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1 GSFC - NASA Goddard Space Flight Center 2 APL - Johns Hopkins University Applied Physics Laboratory Laurel, MD 3 CETP - Centre d-étude des environnements terrestre et planétaires 4 NRL - Naval Research Laboratory 5 Leading Edge Technology, Inc.

Abstract : It has been observed that H+ is the dominant ion species in the plasma sheet and the ring current during quiet times. However, the O+-H+ density ratio increases with increasing geomagnetic storm and substorm activity. Energetic neutral atom ENA images from Imager for Magnetopause-to-Aurora Global Exploration-High Energy Neutral Atom IMAGE-HENA reveal the rapid increase of O+ ring current at substorm expansion. Finding the cause of this substorm-associated O+ enhancement is the main focus of this paper. Two possible sources are suggested: direct injection from the ionosphere and energization of the preexisting oxygen ions in the magnetosphere. We perform numerical simulations to examine these two mechanisms. Millions of O+ are released from the auroral region during a simulated substorm by the Lyon-Fedder-Mobarry MHD model. The subsequent trajectories of these outflowing ions are calculated by solving the full equation of particle motion. A few minutes into the substorm expansion phase, an enhancement in O+ pressure is found on the nightside at ∼12 RE. After careful analysis, we conclude that this pressure peak is coming from energization of the preexisting O+ in the plasma sheet. The direct injection mechanism will introduce a significant time lag between strong ionospheric outflow and magnetospheric enhancement, so that it cannot explain the observed O+ bursts. Using the temperature and density established by the test-particle calculations as boundary conditions to a ring current model, we calculate the O+ fluxes and the corresponding ENA emissions during the model substorm. We are able to reproduce observable features of oxygen ENA enhancements as seen by IMAGE-HENA.

Keywords : substorm injection ion outflow ring current

Autor: Mei-Ching Fok - Thomas E. Moore - Pontus C. Brandt - Dominique Delcourt - Steven P. Slinker - Joel A. Fedder -



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