Evolution of ring current ion energy spectra during the storm recovery phase: Implication for dominant ion loss processes

Abstract

[1] We examine the temporal evolution of spatially averaged energy spectra of ring current ions (27–120 keV for H+, 79–264 keV for O+) during the recovery phase of three magnetic storms. The energy spectra are derived from energetic neutral atom (ENA) imaging provided by the High Energy Neutral Atom (HENA) imager on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite; ENAs generated within HENA lines of sight passing by close to the Earth are excluded in order to distinguish high-altitude emissions from low-altitude emissions. We confirm, by performing a simple model of ENA production, that the retrieved spectra are a good indication of those at 3.5 60 keV H+ was higher than the prestorm level. It is likely that the contribution from >60 keV H+ to the ring current intensity (and H+ pressure) increases as a storm recovers. We suggest that the ring current intensity is dominated by higher-energy (>60 keV) H+ during the late recovery phase.