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

M Nosé,G D Reeves,R J Macdowall,J Goldstein,K Shiokawa,A Matsuoka,Kazuhiro Yamamoto ,Charles W Smith ,Masaaki Teramoto ,Shoji Motomizu ,Iku Shinohara ,Yuki Obana ,Atsushi Kumamoto ,Satoshi Kurita ,Yoshiya Kasahara ,H E Spence ,Fuminori Tsuchiya ,S V Singh ,W S Kŭrth ,L M Kistler,C Kletzing ,Артем Гололобов ,Shun Imajo ,S Oimatsu

doi:10.1186/s40623-020-01235-w

Abstract

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.

Highlights

The inner magnetospheric O+ density enhancement was first discovered by Chappell (1982) employing the retarding ion mass spectrometer (RIMS) instrument onboard the Dynamic Explorer (DE) 1 satellite
Arase flying in the morning sector detected an enhancement of the average plasma mass up to ~ 3.5 amu around L = 4.9–5.2 and magnetic local time (MLT) = 5.0 h, while Probe A flying in the afternoon sector observed no clear enhancements in the average plasma mass
Electromagnetic ion cyclotron waves in oxygen torus We examine whether the local enhancement of O+ density is related to electromagnetic ion cyclotron (EMIC) occurrence

Summary

Introduction

The inner magnetospheric O+ density enhancement (known as the dense oxygen torus) was first discovered by Chappell (1982) employing the retarding ion mass spectrometer (RIMS) instrument onboard the Dynamic Explorer (DE) 1 satellite. (As shown later, the more realistic composition is estimated as 80.6% H +, 3.4% He+, and 16.0% O+.) we suppose that the oxygen torus is identified just outside the plasmapause at L ~ 3.6 on the morning side by Probe B It should be noted, that Probe B flying near the plasmapause on the afternoon side around 09:00 UT did not seem to find a similar enhancement of M. The ions accelerated in the perpendicular direction would have a pitch angle distribution with a larger anisotropy to excite the EMIC wave at 07:10–07:50 UT (Anderson and Hamilton, 1993; Anderson et al 1996) For the former EMIC wave that appeared at approximately 4.5 Hz around 02:40–02:50 UT, no enhancement of the solar wind dynamic pressure occurred (Fig. 3). These results imply a close relation between the background plasma composition and the EMIC wave excitation

Findings

Discussion

Conclusions