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

Abstract

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.

Pitch Angle Dependence of Electron and Ion Flux Changes During Local Magnetic Dipolarization Inside Geosynchronous Orbit

“…At <10 keV, whereas the PADs of all ion species have a strong field‐aligned anisotropy ( A ~−1.0), a mass‐dependent feature is seen in their normalized ion fluxes: heavier ions (He and O) show a strong enhancement in the field‐aligned direction during the dipolarization, unlike the H ions (Figure ). The field‐aligned low‐energy O ion enhancement is consistent with the Van Allen Probes observations of Nosé et al () and Gkioulidou et al (), who interpreted the low‐energy O ion enhancement in terms of the rapid outflow of O + ions from the topside ionosphere during the dipolarization event. Although neither papers examined the low‐energy He ions, it is possible that the field‐aligned low‐energy He ion enhancement occurs in a similar manner to the O ions (Hultqvist, ).…”

“…Whereas the lower energy He and O ion fluxes drop at α = 45–135° similar to the H ion flux, the He and O ion fluxes are enhanced only near the parallel and antiparallel directions. Such a field‐aligned enhancement of the lower energy O ions resembles that reported by Nosé et al () and Gkioulidou et al (), who suggested that the field‐aligned O ion flux enhancement likely arise from a prompt supply of ionospheric origin population O + into the inner magnetosphere after substorm onset. The travel time of 1–5 keV O + from the ionosphere to the inner magnetosphere ( r = 5–6 R E ) is approximately 2–5 min (cf.…”

“…The result (1) suggested that a nonadiabatic local acceleration process is responsible for the 20-50-keV O ion injections, while result (2) suggested that the ionospheric outflow of O + ions occurs almost immediately after the dipolarization onset. A two-dimensional guiding-center test-particle tracing simulation of <1-keV O + ion outflow into the deep inner magnetosphere by Gkioulidou et al (2019) predicted that the energy-dispersed low-energy O ion spectra measured at the Van Allen Probes spacecraft arise from interhemispheric O + ions extracted from the nightside subauroral ionosphere. Liu et al (2016) found 71 sharp dipolarization events in the first tail season, although the events were called "dipolarizing flux bundle (flux tubes with a strong magnetic field)" instead of dipolarization.…”

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

“…The formulation can be gyroaveraged, for instance, for limiting the computational cost for electrons For instance, since the variation of the gyroradius among the particle species varies as 1∶40∶160 (e−:H+:O+), it is necessary to keep the gyrotrajectory when computing particle loss of heavy ions through the magnetopause (e.g., Sorathia et al, ). Global coupled MHD/test particle codes are well adapted, for instance, for azimuthal transport, that is, solving for particle gradient‐curvature drift motion, for rapid particle energization occurring during interplanetary shocks on the front end of coronal mass ejections (e.g., Hudson et al, ; Kress et al, , ), for drift‐orbit bifurcation trajectory (Ukhorskiy et al, ), for acceleration at dipolarization fronts (Ukhorskiy et al, in this collection), for solar wind ion entering the magnetosphere (Sorathia et al, ), for energetic particle injections in the inner magnetosphere during substorms (e.g., Gkioulidou et al, ), or O+ ion outflow directly injected within the radiation belts (Gkioulidou et al, ), or for the sudden depletion (e.g., Ukhorskiy et al, ) and rapid recovery of the outer belt (e.g., Sorathia et al, , in this collection). These codes can also be used to generate diffusion coefficients (e.g., Ukhorskiy & Sitnov, ).…”

Dispersive Alfvén Wave Control of O+ Ion Outflow and Energy Densities in the Inner Magnetosphere

“…In the inner magnetosphere DAWs have been observed coincident with O + outflow at energies reaching 10 keV (Chaston, Bonnell, Wygant, et al, ). Earthward directed Alfvénic Poynting fluxes have also been observed coincident with low‐energy (<1 keV) O + outflow deep in the inner magnetosphere (Gkioulidou et al, ). Recent simulations (Chaston et al, ) show that DAWs can continuously energize preexisting or outflowing O + trapped on closed magnetic field lines.…”