[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-P06] Statistical study of Arase on plasma and field variations in the source region of substorm onset in the inner magnetosphere

*Liwei Chen¹, Kazuo Shiokawa¹, Yoshizumi Miyoshi¹, Shin-ichiro Oyama^1,2, Chae-Woo Jun¹, Keisuke Hosokawa³, Yoichi Kazama⁵, Shiang-Yu Wang⁵, Sunny W. Y. Tam⁴, Tzu-Fang Chang^1,4, Bo-Jhou Wang⁵, Kazushi Asamura⁶, Satoshi Kasahara⁷, Shoichiro Yokota⁸, Tomoaki Hori¹, Kunihiro Keika⁷, Yasumasa Kasaba⁹, Atsushi Kumamoto⁹, Masafumi Shoji¹, Yoshiya Kasahara¹⁰, Ayako Matsuoka¹¹, Iku Shinohara⁶, Fuminori Tsuchiya⁹, Shun Imajo¹¹, Satoko Nakamura¹ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.National Institute of Polar Research, 3.The University of Electro-Communications, 4.Institute of Space and Plasma Sciences, National Cheng Kung University, 5.Institute of Astronomy and Astrophysics, Academia Sinica, 6.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 7.University of Tokyo, 8.Osaka University, 9.Tohoku University, 10.Kanazawa University, 11.Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University)

Keywords:substorm, inner magnetosphere, Arase

Substorm is one of the most frequent dynamic variations in the Earth’s magnetosphere. The study of the substorm is key to grasp the fundamental understanding of magnetospheric disturbances. Previous conjugate observations of substorm auroral brightening have shown characteristic electromagnetic waves in the ULF/Pi2 frequency range in the inner magnetosphere while conjugate substorm auroral activities were observed on the ground (e.g., Chen et al., 2022; 2023). However, general characteristics of electromagnetic and kinetic fluctuations near the substorm onset region in the inner magnetosphere are still not well understood. In this study, we perform a statistical analysis of electromagnetic and pressure variations in the inner magnetosphere under substorm conditions. We focus on the magnetospheric response at +-1 hour of the magnetic local time (MLT) of the substorm onset, based on the observation from the inner-magnetospheric satellite Arase. Using the substorm event list [Newell and Gjerloev, 2011] from April 2017 to October 2022, ~800 substorm events were selected during which the Arase satellite was passing through ±1 MLT to the onset local time. Superposed epoch analysis shows that electromagnetic waves at frequencies of 4-25 mHz are intensified clearly at L ~ 7 during ~-5 to 15 minutes after substorm onset. We found that the electric field oscillation is most prominent in the radial component. The magnetic wave intensification can be seen in both radial and toroidal components with stronger toroidal oscillation. By tracing the Arase location to the Earth along the magnetic field lines, we found that most activities of electromagnetic waves were probably connected to the substorm onset station on the ground within ±5 degrees in magnetic latitudes. Fluctuation in plasma pressure presents a similar distribution at L ~ 7 within 30 minutes after the substorm onset, while fluctuation in magnetic pressure is likely to penetrate further earthward. These results contribute to comprehensive understanding of the characteristics of plasma and field variations in the source region of substorm onset in the inner magnetosphere.