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

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.