First evidence for a Europa plasma torus

Abstract

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.