, Kanazawa University)

Keywords:Low-energy O+ ion, Warm plasma cloak, Oxygen torus, Field-aligned low-energy O+ (FALEO)

In the magnetosphere, the ion composition of background plasma is a critical parameter that significantly influences various in situ electromagnetic phenomena, including solar wind–magnetosphere coupling, magnetic reconnection, the Kelvin-Helmholtz instability, electromagnetic ion cyclotron waves, and magnetohydrodynamic waves. In particular, O⁺ ions, which have 16 times the mass of H⁺ ions, strongly affect the mass density of the background plasma. Therefore, it is essential to investigate when, where, and how their low-energy flux varies. Recent studies have revealed that relatively low-energy (10 eV to a few keV) ions, known as the “warm plasma cloak”, exist in the magnetosphere at L > 6, extending from midnight through the morning and noon sectors [Chappell, 2008; Lee and Angelopoulos, 2014]. Additionally, a localized plasma structure, called the “oxygen torus”, which consists mainly of O⁺ ions with even lower energies (< tens of eV), has been identified around L = 3–5 in the midnight-to-morning sector [Nosé et al., 2018, 2020]. However, these two plasma populations have been studied separately, and thus, their relationship remains poorly understood.
In this study, we investigate where low-energy O⁺ plasma is predominantly observed in the inner magnetosphere, using data from the Arase satellite collected over approximately 6.3 years (April 2017–June 2023). A statistical analysis of 50–300 eV O⁺ ion flux measured by the LEP-i instrument onboard Arase revealed the following key findings: (1) the peak of the O⁺ ion flux is frequently observed near the plasmasphere boundary, (2) the mean O⁺ ion flux is significantly high in the region L = 3–5, 21–12 MLT, (3) as magnetic disturbance increases, the overall O⁺ flux also increases; however, the flux at L = 6 remains unchanged, and (4) the pitch angle distribution of the O⁺ flux exhibits a cigar-type shape, indicating a strong field-aligned component. These statistical observations of low-energy O⁺ plasma are in good agreement with the expected evolution of field-aligned low-energy O⁺ (FALEO) ions, which are believed to flow out from the ionosphere into the inner magnetosphere during substorms and move along magnetic field lines. This suggests that FALEO is the primary source of low-energy O⁺ plasma with energies below 300 eV.