Clues to 'singing comet' blow in on the solar wind

Wednesday, August 19, 2015, 6:22 PM - When the ESA's Rosetta spacecraft detected the "singing" of Comet 67P, it was a shock, but new research may have uncovered the reason for this bizarre behaviour.

An old scifi adage states quite plainly, "In space, noone can hear you scream."

While this is certainly true, on a few occasions, researchers have reported sounds, such as the eerie "music" of interstellar space detected by the Voyager 1 probe, and more recently the "singing" of Comet 67P/Churyumov-Gerasimenko discovered by the Rosetta spacecraft.

These instances aren't true sound, as we would know it here on Earth, though. Rather than pressure waves travelling through air, these sounds from space are simply oscillations in magnetic fields, which can be interpreted as sounds by a computer and played for us to hear.

The case of "singing" Comet 67P presented something of a surprising mystery for scientists, given that they had never encountered a comet emitting waves at these frequencies, and the mystery deepened when it was later reported that Rosetta's instruments had detected no magnetic field from the comet nucleus.

According to new research, though, the answer apparently lies within the solar wind.

As the comet flies through space, it not only moves through the magnetic field of the Sun but also the stream of charged particles that flows away from the Sun in the form of the solar wind.

The dust and ice the comet expels into its coma are mostly made up of neutral atoms and molecules - neither positively nor negatively charged, and if they remained so, there would likely be no "song" to hear from 67P. However, when those atoms and molecules are bathed in ultraviolet rays from the Sun, electrons are stripped away from them, turning the coma into a mixed cloud of positive and negative charges. Moving charges produce magnetic fields, and interaction between the ions in this charged coma and particles of the solar wind churn up the coma, causing the charges, and thus the induced magnetic fields, to oscillate.

The study that details these findings uses data from the first four months of Rosetta's investigation of the comet, from August to November of 2014. However, according to the ESA's Rosetta blog, the comet continued to sing up until February of 2015.

"Around this time, the activity is changing, other features show up, the plasma interaction region becomes much more violent. Singing comet waves are still present, but buried under a variety of other features we are currently trying to understand," said K-H Glassmeier, senior author of the new research paper.

"Whether we also observe the classical type of cometary waves, like those observed at Halley, is very difficult to judge," Glassmeier added. "We are heavily working on further analyzing the dynamics of this region to find out more."

Comet outbound

Rosetta's long-awaited rendezvous with Comet 67P occurred roughly a year before the comet reached perihelion - its closest approach to the Sun. Now that the comet has passed perihelion, on August 13, it is now headed back out to beyond the orbit of Jupiter, with Rosetta in tow.

Rosetta's tiny lander, Philae, has been silent for some time now, after waking up in June and communicating briefly with the team here on Earth. It's suspected that comet activity may have jostled the probe, throwing its communications antenna off, making it unable to contact Rosetta. There is still the possibility of contacting the lander again, once the comet has calmed again.
In the mean time, if you're interested in seeing the comet up-close, check out the ESA's new interactive comet viewer!

Source: ESA