Therefore, aurorae can be created by different processes, according to the planets’ different properties. On Earth, they tend to be external, because the solar wind’s particles pass more easily through the magnetosphere. On the gas giants, the solar wind will have less of an influence, and the aurorae are more likely to be the result of internal processes.

The controversy surrounding Saturn

Although Saturn is smaller than Jupiter, it is nonetheless a gas giant. Its characteristics remind us of those of its neighbour, both in terms of its composition and its behaviour. Just like its large companion, its magnetic field is far more powerful than the Earth’s. The solar wind, given the greater distance separating the planet from the star, is less powerful, and its rotation speed is faster, since it only needs 10 hours to rotate on its axis. And above all, its magnetosphere also contains plasma, whose origin deserves a brief explanation. "Jupiter and Saturn are basically composed of hydrogen and helium in a neutral state", the researchers elaborate. "As regards Jupiter, the main source of plasma is its moon Io, probably the most volcanic object in the solar system. As for Saturn, besides its rings, it's Enceladus, which also has volcanic activity, or cryovolcanic to be more precise. There are massive geysers of water, that turn to ice. The liquid phase doesn’t exist in space". When these eruptions take place, the matter ejected escapes the satellite’s gravitational force (like for Io, Jupiter’s volcanic moon) and is ionised by the Sun’s ultraviolet rays, before being trapped by the planetary magnetic field in plasma form and accumulating in the magnetosphere. A process that applies to both Saturn and Jupiter.

The similarities shared by these two giants still lead a good number of magnetospheric physicists to believe that the origin of Saturn's aurorae is internal. "It’s true that all the conditions for such a process to exist are present on Saturn. We even believe it's been observed in the ultraviolet, but not yet in the infrared. So it’s not something very obvious. Consequently, there’s a big question mark. Even though the two planets are quite similar, why does one present the auroral signature of a huge corotation lag, and not the other one?" A question that has led Aikaterini Radioti’s team to examine a third possibility, whereby the origin of the aurorae shares characteristics particular to those of Earth and Jupiter. "A mixture between the cycles linked to the plasma and those dependent on reconnections with the magnetic field of the solar wind", the scientist continues. "This is the hypothesis we are putting forward, and which seems to us the most probable. At the same time, we haven’t been studying the aurorae on these planets for very long. We’re still at the discovery stage, and we’re continuing to come up with a great many hypotheses, which haven’t been verified. So the controversy continues." "Perhaps", Denis Grodent adds, "when we have the chance to study other giants like Uranus or Neptune, we shall understand physical systematics that will force us to rethink the mechanisms that we apply to Earth. For the moment, our paradigms work for Earth, but if they don’t allow us to explain what is happening elsewhere, many things may be called into question. After all, physics must be applied in the same way everywhere." 

The transpolar arc on Saturn reinstates the solar winds

On Jupiter, the Sun’s magnetic field rebounds and bypasses the planet without penetrating the magnetosphere. Exactly in the same way as water from a river flows around a stone without altering it. However, for reconnection between the field lines to occur, the solar wind must penetrate the magnetosphere, like the water penetrating the porous surface of a stone. Why does the magnetic field slide off Jupiter and could it be trapped by Saturn? The answer probably lies in the intensity of the magnetic field, which is ten times greater than on its cumbersome neighbour, or in the fact that there is less plasma.

But Aikaterini Radioti and Denis Grodent are adament: reconnection processes are at the source of Saturn’s aurorae. It is an interesting discover which allows them to confirm their intuition: the observation of the formation of a transpolar arc for the first time. A frequently observed phenomenon on Earth, but unexpected on Saturn. Remember that it is Saturn’s internal process, linked to the force of its magnetic field and the presence of plasma, that prevents or, at the very least, greatly reduces, the possibility of an external process. Reconnection is therefore far rarer on Saturn than on Earth, where the magnetosphere is less effective at blocking the solar wind.