[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-10] Warm O⁺ plasma in the inner magnetosphere: Future of field-aligned low-energy O⁺ ions (FALEO)

*Masahito Nose¹, Kazushi Asamura², Yoshizumi Miyoshi³, Ayako Matsuoka⁴, Mariko Teramoto⁵ (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.Graduate School of Science, Kyoto University, 5.Department of Space Systems Engineering, Kyushu Institute of Technology)

The ion composition of background plasma in the magnetosphere is a very important parameter, because it significantly alters the nature of various electromagnetic phenomena occurring there, such as solar wind-magnetospheric coupling, magnetic reconnection, Kelvin-Helmholtz instability, electromagnetic ion cyclotron wave, and Alfvén wave. O⁺ ions, which are 16 times heavier than H⁺ ions, have a significant impact on the plasma mass density of the background plasma; thus, it is necessary to investigate when, where, and how low-energy O⁺ ion fluxes change. Recent studies by Chappell et al. [2008] and Lee and Angelopoulos [2014] reported presence of low-energy (10–400 eV) ions, called “warm plasma cloak”, at L>6 from midnight to the dawn and morning sides. Further low-energy (<50 eV) plasma, consisting mainly of O⁺ ions, has also been reported to be localized around L=3–5 on the dawn to morning side [Nosé et al. 2018, 2020]. This lower-energy O⁺ plasma is called “oxygen torus”. However, since these studies were based on observations in the outer magnetosphere at L>6 or event analysis, we need to perform a statistical study to reveal characteristics of the warm plasma cloak and the oxygen torus in the inner magnetosphere.
In this study, we investigated a spatial distribution of low-energy O⁺ plasma in the inner magnetosphere, using data obtained by the Arase satellite over a five-year period from April 2017 to March 2022. We calculated omnidirectional fluxes over 50–300 eV (J) for both H⁺ and O⁺ ions, and selected time intervals, in which J(O⁺) is 5 times larger than J(H⁺), being called “warm O⁺ plasma”. Our findings include that (1) the warm O⁺ plasma appears frequently at 22–07 MLT with an occurrence peak after midnight; (2) its occurrence probability decreases from midnight toward morning, noon, and evening; (3) the occurrence probability has a peak at L=2.5–3.5; (4) it is observed more frequently at higher geomagnetic latitudes than near the geomagnetic equator; and (5) it appears at larger L as the satellite moves from midnight to the morning. Regarding the dependence on the Dst index, the occurrence probability is higher during a disturbed period (Dst=−10~−50 nT) than a quiet period (Dst≧−10 nT), and its spatial distribution extends to smaller L values in the morning to noon. These statistical results of the warm O⁺ plasma are consistent with results expected from the time evolution of “field-aligned low-energy O⁺ ions (FALEO)”, which outflows from the ionosphere into the inner magnetosphere at substorm onset and has the field-aligned distribution, implying that FALEO is an origin for the warm O⁺ plasma in the inner magnetosphere.