Big broadcast: solar radio bursts put a new wrinkle in space weather.

When twisted magnetic fields on the sun snap and unleash their energy, it's the most ostentatious fireworks that grab the headlines. Brilliant explosions in the sun's outer atmosphere can send billion-ton clouds of charged particles speeding towards Earth, where they can short-circuit electrical power grids and cause large-scale blackouts. The sudden release of magnetic energy can also generate solar flares, which pour a torrent of ultraviolet light and X rays into space. Solar flares can disable satellites and harm space-walking astronauts.

But a subtler kind of solar explosion has often gone under the radar. It involves powerful bursts of radio waves that often accompany solar flares. At 2:30 p.m. EST on Dec. 6, 2006, about an hour after a moderately energetic flare erupted, the sun emitted the most powerful burst of radio waves ever recorded. During a high-intensity blitz that lasted more than 10 minutes, the storm swamped the entire sunlit side of Earth with radio noise. Across North and South America and parts of the Pacific, it overwhelmed dozens of radio receivers linked to the Global Positioning System (GPS). The network of GPS satellites provides critical distance and time information for everything from airplane navigation to maintaining the critical alignment of oil rigs as they drill into the seafloor.

The U.S. military reported a "widespread" loss of GPS signals in New Mexico and Colorado.

Although the storm caused no casualties, "the effect on GPS receivers was more profound and widespread than expected," says Paul Kintner, an electrical engineer and computer specialist at Cornell University. The storm was especially surprising because it happened when the sun, near the minimum of its 11-year activity cycle, was relatively calm.

"Now, we're concerned more-severe consequences will occur during the next solar maximum," Kintner adds.

Radio astronomer Dale Gary of the New Jersey Institute of Technology in Newark says that the Dec. 6 event challenges scientists' assumptions about how often, and when, the sun can interfere with GPS and other wireless communications.

Gary, Kintner, and their colleagues described their findings during an April conference on space weather held in Washington, D.C. Says space physicist Anthea Coster of the Massachusetts Institute of Technology's Haystack Observatory in Westford, Mass., "It's obvious that society is increasingly dependent on space-based technology, so it's important to understand why this burst occurred and to be able to quantify the power and potential impact of future radio bursts."

RADIO PUZZLES Solar flares and radio bursts usually occur in tandem, emerging from the same region on the sun. Compared with flares, however, radio bursts are much more difficult to predict and track, notes radio astronomer Don Gurnett of the University of Iowa in Iowa City.

Both kinds of outbursts arise within sunspots, dark areas threaded by strong magnetic fields. The spots look dark because the powerful magnetism acts as a lid, preventing heat and light from rising to the sun's surface.

Magnetic fields twist and tangle in response to the sun's rotation. When they break apart and reconnect, they release vast amounts of heat and radiation. Flares are direct products of that energy discharge. One of the most common types of radio burst, however, results from a more complex process.

First, the strong electric fields associated with a flare accelerate electrons that freely circulate in layers of hot, ionized gas, or plasma, above the sun's surface. Beams of these accelerated electrons, speeding away from the surface, then slam into the plasma that forms the sun's atmosphere, or corona. Just as the stream of air in a flute produces vibrations at specific frequencies, the electron beams striking the background plasma set up oscillations known as plasma waves.

Next, the plasma waves generate bursts of radio waves both at the frequency of the vibrating plasma and at twice that frequency. The bursts gradually shift to lower frequencies as the electron beams travel higher in the sun's atmosphere, where they encounter plasma of progressively lower density.

The Dec. 6 event appears to have followed this general pattern, but data show little correlation between the strength of a flare and the severity of the subsequent burst. The radio storm caught everyone by surprise.

"We don't know the physics well enough to be able to predict when bursts are going to happen and how large they can be," says Gary. But he points out one clue to the strength and duration of the burst: The so-called active region from which the burst emerged appeared to have contained several adjoining bundles of magnetic fields. Gary speculates that the north and south poles in some of those bundles disconnected and then connected to opposite poles in adjoining regions, unleashing energy.

The bundle of fields may also have served as a highly efficient trap for the energized electrons within the region. Bottled up within these tangled fields, the electrons would be accelerated for an extended period before escaping and might end up creating radio waves of higher intensity and longer duration. He notes that on Dec. 13 and Dec. 14, the same sunspot region that produced the Dec. 6 event generated significant but weaker radio bursts.

ANATOMY OF A BURST During the Dec. 6 outburst, the radio output from the sun increased by a factor of 20,000 over its intensity just before the storm began, according to measurements from the Owens Valley Solar Array near Bishop, Calif., a network of radio telescopes devoted to observations of the sun. Unlike most other networks, the array monitors solar radio signals that have frequencies similar to those broadcast by GPS satellites and also the same polarization. The array therefore provides one of the best measures of how radio emissions from the sun can interfere with--or overwhelm--the relatively weak GPS radio signals.

Still, scientists didn't immediately recognize the storm as a record breaker, notes Gary. That's in part because the Air Force's Radio Solar Telescope Network reported much lower values than did the Owens Valley array. Only later did scientists realize that because of a software error, the Air Force array had measured only the beginning of the storm, Gary says. Previous errors have caused the Air Force to underestimate solar radio storms at least twice in the recent past, he adds.

The Dec. 6 event interfered with the reception of both frequencies transmitted by GPS satellites: 1,575.42 megahertz (MHz), dubbed L1, and 1,227.6 MHz, dubbed L2. Many receivers that use only the L1 frequency continued to provide accurate guidance for navigation, notes Richard Langley of the University of New Brunswick in Fredericton, because that signal is stronger and less sensitive to noise.

Systems that rely on both frequencies for high-precision distance, time, and navigation measurements suffered the greatest losses. To obtain complete navigational information, a GPS receiver must collect signals from at least four satellites. Within the International Global Satellite System (IGS) Service, a network devoted to scientific research, the number of receivers able to lock on to the signals from four satellites declined from 120 to 60.

The military reported that in the Four Corners area of New Mexico and Colorado, several aircraft lost GPS signals. The number of satellites that these aircraft tracked dropped from between seven and nine to only one or none, Langley reports in the May GPS World.

NAVIGATIONAL FUTURE It's difficult to know the extentto which radio bursts have hampered communications over the past 2 decades, says Gary. That's because scientists have had only limited access to GPS data during previous solar storms. Gary and other researchers are trying to correlate past solar activity with measurements taken by solar arrays.

Solar radio bursts can also hamper cell phone communications. But they affect only cell-tower broadcasts occurring at sunrise or sunset, when the sun is low on the horizon. That's because the antennas on those towers direct their signals horizontally, from one tower to another, and don't detect noise coming from high in the sky. Therefore, the researchers are planning to review the performance of cell-phone towers that experienced solar radio bursts during sunrises and sunsets.

Alessandro Cerruti of Cornell University raised a red flag about the dangers of radio bursts nearly 2 years ago. On Sept. 7, 2005, the Owens Valley array recorded a relatively low-level radio storm, and Cerruti documented for the first time the loss or degradation of signals by several GPS receivers. But he never expected that a storm as large as the Dec. 6, 2006 event would come so soon, during a period of minimum solar activity.

What can be done to lessen the impact of radio bursts? Patricia Doherty of Boston College has noted that a civilian aviation-navigation system, the Wide Area Augmentation System (WAAS), operated successfully throughout the burst's duration, albeit with greater-than-usual noise levels. WAAS receivers are often installed in radio-noisy environments, such as airports and air-traffic-control centers. Unlike the receivers used for GPS, WAAS receivers are built to reject extraneous radio signals, says Doherty.

"Apparently, the radio-noise rejection in the WAAS receivers made them more stable under the influence of the solar radio burst, Doherty says. "The stability during this event provides a very good lesson for receiver manufacturers and GPS-based networks that must maintain continuous operation," she adds. It would be a large and costly job to upgrade all 347 active IGS stations worldwide, but Doherty argues that some kind of long-range upgrade could be a solution. That's because the receivers have limited lifetimes, so must be replaced eventually.

Another possibility would be to increase the signal strength of GPS satellites so that they would be less easily drowned out by the sun, notes Langley.

Broadcasting over a selection of frequencies greater than just L1 and L2 could also increase the odds that a radio storm would not dramatically interfere with GPS operations. A U.S. satellite transmitting a third GPS signal, in addition to L1 and L2, is scheduled for launch in 2008.

Increased reliance on a navigational system that broadcasts radio signals from the ground rather than from satellites could also lessen the impact of solar storms. Such a system, currently known as LORAN-C (Long-Range Navigation), has been in place since World War II. The U.S. government had been considering a phaseout of this system, but Langley says that if it's retained, it could play a key role in future navigational systems.

At Owens Valley, Gary has proposed building a more sensitive set of radio telescopes that would better pinpoint the location of radio bursts and their development within the morass of sunspots, dense magnetic fields, and flares from which the bursts emerge. By determining where within a sunspot a radio burst is formed, and by mapping the burst's structure, astronomers may be able to warn GPS users when an intense storm is about to erupt, Gary says.

That capability would come none too soon. The next solar cycle is expected to peak in 2012, and some experts say that it could be 30 to 50 percent stronger than the current cycle.

"I'm not trying to be Chicken Little," says Langley. He notes that the Dec. 6 event was of little consequence for most GPS users and the public. Even so, he adds, the storm's 10-minute duration and strength highlight the vulnerability of a world increasingly dependent on space-based technology.

GPS Network: Satellites know exactly where they are

The Global Positioning System (GPS) network provides information on position that is accurate to within 10 to 20 centimeters anywhere in the world. Orbiting about 20,000 kilometers above Earth, each of the 30 GPS satellites transmits a unique digital code--a pattern of 1s and 0s--that is timed by an atomic clock. A support system of ground stations stays in constant communication with the satellites. By measuring the time that it takes for radio signals to travel between the satellites and the ground stations, the system keeps track of the satellites' exact positions. A GPS receiver that tunes into signals from the satellites-normally, from four simultaneously--can then work out its own position.

--R.C.