Researchers have known for three decades that the Earth is a potent radio transmitter, but they were never able to pinpoint where the noise was coming from. By using data from the four spacecraft of the European Space Agency’s Cluster mission, NASA-funded scientists have now precisely located the source of that radio noise along magnetic field lines several thousand miles above bright regions in Earth’s northern lights.

Dr. Robert Mutel, Dr. Donald Gurnett, and colleagues from the University of Iowa have used simultaneous measurements from the quartet of spacecraft to pinpoint the location in space where auroral kilometric radiation (AKR) is emitted. AKR is a form of radio wave with a frequency just below the AM radio band (about 540-550 kilohertz).

AKR is generated by the movement of high-energy particles through Earth’s magnetic field (or magnetosphere) in connection with auroras. In preliminary findings, Mutel and colleagues have noted that the most intense AKR emissions seem to occur above the bright spots in the auroral ovals around Earth’s polar regions, where high-energy particles slam into the upper atmosphere and create the northern lights.

Mutel presented his results on December 10 at the Fall Meeting of the American Geophysical Union in San Francisco during a joint press conference with colleagues from ESA.

Using the Wideband (WBD) Plasma Wave Instruments that they built for Cluster, along with a modified form of the astronomical technique known as very-long baseline interferometry, Mutel and colleagues have been able to detect bursts of radio waves being emitted by electrons sliding down Earth’s magnetic field lines toward the auroral zones. Researchers proposed in the 1970s that AKR was likely originating about 7,000 to 8,000 miles (11,000 to 13,000 km) above Earth’s surface in the magnetosphere, but until now they were not able to make the measurements to confirm the theory.

"It is only by using independent, simultaneous measurements from multiple spacecraft that we are able to make these sorts of observations," said Dr. Melvyn Goldstein, NASA’s project scientist for the Cluster mission. "This research is exactly the sort of thing we were hoping to do with the Cluster mission."

Though AKR is not detectable from the Earth’s surface — the ionosphere blocks most radio waves from space at those frequencies — it is the most important and intense naturally occurring radio emission from Earth. Sounding like sporadic bursts of high-pitched whistles and squawks, AKR is emitted by Earth about one-third to one-half of every day at a signal strength as high as one billion Watts. The most potent commercial radio signals on Earth are only 100,000 Watts, meaning that AKR would drown out much of our AM radio signals were it not for the ionosphere.

The emission of AKR by the magnetosphere should help researchers understand the wider picture of how charged particles move between the solar wind, the magnetosphere, and Earth’s ionosphere, Mutel notes. "AKR is a tracer of the propagation of electrons from the tail of Earth’s magnetosphere and might someday help us map the diffusion of particles inward from that tail."

Studies of AKR might have applications for astronomy as well. "This is a basic physical phenomenon of many planets and planetary systems," says Mutel. Essentially, almost every planet with a global magnetic field emits some form of AKR. "The detection of AKR emission may prove to be a common, detectable property of extra-solar systems."

The four Cluster spacecraft were built and launched in 2000 by the European Space Agency. NASA has contributed hardware and research funding to the mission -- including the WBD set of instruments. More than 75 co-investigators from the United States are participating in the program.