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First composition measurements of energetic neutral atoms

First composition measurements of energetic neutral atoms. A. T. Y. Lui et al., GRL, Vol 23, pages: 2641-2644, 1996. Abstract.

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First composition measurements of energetic neutral atoms

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  1. First composition measurements of energetic neutral atoms A. T. Y. Lui et al., GRL, Vol 23, pages: 2641-2644, 1996.

  2. Abstract • Energetic neutral atoms (ENAs) were detected by the Energetic Particle and Ion Composition (EPIC) instrument on Geotail during a magnetic storm on October 29-30, 1994. • The energetic particles could be identified as neutrals: • - the direction of the particle flux steadily tracked the direction of the Earth. • - uncorrelated with the changing orientation of the ambient magnetic field. • First composition measurements of ENAs; storm-time evolution of ENA fluxes of hydrogen, helium, and oxygen. • ENA fluxes and the rate of recovery of Dst are both roughly steady. • So, charge exchange is a significant energy loss process for the storm-time ring current.

  3. Abstract • For total energies above 200 keV, the intensity of ENA oxygen is the highest, followed by hydrogen, and then by helium. • Geotail observations enable us to deduce the line-of-sight (LOS) integrals of the products nHjion and nHfion, where nH = hydrogen geocorona density, jion = differential ion flux, and fion = phase space density (PSD), for H+, He+, and O+ at energies between ~60 and ~600 keV. • A ~50-60 keV Maxwellian fits the O+ LOS PSD fairly well but fits the He+ and the H+ only poor.

  4. Observations (a) Dst index for the storm period, with a representative recovery rate indicated by the dashed line. (b) A schematic diagram showing the different EPIC/ICS sectors responding to the incoming ENA flux at different positions along the Geotail trajectory.

  5. GEOTAIL EPIC (Oct 29, 1994) • P2 channel usually measures protons but in their absence measures neutral hydrogen atoms at 58-77 keV. • CNO is dominated by oxygen. • The circles mark the magnetic field direction projected on the equatorial plane. Dawn P2: 58-77 keV Tail Dusk Sun CNO: 187-221 keV

  6. GEOTAIL EPIC (Oct 29, 1994) Dawn 58-77 keV Angular anisotropy of ENA intensity of 58-77 keV hydrogen Tail Dusk Sun 187-221 keV

  7. Comparison Polar obs. and Akebono obs. UT = 16 UT = 16

  8. Comparison Polar obs. and Akebono obs.

  9. Discussion and summary • The observed ENA fluxes for hydrogen, oxygen, and helium show similar and very slow decays over the 12-hour period during the monotonic recovery of the Dst index (average rate ~8 nT/hour). • Roelof et al. [1985] suggested that if the active ring current is decay by charge exchange, then the rate of ring current energy loss (as measured by dDst/dt) is proportional to the total energy flux carried by ENAs escaping from the ring current. • The portion of the ENA flux observed by Geotail was nearly steady (very slow decay) while Dst was recovering at roughly 8 nT/hour. • This is qualitatively consistent with the theoretical relationship expected if charge exchange is a significant loss mechanism for the ring current energy. • Oxygen had the highest intensity at a common energy of 200 keV, followed by hydrogen and then by helium.

  10. Discussion and summary • This result supports the earlier conclusion [Roelof, 1985]: the dominance of neutral oxygen intensity over intensities of other species from IMP-7/8 and ISEE-1 observations. • The angle-averaged LOS-geocorona-weighted phase space densities for O+ could be fitted approximately to a ~60 keV Maxwellian, but the He+ and H+ had a different spectral shape (the H+ appearing more like a two-component spectrum that hardens at higher energies).

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