Between the arrival of the MAW and the RAW in each hemisphere, the currents are mainly on the side of Io closest to Jupiter (i.e., lower L-shell); however, after the passage of the RAW, the pattern becomes more complicated and the regions of upward current migrate across the footprint and appear at higher L-shells. This is due to phase mixing as the wave propagates. Phase mixing (e.g., Mann et al., 1995) occurs since the different field lines have slightly different lengths, so that Alfvén waves propagating on adjoining field lines become out of phase. This effect becomes more pronounced after the passage of the reflected Alfvén wing, which occurs after the Alfvén wave has passed through the whole plasma torus. At this point, the waves reflected from the torus boundary and from the conjugate ionosphere interfere with one another, producing the more complicated pattern as in the Poynting flux plot. Figure 4b for the high-density case shows similar features, with the MAW and RAW currents farther apart, as in Figure 2.

The situation is modified when Io is near the boundary of the torus. Figures 5a (low-density case) and 5b (high-density case) show the Poynting flux in the region where Io is at its most northerly latitude. In this case, the main Alfvén wing arrives at the northern ionosphere before the waves arrive at the southern ionosphere since they spend less time in the plasma torus. In this case, the MAW and RAW in the northern hemisphere are farther apart (about 9° of longitude) while they are closer together (3° of longitude) in the southern hemisphere. In addition, the reflection from the northern torus boundary and the northern ionosphere are very close together since both of these waves must traverse nearly the entire plasma torus. The secondary waves between the ionosphere and torus are weaker in the north and stronger in the south, particularly between the MAW and RAW in that hemisphere. This pattern is reversed between the hemispheres when Io is near the southern boundary of the torus (not shown). The run shown in Figure 5a is shown as an animation in Supporting Information S2.