Earth Sings with Mysterious Chorus Waves—and Deep Space Does, Too

Earth Sings with Mysterious Chorus Waves—and Deep Space Does, Too

By Jonathan O’Callaghan edited by Lee Billings

Thousands upon thousands of kilometers overhead, two powerful belts of radiation encompass our world. Here particles trapped in Earth’s sprawling magnetic field whiz around at close to the speed of light—fast enough to pose grave dangers for any spacecraft or astronauts that hope to traverse them. Some of the deadliest particles, known as “killer electrons,” reach such high speeds from acceleration by peculiar perturbations in Earth’s magnetic field called chorus waves, so named for their sonic resemblance to birdsong. These chorus waves have long been thought to occur only close to Earth and other planets. And in principle, steering clear of them could allow safer, less radiation-riddled space voyages— except that new results suggest such waves are far more common in deep space than anyone realized.

Writing in the journal Nature, Chengming Liu of Beihang University in China and his colleagues report that they found chorus waves using NASA’s Magnetospheric Multiscale (MMS) mission, four satellites flying in formation that the agency launched in 2015 to study Earth’s magnetic field. These waves weren’t very close to Earth at all, however. Instead they appeared at a distance of 165,000 kilometers (100,000 miles) from our planet, about three times as far from Earth as any previously detected chorus waves. That places them in the trailing tail of our planet’s bubblelike magnetosphere, far from where many researchers had assumed they must form.

“It’s a very important paper,” says James Burch of the Southwest Research Institute, who is principal investigator of the MMS mission and a co-author of the study. “This could be occurring anywhere in the universe where there’s a magnetic field, which is just about everywhere.”

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Listen to an audio reconstruction of a deep-space chorus wave here:

Chorus waves, or more specifically, whistler-mode chorus waves, are as fascinating as they are confusing. They are small bursts of energy that last just a few tenths of a second and produce an unusual “chirp” in frequency when they are converted to audio. “They sound very much like birdsong at dawn,” says Richard Horne, a space weather expert at the British Antarctic Survey, who studies the phenomenon. “That’s how they got their name.” The tiny fluctuations we perceive as chorus waves are produced by plasma instabilities, unstable distributions of charged particles flowing along Earth’s magnetic field lines. Because they can interact with the high-energy particles trapped in our planet’s geomagnetic grip, “we realized in the late 1990s and early 2000s that they play a major role in forming Earth’s radiation belts,” says Horne, a peer reviewer of the new Nature paper who wrote an accompanying commentary.

The discovery of chorus waves happened by accident—and not in space but on Earth—when World War I radio operators overheard them emanating from thunderstorms. “People were listening for enemy transmissions, and they instead heard this chorus of ‘birds,’” says Allison Jaynes, a space weather physicist at the University of Iowa, who was not involved with the study. “It turned out later they were listening to chorus waves produced by lightning.”

Since then the waves have been spotted on every other planet of the solar system that possesses any semblance of a magnetic field: Mercury, Mars, Jupiter, Saturn, Uranus and Neptune. They have even been found on Venus, which lacks a magnetic field; there they have formed from transient fields created by solar wind barreling into the planet’s atmosphere.

Collectively, all these previous detections suggested a rather simple prerequisite for making chorus waves: a dipolar magnetic field—one with “north” and “south” directionality, like the two ends of a bar magnet—that curves its magnetic field lines around a planet, such as Earth. This curved dipolar configuration allows for chorus waves to propagate from pole to pole, producing a “chirp.” Because of their great distance from Earth, however, the chorus waves in the latest results “sort of remove the curvature element,” says Daniel Ratliff, a plasma physicist at Northumbria University in England, who was not part of the study. “And yet you still get these very clear, rising tone features.”

That points to another mechanism for chorus wave production—namely, changes in the frequency of the magnetic field. These frequency changes can give rise to—and can also arise from—high-speed electrons that move through a magnetic field with minimal curvature, spawning chorus waves as a result. “This paper suggests the origin of chorus emissions is frequency variation,” says Yoshiharu Omura of Kyoto University in Japan, who was not involved with the study. Even so, Omura and Ratliff note, both processes might still play a role.

This alternate pathway is important because it would mean that chorus waves were not limited to the curved magnetospheres of planets and stars but were free to form anywhere out in space with a magnetic field. “Space is full of high-energy particles [such as cosmic rays], but this could be contributing to those that are already there,” Burch says. “If you try to go from Earth to Mars, you need a lot of shielding [from radiation]. This is a new source of energetic electrons that we didn’t know about that can occur everywhere. So it should be looked for.”

Liu and his team also found evidence for associated effects called “electron holes,” essentially gaps in a chorus wave caused by electrons bunching together as it propagates along Earth’s magnetic field. “The resonance generates waves, and you get these kinds of holes,” Horne says. “And that is a critical observation,” made possible thanks to the unique data available from the MMS mission.

Magnetic reconnection—a process in which the magnetic field lines of Earth and the sun snap together, releasing bursts of radiation—is also thought to be linked to chorus waves, supplying some of the high-energy particles that the waves can then essentially supercharge. Liu’s results suggest this process is taking place “rather far away from Earth,” Omura says. This linkage may mean that closely monitoring the incoming solar wind could help scientists better predict the production of chorus waves around Earth and other planets, potentially improving space weather forecasts.

That means understanding chorus waves could be crucial for ensuring future missions to the moon, Mars and other deep-space destinations aren’t embarking on doomed swan songs. “If you’re pumping electrons up to very high energies, you want to know, for crewed spaceflight and spacecraft assets, how many of these killer electrons are in the magnetosphere,” Jaynes says. “Chorus waves are very important to understand that.” Knowing more about them could tell us more about when it is safe to fly in these regions of space. “We want to predict when and where they’re going to happen,” Ratliff says, “so that we know when and where it might be too dangerous for operations.”