PLASMA OBSERVATIONS AT IO WITH THE GALILEO SPACECRAFT

Plasma measurements made during the flyby of Io on 7 December 1995 wit h the Galileo spacecraft plasma analyzers reveal that the spacecraft u nexpectedly passed directly through the ionosphere of Io. The ionosphe re is identified by a dense plasma that is at rest with respect to Io. This plasma is cool relative to those encountered outside the ionosph ere. The composition of the ionospheric plasmas includes O++, O+ and S ++, S+, and SO2+ ions. The plasma conditions at Io appear to account f or the decrease in the magnetic field, without the need to assume that Io has a magnetized interior.