An Overview of Galileo Plasma Wave Observations During the I31 and I32 Flybys of Io

An overview is given of results from the Galileo plasma wave instrument during the I31 and I32 flybys of Io, which occurred on August 6, 2001 and October 15, 2001, respectively. The I31 flyby passed over the north pole of Io at a closest approach altitude of 194 km, and the I32 flyby passed over the south pole of Io at a closest approach altitude of 180 km. The plasma wave electric field data from the I31 flyby show a very clear upper hybrid emission line during the entire flyby, and a region of intense low frequency electric noise lasting about ten minutes centered on closest approach. The upper hybrid emission line can be used to make very accurate measurements of the electron density. As the spacecraft entered the geometric boundary of the Io magnetic flux tube at about 04:55 UT, the electron density abruptly increased from the ambient level of about 1.9 x 10³ cm^-3 in the Io plasma torus to values ranging from about 1.2 to 1.8 x 10⁴ cm^-3 as it passed over the polar region. As the spacecraft exited the geometric boundary of the Io flux tube, the electron density abruptly decreased down to the preexisting ambient level in the Io torus. These boundaries coincide within about one minute of the times when the low frequency noise occurs. At the time that this abstract was written no data were available from the I32 flyby.