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.