This paper summarizes the evidence from the Pioneer 10 plasma analyzer that plasma derived from Europa was present in the Jovian magnetosphere in December 1973. Plasma detected between 1900 UT and 2100 UT on December 3, 1973, shows a number of significant phenomena near the expected position of Europa's L shell. Mass addition to the magnetospheric plasma is suggested by a local increase in density apparently superimposed on the density gradient of Iogenic plasma. This increase in plasma density is unlike any phenomenon observed when the spacecraft is not close to a lunar L shell. The density shows fluctuations which allow an estimate of the net outflow speed of magnetospheric ions per Jovian rotation. We estimate a radial flow speed in 1973 of 0.37 km/s from the Pioneer data and we estimate 1 km/s in 1979 from Voyager 2 data, thus indicating a significant change. The mass addition from Europa is consistent with the expected derivation of oxygen ions (or OH+ or H2O+) from the icy surface of Europa by sputtering or other processes or with the derivation of recycled sulphur ions from Europa. Evidence from other peaks in the plasma spectra also argues for the identification of the most prominent peak in these spectra as having M/Q in the range of 16–18. The estimate of the bulk plasma speed obtained by combining this identification with the absolute energies of the peaks and the spacecraft trajectory information indicates that at this time the corotation speed is substantially higher than the expected corotation speed for a large number of spectra. If the most prominent peak in the spectra were associated with M/Q values of 32, then the corotation speed would be consistent with rigid corotation. These results contrast with the Voyager 1979 observations and are more evidence for a large change in magnetospheric conditions between the 1973 and 1979 observations. We can account partially for the increased corotation speed of the plasma during the Pioneer 10 encounter by use of momentum conservation and compression of the magnetosphere by the solar wind.
Analysis of Pioneer 10 plasma and magnetic field observations at 8.3 AU in 1975 provides new evidence for the presence of interstellar pickup hydrogen (H+) ions. Use of plasma sensors that look far from the solar wind direction confirms the spherical shell distribution of the pickup ions in velocity space. Phase space density and flux estimates are closely consistent with those from Ulysses measured under similar conditions. Power spectral analyses of the magnetic field data show a distinct signal a little above the proton gyrofrequency, consistent with the presence of Doppler‐shifted ion‐cyclotron waves generated by H+ pickup ions. These results show that the Pioneer data set has the potential for systematic studies of the global properties of interstellar pickup ions.
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