Abstract
The plasma instrumentation (PLS) for the Galileo Mission comprises a
nested set of four spherical-plate electrostatic analyzers and three
miniature, magnetic mass spectrometers. The three-dimensional velocity
distributions of positive ions and electrons, separately, are determined
for the energy-per-unit charge (E/Q) range of 0.9 V to 52 kV. A large
fraction of the 4-pi-steradian solid angle for charged particle velocity
vectors is sampled by means of the fan-shaped field-of-view of 160°,
multiple sensors, and the rotation of the spacecraft spinning section.
The fields-of-view of the three mass spectrometers are respectively
directed perpendicular and nearly parallel and antiparallel to the spin
axis of the spacecraft. These mass spectrometers are used to identify
the composition of the positive ion plasmas, e.g., H+,
O+, Na+ and S+, in the Jovian
magnetosphere. The energy range of these three mass spectrometers is
dependent upon the species. The maximum temporal resolutions of the
instrument for determining the energy (E/Q) spectra of charged particles
and mass (M/Q) composition of positive ion plasmas are 0.5 second.
Three-dimensional velocity distributions of electrons and positive ions
require a minimum sampling time of one spacecraft rotation, typically
18.3 to 19.8 seconds. The two instrument microprocessors provide the
capability of inflight implementation of operational modes by
ground-command that are tailored for specific plasma regimes,
e.g., magnetosheath, plasma sheet, cold and hot tori, and
satellite wakes, and that can be improved upon as acquired knowledge
increases during the tour of the Jovian magnetosphere. Because the
instrument is specifically designed for measurements in the environs of
Jupiter with the advantages of previous surveys with the Voyager
spacecraft, first determinations of many plasma phenomena can be
expected. These observational objectives include field-aligned currents,
three-dimensional ion bulk flows, pickup ions from the Galilean
satellites, the spatial distribution of plasmas throughout most of the
magnetosphere and including the magnetotail, and ion and electron flows
to and from the Jovian ionosphere.