In 2014, shortly after Rosetta's arrival at Comet 67P/Churyumov-Gerasimenko, the magnetometer on the Rosetta Plasma Consortium (RPC) suite of instruments, RPC-Mag, detected some surprising oscillations in the plasma surrounding the nucleus, revealing the comet's mysterious “song”. 

Now, after two years of monitoring the plasma around the comet, the RPC team present a new song based on data collected during the entire mission, describing the comet's evolution from the point of view of Rosetta's magnetometer.

While the nucleus of Comet 67P/C-G is itself not magnetised, as measured by plasma instruments on both Rosetta and the lander Philae, it is embedded in the interplanetary magnetic field carried throughout the Solar System by the solar wind – a continuous flow of electrically charged particles streaming from the Sun. As the comet pours water vapour and other molecules into space, disturbing the solar wind, interesting phenomena take place in the surrounding plasma.

The interaction between comets and the solar wind had been studied in the past during the flybys of previous cometary missions, but Rosetta was the first probe to observe this phenomenon for an extended period of time, following its evolution as the comet swung around the Sun.

“When Rosetta arrived at the comet, in 2014, the activity was very low,” explains RPC-Mag principal investigator Karl-Heinz Glassmeier from the Institute for Geophysics and extraterrestrial Physics at Technische Universität Braunschweig, Germany.

At the interface between the solar wind and the comet's fuzzy atmosphere, or coma, gas molecules released by the comet may lose one or more electrons. As observed at other comets, these ionised molecules are normally picked up by the solar wind, building up boundaries in the plasma environment of the comet.

However, no spacecraft had ever visited a comet when its activity was as low as that of Comet 67P/C-G at the time of Rosetta's arrival and, when the RPC magnetometer measured some unexpected waves, it took the scientists by surprise.

Shortly after the first detection in September 2014, the RPC-Mag team collaborated with musician and composer Manuel Senfft to create a sonification of these puzzling measurements, representing the data in an audible form and creating 'A singing comet' (read more about it in the original blog post and in the 'Behind the scenes' follow up). The audio track, released on the day before Philae landed on the comet, became a worldwide sensation, providing an extra element to the experience of “being there” at the comet with Rosetta and Philae.

After several months analysing the data, the scientists figured out the physical processes that led to the comet's song.

“Because of the comet's low activity, the ions are not fully picked up by the solar wind and move perpendicularly to the magnetic field, forming what is called a cross-field electric current,” adds Karl-Heinz.

“But this electric current is unstable, giving rise to the oscillations we measured back in 2014 – that's what made the comet sing.”

The details of the 'singing comet' are described in a paper by Ingo Richter et al, which was published in Annales Geophysicae in August 2015. By then, however, the music at Comet 67P/C-G had already changed.

“As the comet moved closer to the Sun in 2015, it was pouring increasingly larger amounts of gas into its surroundings and, with part of this gas becoming ionised, interaction with solar wind particles intensified,” says Charlotte Goetz of the Institute for Geophysics and extraterrestrial Physics at Technische Universität Braunschweig Braunschweig, Germany.

Between April and June 2015, plasma regions with different properties – velocity, temperature, density – started to form around the nucleus along with boundaries separating the different regions. Rosetta observed at least one such boundary, which was likely moving, as described in a recent paper by Kathy Mandt and collaborators.

As the comet's activity was not strong enough yet, these boundaries could not last long, leading to a phase of chaotic variations in the magnetic field, which was jumping up and down by 30–40 nT over time scales of seconds to hours according to Rosetta's measurements.

“We like to think that, in this period, the comet was 'confused' much like a teenager: the activity was not as weak as when Rosetta first arrived and detected the 'singing comet' waves, but it was neither strong enough to create stable boundaries,” adds Charlotte.

The situation changed when 67P/C-G was approaching perihelion – the closest point to the Sun along its orbit – and finally achieving full 'cometary maturity'. Ever since June 2015, only a few weeks before the comet's perihelion on 13 August, Charlotte and her colleagues started observing a diamagnetic cavity: a region near the comet nucleus where the solar wind cannot penetrate and the magnetic field is practically zero.

“We were really happy to detect the diamagnetic cavity because we had almost given up on finding it: at the time, because of the intense comet activity, Rosetta was flying at large distances from the nucleus – farther away than where we thought the cavity would be. Fortunately, the cavity turned out to be much bigger and dynamic than we had expected, so we were able to measure it,” says Charlotte.

The RPC-Mag team found several hundreds of such magnetic-field free regions until February 2016. The discovery of a diamagnetic cavity at Comet 67P/C-G was first described in a paper by Charlotte Goetz et al, published in March 2016 in Astronomy & Astrophysics.

After that, as the comet moved away from the Sun and its activity declined, plasma boundaries became unstable again and the magnetic field turned chaotic one more time. Eventually, the activity became as low as it had been at the time of Rosetta's first measurements: the oscillations returned and, with them, the 'singing comet'.

“Having detected the waves early in the mission, we later predicted they would return as soon as the activity would reduce... and they did! This was definitely a pleasant surprise,” says Karl-Heinz.

To convey the life of the comet as monitored by the RPC magnetometer, the scientists teamed up again with Manuel, who could now transform into sound not just one day's worth of measurements, but a selection of data from two years.

“I was very fascinated by the symmetry of the magnetic field readings: first the oscillations, then chaos, then the magnetic field was gone... then chaos was back and then, in the end, also the oscillations,” says Manuel.

The composer used this symmetry to construct the dramaturgy of the new piece, called 'A comet's life' and centred on the perihelion of Comet 67P/C-G. The piece starts with the famous 'singing comet' waves, building up to a climax as the comet's activity reaches its maximum, then slowly peters out, coming to an end with the return of the waves.

Manuel employed a similar technique to that developed for his 2014 sonification, using one of the three components of the magnetic field to modulate the pitch of the sound itself, and using variations in the other two components to control the location of the sound in a ‘sound stage’ with respect to the listener, moving it back and forth and left and right.

This time, given the richness of the data, he also made use of a new synthesiser to alter the 'character' of the sound.

“For example, I added distortion to the sound, making it more noisy, to better represent the period of chaotic variations, while I used silence to reflect the absence of magnetic field in the cavity,” Manuel explains.

The result is a spacey, 1:30 minute-long, binaural piece that takes the listener along and lets it fly with Rosetta, following the comet as its activity rose and declined while its Sun-bound journey unfolded.

“The first 'singing comet' was as exciting as hearing a baby's first cry, but the new track is even better: it's a lifetime experience, and it's our experience, living with this incredible comet over the past two years,” concludes Karl-Heinz.

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The results from the RPC-Mag instrument and the audio track, 'A comet's life', were presented by Charlotte Götz during today's science briefing at the European Space Operations Centre (ESOC) in Darmstadt, Germany.

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THE COMETARY ZOO