Intense Solar Flare Worries Scientists

Blast of Radio Waves Larger Than Thought Possible Caused GPS Disruptions

Global Positioning System devices around the world were briefly but significantly disrupted late last year by an unexpectedly large blast of radio waves during a solar flare, researchers reported at a conference last week.

The Dec. 6 solar flare spawned an intense burst of radio wave radiation, including some at the same frequencies used by GPS hardware -- creating a "noise" that made it impossible for many receivers to continue receiving signals from GPS satellites. The effect, said Cornell University professor Paul Kintner, was "more profound and widespread than we thought possible."

The event is considered especially worrisome because it occurred during a "solar minimum," part of an 11-year cycle when sunspots and solar flares are least frequent. When the next "solar maximum" arrives around 2011, researchers said, the disruptions to GPS devices and other satellite-based navigation instruments could be far more severe.

"This radio event was 10 to 20 times bigger than anything we had measured before, or thought would reach Earth from the sun," said William Murtagh of the National Oceanic and Atmospheric Administration's Space Environment Center, an organizer of the Washington conference on the increasingly important field of space weather.

"It told us that when it comes to preparing for big solar events, we can never let down our guard," Murtagh said. "We really don't know what might be coming at us."

The impact of the December disruption on transportation and other systems remains unclear, in part because military and commercial GPS users do not like to discuss their vulnerabilities. But the researchers said many receivers were useless for 10 minutes, and some for longer periods.

GPS is widely used in navigation, surveying, siting sensitive equipment such as deep-sea oil rigs and military operations of all kinds, as well as in organizing the flow of electricity through power grids and in international financial transactions. Its use is growing quickly without much awareness of its vulnerability, several space-weather researchers said.

Scientists have known for some time that GPS -- which relies on receiving precisely timed radio signals from several of 24 orbiting satellites -- can be disrupted by electromagnetic storms created in Earth's ionosphere by solar flares and eruptions. A major electromagnetic storm at the end of October 2003, for instance, had some GPS units down for as much as 19 hours.

These storms are caused by electrically charged particles and electromagnetic fields spewed by the sun that travel relatively slowly toward Earth. As a result, space-weather forecasters can usually give GPS users several hours' to several days' warning that a disruption may be coming.

The radio waves from the Dec. 6 flare, however, traveled at the speed of light and passed quickly through the region of the atmosphere that usually blocks incoming radiation. As a result, they began jamming GPS receivers with no warning.

"The GPS signal is a relatively weak one," said Kintner, who, along with a Cornell graduate student, first described the effect of solar radio waves on GPS. "What happens when there's a solar radio blast is that the background noise to the signal increases -- like at an increasingly noisy cocktail party. Eventually, the background noise makes it so you can't hear the person you're next to."

Anthea Coster of the Massachusetts Institute of Technology's Haystack Observatory said the December radio flare was so strong that it disrupted the most sophisticated GPS receivers -- ones far more powerful than those used in many automobiles.