Low-energy ions at low equatorial altitudes

Abstract

Williams and Frank (1984, J. geophys. Res. 89, 3903) have reported the observation of intense low-energy ion populations in the low-altitude, equatorial magnetosphere. They reported observing two narrow energy peaks in the total ion flux that increased in energy from ∼ 8 keV to > 24 keV as the altitude decreased from ≈ 4 R E to 2.1 R E. They analyzed one pass (29 November 1977) in detail and described the difficulties caused by the fact that the analysis had to use data from the limits of the response functions of the plasma and energetic particle instruments. Because of these difficulties, we have analyzed additional data and incorporated ion mass composition data from the Lockheed mass spectrometer. The new data agree in general with the earlier results and show that complex spectral features occur in the total ion observations. The observed ions are now identified as primarily H +, O +, and He +, whose spectra also show complex structures. Observed peak energies increase with decreasing altitude and can vary by at least 30% over time. An acceleration mechanism appears to be continuously present throughout the plasmasphere. As discussed earlier by Williams and Frank (1984), the most likely mechanism for the energization of the observed ions is a resonant wave-particle interaction, probably with ion cyclotron waves generated by an anisotropic energetic ion population. It has been estimated that such a mechanism may act in a two-phase mode resulting in the energization of minor ions up to the local Alfvén energy of the ambient plasma, E A = B 2 8πN total . Although this energy (~ 10 keV at L ∼ 2.5) is similar to the peak energies observed, the radial dependence of the peak energy is not consistent with that expected for E A.