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 tonne per second), much of it in the form of ions¹; the motion of Io through Jupiter's powerful magnetic field in turn generates a million-ampere current² between the charged near-Io environment and the planet's ionosphere. This current is presumably carried by Alfvén waves³, the electromagnetic equivalent of sound waves. Here we present far-ultraviolet observations of the atmospheric footprint of this current, which demonstrate that most of the energy is dissipated rapidly when the waves first encounter Jupiter's ionosphere; the position of the footprint varies with time. We see no evidence for the multiple ionospheric interactions that have been proposed to explain the structure of the radio emissions associated with these waves⁴.