GEOMETRY OF THE PLASMA SHEET IN THE MIDNIGHT-TO-DAWN SECTOR OF THE JOVIAN MAGNETOSPHERE - PLASMA OBSERVATIONS WITH THE GALILEO SPACECRAFT

From August 10 to October 22, 1996, the Galileo spacecraft made observ ations in the midnight-to-dawn sector of the magnetosphere of Jupiter out to a distance of 124R(J), encompasing the region previously explor ed by the outbound passes of the Pioneer 10, Voyager 1, and Voyager 2 spacecraft. We use the count rates of electrons with energy between 1 and 4 keV, measured by the PLS experiment, to identify the plasma shee t and to study its configuration. We find a pronounced dependence of t he count rates on the System III longitude of the spacecraft, with one or two maxima per Jovian rotation, consistent with the generally acce pted assumption of a thin plasma sheet in the magnetic equatorial regi on, which propagates outward as a surface wave generated by the rotati on of the tilted magnetic dipole of Jupiter. The inferred effective pr opagation speed agrees approximately with previous estimates. With Gal ileo located essentially in the equatorial plane, the plasma sheet can be traced unambiguosly to distances of 100R(J) and beyond. A new resu lt, made possible by the long duration of the Galileo observations, is that the observed rotational variation of the count rates is subject to a longer-term, approximate to 5-7 day modulation, indicating change s in the mean location, or the surface-wave amplitude, or the thicknes s of the plasma sheet, or a combination of these. In at least one inst ance, only a change of mean location can account for what is observed. The cause of this modulation has not been yet definitively identified . Solar wind effects, dynamics of plasma supply from the Io torus to t he plasma sheet, variability of the plasma source due to volcanic acti vity of Io, and drifting longitudinal asymmetry of the hinging distanc e for the plasma-sheet surface wave are among the possibilities.