Jovian auroras can be broken down into three parts, each having its own mechanism of formation: a main oval area around the magnetic poles, polar emissions that are quite variable, and the footprints of the satellites Ganymede, Europe and Io, located closer to the equator than the main oval. The satellite footprint that is most clearly visible is that of Io. This is formed by a large spot followed by a streak, and sometimes one or more smaller spots appear in front of the streak. The physics of the origin of the main oval is relatively well known, as Bertrand Bonfond explains: “Because of centrifugal force, charged particles within the plasma torus migrate outside the torus and get progressively further away from the planet, which makes necessary a regular increase in their speed in order for them to follow the magnetic field as it rotates every 10 hours. In the beginning they manage it, but at some point they aren’t able to accelerate enough. Electric currents thus are established between the atmosphere of Jupiter and these particles. These currents cause electrons to accelerate along the lines of the magnetic field. Then they strike the upper atmosphere of Jupiter with great speed. The resulting shock causes light to be emitted in the form of an oval aurora appearing at the foot of these currents.”

A scenario was also proposed to model the auroral imprint of Io. The relative movement of Io within the plasma torus also generates a current that circulates in Io and in each of the poles of Jupiter, along the lines of force of the magnetic field. “An analogy will help us understand what is happening, “ Bonfond suggests. “Imagine a rock in the middle of a current of water. The rock will cause waves that will propagate over the surface of the water, and touch the banks downstream from the rock. If the rock is closer to the right bank than the left, the waves that are produced that travel toward the left bank will reach that bank later and further downstream than the waves that were heading toward the right bank (which is nearer). Similarly, since Io is not gravitating around Jupiter at the same speed as the Jovian magnetic field, the satellite is an obstacle for part of the particle flux in the plasma torus, and this creates a perturbation that gets propagated the length of the lines of force of the magnetic field in the form of plasma waves (Alfvén waves), all the way to the poles. Once they get close to Jupiter, these waves accelerate electrons toward the planet, which create the main spot in Io’s footprint. As with the rock in the stream, if Io is closer to the northern part of the plasma torus, the southern spot will form in a leading position, further downstream than the spot in the north.” The leading position of the largest spot can be explained by the fact that the gas that has just been ionized close to Io is still turning at the same speed as Io, and not yet at the speed of the magnetic field. Streaks then form, as in the case of the main oval.