There are still many mysteries surrounding the polar aurorae of the other planets in the solar system. They are a source of fascination for atmosphere specialists, and sometimes antagonistic hypotheses on their origins trigger heated debates amid lively controversies. On Earth, they mainly depend on the solar wind. A solar wind which, according to many theories, is incapable of penetrating the magnetosphere of gas giants such as Saturn or Jupiter. A team led by two researchers from the University of Liège could however make the scales tip the other way. Using the Cassini probe, it recently observed the formation of a transpolar arc on Saturn for the first time. A frequent phenomenon on Earth, but unsuspected on the giant planets, since it is very unlikely unless the planet’s magnetic field captures particles from the solar wind.

Some travel to Norway, or to the great Canadian north to observe polar aurorae, and marvel at the incredible beauty of these displays of energy. However, these destinations are no longer exotic enough for planetologists specialising in atmospheric physics. Because space observatories, such as the Hubble Space Telescope or more recently the Cassini spacecraft, have allowed us to discover and observe the formation of aurorae on other planets in the solar system, i.e. Jupiter and Saturn. Theoretically, if a planet has a magnetic field and an atmosphere, aurorae can form in its atmosphere around the magnetic poles.

For Aikaterini Radioti and Denis Grodent, from the University of Liège’s Laboratory for Planetary and Atmospheric Physics, Saturn, in particular, was once again the source of their attention, and was able to satisfy their curiosity. "Thanks to the UVIS spectograph on board Cassini", Aikaterini Radioti explains, "we observed the formation of a transpolar arc for the first time. It is a common occurrence on Earth, formed by repeated interactions between the magnetosphere and solar wind. But we didn’t think it was possible on Saturn. Because its system has internal processes that play very important roles in the behaviour of the magnetosphere, which could prevent the formation of these arcs." The discovery could change what we know about Saturn.  For instance, recognising that the solar wind strongly influences the deep regions of its magnetosphere, and finding similarities between these mechanisms and those observed on Earth, despite the huge differences that characterise these two planets. 

Terrestrial aurorae, an external process

Before turning to the observations on Saturn, we should perhaps take a closer look at the phenomenon of polar aurorae. On Earth, they can be observed at both poles. There is the aurora borealis at the North Pole, and the aurora australis at the South Pole. But what we observe when we see these curtains of light in the sky is, in fact, nothing other than the signature of a mechanism that is happening much further away. It is the residue of an interaction between the Earth’s magnetic field and the solar wind.

(1) Radioti, Aikaterini and al, Saturn’s elusive nightside polar arc, Geophysical Research Letters, Sept 2014