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.A. T. Y. Lui et al., GRL, Vol 23, pages: 2641-2644, 1996.

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.

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.

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.

Dawn

Dusk

Tail

Sun

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.

P2: 58-77 keV

CNO: 187-221 keV

Dawn

Tail

SunDusk

GEOTAIL EPIC (Oct 29, 1994)

Angular anisotropy of ENA intensity of 58-77 keV hydrogen

58-77 keV

187-221 keV

Comparison Polar obs. and Akebono obs.

UT = 16

UT = 16

Comparison Polar obs. and Akebono obs.

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.

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).