The space environment around the Earth is not empty. It is filled with ionized particles that are known as plasmas. Space is not ‘silent’ either. Fluctuations of magnetic and electric fields and moving particles produce various kind of electromagnetic waves, which early scientists studied by listening to their audio signatures.
In the Earth’s outer radiation belt, which consists of relativistic particles moving with speeds that are close to the speed of light, one of the most beautiful “musical sounds” in space is produced, namely, chorus waves. Their existence was known even before the launch of the first satellite, because they can propagate roughly following the magnetic field to the ground. Their audio sounds like the dawn chorus of birds and thus got its name. In the space age, satellites were able to directly detect those emissions in space. Early satellite measurements 50 years ago discovered that chorus emissions typically consist of two frequency bands, the upper-band and the lower-band, separated by a gap at half electron gyro-frequency (electrons gyrate around the magnetic field). The origin of the two-band signature of chorus has since then been intensively studied by many scientists.
In 2012, NASA launched the twin spacecraft known as the Van Allen Probes, named after the discoverer of the radiation belts, James Van Allen. As part of the Living With a Star program, this mission was designed to explore the radiation belt environment and its variability. Soon after the launch, the satellite data shows that the relativistic electrons are accelerated by chorus emissions in the heart of the outer radiation belt. The origin of banded chorus has thereafter become one important question.
Recently, Dr. Jinxing Li, Prof. Jacob Bortnik and their collaborators at UCLA analyzed the data collected by Van Allen Probes, and reported that the two-band chorus waves are commonly accompanied by two separated unstable electron populations. Using computer simulations, they demonstrated that freshly injected electrons can generate one single chorus band. But the initial chorus emissions can quickly feed back to the electron populations and stabilize the electrons at medium energies (usually of 1/10 of the speed of light). This naturally divides the unstable electrons into a low and a high energy components, which then excite the upper-band and lower-band chorus, respectively.
Two-band chorus waves are also detected in the space of other magnetized planets including Jupiter and Saturn. Chorus waves are believed to play a critical role in accelerating seed electrons to relativistic energies. They also precipitate warm electrons from space into the atmosphere, and thereby contribute to diffuse and pulsating aurorae on the Earth and other planets.