AMPTE Ion Composition Results

Abstract

With the CHEM and SULEICA experiments on the AMPTE/CCE and IRM spacecraft respectively it is now possible to identify ion species of solar wind as well as ionospheric origin and to measure their distribution functions in the energy range of ∼ 1 to 315 keV/e. Since the launch of the AMPTE mission in August 1984 we have obtained ion composition data with the CHEM instrument in the equatorial region at L values ≲ 9 RE covering all local times and a range of geomagnetic activities. SULEICA on IRM has discovered interstellar He pick-up ions, and has sampled the upstream regions and the near-earth plasma sheet to ∼ 19 RE. We review our current observations of the composition of magnetospheric ions in the bulk of the ring current and in the near-earth plasma sheet during both quiet and disturbed times, and in the near-noon magnetosheath at times when the magnetosphere is compressed. We find that (a) the relative abundances of high-charge state heavy ions of solid wind origin in the ring current population are similar to the respective solar wind abundances, implying little or no mass-dependent bias in the processes governing entry, transport and acceleration of these ions; (b) the ion distribution functions have similar shapes in terms of energy per charge, implying energy/charge dependent processes (such as, e.g., acceleration by an electric field); and (c) the radial profiles and plasma sheet distribution functions of solar wind origin ions are different from those of ionospheric origin ions indicating differences in acceleration and/or transport histories for these two plasmas. Our measurements of the relative abundances of various ion species with different charge states throughout the magnetosphere allow us to establish that the ionospheric and the solar wind sources contribute roughly equal numbers of particles to the magnetosphere.