Oxygen torus and its coincidence with EMIC wave in the deep inner magnetosphere: Van Allen Probe B and Arase observations

Earth, Planets and Space

Y. Kasahara¹  C. A. Kletzing²  W. S. Kurth²  M. Teramoto³  Y. Obana⁴  A. Matsuoka⁵  S. Oimatsu⁵  K. Yamamoto⁶  F. Tsuchiya⁷  A. Kumamoto⁷  S. Singh⁸  Y. Miyoshi⁹  M. Shoji⁹  K. Shiokawa⁹  S. Imajo⁹  M. Nosé⁹  L. M. Kistler^1,10  H. Spence^1,11  C. W. Smith^1,11  I. Shinohara^1,12  A. Gololobov^1,13  S. Kurita^1,14  R. J. MacDowall^1,15  J. Goldstein^1,16  G. D. Reeves^1,17 

[1] Advanced Research Center for Space Science and Technology, Kanazawa University, Kanazawa, Japan;Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA;Department of Space Systems Engineering, Kyushu Institute of Technology, Kitakyusyu, Japan;Faculty of Engineering, Osaka Electro-Communication University, Neyagawa, Japan;Graduate School of Science, Kyoto University, Kyoto, Japan;Graduate School of Science, The University of Tokyo, Tokyo, Japan;Graduate School of Science, Tohoku University, Sendai, Japan;Indian Institute of Geomagnetism, Navi Mumbai, India;Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan;Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Japan;Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA;Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA;Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan;North-Eastern Federal University, Yakutsk, Russia;Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan;Solar System Exploration Division, Goddard Space Flight Center, Greenbelt, MD, USA;Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA;University of Texas at San Antonio, San Antonio, TX, USA;Space Sciences and Applications Group, Los Alamos National Laboratory, Los Alamos, NM, USA;

关键词: Oxygen torus;    EMIC wave;    ULF wave;    Ion composition;    Inner magnetosphere;   

DOI  :  10.1186/s40623-020-01235-w

来源: Springer

【 摘 要 】

We investigate the longitudinal structure of the oxygen torus in the inner magnetosphere for a specific event found on 12 September 2017, using simultaneous observations from the Van Allen Probe B and Arase satellites. It is found that Probe B observed a clear enhancement in the average plasma mass (M) up to 3–4 amu at L = 3.3–3.6 and magnetic local time (MLT) = 9.0 h. In the afternoon sector at MLT ~ 16.0 h, both Probe B and Arase found no clear enhancements in M. This result suggests that the oxygen torus does not extend over all MLT but is skewed toward the dawn. Since a similar result has been reported for another event of the oxygen torus in a previous study, a crescent-shaped torus or a pinched torus centered around dawn may be a general feature of the O+ density enhancement in the inner magnetosphere. We newly find that an electromagnetic ion cyclotron (EMIC) wave in the H+ band appeared coincidently with the oxygen torus. From the lower cutoff frequency of the EMIC wave, the ion composition of the oxygen torus is estimated to be 80.6% H+, 3.4% He+, and 16.0% O+. According to the linearized dispersion relation for EMIC waves, both He+ and O+ ions inhibit EMIC wave growth and the stabilizing effect is stronger for He+ than O+. Therefore, when the H+ fraction or M is constant, the denser O+ ions are naturally accompanied by the more tenuous He+ ions, resulting in a weaker stabilizing effect (i.e., larger growth rate). From the Probe B observations, we find that the growth rate becomes larger in the oxygen torus than in the adjacent regions in the plasma trough and the plasmasphere.

【 授权许可】

CC BY