Plasma Observations at Io with the Galileo Spacecraft

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Frank L. A., Ackerson K. L., Lee J. A., English M. R., Pickett G. L., Space Sci. Rev. 60, 283 (1992). The plasma analyzer consists of a set of nested spherical-segment plates supplied with a programmed series of voltages that provide energy-per-unit charge (E/Q) analyses of positive ions and electrons, separately and simultaneously. Multiple sensors at the exit apertures of these electrostatic analyzers allow determination of the latitude of the direction of arrival of the charged particles with respect to the spacecraft axis. The rotation about the spacecraft axis allows the analyzers to sample the full range of azimuths for the arriving particle trajectories. Thus, all directions of arrival for the charged particles are covered. Typically, the number of directions for arrival is programmed to be 56. For each direction of arrival E/Q, samples at 32 to 64 logarithmically spaced values are taken in the E/Q range from 0.9 V to 52 kV. In addition, three miniature magnetic spectrometers are positioned at the exit apertures of the electrostatic analyzers for the positive ions. The angular coverage of these mass spectrometers is three partial ranges of particle arrival latitudes. For a given value of E/Q, the magnetic fields in the mass spectrometers are varied in order to obtain a mass-per-unit charge (M/Q) analysis of the composition. O⁺ and S⁺⁺ ions, two of the major ions in Io's torus, cannot be identified separately with the plasma instrumentation. The temporal resolution for gaining complete determination of the plasma distributions is limited by the rotation period of the spacecraft platform, about 20 s.

The gyrofrequency in the ambient magnetic field during closest approach to Io is about 1 s^-1. The ion-neutral collision frequency for momentum transfer for SO₂ is about 10^-9 n, where n is the neutral density, independent of temperature [as given by Banks P. M., Kockarts G., Aeronomy, Part A (Academic Press, London, 1973), pp. 217–219]. With the assumption that the gyrofrequency is about equal to the collision frequency as the minimum requirement for decoupling the ion motion from the magnetic field, the corresponding neutral density is 10⁹ cm^-3

For the Na observations, see Schneider N. M., Hunten D. M., Wells W. K., Schultz A. B., Fink U., Astrophys. J. 368, 298 (1991) and Wilson J. K., Schneider N. M., Icarus 111, 31 (1994); for the SO₂ observations, see Strobel D. F., Zhu X., Summers M. E., ibid., p. 18, and Ballester G. E., et al., ibid., p. 2.

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We thank the Galileo project personnel at the Jet Propulsion Laboratory for obtaining the plasma observations. The research at the University of Iowa was supported in part by the Jet Propulsion Laboratory under contract JPL-958778