Impulsive enhancements of oxygen ions during substorms.

Mei-Ching Fok (1) , Thomas E. Moore (1) , Pontus C. Brandt (2) , Dominique C. Delcourt (3) , Steven P. Slinker (4) , Joel A. Fedder (5)

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.

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.

Mots clés: substorm injection; ion outflow; ring current

Domaines: Astrophysique [astro-ph]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Physique des plasmas [physics.plasm-ph]

Mei-Ching Fok, Thomas E. Moore, Pontus C. Brandt, Dominique C. Delcourt, Steven P. Slinker, et al.. Impulsive enhancements of oxygen ions during substorms.. Journal of Geophysical Research Space Physics, 2006, 111 (A10), pp.A10222. ⟨10.1029/2006JA011839⟩. ⟨hal-00156492⟩