Observations and modeling of global O+ substorm injections

M. Fok, P.C. Brandt, D.G. Mitchell, S.T. Jones, E.C. Roelof, S. Ohtani, 
and R. DeMajistre

The High Energy Neutral Atom (HENA) camera on board IMAGE has revealed
significant increases in the Oxygen energetic neutral atom (ENA) in the 50-200
keV range, coincident with storm-time substorm onsets. Preliminary analysis
shows that at the time of auroral onset there is an <= 2 min enhancement of ENA
emissions originating from high altitudes. 5-10 min after, low-altitude ENA
emissions increase dramatically. Neither of these signatures are present in the
hydrogen ENA images. One scenario that may explain these observation relates to
the different gyroperiods of oxygen ions and protons. When a geomagnetic storm
commences, O+ flows out from the polar and auroral ionosphere and reaches the
plasmasheet with roughly ~1 keV energy. As the geomagnetic field dipolarizes at
substorm onset, the duration of the induced electric field is of the same order
as the O+ gyroperiod as (~min), while the proton gyroperiod is sixteen times
shorter. Therefore, O+ ions behave non-adiabatically in the dipolarization
process and can be shown to reach high energies. This scenario would explain the
sudden occurence of high-energy oxygen ENAs. However, high-energy O+ ions have
been observed at in the auroral region at ionospheric heights by the FAST
satellite in relation to substorm onsets. A 3D particle code has been developed
with a time-varying model magnetic field obtained from an MHD model during a
substorm dipolarization. We will present and discuss the analysis of the ENA
images and the in-situ data in light of the new model results.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004