Alfvén wave propagation in the Io plasma torus

Abstract

Gurnett and Goertz (1981) proposed that the large number of discrete arcs observed in the Jovian decametric radio emission is caused by multiple reflections of Alfvén waves excited by Io. In this paper the plasma measurements that were made by the Voyager 1 plasma science experiment have been combined with a model of Jupiter's magnetic field to calculate the time an Alfvén wave takes to travel between Io and Jupiter's ionosphere and the period of subsequent bounces between the northern and southern hemispheres. The result is a wave pattern extending around Jupiter as the multiply reflected Alfvén waves are carried away from Io by the corotating magnetospheric plasma. Although the whole pattern continually changes over the 13 hours Io takes to move through 360° of Jovigraphic longitude, a general longitudinal structure is exhibited independent of the position of Io, due to the geometry of the magentic field and the distribution of plasma in the Io torus. If the Alfvén waves stimulate the decametric radio emission, then the wave pattern predicts specific properties of the decametric emission which can be compared with radio observations.