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.

中文翻译：

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氧环面及其与深内磁层中的 EMIC 波的重合：Van Allen Probe B 和 Arase 观测

我们使用 Van Allen Probe B 和 Arase 卫星的同时观测，研究了 2017 年 9 月 12 日发现的特定事件的内磁层中氧环的纵向结构。发现探针 B 在 L = 3.3-3.6 和磁本地时间 (MLT) = 9.0 h 时观察到平均等离子体质量 ( M ) 明显增强高达 3-4 amu。在 MLT ~ 16.0 h 的下午部分，Probe B 和 Arase 都没有发现 M 有明显的增强。这一结果表明，氧环面并没有延伸到整个 MLT，而是向黎明倾斜。由于在之前的研究中对另一个氧环面事件报告了类似的结果，以黎明为中心的新月形环面或收缩环面可能是内磁层中 O + 密度增强的一般特征。我们新发现H + 波段的电磁离子回旋（EMIC）波与氧环同时出现。从 EMIC 波的较低截止频率，氧环的离子组成估计为 80.6% H + 、3.4% He + 和 16.0% O + 。根据EMIC波的线性色散关系，He + 和O + 离子均抑制EMIC 波的生长，He + 的稳定作用强于O + 。因此，当H + 分数或M 恒定时，密度越大的O + 离子自然伴随着更稀薄的He + 离子，导致稳定作用较弱（即生长速率较大）。从探测器 B 的观察中，我们发现氧环面的生长速率比等离子体槽和等离子体球中的相邻区域的生长速率更大。