3-D illustration of the mechanism that researchers think is responsible for the formation of a smaller spot above the imprint of Io (see text).
The plasma torus (yellow) is made up of ionized particles resulting from volcanic activity on Io. The direct current (dark blue) responsible for the main spot of the footprint of Io comes from plasma waves generated by the relative movement of Io within the torus. When these waves reach the edge of a torus, one part is reflected (light blue) because of a difference in density. The longer course followed within the plasma torus by the reflected waves explains why they reach the poles later than the direct waves, and thus why the smaller spot necessarily must occur later according to this scenario.
