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

<jats:title>Abstract</jats:title><jats:p>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 (<jats:italic>M</jats:italic>) up to 3–4 amu at <jats:italic>L</jats:italic> = 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 <jats:italic>M</jats:italic>. 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<jats:sup>+</jats:sup> density enhancement in the inner magnetosphere. We newly find that an electromagnetic ion cyclotron (EMIC) wave in the H<jats:sup>+</jats:sup> 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<jats:sup>+</jats:sup>, 3.4% He<jats:sup>+</jats:sup>, and 16.0% O<jats:sup>+</jats:sup>. According to the linearized dispersion relation for EMIC waves, both He<jats:sup>+</jats:sup> and O<jats:sup>+</jats:sup> ions inhibit EMIC wave growth and the stabilizing effect is stronger for He<jats:sup>+</jats:sup> than O<jats:sup>+</jats:sup>. Therefore, when the H<jats:sup>+</jats:sup> fraction or <jats:italic>M</jats:italic> is constant, the denser O<jats:sup>+</jats:sup> ions are naturally accompanied by the more tenuous He<jats:sup>+</jats:sup> 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.</jats:p>