The Van Allen radiation belts are a key component of Earth’s magnetosphere, capturing energetic particles from the solar wind that would otherwise impact the planet, and constraining them to a narrow band of space. Although they protect Earth’s surface, the belts form a hazard for passing spacecraft electronics and crews. The effects from the concentrated radiation can damage such orbital bodies in short order. Understanding these belts is therefore key and the motivation behind NASA’s Radiation Belt Storm Probe (RBSP) twin spacecraft that launched at the end of August.
Even before RBSP, which needs a couple months to startup all its instruments, becomes fully operational, the probes have been sending tantalizing snippets of data back to Earth. The latest is a recording from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) of peak radio wave events, commonly known as chorus.
“People have known about chorus for decades,” says EMFISIS principal investigator Craig Kletzing, of the University of Iowa. “Radio receivers are used to pick it up, and it sounds a lot like birds chirping. It was often more easily picked up in the mornings, which along with the chirping sound is why it’s sometimes referred to as ‘dawn chorus.’”
Chorus is not the only sound to come out of the belts’ plasma waves. Whistlers are a well-known phenomenon first detected during World War I. Whistlers occur when lightning-produced electromagnetic waves propagate along Earth’s magnetic field lines, resulting in a high pitched whistling sound in the audible range. Chorus emissions travel in a similar way, but occur near continuously. These emissions are among the most intense plasma waves in the magnetosphere. Auroral Hiss is more localized and occurs as broad, intense emissions centered on the auroral zone. Insights into these and other radiation belt phenomenon are sure to arise as RBSP and its investigators really get to work.
Listen to Chorus recorded by RBSP on September 5: