Direct Outflow and Subsequent Transport of Low Energy (<keV) O+ Ions into the Inner Magnetosphere During Geomagnetically Active Times and Implications for Radiation Belt Dynamics.

Abstract:

The low-energy (eV to hundreds of eV) ion population in the inner magnetosphere, the warm plasma cloak, and in particular its heavy ion component, the O+ torus, is crucial to inner magnetosphere dynamics. Thermal O+ torus can be locally accelerated to ring current energies due to dipolarizations inside geosynchronous orbit. Furthermore, heavy ion mass loading is crucial with respect to meaningful calculations of ULF wave field line resonance radial profiles in the plasmasphere. Yet, although the effects of high latitude and cusp ionospheric O+ outflow and its subsequent transport and acceleration within the magnetotail and plasma sheet have been extensively studied, the source of low-energy O+ within the inner magnetosphere (already observed by the DE1 spacecraft in the 80s) remains a compelling open question. The HOPE instrument aboard each of the Van Allen Probes, moving in highly elliptical, equatorial orbits with apogee of 5.8 R_E, has repeatedly detected low-energy O+ field-aligned enhancements. We present a comprehensive study of such events, where low energy O+ field-aligned intensity enhancements were observed, both at small and large pitch angles, during geomagnetically active times. The energy spectrogram exhibited a dispersive signature and a banded structure, features that our simple particle tracing simulation demonstrated are due to O+ ions outflowing from both hemispheres of the night-side ionosphere directly into the magnetosphere within L = 4, and subsequently bouncing from one hemisphere to the other. These outflows are associated with field-aligned Poynting flux enhancements and field-aligned electron beams, as observed at the Van Allen Probes location, revealing energy transport from the magnetosphere to ionosphere as well as simultaneous field-aligned electron heating. The ubiquity of such events in the Van Allen Probes data and their relation to geomagnetic activity might reveal one of the main processes for O+ mass loading of the inner magnetosphere during active times.