Abstract
The Suprathermal Ion Mass Spectrometer (SMS) was developed to study the thermal (0-25eV) and suprathermal (25eV-several keV) ion distributions in the low altitude magnetosphere. The instrument has a mass (1-70amu/e) and plasma density (10-3 to 105cm-3) dynamic range sufficient to measure, on a regular basis, the major and the minor ion distribution functions at apogee (≈104km) as well as at perigee (≈300km). The instrument is a radio frequency type mass spectrometer and has a programmable mass resolution (Δm/m from 0.06 to 0.20) which is independent of energy and mass selected.
We present here measurements of the thermal ion mass composition near apogee which show that the composition varies from one dominated by H+ and O+ to one in which the density of “minor” ions (O++, He+, N+) is comparable to or greater than that of H+ and O+. We also present high time resolution mass/energy/angle observations in the topside ionospheric plasma in or near the cusp region where rapid heating of the bulk of the plasma and the formation of conic distributions is clearly evident. These unique observations will be compared with other plasma diagnostics on the spacecraft to increase our understanding of ionospheric ion production, convection and loss processes as well as low altitude ion energization phenomena.
