RAPID ENERGY-DISSIPATION AND VARIABILITY OF THE IO-JUPITER ELECTRODYNAMIC CIRCUIT

THE electrodynamic interaction between Jupiter and the closest of its large moons, Io, is unique in the Solar system. Io's volcanoes eject a considerable amount of material into the inner jovian system (>1 tonn e per second), much of it in the form of ions(1); the motion of Io thr ough Jupiter's powerful magnetic field in turn generates a million-amp ere current(2) between the charged near-Io environment and the planet' s ionosphere. This current is presumably carried by Alfven waves(3), t he electromagnetic equivalent of sound waves. Here we present far-ultr aviolet observations of the atmospheric footprint of this current, whi ch demonstrate that most of the energy is dissipated rapidly when the waves first encounter Jupiter's ionosphere; the position of the footpr int varies with time, We see no evidence for the multiple ionospheric interactions that have been proposed to explain the structure of the r adio emissions associated with these waves(4).