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Plasma Detector (PLS)

NSSDC ID: 1989-084B-04
Mission Name: Galileo Orbiter
Principal Investigator: Prof. Louis A. Frank


The plasma instrument (PLS) on Galileo had as its objectives to: (1) establish the sources of magnetospheric plasma; (2) investigate the interaction of ambient plasma with the Jovian satellites; (3) examine the role of plasma as a source of energetic charged particles in the radiation zones; (4) determine the nature of the equatorial current sheet; and, (5) evaluate the roles of magnetic merging, co-rotational forces, and field-aligned currents in the dynamics of the Jovian magnetosphere.

To achieve these goals, the instrument consisted of a nested set of four spherical-plate electrostatic analyzers and three miniature, magnetic mass spectrometers. The overall dimensions of the instrument package were 20.3 x 38.1 x 44.7 cm. Each analyzer consisted of three 70-degree spherical-segment plates. The inner and outer plates were grounded while the center plate was supplied with a programmed series of voltages to obtain energy spectra of incident electrons and ions. A charged particle with a particular energy per unit charge (E/Q) passed through the plates only when certain voltages were applied to the center plate. Positioned at the exit apertures of the analyzers were continuous-channel electron multipliers. Which of the seven exit apertures the particle would arrive at was determined by the direction of arrival of the particle at the entrance aperture. In this manner, because the PLS was mounted on the magnetometer boom on the spun section of the spacecraft, the instrument was able to attain a coverage of nearly the entire unit sphere and obtain angular distributions as well.

The analyzers were capable of measuring plasma energies ranging from 0.9 eV-52 keV in 64 different passbands with a typical energy resolution of delta-E/E ~ 0.11 in each of the fourteen detectors (seven for ions, seven for electrons). The temporal resolution of the instrument was 0.5 s. The mass spectrometers allowed the determination of the mass per unit charge (M/Q) of the incident ions. Each spectrometer included both an integral and a differential sensor. The average mass resolution of the differential sensors was delta-M/M=0.24 and for the integral sensors was delta-M/M ~ 0.5. The resolutions were chosen to permit the identification of H+, H2+/He++, He+, O++, O+, Na+, S+, and K+ with the differential sensors and of H+, H2+/He++, O++, O+, S+, and SO2+ with the integral sensors. The spectrometers could not distinguish between two ion species with the same M/Q (e.g., H2+/He++ or O+/S++).

The PLS operated in one of five spin modes during each spacecraft rotation. Each spin mode was dedicated to one type of plasma measurement: a survey of ion and electron velocity distributions, the determination of the velocity distribution of an ion beam, a survey of the ion composition (two modes with different spectrometers), and the determination of the composition of an ion beam. Each instrument cycle was composed of a sequence of twelve of these spin modes. The ordering of these modes in a cycle was controlled by an on-board lookup table, the values of which could be set by ground command or could use default values stored in ROM.

Alternate Names

  • PLS
  • Plasma Science Experiment

Facts in Brief

Mass: 13.2 kg
Power (avg): 10.7 W
Bit rate (avg): 0.612 bps

Funding Agency

  • NASA-Office of Space Science Applications (United States)


  • Planetary Science: Fields and Particles
  • Space Physics: Heliospheric Studies

Additional Information

Questions or comments about this experiment can be directed to: Dr. Edwin V. Bell, II.



NameRoleOriginal AffiliationE-mail
Dr. Kent L. AckersonCo-InvestigatorUniversity of
Prof. Edward C. Stone, Jr.Co-InvestigatorCalifornia Institute of
Prof. Louis A. FrankGeneral ContactUniversity of
Dr. Ferdinand V. CoronitiCo-InvestigatorUniversity of California, Los
Prof. Vytenis M. VasyliunasCo-InvestigatorMax-Planck-Institut fur

Selected References

Frank, L. A., et al., Plasma observations at Venus with Galileo, Science, 253, No. 5027, 1528-1531, Sept. 1991.

Frank, L. A., et al., The plasma instrumentation for the Galileo mission, Space Sci. Rev., 60, No. 1/4, 283-307, May 1992.

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