Postmidnight storm-time enhancement of tens-of-keV proton flux

We investigated the global morphology of the storm-time distribution of ring current protons and energetic neutral atoms (ENAs) observed by the High-Energy Neutral Atom (HENA) imager on the IMAGE satellite. The postmidnight enhancements of the proton and ENA fluxes were in particular focused on in this study, and the following six possible mechanisms causing the postmidnight enhancements were tested by using a self-consistent kinetic simulation of the ring current protons: (1) shielding electric field, (2) gap between the region 2 field-aligned current and the auroral oval, (3) strong gradient of the ionospheric conductivity near the terminator, (4) plasma sheet density, (5) plasma sheet temperature, and (6) local-time dependence of the plasma sheet density. When the ring current is self-consistently coupled with the ionosphere through the region 2 field-aligned current, the simulated postmidnight enhancements are found to agree well with the IMAGE/HENA observations, even though effects other than shielding fields were not included. The overall convection strength is found to have a substantial influence on the morphology of the distribution of the ring current protons. The MLT of the flux peak is also shown to depend slightly on the plasma sheet density and solar activity. A local-time dependence of the plasma sheet density can produce a pronounced postmidnight enhancement without introducing the self-consistent electric field. Other possible mechanisms causing the postmidnight enhancements are also discussed in detail.