The evolution of Io's Alfv¿n waves is modeled in a realistic magnetic field and torus density distribution. This is performed by calculating the normal modes of the field lines disturbed by Io and synthesizing the waves near Io from a complex sum over the eigenmodes. The wave pattern produced downstream from the satellite exhibits periodic structure of a range of scales. In terms of the Jovian longitude (or CML) of a stationary observer, we expected large-scale structure (>60¿), small-scale structure (<6¿), and intermediate periods too. These are close to observed intervals in decametric (DAM) emissions, such as the length of a DAM storm, bunching of arcs within a storm, and individual arc separation. ¿ American Geophysical Union 1990