[P-EM16] Dynamics of Earth's Inner Magnetosphere and Initial Results from Arase

[PEM16-P15] Oxygen torus near the plasmapause observed by Arase

*Masahito Nose¹, Ayako Matsuoka², Atsushi Kumamoto³, Yoshiya Kasahara⁴, Mariko Teramoto⁵, Fuminori Tsuchiya³, Shoya Matsuda⁵, Masafumi Shoji⁵, Yuki Obana⁶, Shun Imajo¹, Oimatsu Satoshi¹, Kazuhiro Yamamoto¹, Reiko Nomura⁷, Akiko Fujimoto⁸, Iku Shinohara², Yoshizumi Miyoshi⁵ (1.Graduate School of Science, Kyoto University, 2.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3.Graduate School of Science, Tohoku University, 4.Information Media Center, Kanazawa University, 5.Institute for Space-Earth Environmental Research, Nagoya University, 6.Faculty of Engineering, Osaka Electro-Communication University, 7.Tsukuba Space Center, Japan Aerospace Exploration Agency, 8.International Center for Space Weather Science and Education, Kyusyu University)

Using the magnetic field and plasma wave data obtained by the fluxgate magnetometer (MGF) and plasma wave experiment/high-frequency wave receiver and onboard frequency analyzer (PWE/HFA and OFA) instruments on board the Arase satellite, we search for enhancements of O⁺ ion density in the deep inner magnetosphere known as “the oxygen torus”. We estimate the local plasma mass density (rho_L) and the local electron number density (n_eL) from the resonant frequencies of standing Alfvén waves and the upper hybrid resonance band, respectively. The average ion mass (M) can be calculated by M~rho_L/n_eL under the assumption of quasi-neutrality of plasma. Preliminary analysis of some events at dawn, in which toroidal standing Alfvén waves appear clearly during the storm recovery phase, reveals that M is enhanced in the vicinity of the plasmapause, implying formation of the oxygen torus. In presentation, we will show more detailed analysis results and discuss the formation mechanism of the oxygen torus.