"In reality, there are two stages to the reconnection that takes place in the cycle of the external process. On the day side, i.e. the side lit by the Sun, the Earth’s magnetic field lines, or those of Saturn, go in one direction, and the Sun’s magnetic field in another direction. The lines closest to each other will open, and allow what is known as dayside reconnection. The Earth’s magnetic field opens and its field lines connect with the Sun’s. This is the Dungey cycle, named after the English physicist who discovered it. These field lines will then move back and, only once they are much further away, in the magnetotail, will they reconnect again, on the night side this time. They then close up, and the Sun’s field lines will be able to continue on their way. In fact, it’s a way for the Sun’s magnetic field to bypass an obstacle." During this cycle, several field lines associated with the night side sometimes close one after the other, while few field lines open on the day side. Therefore, there is an accumulation of a magnetic flux on the night side, which remains stuck in the tail and can no longer get out. It is this instability in the auroral footprint that will advance into the polar region, during the accumulation of the flux, and form an arc.

"It’s only a theory on the formation of arcs, but it’s the most likely one as regards both Earth and Saturn. And yet, on Saturn, this process coexists with an internal process, which makes the Dungey cycle quite rare. Therefore, an accumulation of flux, which means several reconnections one after the other, must be an event that is even less frequently observed. We were really lucky to spot it. Before we observed it, we didn’t even think such an event could exist. In any case, this observation allows us to believe that if these arcs are indeed the effect of a reconnection linked to the solar wind on Earth, this is also the case on Saturn, and they are greater than we thought."

More and more observations

Hubble took the first images of aurorae on the gas giants. In 2004, the space probe Cassini, developed by NASA and ESA, went into orbit around Saturn leading to more precise observations, which fulfilled the expectations of research groups from a wide range of areas of study. In particular, it led to the discovery of the existence of geysers on Enceladus, the collection of a great deal of data on the composition of Saturn’s rings, a better understanding of its atmosphere, its physical and climatic properties, but also its magnetic field, its magnetosphere, and the effect it could have on the formation of its aurorae.

But Cassini still has many wonders in store for us. "The mission was extended until the end of 2017", the researchers tell us enthusiastically. "Normally, Cassini turns around Saturn in the equatorial plane. But when the probe passes close to a satellite, we can use its swing-by to modify the orbit and tilt it towards the poles. And for the mission’s "Grand Finale", we expect to collect some amazing data. We’ll have a near polar orbit. This course change will monopolise the probe’s last resources. It will move closer to Saturn, enter its atmosphere and completely disintegrate there. But during the whole period leading up to its end, we will be able to observe the two poles from very close up, in detail, with a definition that is almost better than the one we have for Earth observations. We’ll have the day side and night side in the same frame, and we shall observe phenomena we aren’t yet familiar with, which we probably still can’t explain, that will allow us to confirm or abandon some of our hypotheses."  

And the end of Cassini doesn’t mean an end to the observation of the giant planets' aurorae. In 2016, Juno, one of NASA’s new, and even more high-performance probes (with an instrument on board that the CSL (Liège Space Centre)helped to develop), will be put into orbit around Jupiter to carry on the work. These observations will simply be an initial area of research.  Because the researchers are still at the discovery stage.