Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

M Nosé; S Oimatsu; K Keika; C. A Kletzing; W. S Kurth; S De Pascuale; C. W Smith; R. J MacDowall; S Nakano; G. D Reeves; H. E Spence; B. A Larsen

doi:10.1002/2014JA020593

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Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

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Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

M Nosé, S Oimatsu, K Keika, C. A Kletzing, W. S Kurth, S De Pascuale, C. W Smith, R. J MacDowall, S Nakano, G. D Reeves, …

Journal of geophysical research. Space physics, Vol.120(2), pp.1182-1196

02/2015

DOI: 10.1002/2014JA020593

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Abstract

We study the formation process of an oxygen torus during the 12–15 November 2012 magnetic storm, using the magnetic field and plasma wave data obtained by Van Allen Probes. We estimate the local plasma mass density (ρL) and the local electron number density (neL) from the resonant frequencies of standing Alfvén waves and the upper hybrid resonance band. The average ion mass (M) can be calculated by M ∼ ρL/neL under the assumption of quasi‐neutrality of plasma. During the storm recovery phase, both Probe A and Probe B observe the oxygen torus at L = 3.0–4.0 and L = 3.7–4.5, respectively, on the morning side. The oxygen torus has M = 4.5–8 amu and extends around the plasmapause that is identified at L∼3.2–3.9. We find that during the initial phase, M is 4–7 amu throughout the plasma trough and remains at ∼1 amu in the plasmasphere, implying that ionospheric O+ ions are supplied into the inner magnetosphere already in the initial phase of the magnetic storm. Numerical calculation under a decrease of the convection electric field reveals that some of thermal O+ ions distributed throughout the plasma trough are trapped within the expanded plasmasphere, whereas some of them drift around the plasmapause on the dawnside. This creates the oxygen torus spreading near the plasmapause, which is consistent with the Van Allen Probes observations. We conclude that the oxygen torus identified in this study favors the formation scenario of supplying O+ in the inner magnetosphere during the initial phase and subsequent drift during the recovery phase. Key Points An oxygen torus is identified at L = 3.0–4.5 during the storm recovery phase The oxygen torus extends around the plasmapause The formation process of the oxygen torus is examined with numerical simulation

inner magnetosphere

magnetic storm

oxygen torus

plasmasphere

ring current

ULF waves

Details

Title: Subtitle: Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations
Creators: M Nosé - Kyoto University
S Oimatsu - Kyoto University
K Keika - Nagoya University
C. A Kletzing - University of Iowa
W. S Kurth - University of Iowa
S De Pascuale - University of Iowa
C. W Smith - University of New Hampshire
R. J MacDowall - Goddard Space Flight Center
S Nakano - Research Organization of Information and Systems
G. D Reeves - Los Alamos National Laboratory
H. E Spence - University of New Hampshire
B. A Larsen - Los Alamos National Laboratory
Resource Type: Journal article
Publication Details: Journal of geophysical research. Space physics, Vol.120(2), pp.1182-1196
DOI: 10.1002/2014JA020593
ISSN: 2169-9380
eISSN: 2169-9402
Number of pages: 15
Grant note: Ministry of Education, Culture, Sports, Science and Technology (MEXT) (25287127) JHU/APL (921648) RBSP‐ECT (JHU/APL contract 967399) National Aeronautics and Space Administration (NAS5‐01072; NAS5‐01072) U.S. Department of Energy
Language: English
Date published: 02/2015
Academic Unit: Physics and Astronomy
Record Identifier: 9984199681502771

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