Thermal electron density in the Jovian magnetosphere

Observations of the portions of Io's sodium cloud which are far from Io are presented. We demonstrate that the size of the sodium cloud is highly variable and relate this variability to fluctuations in the state of the thermal electron plasma in the magnetosphere of Jupiter. We show that estimates of the plasma density based on the assumption that the main sodium sink is electron impact ionization are consistent with those based on the intensities of the forbidden doublet of ionized sulfur which is excited by electron collisions. We discuss briefly the implications of our results for the magnetosphere of Jupiter.