Observations and Modeling of Global O+ Substorm Injections

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.