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

The earth sings with mysterious chorus waves, and so does deep space

Who Jonathan O’Callaghan edited by Lee Billings

Thousands upon thousands of kilometers above, two powerful radiation belts embrace our world. Here, particles trapped in Earth’s vast magnetic field travel close to the speed of light, fast enough to pose serious risks to spacecraft or astronauts hoping to pass through them. Some of the deadliest particles, known as “killer electrons,” from acceleration they reach such high speeds due to special perturbations in the Earth’s magnetic field that they are called chorus waves, named for their similarity to sound. birdsong. These chorus waves have been thought to exist only near Earth and other planets. And in principle, moving away from them can make it safer, less radiation-filled space travel—except the new results suggest that such waves are much more common in deep space than anyone thought.

Writing in a journal nature, Chengming Liu of Beihang University in China and his colleagues report that they discovered chorus waves using NASA’s Magnetospheric Multiscale (MMS) mission, four satellites flying in the agency’s formation. It was launched in 2015 To study the Earth’s magnetic field. These waves were not very close to Earth at all, however. Instead, they appeared 165,000 kilometers (100,000 miles) from our planet, three times farther from Earth than previously detected chorus waves. That places them at the tail end of our planet’s bubble-like magnetosphere, far from where many researchers thought they must have formed.

“It’s a very important role,” says James Burch of the Southwest Research Institute, principal investigator of the MMS mission and co-author of the study. “This could happen anywhere in the universe where there is a magnetic field, which is almost anywhere.”

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Listen to an audio reconstruction of a chorus wave here:

Chorus waves, or more specifically, whistle-like chorus wavesthey are as confusing as they are fascinating. These are small bursts of energy that last only a few tenths of a second and when converted to audio produce a “chirp” of an unusual frequency. “It’s 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 small fluctuations that we perceive as coronal waves are caused by plasma instabilities, unstable distributions of charged particles moving along the Earth’s magnetic field lines. Because they can interact with high-energy particles trapped in our planet’s geomagnetic grip, “we realized in the late 1990s and early 2000s that they play an important role in shaping Earth’s radiation belts,” says Horne, a co-author of the new review. nature paper, with which he wrote a commentary.

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

Since then waves have been observed on every other planet in the solar system any magnetic field appearance: Mercury, Mars, Jupiter, Saturn, Uranus and Neptune. They have been Found on Venushaving no magnetic field; they were created there from the transient fields created by the solar wind entering the planet’s atmosphere.

Collectively, all of these previous detections suggested a relatively simple prerequisite for chorus waves: a dipolar magnetic field—one with a “north” and “south” direction, 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 chorus waves to propagate from pole to pole, creating a “chirp”. Because of their great distance from Earth, however, chorus waves “kind of remove the curvature element” in the final results, says Daniel Ratliff, a plasma physicist at England’s Northumbria University who was not part of the study. “And yet you get these very clear rising tone characteristics.”

This indicates another mechanism for the production of chorus waves, namely changes in the frequency of the magnetic field. These frequency changes can—and do—produce high-speed electrons that move through a magnetic field with minimal bending, resulting in chorus waves. “This observation suggests that the origin of chorus emissions is frequency variation,” says Yoshiharu Omura of Kyoto University in Japan, who was not involved in the study. However, Omura and Ratliff say both processes will still play a role.

This alternative pathway is important because chorus waves were not confined to the curved magnetospheres of planets and stars, but were free to form anywhere in space with a magnetic field. “Space is full of high-energy particles (such as cosmic rays), but that can help those already there,” says Burch. “If you try to go from Earth to Mars, you need a lot of shielding (from radiation). It’s a new source of energetic electrons that we didn’t know about, that can happen everywhere. So you have to look for it.’

Liu and his team also found evidence of associated effects called “electron holes,” essentially holes in a propagating chorus wave that propagate by stacking electrons together. “Resonance creates waves, and you get these kinds of holes,” says Horne. “And that’s a critical observation,” made possible by the unique data available to the MMS mission.

Magnetic reconnection—the process by which Earth’s and the sun’s magnetic field lines bind together, releasing bursts of radiation—is thought to be linked to chorus waves, supplying some high-energy particles that the waves can essentially supercharge. Liu’s results suggest that this process is happening “quite far from Earth,” says Omura. This link means that close monitoring of the incoming solar wind will help scientists better predict the production of chorus waves around Earth and other planets, improving their performance. space weather forecasts.

This means that understanding chorus waves could be crucial to securing future missions the moon, Mars and other deep space destinations not starting the doomed swan song. “If you’re pumping electrons to very high energies, you want to know, for crewed flights and spacecraft, how many of these killer electrons are in the magnetosphere,” Jaynes says. “Corona waves are very important to understanding this.” Knowing more about them could tell us more about when it’s safe to fly in these regions of space. “We want to predict when and where they’re going to happen,” says Ratliff, “so we know when and where it might be too dangerous to operate.”