First composition measurements of energetic neutral atoms

Energetic neutral atoms (ENAs) were detected by the Energetic Particles and Ion Composition (EPIC) instrument on the Geotail spacecraft during a magnetic storm on October 29-30, 1994. The energetic particles could be identified as neutrals because the direction of the particle flux steadily tracked the direction of the Earth and was uncorrelated with the changing orientation of the ambient magnetic field as the spacecraft viewed the dayside magnetosphere for similar to 16 hours. These observations provide the first composition measurements of ENAs, yielding the storm-time evolution of ENA fluxes of hydrogen, helium, and oxygen separately as well as their energy spectra. ENA fluxes and the rate of recovery of D-st are both roughly steady, consistent with charge exchange being a significant energy loss process for the storm-time ring current. For total energies above 200 keV, the intensity of ENA oxygen is the highest, followed by hydrogen, and then by helium. These observations enable us to deduce the line-of-sight (LOS) integrals of the products n(H)j(ion) and n(H)f(ion), where n(H) = hydrogen geocorona density, j(ion) = differential ion flux, and f(ion) = phase space density (PSD), for ion species H+, He+, and O+ at energies between similar to 60 and similar to 600 keV. A similar to 50-60 keV Maxwellian fits the O+ LOS PSD fairly well but fits the He+ and the H+ only poorly.