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Release: Dec. 19, 2001

Photo: A computer-rendered image of the European Space Agency's revolutionary Cluster mission to explore near-Earth space and study the interaction between the Sun and Earth.

UI researchers capture 'bird's-eye' view of Earth's radio noise

IOWA CITY, Iowa -- Two University of Iowa researchers and their colleagues have found a novel way to remotely pinpoint the source of Earth's most intense, naturally occurring radio noise.

Robert Mutel and Donald Gurnett, professors in the UI College of Liberal Arts and Sciences department of physics and astronomy, presented their findings on the radio noise, called auroral kilometric radiation (AKR), at the American Geophysical Union (AGU) Dec. 10-14 annual meeting in San Francisco.

Mutel and Gurnett used University of Iowa-designed-and-built wideband plasma wave (WBD) instruments carried aboard the European Space Agency’s four-satellite mission called Cluster. The four UI instruments, each roughly the size of a toaster, simultaneously collect data as though they are a single, large telescope -- a technique called long baseline interferometry. The results show AKR being emitted along magnetic field lines about 3,000 miles above bright regions in the Earth's northern lights.

"Our new technique, using multiple spacecraft from above, allows us to image the sources of AKR as if taking a picture from well above the AKR region," says Mutel, who notes that a previous satellite detected AKR at the source several years ago. "This allows us to study many aspects of AKR emission that cannot be done with observations at the source, such as determining the motion of AKR sources."

Mutel says that the emission of AKR by the Earth's magnetosphere should help researchers better understand how charged particles move between the solar wind, the magnetosphere, and Earth’s ionosphere, as well as learn more about other planets. "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."

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 and having a frequency just below the AM radio band (about 540-550 kilohertz), 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 short-wave radio signals on Earth are about one million watts, meaning that AKR would drown out much of our AM radio signals were it not for the ionosphere.

The movement of high-energy particles through Earth's magnetic field (or magnetosphere) generates AKR in connection with auroras. In preliminary findings, Mutel and his colleagues note 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.

The UI instruments collect data on plasma waves generated in the Earth's magnetosphere, a teardrop-shaped region of energetic particles trapped in the magnetic field surrounding the Earth. Each second, the sun releases more than one million tons of electrons, protons and other particles to form a very thin gas (called plasma) that makes up the solar wind. When a large cloud of these particles reaches the Earth following a solar storm, the "space weather" they create can cause such phenomena as magnetic storms and the northern lights, as well as electrical surges in power lines. In addition to principal investigators Mutel and Gurnett, the UI team includes professors Craig Kletzing and Steven Spangler, as well as project manager Rich Huff, project engineer Don Kirchner and science manager Jolene Pickett. As the only U.S. experiment aboard Cluster, the UI WBD project represents over 10 years of work and more than $4 million in NASA funding.

The 10 other scientific experiments carried by Cluster were built by Great Britain, France, Germany, Austria and Sweden. The four Cluster spacecraft were built and launched in 2000 by the European Space Agency. More than 75 co-investigators from the United States are participating in the program.

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