Low-Energy (<keV) O + Ion Outflow Directly Into the Inner Magnetosphere: Van Allen Probes Observations

Matina Gkioulidou, S. Ohtani, A. Y. Ukhorskiy, D. G. Mitchell, K. Takahashi, H. E. Spence, J. R. Wygant, C. A. Kletzing, R. J. Barnes

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The heavy ion component of the low-energy (eV to hundreds of eV) ion population in the inner magnetosphere, also known as the O + torus, is a crucial population for various aspects of magnetospheric dynamics. Yet even though its existence has been known since the 1980s, its formation remains an open question. We present a comprehensive study of a low-energy (<keV), bidirectional O + outflow event, which occurred deep into the inner magnetosphere (inside L = 4), and was observed by the Helium, Oxygen, Proton and Electron (HOPE) instrument aboard the Van Allen Probe B. The observed spectrogram exhibited multiple bands of field-aligned intensity enhancements with energy dispersion. A 2-D guiding-center test-particle tracing simulation demonstrates that the observed spectral features can be attributed to O + ions exiting both hemispheres of the nightside ionosphere over L ~ 3–4 latitudinal and magnetic local time (MLT) ~ 21 to 23 hr longitudinal extent, directly entering the inner magnetosphere, and subsequently bouncing from one hemisphere to the other. The outflow is associated with earthward field-aligned Poynting flux enhancement and field-aligned electron beams, as observed at the Van Allen Probes location, as well as with strong upward field-aligned current, as revealed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) at the ionospheric footpoint of the spacecraft. O + partial density in the region outside plasmapause was significantly enhanced by the outflow population, exceeding the H + density and indicating the possible formation of an O + torus.

Original languageEnglish (US)
Pages (from-to)405-419
Number of pages15
JournalJournal of Geophysical Research: Space Physics
Issue number1
StatePublished - Jan 2019

Bibliographical note

Funding Information:
The authors thank the extended Van Allen Probes team and Solene Lejosne for useful discussions. M. G. was supported by JHU/APL subcontract 131803 to the New Jersey Institute of Technology under NASA Prime contract NNN06AA01C and by NASA grant NNX17AI54G. S. O. was supported by NASA grant NNX16AF74G. Data from the Van Allen Probes spacecraft used in this study are publicly available through the following instrument websites: EMFISIS (https://emfisis.physics. uiowa.edu/), EFW (http://www.space. umn.edu/rbspefw‐data/), and HOPE (https://rbsp‐ect.lanl.gov/rbsp_ect.php, Release 3). We thank the AMPERE team and the AMPERE Science Center for providing the Iridium derived data products (http://ampere.jhuapl.edu/). SuperMAG indices as well as polar diagrams of equivalent currents were provided through the SuperMAG website (http://supermag.jhuapl.edu/). SuperMAG is an international collaboration with many organizations and institutes funded by National Science Foundation (NSF) grant 1417899.


  • O outflow
  • inner magnetosphere

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