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Cassini scoops
Saturn's aurora

Posted: 24 September 2010

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A stunning animation of Saturn's aurora created from 1,000 images, and the first observations from within the planet's radio aurora, were presented today at the European Planetary Science Congress.

Composite image made up of 65 individual VIMS observations, each six minutes long (1 November 2008). The mosaic shows the entire planet, including the rings as seen by Cassini from the south. Image: NASA/JPL/University of Leicester/University of Arizona.

Aurorae are the beautiful manifestation of solar wind particles as they are bounced around a planet's magnetic field and channeled towards its poles, where they interact with gases in the upper atmosphere. On Earth we are treated to predominantly green and red displays as the solar wind interacts with oxygen and nitrogen atoms. At Saturn – and in the latest images and movies presented – blue represents sunlight reflected off the rings and clouds in Saturn's atmosphere; green is auroral emission from the H3+ molecule and red is thermal emission from below the clouds. Auroral features on Saturn are also generated when its moons move through Saturn's magnetosphere – the protective bubble blown out by the planet's magnetic field which deflects the majority of the solar wind particles.

“Cassini’s instruments have been imaging the aurora in magnificent detail, but to understand the overall nature of the auroral region we need to make a huge number of observations,” says Tom Stallard of the University of Leicester, who presented the data. “However, there are VIMS (visual and infrared mapping spectrometer) observations of numerous other scientific targets that also include auroral information. Sometimes the aurora can be clearly seen, sometimes we have to add multiple images together to produce a signal. But as a whole, this wide set of observations will allow us to understand the aurora in general, rather than the beautiful specific cases that dedicated auroral observations allow.”

These images were taken between 2:15 p.m. UTC on Sept. 22, 2007 and 10:53 a.m. on Sept. 23, 2007. In this video, the two separate days are shown one above the other, with features from the planet’s interior (in red) lined up. This clearly shows that the auroral emission follows a similar pattern on each day, with an auroral brightening rotating around with the planet. Image: NASA/JPL/University of Leicester/University of Arizona.

Stallard and colleagues have so far investigated around 1,000 images from the 7,000 that VIMS has collected that contain information about Saturn's auroral regions. Variations in the intensity of the aurora over the course of the 10 hour 47 minute Saturnian day are clearly seen in the data, with brightening occurring on the noon and midnight sides that suggests this behaviour is linked to the direction of the Sun. Other features appear to rotate with the planet, reappearing at the same time and the same place on the second day, suggesting that these are directly controlled by the direction of Saturn’s magnetic field.

“Saturn’s aurora are very complex and we are only just beginning to understand all the factors involved,” says Stallard. “This study will provide a broader view of the wide variety of different auroral features that can be seen, and will allow us to better understand what controls these changes in appearance.”

Image of Saturn’s aurora seen at ultraviolet wavelengths.  The spiral shape seen here is similar to the distorted radio aurora visualised by the team and also indicates enhanced auroral activity. Image: ESA/NASA/Hubble.

In a separate talk, Laurent Lamy from the Observatoire de Paris described the first observations from within the radio aurora of Saturn, the first time this has been achieved for any planet other than the Earth. The observations were made when the spacecraft flew through an active auroral region in 2008.

"So far, this is a unique event," says Lamy. "Whereas the source region of Earth's radio aurora has been studied by many missions, this is our first opportunity to observe the equivalent region at Saturn from the inside.  From this single encounter, we have been able to build up a detailed snapshot of auroral activity using three of Cassini's instruments.  This gives us a fascinating insight into the processes that are generating Saturn's radio aurora."

Cassini crossed the radio aurora of Saturn on 17 October 2008, at a distance of 4 Saturn's radii above the atmosphere. These radio emissions, generated by fast electrons, are strongly beamed. They were characterised by simultaneous observations of three different experiments. Image: NASA/JPL/University of Iowa/CNES/Observatoire de Paris.

The spacecraft encountered the auroral region at a distance of 247 million kilometres from Saturn's cloud tops. This is much higher than the visible light displays, but emissions occur here at radio wavelengths, generated by fast moving electrons spiralling along Saturn's magnetic field lines. Three successive auroral curtains were detected by Cassini's magnetic, radio and particle detectors on 17 October 2008, which has enabled scientists to build up a picture of the local environment. "The instrument that measures radio waves can tell us the direction that each radio wave detected is travelling," explains Lamu. "By mapping this information onto magnetic field lines, we can work out the location of each radio source. In addition, we can project the source locations along the field lines that curve down to Saturn’s southern pole and visualise a radio oval comparable to the auroral features commonly seen at ultraviolet wavelengths. Unusually, the oval observed during this event is strongly distorted, which indicates a particularly enhanced auroral activity."

Cassini crossed high latitude auroral field lines during 40 orbits in 2008, but this is the only time that the instruments detected unusually strong electric currents in that region in space with in situ evidence of an active aurora. "We think that the unusual conditions responsible for these intense electric currents might have been triggered by a solar wind compression squeezing Saturn's magnetic field and producing the observed auroras," says Emma Bunce, a team member from the University of Leicester in the UK.

The observations show that the process that generates radio aurora on Saturn is the same as it is for Earth, but there are also some differences. On Earth there is a cavity in the plasma above the auroral oval that rises for several thousand kilometres, which is not seen at the gas giant, and the radio sources were crossed at much further distances from Saturn than at Earth, reflecting intrinsic differences between the two planetary magnetospheres.