Whispers of the Cosmos: The Enigmatic Harmonies of Earth and Beyond

Earth Resonates with Enigmatic Chorus Waves—and So Does Deep Space

By Jonathan O’Callaghan edited by Lee Billings

Thousands of kilometers above, two robust radiation belts envelop our planet. Within these areas, particles entrapped in Earth’s extensive magnetic field zip around nearly at light speed—rapid enough to create serious hazards for any spacecraft or astronauts aiming to navigate through them. Some of the most perilous particles, termed “killer electrons,” achieve such velocities due to amplification from unusual disturbances in Earth’s magnetic field known as chorus waves, named for their acoustic similarity to the sound of birdsong. Historically, these chorus waves were believed to manifest solely near Earth and other celestial bodies. In theory, avoiding them could facilitate safer, less radiation-intensive space travels—except that recent data indicates these waves are much more prevalent in the vastness of deep space than previously acknowledged.

In an article published in the journal Nature, Chengming Liu from Beihang University in China and his team report their discovery of chorus waves through NASA’s Magnetospheric Multiscale (MMS) mission, comprising four satellites in formation that the agency launched in 2015 to investigate Earth’s magnetic field. Notably, these waves were not located near Earth at all. Instead, they emerged at a distance of 165,000 kilometers (100,000 miles) from our planet, approximately three times farther than any previously recorded chorus waves. This positions them within the trailing tail of Earth’s bubble-shaped magnetosphere, far from the regions where many researchers presumed such phenomena would occur.

“This is a highly significant paper,” states James Burch from the Southwest Research Institute, principal investigator of the MMS mission and a co-author of the article. “Such occurrences could be happening anywhere in the universe where there exists a magnetic field, which is just about everywhere.”

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

Chorus waves, or more precisely, whistler-mode chorus waves, captivate our attention with their complexity. They consist of rapid bursts of energy lasting mere tenths of a second, generating a distinctive “chirp” in frequency when transformed into sound. “They resemble birdsong at dawn,” notes Richard Horne, a space weather authority from the British Antarctic Survey who investigates the phenomenon. “That’s the origin of their name.” The minute variations that we interpret as chorus waves arise from plasma instabilities—unsteady distributions of charged particles moving along Earth’s magnetic field lines. As they can engage with the high-energy particles confined within our planet’s geomagnetic lock, “it became apparent in the late 1990s and early 2000s that they significantly contribute to the formation of Earth’s radiation belts,” explains Horne, who reviewed the new Nature publication and penned an accompanying commentary.

The identification of chorus waves was serendipitous—not in the cosmos but on Earth—when World War I radio operators intercepted them radiating from thunderstorms. “Individuals were tuned to enemy signals, and instead they detected this chorus of ‘birds,’” recalls Allison Jaynes, a space weather physicist at the University of Iowa, not associated with the research. “Later realizations indicated they were eavesdropping on chorus waves generated by lightning.”

Since then, these waves have been observed on every planet in the solar system with any form of a magnetic field: Mercury, Mars, Jupiter, Saturn, Uranus, and Neptune. They have even been detected on Venus, which does not have a magnetic field; there, they developed from transient fields produced by solar wind colliding with the planet’s atmosphere.

Collectively, all these prior observations suggested a straightforward prerequisite for the formation of chorus waves: a dipole magnetic field—characterized by “north” and “south” directional points, similar to the ends of a bar magnet—that bends its magnetic field lines around a planet, like Earth. This curved dipole arrangement permits chorus waves to travel from pole to pole, generating a “chirp.” However, due to their significant distance from Earth, the chorus waves recorded in the most recent findings “essentially eliminate the curvature aspect,” remarks Daniel Ratliff, a plasma physicist at Northumbria University in England, who was not part of this study. “Yet one still observes…

these exceptionally distinct, ascending tone characteristics.”

This indicates another process for the generation of chorus waves—specifically, alterations in the frequency of the magnetic field. Such frequency modifications can lead to—and can also stem from—high-velocity electrons that traverse a magnetic field with slight curvature, producing chorus waves in the process. “This paper implies that frequency fluctuations are the source of chorus emissions,” states Yoshiharu Omura from Kyoto University in Japan, who was not part of the research. Nonetheless, Omura and Ratliff emphasize that both mechanisms could still be significant.

This alternative pathway holds significance because it suggests that chorus waves are not confined to the curved magnetospheres of celestial bodies but can emerge anywhere in space where a magnetic field exists. “The cosmos is abundant with high-energy particles [like cosmic rays], but this might influence those already present,” Burch remarks. “When planning a journey from Earth to Mars, substantial shielding [from radiation] is necessary. This presents a newfound source of energetic electrons that we were previously unaware of, which can occur universally. Therefore, it should be actively searched for.”

Liu and his colleagues also discovered signs of related phenomena known as “electron holes,” which are essentially voids in a chorus wave caused by the clustering of electrons as it travels along Earth’s magnetic field. “The resonance produces waves, resulting in these types of holes,” Horne explains. “And that represents a significant finding,” made feasible by the exceptional data provided by the MMS mission.

Magnetic reconnection—a mechanism through which the magnetic field lines of Earth and the sun interconnect, discharging bursts of radiation—is also believed to be associated with chorus waves, supplying a portion of the high-energy particles that the waves can subsequently energize. Liu’s findings indicate that this phenomenon occurs “considerably far from Earth,” according to Omura. This connection might imply that closely observing the incoming solar wind could assist scientists in more precisely forecasting the formation of chorus waves around Earth and other celestial bodies, potentially enhancing space weather predictions.

This signifies that comprehending chorus waves could be vital for ensuring that future missions to the moon, Mars, and other deep-space locations do not embark on fated ventures. “If you’re elevating electrons to extremely high energies, it’s crucial to understand, for crewed spaceflight and spacecraft assets, how many of these hazardous electrons exist within the magnetosphere,” Jaynes remarks. “Grasping the nature of chorus waves is essential for that.” Gaining further knowledge about them could provide insights regarding the safety of operations in these spatial regions. “We aim to foresee when and where these events will occur,” Ratliff adds, “ensuring we know when and where operations might become exceedingly perilous.”