Planets which have a magnetic field, like the Earth or the giant planets, generate luminous emission in the neighborhood of their magnetic poles : this emission is referred to as polar aurorae. They are the product of a flow of energetic particles, essentially electrons accelerated in the magnetosphere - the planetary magnetic environment – via various complex mechanisms, and subsequently channeled towards the planet along the high latitude magnetic lines of force. When they plunge into the atmosphere, these electrons generate radiation via collisions, a radiation which is observed in the optical domain (visible, ultra-violet or infra-red).

Above the atmosphere, as far out as several planetary radii, these same electrons also amplify radio waves. This radio emission is extremely intense. It is essential to studying this radiation in situ in in order to understand just how it is produced and how its study from a distance enhances our understanding of the magnetosphere. Moreover, a study of the magnetospheres of the planets in our solar system will lead to the establishment of a standard reference which will help us to interpret the radio emission of exo-planets, brown dwarves and stars, whose research is expanding rapidly [1].

During the « Grand Finale », the final stage of the Cassini mission, the space probe repeatedly flew over the magnetic poles at low altitude, just where Saturn’s auroral radio emission is born. By analyzing the in situ data acquired by the probe’s radio instrument [2] and magnetometer, the authors of the paper have identified the « sources » of the far radio aurorae out to about 3 planetary radii (180 000 km) above the atmosphere. They have thus been able to characterize the properties of the radio waves and whence they are born, and thereby compare successfully observation with theoretical prediction.

The result : Saturn’s auroral radio emission is produced by the same process as that which has been identified on the Earth and recently on Jupiter [3] – a plasma instability known as the Maser Cyclotron instability – which enables electrons, in this case having energies of several kilo-electron volts, to give up a part of their energy to radio waves as they gyrate around the magnetic lines of force.
Nevertheless, this mechanism functions under very different conditions than those known on the Earth. The regions responsible for the radio emission are very much farther out from the planet in the case of Saturn, and the electrons involved, measured locally, have had to have been accelerated towards the planet much farther out than the emission region, which somewhat shakes up our understanding of the acceleration mechanisms operating in the magnetosphere.

Furthermore, the radio sources were found on magnetic field lines connected in fact to specific regions where ultra-violet aurorae were observed at the same time by the Hubble space telescope in orbit around the Earth [4]. The authors have shown that this partial association of distant radio sources and ultra-violet aurorae could be explained by a very variable local plasma density, whose origin has still to be identified and which is sometimes too high for the instability to operate.

These results confirm that one and the same universal mechanism can produce auroral radio waves in the environments of very different magnetic bodies.

Video

Paris Observatory’s Youtube animation station : Animated presentation of Saturn’s kilometric radiation observations

Reference

• The research is published as a paper entitled “The low frequency source of Saturn’s Kilometric Radiation," by L. Lamy, et.al. in the October 5th 2018 issue of Science
• The team includes four french researchers : Laurent Lamy (Observatoire de Paris – PSL), quatre chercheurs français : L. Lamy (astronome de l’Observatoire de Paris – PSL), P. Zarka (directeur de recherche CNRS), B. Cecconi (astronome de l’Observatoire de Paris – PSL), R. Prangé (directeur de recherche CNRS), and six others researchers : W. S. Kurth, G. Hospodarsky, A. Persoon, M. Morooka, J.-E. Wahlund, G. J. Hunt.