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Title: | Oxygen torus and its coincidence with EMIC wave in the deep inner magnetosphere: Van Allen Probe B and Arase observations |
Authors: | Nosé, M. Matsuoka, A. Kumamoto, A. Kasahara, Y. Teramoto, M. Kurita, S. Goldstein, J. Kistler, L.M. Singh, S. Gololobov, A. Shiokawa, K. Imajo, S. Oimatsu, S. Yamamoto, K. Obana, Y. Shoji, M. Tsuchiya, F. Shinohara, I. Miyoshi, Y. Kurth, W.S. Kletzing, C.A. Smith, C.W. MacDowall, R.J. Spence, H. Reeves, G.D. |
Keywords: | Oxygen torus EMIC wave ULF wave Ion composition Inner magnetosphere |
Issue Date: | 2020 |
Citation: | Earth, Planets and Space, v. 72, https://doi.org/10.1186/s40623-020-01235-w |
Abstract: | We investigate the longitudinal structure of the oxygen torus in the inner magnetosphere for a specifc 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 fnd that an electromagnetic ion cyclotron (EMIC) wave in the H+ band appeared coincidently with the oxygen torus. From the lower cutof 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 efect 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 efect (i.e., larger growth rate). From the Probe B observations, we fnd that the growth rate becomes larger in the oxygen torus than in the adjacent regions in the plasma trough and the plasmasphere. |
URI: | http://library.iigm.res.in:8080/xmlui/handle/123456798/138 |
Appears in Collections: | UAS_Reprints |
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Singh,S_etal_EarthPlanets&Sc_2020.pdf Restricted Access | 10.96 MB | Adobe PDF | View/Open Request a copy |
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