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

Document Type

Article

Publication Date

1-1-2011

Abstract

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 < L < 5. The spectral evolution during the early and rapid recovery phase of the examined intense and weak storms shows that higher-energy H + were lost more than lower-energy H+. The loss rate of ∼100 keV H+ is higher than or comparable to that of ∼100 keV O+. During the early and rapid recovery of the examined moderate storm, the O+ loss rate was higher for lower energies. These results suggest that charge exchange makes a small contribution to the ring current rapid recovery. We conclude that ion precipitation through pitch angle scattering (probably due to interactions with electromagnetic ion cyclotron waves) plays a significant role in ion loss during the early and rapid recovery phase. For the late and slow recovery phase, on the other hand, the spectral evolution is consistent with that expected from charge exchange. The retrieved phase space density of <60 keV H+ decreased down to around the prestorm level when Dst did not yet recover to the prestorm value, while the phase space density of >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. Copyright 2011 by the American Geophysical Union.

Identifier

79953006200 (Scopus)

Publication Title

Journal of Geophysical Research Space Physics

External Full Text Location

https://doi.org/10.1029/2010JA015628

e-ISSN

21699402

Issue

3

Volume

116

Grant

22740322

Fund Ref

Japan Society for the Promotion of Science

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