The final flyby of the northern pole of Saturn by the Cassini spacecraft was the last opportunity to take images of the aurorae before the end of the mission. Unexpectedly, the ultimate image is very attractive, displaying an explosion of polar lights, all connected to different phenomena in the magnetosphere of Saturn. A bright polar emission never seen before has even been highlighted.

On September 15, the Cassini spacecraft plunged into the atmosphere of Saturn, making an end to the NASA-ESA Cassini-Huygens mission after more than 13 years of observations of Saturn, its rings, its moons and its magnetosphere. The magnetosphere is the cavity bathing the planet where the magnetic field is dominant for the physical processes taking place inside it and shields the planet from the particles coming from the Sun. The magnetosphere is filled with neutral particles and plasma, a mixture of charged particles. The dynamics of the plasma within Saturn’s magnetosphere and the interaction of charged particles with the magnetic field, the atmosphere and the moons are highly complex. The magnetospheric dynamics can be investigated using the observations of the auroral emissions in the upper atmosphere of Saturn, in the polar regions. Indeed, these polar lights are produced by atmospheric precipitation of particles guided by the magnetic field lines. Since these field-aligned electrons come from various magnetospheric regions, the auroral emissions provide a global picture of the magnetospheric processes all around the planet and the evolution of the shape and the brightness of the aurorae display the plasma dynamics in Saturn’s magnetosphere.

Images of the ultraviolet auroral emissions at Saturn were acquired by the Ultraviolet Imaging Spectrograph (UVIS) onboard the Cassini spacecraft. During the whole mission, hundreds of UVIS auroral images have been analyzed by the scientists at the Laboratory for Planetary and Atmospheric Physics (LPAP) of the STAR Institute of the University of Liege (Belgium), in collaboration with the UVIS Team at the Laboratory for Atmospheric and Space Physics (LASP) of the University of Colorado (USA), gave rise to important advances in the knowledge of Saturn’s magnetosphere. One day before the final plunge of Cassini in the atmosphere, the UVIS spectrograph captured its ultimate image of the ultraviolet auroral emissions in the northern polar region of Saturn. Although it was taken from a fairly large distance from the planet, this very last UVIS image exhibits an explosion of polar lights and displays several individual auroral structures. A polar projection of this image is reproduced below, with the north pole at the center of the figure and a longitude-latitude grid including meridians every 40° and the parallels at 80, 70 and 60° of latitude. The emissions on the lower part of the projection face the Sun while the upper part is on the nightside of the planet. This ultimate UVIS image was preceded about one hour earlier by a series of observations assembled in a movie shown below as well. This movie enables us to track the evolution of the different components of the aurora.

Final image of Saturn’s northern aurorae taken by the UVIS spectrograph on board Cassini on September 14, 2017, one day before the end of the mission. Credit: University of Liege/LPAP

This final image displays many interesting auroral structures. On the evening side, the emissions are organized as a broad arc which exhibits large auroral spots indicated by the letter (a) on the image. These bright spots are likely an auroral signature of the presence in the dayside magnetosphere of bubbles of plasma with an energy greater than the one of the surrounding plasma. Electric currents connect the boundaries of this hot plasma bubble to the planet and the atmospheric precipitation of the electrons carried by these currents produce the auroral spots observed in the atmosphere.

The magnetic field of Saturn includes closed magnetic field lines going from one hemisphere to the other and the so-called “open” field lines, at high latitude, connected to the interplanetary magnetic field generated by the Sun. Electric currents flow at the interface between the closed field lines and the open field lines. The faint arc-shaped auroral emission visible on the nightside along the inner edge of the broad arc and indicated by a (b) could be produced by these currents.

The closed field lines at the front of the magnetosphere can become open by combining with the interplanetary magnetic field while the open field lines can close again in the nightside magnetosphere, a region called magnetotail. The closing of the field lines in the magnetotail generate a high-latitude auroral arc on the nightside, distinguishable on the last UVIS image and identified by the letter (c), although the arc is short and faint here.

Due to the planetary rotation, closed field lines tend to stretch out in the magnetotail. When the stretching becomes too large, the field lines break off then reconnect to recover the initial configuration. This process leaves an auroral signature in the atmosphere, as a morning auroral arc (emission (e) on the image) which then extends and brightens while rotating towards the dayside (emission (d)).

Sequence of 8 images of Saturn’s northern aurorae taken by the UVIS spectrograph on September 14, 2017, 80 min before the final UVIS image discussed in the article.

Finally, on this last image, the auroral emissions display an extended structure on the morning side (f), almost at the pole, which has never been observed in the past. This polar emission still needs to be analyzed in order to understand the magnetospheric process causing it. The morphology of this auroral arc resembles the dayside transpolar arc commonly observed at Earth. The magnetospheric mechanism producing this dayside polar arc could then exist in both Earth and Saturn’s magnetospheres. Alternatively, this auroral structure might be associated with the same process as for emission (d), namely a rearrangement of the magnetic field lines in the magnetotail.

Several possible scenarios have been postulated over the years to explain Saturn’s changing auroral emissions but we are still far from a global solution to this complicated puzzle. It will probably take another decade to fully (re)analyze the hundreds of sequences of UVIS spectral images acquired during the 13 years of the Cassini mission and to combine them with the numerous observations obtained with the other instruments onboard Cassini or gathered by other space-based and Earth observatories. Undoubtedly, many amazing discoveries await us.

Benjamin Palmaerts  is a Post-doc researcher at the Laboratory for Planetary and Atmospheric Physics (LPAP)  – in the STAR Research Institute –  counting a lot of specialist of the study of the UV aurora emissions on Jupiter and Saturn .