The Cassini spacecraft has made the first observations from within the radio aurora of another planet than Earth. The measurements, which were taken when the spacecraft flew through an active auroral region in 2008, show some similarities and some contrasts between the radio auroral emissions generated at Saturn and those at Earth. Results were presented this week by Dr Laurent Lamy at the European Planetary Science Congress in Rome, and recently published in Geophysical Research Letters.
"So far, this is a unique event," said Lamy (Observatoire de Paris, France). "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 encountered the auroral region at a distance of 247 million kilometres from Saturn's cloud tops (about 4 times Saturn's radius). High above the spectacular visible-light displays of Saturn's Northern and Southern Lights, auroral emissions occur this far from the planet at radio wavelengths. The emissions are generated by fast moving electrons spiralling along Saturn's magnetic field lines, which are threaded through the auroral region.
On 17 October 2008, Cassini's MAG (magnetometer), RPWS (radio) and CAPS (electrons) instruments detected three successive curtains of active auroras. An international team of scientists has now combined magnetic, radio and particle in situ observations to build up a picture of the local radio source properties and the surrounding auroral plasma. They also identified the magnetic field lines along which radio aurora are emitted.
"The instrument that measures radio waves, RPWS, can tell us the direction that each radio wave detected is travelling. 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," said Lamy.
Earth also has radio auroral emissions and these new results show that the process that generates radio aurora appears to be the same at both planets. Interestingly, there are two minor differences between the aurora at Earth and Saturn. At Earth, there is a cavity in the plasma above the auroral oval that rises for several thousand kilometres. The new observations show that this is not seen at Saturn. Secondly, radio sources were crossed at much further distances from the planet. These discrepancies reflect intrinsic differences between the two magnetospheres, in terms of dimensions and planetary rotation speed.
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," said Emma Bunce, a team member from the University of Leicester in the UK.
Animation: Visualisation of the sources of a radio aurora at Saturn. The animation is based on Cassini observations using the RPWS instrument. On the left hand side are the radio sources as seen from Cassini. The right hand side shows the projection of the radio sources down onto the southern pole of the planet.
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