Global imaging of O+ from IMAGE/HENA

The magnetospheric O⁺ population in the 52–180 keV range during storms is investigated through the analysis of energetic neutral atom (ENA) images. The images are obtained from the high energy neutral atom (HENA) imager onboard the IMAGE satellite. At each substorm onset following the commencement of a geomagnetic storm the oxygen ENA display ～30 min intense bursts. Only very weak corresponding features in the 60–119 keV hydrogen ENA can be occasionally seen. The dominating fraction of the oxygen ENA emissions are produced when O⁺ ions mirror/precipitate at low altitudes, where the number density of the neutral atmosphere is high. During the storm we observed several bursts of oxygen ENA, but it is still not clear how much the O⁺ content of the ring current increases during the storm main phase. Our observations suggest that the responsible injection mechanism is mass-dependent and scatters the pitch angles. This leads us to favor a non-adiabatic mechanism proposed by (Delcourt, 2002).