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 similar to 3-4 latitudinal and magnetic local time (MLT) similar to 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.