RADIO EMISSIONS OBSERVED BY GALILEO NEAR IO

The Galileo spacecraft observed spin modulation of radio wave emission s near Io in the frequency range from about 600 kHz to about 1.2 MHz. Assuming transverse EM radiation, we have used the modulation of the h igh-frequency sweep-frequency receiver signals of the electric dipole antenna over many spins to estimate the plane through the source. The emission has a range of frequencies close to the local upper hybrid fr equency of the plasma near Io. We conclude that the emission may be as sociated with either the plasma torus or magnetic flux tubes in the wa ke of Io (the Alfven current system). We postulate this emission may b e associated with a free-energy source such as density gradients, ener getic plasma beams and/or an electron distribution with a temperature anisotropy. All of these free-energy sources are observed or expected in the torus near Io. The observations are the first in the hectometri c frequency range that have a source associated with Io or in the Io t orus.