A magnetic ion-mass spectrometer was flown to measure in situ the concentrations of the
ambient ion species in the mass range from 1 to 90 atomic mass units (u). It was mounted on the satellite
equator normal to the spin axis, and the entrance aperture faced forward when the spacecraft was in the
despun mode. The electric and magnetic fields were arranged to produce a mass spectrum along the focal
plane following the magnetic analyzer. Three slits were placed along the focal plane in appropriate places
to simultaneously collect ions in the mass ratios 1 to 4 to 16. Ionospheric ions were accelerated into the
analyzer system by a negative voltage that varied from -1060 to -225 V. The three mass ranges measured
simultaneously were 1 to 4, 4 to 16, and 16 to 90 u. Following each slit was an electron multiplier and
a logarithmic electrometer-amplifier detector. The detector output could be measured directly for an
analog output, or it could be fed to a "peak" circuit that determined the amplitude of each peak in the
spectrum. Only the amplitude of each peak was telemetered in the primary peaks mode, and in this mode
the time required to simultaneously sweep all three mass ranges was 1 s. Other modes of operation were
possible. In the analog short mode, the three mass ranges were swept in 4 s, alternating with 1-s "peaks"
mode scans. An 8-s sweep was required in the analog long mode, again alternating with 1-s peaks mode scan.
An option existed in the locked mode to continuously measure any set of mass numbers in the ratio 1 to 4
to 16 to give high spatial resolution. More experiment details can be found in
J. H. Hoffman et al., Radio Sci., v. 8, n. 4, p. 315, 1973. NSSDC has all the useful data that exist
from this experiment.