Advantages of the Galileo Plasma Measurements

The Galileo Mission advantages for plasma investigations in the Jovian magnetosphere are (1) the spinning section of the spacecraft, (2) an instrument microprocessor to restructure the instrument operation by ground command, and (3) a series of orbits that allow close flybys of the Galilean satellites, a survey of the Jovian magnetotail, and a substantial local-time survey of the magnetosphere. The spinning section of the spacecraft provides the important capability for a suitably designed instrument to view the entire 4-pi-steradian solid angle for particle velocity vectors at the spacecraft position. The instrument microprocessor can be used to tailor the operation of the plasma instrument for the most effective measurements in each of the diverse plasma regimes of the magnetosphere and its environs, e.g., magnetosheath, plasma sheet, satellite wake or flux tube, or magnetospheric wind. Targeted encounters with the satellites and a tour of the magnetosphere and magnetotail offer exceptional opportunities for studies of most of the important plasma regions and their temporal responses to variations of Iogenic and solar wind plasmas, and the interactions of magnetospheric plasmas with the satellites.

The Galileo plasma instrumentation (PLS) is substantially more capable for measurements of the Jovian plasmas than those of the Pioneer and Voyager spacecraft because it is specifically designed for this purpose. The basic advantages are in the performance areas of (1) extended energy range, (2) coverage of the angular distributions of plasmas, (3) angular resolution, (4) temporal resolution, and (5) ion composition.

The energy-per-unit charge ranges of the Pioneer and Voyager plasma instruments are 100 to 4800 V and 10 to 5920 V, respectively. The corresponding range of the Galileo plasma analyzer is 0.9 to 52,000 V. This extended energy range spans the important energy gap between 5920 V and 30,000 V in the combined performances for the Voyager plasma instrument and medium-energy particle detectors. The 4-pi-steradian solid angle for particle velocity vectors at the spacecraft position is sampled adequately to provide determinations of the three- dimensional velocity distributions for positive ions and electrons. Thus such important plasma parameters as field-aligned currents, cross-field currents, plasma bulk flow velocities, heat fluxes, and free energy are to be determined for the first time with the Galileo instrument. The angular resolution is sufficient to provide definitive measurements of the above plasma parameters. Temporal resolutions for obtaining electron and positive ion spectra are about 200 seconds for the Pioneer analyzer (ions only) and about 100 seconds for the Voyager Faraday cups. The corresponding temporal resolution for the Galileo plasma analyzer is about 0.5 second; complete three-dimensional velocity distributions for positive ions and electrons can be telemetered once each 20 seconds. These improved temporal resolutions are particularly important during the brief encounters with the satellites and the traversals of plasma boundaries such as those of the plasma sheet and current sheet in the middle and outer magnetospheres.

Three miniature mass spectrometers which are positioned at the exit apertures of the electrostatic analyzers in the Galileo instrument provide determinations of the positive ion composition. The Voyager determinations of ion composition from E/Q spectra are model dependent and are possible when the Mach number of the corotational flow is greater than 5 or 6. This method is acceptable generally near the Io orbit but as the Jovian radial distance increases, ion thermal speeds rapidly increase and prevent decisive identification of ion species. The Galileo mass spectrometers provide a direct determination of ion composition, specifically the mass-per-unit charge.

In addition to the above performance features, the Galileo plasma analyzer can be operated flexibly via electronic reconfiguration by ground command. The operational configuration of energy-per-unit charge (E/Q) passbands, mass-per-unit charge (M/Q) channels, sensors, and angular sectors can be tailored for a specific plasma region. The temporal resolution for a given measurement can also be selected. The Galileo plasma analyzer is equipped with sufficient onboard hardware and software to implement automated beam capture modes for ion velocity distributions and for determination of ion composition.