SAN FRANCISCO--The northern lights are just one manifestation of the magnetic and electrical frenzy sparked in Earth's upper atmosphere by solar storms; most of those intense currents were always to remain invisible. But now, researchers have devised a way to map the electrical power flowing through the planet's ionosphere. The technique, described here on 15 December at a meeting of the American Geophysical Union (AGU), relies on a costly network of communication satellites.
Earth's magnetic field creates a comet-shaped cavity in the solar wind, a constant stream of charged particles from the sun. Changes in the wind's intensity ripple this vast bubble "like a wind sock," says atmospheric physicist Brian Anderson of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. Those surges generate 200,000-volt currents in the ionosphere that surrounds Earth; most of that energy funnels toward the north and south poles. To calculate how much power surges through these circuits, researchers must measure both the electric and magnetic fields far overhead. A network of a dozen ionospheric radars tracks the electric fields over the poles, but nothing could measure the magnetic fields at high latitudes.
The solution came from the Iridium network, a $5 billion constellation of 72 satellites that provided global telephone service to Motorola customers. The network failed to attract enough users, and the operating company--Iridium LLC--declared bankruptcy. Last week a new company, called Iridium Satellite LLC, revived the network for at least two years with a $72 million contract from the Department of Defense. That's good news for Anderson and his colleague Colin Waters of the University of Newcastle in Australia, who had figured out how to use Iridium data to map the polar magnetic fields.
Each satellite orbits from pole to pole at an altitude of 780 kilometers and carries a magnetometer, which detects magnetic-field strength. When Anderson and Waters combined those readings with the radar data, they could chart maps of the electrical power flowing into and out of the ionosphere. Typical power levels are 40 billion watts, although they flicker wildly depending on the sun's mood. Much power focuses within "hot spots" that ebb and flow from hour to hour. Faster data transfer would allow new power maps every 10 minutes or so, Anderson says, possibly leading to a system to warn electrical utilities and satellite operators about approaching hot spots.
"This demonstrates the feasibility of detecting the magnitudes and locations of electrical currents on a global basis," says Robert Robinson, program director for magnetospheric physics at the National Science Foundation. "We're very pleased with this clever and innovative use of the Iridium satellite system."