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

Journal Article · Mon Aug 03 00:00:00 EDT 2020 · Earth, Planets and Space (Online)

DOI:https://doi.org/10.1186/s40623-020-01235-w· OSTI ID:1804382

Nosé, M. ^[1]; Matsuoka, A. ^[2]; Kumamoto, A. ^[3]; Kasahara, Y. ^[4]; Teramoto, M. ^[5]; Kurita, Satoshi ^[2]; Goldstein, J. ^[6]; Kistler, L. M. ^[7]; Sing, S. ^[8]; Gololobov, A. ^[9]; Shiokawa, K. ^[1]; Iamojo, S. ^[1]; Oimatsu, S. ^[2]; Yamamoto, K. ^[10]; Obana, Y. ^[11]; Shoji, M. ^[1]; Tsuchiya, F. ^[3]; Shinohara, I. ^[12]; Miyoshi, Yoshizumi ^[1]; Kurth, W. S. ^[13] ; Kletzing, C. A. ^[13]; Smith, C. W. ^[14]; MacDowall, R. J. ^[15]; Spence, H. E. ^[14]; Reeves, Edmond Geoffrey David ^[16]

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1. Nagoya Univ. (Japan)
2. Kyoto Univ. (Japan)
3. Tohoku Univ., Sendai (Japan)
4. Kanazawa Univ. (Japan)
5. Kyushu Institute of Technology (Japan)
6. Southwest Research Institute, San Antonio, TX (United States); Univ. of Texas at San Antonio, TX (United States)
7. Nagoya Univ. (Japan); Univ. of New Hampshire, Durham, NH (United States)
8. Indian Institute of Geomagnetism, Navi Mumbai (India)
9. North-Eastern Federal University, Yakutsk (Russia)
10. Univ. of Tokyo (Japan)
11. Osaka Electro-Communication University, Neyagawa (Japan)
12. Japan Aerospace Exploration Agency (ISAS/JAXA), Sagamihara, Kanagawa (Japan)
13. Univ. of Iowa, Iowa City, IA (United States)
14. Univ. of New Hampshire, Durham, NH (United States)
15. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
16. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

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.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: Japan Society for the Promotion of Science (JSPS); National Aeronautics and Space Administration (NASA); USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: 89233218CNA000001
• OSTI ID: 1804382
• Report Number(s): LA-UR--21-23196
• Journal Information: Earth, Planets and Space (Online), Journal Name: Earth, Planets and Space (Online) Journal Issue: 1 Vol. 72; ISSN 1880-5981
• Publisher: Springer
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
EMIC wave
Heliospheric and magnetospheric physics
Inner magnetosphere
Ion composition
Oxygen torus
ULF wave