The four Galiean satellites in the Jovian magnetosphere have orbital periods that are much longer than the rotational period of Jupiter (10 hrs). Therefore, these moons are constantly overtaken by the magnetoplasma of the Jovian magnetosphere, and interact with it. This interaction has, at least, two effects:

1.  it creates a wake region in the magnetoplasma downstream of the moon, which is not important for this topic; and

2.  the interaction of the magnetic field with the moon creates so-called Alfvén wings, propagating away from the moon along the magnetic field of Jupiter.

The interaction of the Jovian magnetic field with the Galilean moons, i.e. when a magnetic field line gets into contact with the moon?s ionosphere, results in the emission of an Alfvén wave that propagates along the magnetic field line. At the same time the magnetic field line moves with the rotation of Jupiter and thus, in the frame of the moon, the signal propagates at an angle away, like a magnetic wake. The distortion of the magnetic field is created by magnetic field aligned currents.

The field aligned currents, basically driven by the motional electric field in the moon?s frame. At Io it was expected, and confirmed by Galileo measurements, that a few million amps are flowing in the flux tube. These currents, interact with the ionosphere of Jupiter, where the energetic particles deposit their energy. This shows up as bright spots of aurora at the location of the foot points of the moon flux tubes. Europa and Ganymede show up as bright points in the diffuse aurora region, whereas Io shows up as a bright spot with tail below the main auroral oval. The tail created downstream of Io?s foot point is caused by the enormous mass loading that takes place near Io.