Abstract
The pattern of arcs made by bursts of Io-modulated decametric (DAM) emission in frequency time spectrograms has been examined for the A and B sources and compared with predictions of the Alfven wave model. Previous studies found the spacing between individual arcs to be much less than predicted. We have looked at the Planetary Radio Astronomy data from the Voyager 1 and 2 Jupiter encounters and organised the observations of the A and B sources with respect to the Io phase for fixed 30° longitude intervals. We found a clear pattern of regions of strong emission separated by “holes”: regions with weak or no emission. This preliminary study suggests the bunching of decametric arcs matches the Alfven wave pattern generated by Io. This implies that each Alfven current generates multiple beams of DAM emission. By comparing the viewing geometry of the A and B sources we were able to determine the geometry of the source. The axis of the emission cone is found to be ∼ 19° from the meridional plane with a full cone angle of ∼ 70–72°. The duration of the A-source pattern suggests that less than 20 % of the Alfven wave energy is dissipated per traversal of the torus.
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