Electron densities near Io from Galileo plasma wave observations

This paper presents an overview of electron densities obtained near lo from the Galileo plasma wave instrument during the first four flybys of lo. The se flybys were IO, which was a downstream wake pass that occurred on Decemb er 7, 1995; 124, which was an upstream pass that occurred on October 11, 19 99; 125, which was a south polar pass that occurred on November 26, 1999; a nd 127, which was an upstream pass that occurred on February 22, 2000. Two methods were used to measure the electron density. The first was based on t he frequency of upper hybrid resonance emissions, and the second was based on the low-frequency cutoff of electromagnetic radiation at the electron pl asma frequency. For three of the flybys, 10, 125, and 127, large density en hancements were observed near the closest approach to lo, The peak electron densities ranged from 2.1 to 6.8 X 10(4) cm(-3). These densities are consi stent with previous radio occultation measurements of To's ionosphere. No d ensity enhancement was observed during the 124 flyby, most likely because t he spacecraft trajectory passed too far upstream to penetrate lo's ionosphe re. During two of the flybys, 125 and 127, abrupt step-like changes were ob served at the outer boundaries of the region of enhanced electron density. Comparisons with magnetic field models and energetic particle measurements show that the abrupt density Steps occur as the spacecraft penetrated the b oundary of the lo flux tube, with the region of high plasma density on the inside of the flux tube. Most likely the enhanced electron density within t he lo flux tube is associated with magnetic field lines that are frozen to lo by the high conductivity of lo's atmosphere, thereby enhancing the escap e of plasma along the magnetic field lines that pass through lo's ionospher e.