Illustration in 3-D of the mechanism proposed by Bertrand Bonfond to explain the existence of a smaller spot that precedes the footprint of Io.

The plasma torus (yellow) is made up of ionized particles resulting from volcanic activity on Io. The direct current (blue) that is responsible for the main spot in the footprint of Io comes from plasma waves generated by the movement of Io within the plasma torus. The speed of these waves is less within the torus than outside it. The bundles of electrons (red) that go back and forth between the two poles are responsible for the smaller spots. These bundles are not slowed down by their passage within the torus, and this, in accordance with various configurations, causes the appearance of a smaller spot that precedes the main spot.

©LPAP/Université de Liège

(Representation not to scale)