Mass-dependent evolution of energetic neutral atoms energy spectra during storm time substorms: Implication for O+ nonadiabatic acceleration

We examine the temporal variations of energy spectra of energetic neutral atoms (ENAs) detected by the High Energy Neutral Atom imager (HENA) onboard the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite during three substorms on 21 October 2001 and three substorms on 19 March 2002; the substorms occurred during the storm main phase. The ENA energy used in the present study ranges from 10 to 198 keV for hydrogen and from 29 to 222 keV for oxygen. We use ENA data obtained from two independent areas of a HENA image, for which HENA lines of sight pass through the inner magnetosphere (similar to-6 RE < X < similar to-3 R-E around the magnetic equator) and the outer magnetosphere (X < similar to-6 R-E around the magnetic equator). The analyses of the selected ENA data yield the following results: (1) The oxygen ENA flux displays 20-30 min bursts during all substorms, while the hydrogen ENA flux did not increase or less significantly increased than the oxygen flux. (2) The temporal evolution of energy spectra is mass dependent for all examined substorms. (3) For two of the substorms, the oxygen flux ratio between before and after a substorm increases with increasing energy, indicating the hardening of an O+ energy spectrum. (4) The flux ratio for the inner image area is comparable to or higher than that in the outer area. The results confirm that nonadiabatic acceleration with regard to the first adiabatic invariant did occur in the near-Earth magnetotail (X > -8 R-E). Although the influence of the nonadiabatic acceleration appears in the inner magnetosphere (similar to-6 R-E < X < similar to-3 R-E) as well as the outer magnetosphere (similar to-8 R-E < X < similar to-6 R-E), it is not clear from the present results whether O+ energization in the inner magnetosphere is due to nonadiabatic acceleration in the inner magnetosphere or adiabatic transport of O+ nonadiabatically accelerated in the outer magnetosphere. It is likely, for at least the two substorms, that the nonadiabatic acceleration makes a more significant contribution to O+ energization than increase of the O+ density in the plasma sheet.