It’s a world where everything is invisible. A magnetosphere forms where the solar wind, a stream of electrically charged particles we call ‘plasma’, encounters the magnetic field of a planet like Saturn, Jupiter, Neptune, Uranus, Mercury or Earth. Inside the magnetosphere, plasma motions are dominated by the magnetic field of the host planet. The magnetosphere’s intense electronic activity accelerates electrons, which dissipate their kinetic energy as they’re conducted along magnetic field lines to the poles. When these electrons hit the upper atmosphere, they generate aurorae that can be observed in the ultraviolet, visible and infrared.
But thousands of kilometres higher up, auroral radio emissions are also produced: these are low-frequency electromagnetic waves emitted at between 1 and 1,300 KHz, in other words, at wavelengths of several kilometres. This is known as Saturn’s kilometric radiation (SKR).
“These radio emissions belong to the family of aurorae and offer the advantage of being observable continuously with electrical antennas that don’t need to be pointed at Saturn, unlike most conventional spectro-imaging instruments,” explains Laurent Lamy. This radiation is a mine of information about Saturn’s magnetosphere. For example, its periodic pulsation is directly related to the rotation of the magnetic field, whereas its sudden bursts—which can last several days—are a sign of auroral storms triggered by solar wind activity.
Flying through radiation source regions
Orbits of the Cassini spacecraft in 2016-2017, F-Ring and then ‘Grand Finale’ (proximal).
One of the Cassini spacecraft’s suite of plasma instruments was RPWS (Radio and Plasma Wave Science), packed with sensors and receivers. In particular, it featured three 10-metre-long antennas connected to the High Frequency Receiver (HFR), built in 1995 at LESIA (formerly the DESPA space research department of the Paris Observatory) in France with support from CNES. This receiver delivered data of exceptional quality almost continually from the spacecraft’s launch late in 1997 to the last day of the mission at the end of 2017.
“This type of radio instrument acquires two kinds of additional information. It measures electromagnetic emissions from a distance and can analyse the plasma encountered by the probe close up,” notes Laurent Lamy. “During its ‘Grand Finale’, Cassini flew a series of polar orbits grazing the planet’s rings, taking it through the source regions where Saturn’s kilometric radiation is formed and collecting unique in-situ readings. Analysis of the preceding orbits focused on the low-frequency sources of this kilometric radiation that are further away from the planet, between 2½ and 3½ planetary radii.”
Analysis of the mechanism driving these radio emissions has yielded its first conclusions, which confirm it’s the same as the one that generates auroral radio emissions on Earth—a plasma electron/wave instability known as the cyclotron maser instability (CMI). This mechanism requires two conditions: the presence of energetic electrons and a low-density but highly magnetized plasma, like that found in auroral regions.
More still to come from the ‘Grand Finale’
“Our first surprise was the small number of radio sources the spacecraft encountered, despite making 30 very similar passes above each of the magnetic poles. That shows these low-frequency radio sources vary greatly with time and with the spacecraft’s position,” explains Laurent Lamy. “What’s more, we discovered that it’s the density of the plasma, when it suddenly increases locally by several orders of magnitude, that ‘quenches’ the CMI. That tells us some very interesting things about Saturn’s inner magnetosphere.” The challenge now is to determine the origin of this variation in plasma density near the poles. A year after Cassini’s final daring dive into Saturn, most of the data from its Grand Finale are still to be analysed and will be keeping scientists busy for years yet, no doubt with more unexpected results to come…
“By a happy scheduling coincidence, the Juno mission arrived in polar orbit around Jupiter mid-2016 with the same science goals as Cassini’s Grand Finale, and has already acquired very complementary plasmas measurements that have enabled us to do some comparative planetology on two very different giant planets. It’s a golden age for planetary auroral physics!” says Laurent Lamy with a smile. And advancing our understanding of the complex physics of auroral radio emissions at planets within reach of exploration missions will be vital to unlock the secrets of similar emissions on exoplanets that we will never be able to visit.
« The Low Frequency Souce of Saturn’s Kilometric Radiation » L.Lamy et Al, Science, 5/10/2018
- Laurent Lamy, LESIA astrophysicist: laurent.lamy at obspm.fr
- Francis Rocard, head of CNES’s solar system exploration programme: francis.rocard at cnes.fr