Jupiter Revisited: First Results from the University of Chicago Charged Particle Experiment on Pioneer 11

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Science  02 May 1975:
Vol. 188, Issue 4187, pp. 455-459
DOI: 10.1126/science.188.4187.455


During the December 1974 Pioneer 11 Jupiter encounter our experiment provided measurements of Jovian energetic protons and electrons both in the magnetic equatorial zone and at previously unexplored high magnetic latitudes. Many of the observations and conclusions from the Pioneer 10 encounter in 1973 were confirmed, with several important exceptions and new findings. We report evidence from Pioneer 11 for protons (∼ 1 million electron volts) of Jovian origin in interplanetary space. In the outer magnetosphere particle intensities at high magnetic latitudes were comparable to those observed in the equatorial zone, and 10-hour variations in particle intensities and spectra were observed at both high and low magnetic latitudes. Therefore, confinement of particles in the outer magnetosphere to a thin equatorial magnetodisc is adequate neither as a description of the particle distribution nor as a complete explanation of the 10-hour variations. Pioneer 11 data support a model in which the intensity varies with a 10-hour period in phase throughout the sunward side of the magnetosphere and is relatively independent of position within the magnetosphere. Transient, highly anisotropic bursts of protons with energies of ∼ 1 million electron volts observed near the orbit of Ganymede suggest local acceleration in some regions of the magnetosphere. In the inner core where particles are stably trapped, a maximum in the high-energy nucleonic flux was again found, corresponding to the Pioneer 10 maximum at ∼ 3.4 Jupiter radii (RJ), which is apparently a persistent feature of, the inner radiation zone. In addition, Pioneer 11 data indicate two more local maxima in the nucleonic flux inside 3.4 RJ, one of which may be associated with absorption by Amalthea, and a maximum intensity at 1.9 RJ more than 20 times that at 3.4 RJ, The flux of relativistic electrons reached a maximum on the magnetic equator at 1.8 RJ, only slightly less its closest approach at 3.1 RJ.