The interest of such measurements pertains to compared planetology. The size of Jupiter makes it possible to better distinguish the different mechanisms than on Earth, where everything is mixed together. We can then verify whether the theories that work on our planet also apply elsewhere. With regard to the processes that are to be found only on Jupiter and to a lesser extent on Saturn, it is not just a question of understanding unusual physical phenomena. “Jupiter has a unique profile in our solar system. On the other hand, most of the known exoplanets are gas giants. We are beginning to wonder if, in systems with even more powerful magnetic fields and composed of even more volcanic moons, it might be possible to directly detect auroral emissions in the infrared or ultraviolet. If these processes reveal themselves to be common in the universe, Jupiter becomes a precious tool for understanding the way this works. Given the dimensions and the magnetic fields involved, it is the closest analogy we have at our disposal”. 

The different aurorae of Jupiter

Distinguishing the influence of the solar wind from that of internal processes is not the only challenge. This is because many internal processes are involved in the circulation of plasma in the magnetosphere of Jupiter and each one possesses its own auroral signature. Among the different structures which form the aurorae of Jupiter, the easiest one to identify is in the form of an almost continuous contour known as main emission or main oval. This depends on one of these mechanisms. “Once the gas from Io has been ionized while escaping from the moon, it is captured by Jupiter’s magnetic field and begins to rotate around Jupiter at the same speed as the planet rotates on its axis. This ionized gas rotates around Jupiter 4 times faster than Io. The magnetic tension prevents these particles from being ejected immediately but the centrifugal force still allows the plasma to progressively migrate toward the exterior. The further it migrates, the greater the distance it has to travel in order to complete a full circle. If it stays at the same speed, it does not rotate as fast as Jupiter. In order to maintain what is called the corotation, that is to say, an angular velocity that is equal everywhere, the plasma has to be accelerated”. On Jupiter, the further the plasma moves away, the more it loses its angular velocity. This loss generates a torsion of the magnetic field and the associated electric current, as is always the case in electromagnetism, circulates in a direction such that its effects will be opposite to the cause which gave rise to it in the first place. In simple terms, the current will accelerate the plasma so that it reaches a speed close to that of the corotation again. But another consequence of this intense electric current is the acceleration of charged particles in another place: along the magnetic field lines. When the particles strike Jupiter’s atmosphere, they give rise to the main oval.