Pioneer 11 observations of effects of Ganymede and Callisto on Jupiter's trapped radiation

Abstract

Beyond about 10 RJ from Jupiter the configuration of magnetic field lines in Jupiter's magnetosphere is strongly affected by the presence of an equatorial current sheet. Using a Voyager 1 magnetic field model that explicitly includes the effects of this current sheet to describe the magnetic field line geometry, we have reexamined charged particle observations from Pioneer 11's high‐latitude flyby of Jupiter in 1974. We find that if the trajectory of Pioneer 11 is mapped to the equatorial plane along the model magnetic field lines, significant features in the time‐intensity profiles of trapped protons and electrons, including one microsignaturelike feature, are found to correspond to shells of closed field lines crossed by the orbits of the satellites Ganymede and Callisto. Although many of the characteristics of the features, including their magnitude, persistence, and similarity of appearance for protons and electrons, are difficult to explain, we suggest that these features are signatures of interaction of the trapped particles with Ganymede and Callisto. If this interpretation is correct, it offers explanations for several large features in the time‐intensity profile of trapped particles observed during Pioneer 11's flyby that could not be explained using earlier magnetic field models which did not include effects of the current sheet, and it leads to reinterpretation of some features previously associated with Ganymede as signatures of Callisto. It also implies that the configuration of the current sheet during the Pioneer 11 flyby in 1974 was very similar to that observed by Voyager 1 in 1979 and provides confirmation of the essential accuracy of current sheet magnetic field models for describing the magnetic field configuration out to radii of about 30 RJ from Jupiter. Finally, the characteristics of the signatures described suggest interesting problems for future research on the dynamics and interactions of trapped particles with satellites in the outer regions of the Jovian trapped radiation zone.