An international research team led by Chinese scientists from Beihang University has discovered chorus waves more than 160,000 km away from Earth -- a space phenomena previously believed to occur only near Earth's dipole magnetic field regions, according to a study paper recently published in Nature.
The scientists observed chorus waves with frequencies below 100 Hz. When they converted the chorus waves into audio output, they obtained a piece of "space chorus" that human can hear. The scientists described the sound like "the chirping of birds".
"The space chorus is a kind of low-frequency electromagnetic waves found in space, with frequencies ranging from 100 Hz to several thousand Hz. Its frequency spectrum resembles the chorus of bird vocalizations we usually hear on Earth, and that's why we call it chorus," said Cao Jinbin, dean of the School of Space and Environment, Beihang University.
The Earth's magnetic field extends into space. When charged particles in the cosmos pass the magnetic field, they can excite chorus waves, or the electromagnetic waves with frequency characteristic similar to birdsongs in the morning.
As one of the most intense electromagnetic fluctuations in space, chorus waves have been at the forefront of space physics research. It was widely believed that they only occur near Earth's dipole magnetic field regions.
"Over the past seven decades, it was widely accepted that the wave exists only in near-Earth space, in the Earth's dipole magnetic field regions near the geosynchronous orbit. No one had ever considered the possibility that the wave could exist at a greater distance," said Liu Chengming, associate professor of the School of Space and Environment.
The team, which consists of researchers from China, the United States and Sweden, has analyzed vast amounts of data collected by the Magnetospheric Multiscale Mission, a solar-terrestrial probe mission.
Chorus waves play a crucial role in understanding fundamental questions in space and have wide practical implications. They are key to the acceleration of high-energy electrons in Earth's radiation belts and the generation of pulsating auroras in the polar regions. They can also influence space weather changes, endangering the stable operation of spacecraft and the health of astronauts.
These findings provide essential theoretical support for the precise modeling and forecasting of space weather, according to the study.
Chinese scientists discover "space chorus" far from Earth
