"Chirping" chorus waves, posing a threat to spacecraft, have been detected far from Earth.

Outer space is filled with electromagnetic waves and charged particles. Due to the vast distances between these particles, they rarely collide with one another. As a result, electromagnetic waves are considered the most significant "transmitters" of energy in space, accelerating and scattering particles.

“Whistling” chorus waves are among the strongest naturally occurring electromagnetic waves in outer space. Scientists have been studying them for over 70 years, not only near Earth but also around Mars, Jupiter, and Saturn. If these waves are converted into sound frequencies that our ears can detect, they resemble the chirping of birds.

Years of observation have revealed that chorus waves play a crucial role in accelerating the formation and dynamics of Earth's radiation belts, the emergence of pulsating auroras, and the transfer of energy from particles into the planet's atmosphere.

Despite decades of research, scientists remain uncertain about how these chorus waves are formed. One of the main challenges is the lack of direct measurements of energy exchange within the waves. Instruments on spacecraft have struggled to gather suitable data due to the high energies of particles and the strong magnetic field near Earth. Moreover, chorus waves have only been searched for within a distance of 10 Earth radii, as it was believed that Earth's dipole magnetic field plays a role in their formation.

In a new study, published in the journal Nature, researchers have for the first time described chorus waves at a distance of 165,000 kilometers from Earth, in the "tail" of the magnetosphere. The scientists utilized observational data collected by four MMS (NASA) satellites in December 2019.

At a distance of 26 Earth radii, the magnetic field is already significantly stretched and loses its dipole effect. The waves detected there lasted approximately 0.1 seconds, exhibited a growing nature, and had a “chirping” frequency close to 100 hertz. In terms of parameters, they are very similar to waves near Earth, indicating that a strong dipole field is not a necessary condition for their formation.

According to the new observational data, the authors of the study managed to measure interactions within the waves. They demonstrated that chorus radiation occurred “in situ,” fueled by energy from electrons—likely thermal electrons with energies around three kiloelectronvolts.

The impulses of chorus waves effectively accelerate electrons, increasing the risk associated with Earth's radiation belts. These belts can damage the instruments of passing spacecraft and pose dangers to astronauts. Understanding the nature of chorus waves will aid in finding solutions to minimize their negative effects.