Of the four Galilean satellites, Io orbits closest to its host planet Jupiter. The satellite moves so rapidly through Jupiter's intense magnetic field, that it acts as an electric generator, creating powerful electric currents that flow toward Jupiter's poles along magnetic field lines. The charged particles comprising these currents originate from Io's thin atmosphere which, in turn, is replenished by the satellite's volcanic activity. The particles crash into Jupiter's upper atmosphere and cause bright aurora-like spots, one in the north and one in the south. For more on Io's interaction with Jupiter's magnetic field, click here.