Parting shots: just as the sun was calming down, it flared with a vengeance

Science News, July 31, 2004 by Sid Perkins

Oct. 14, 2003, dawned with a virtually spotless sun. It turned out to be the calm before the storm. During the next few days, the number and size of sunspots increased. On Oct. 18, a massive solar flare spewed from a newly visible sunspot near the eastern edge of the sun. The sun's rotation carded this Jupiter-size zone of intense magnetic activity across the solar face and, on Oct. 23, another hyperactive spot of similar size spun into view. A third blemish popped up on the sun's left cheek on Oct. 27, tripled in size by the next day, and then quickly doubled. At their largest, these three dark pocks and their smaller satellite blotches covered enough of the sun's surface to cause a dip in solar radiation measurable by Earth-orbiting satellites.

Over an exceptionally active 18-day period that ended Nov. 5, these sunspots spewed almost a year's number of flares, ejecting massive torrents of charged particles into space to race outward along the sun's magnetic field lines. Space probes throughout the solar system detected these gusts in the solar wind (SN: 6/5/04, p. 366).

Material from several flares swept past Earth, triggering geomagnetic storms that disrupted high-frequency radio communications, knocked out satellites, and caused a power outage in Sweden. Passengers on some high-flying aircraft received increased amounts of radiation, and astronauts on the International Space Station had to occasionally take shelter in protective areas of the facility's service module. The northern lights, which usually unfold only at high latitudes, enthralled sky gazers as far south as Houston.

Nine months later, scientists are compiling their initial analyses of the record-setting barrage of flares. They're beginning to present their findings at scientific meetings. A fleet of instrument-laden spacecraft recorded the reams of raw data. Staring unblinkingly into the sun's glare, these probes collect information that's vital for creating and refining models of solar activity.

Getting better at predicting how the sun will behave has become far more than a goal of academic solar physicists. Never before have so many people, services, and assets depended on the reliable operation of satellites. And better predictions of space weather may protect space travelers from solar tantrums, an important factor if people are to travel to the moon and Mars.

SOLAR WALLOP Although they appear dark, sunspots glow brighter than an arc welder's torch. At a temperature of around 3,500[degrees]C, they look dim merely in comparison with the rest of the sun's surface, which radiates more energy because it's about 2,000[degrees]C hotter.

Sunspots usually occupy only a minuscule portion of the sun's surface. Not so for the three massive sunspots that appeared late last October and were dubbed 484, 486, and 488 by scientists who track such blotches and their activity. Together, the spots covered nearly 0.8 percent of the sun's face, says Thomas N. Woods of the University of Colorado in Boulder. According to satellite data, total solar irradiance--the amount of the sun's radiation reaching the top of Earth's atmosphere--dropped as much as 0.34 percent, Woods and his colleagues report in the May 28 Geophysical Research Letters.

The sunspot trio also spawned an extraordinary number of large solar flares. During the previous two solar cycles, which together stretched from mid-1976 to mid-1996, one modest-size, or M-class, solar flare occurred every 2 days on average, and one extreme, or X-class, flare burst forth each month or so. During the most intense phase of last fall's barrage, the sun belched out 44 M-class fares and 11 X-class ones in just 18 days.

"When the [solar] activity took off, it took off with a vengeance," says Woods.

The flare that had the largest effect on Earth emanated from Sunspot 486, which was near the center of the sun's disk when it erupted on Oct. 28. Although visible radiation from the sun didn't rise appreciably, emissions at shorter, high-energy wavelengths increased substantially. At extreme ultraviolet wavelengths between 27 and 115 nanometers, radiation doubled. At UV wavelengths below 10 nm, irradiance was about 50 times the amount measured just before the flare. At X-ray wavelengths between 0.1 and 0.8 nm--the window of radiation that researchers traditionally analyze to rate solar flares--radiation skyrocketed to about 570 times the pre-flare value. That radiation intensity placed the Oct. 28 event in the category of X-17 flares. Only two flares in the two previous solar cycles had been that strong.

Traveling from the sun at the speed of light, the X-ray and extreme ultraviolet radiation resulting from the flare from sunspot 486 took a little more than 8 minutes to reach Earth. Unlike visible light, photons at those wavelengths don't penetrate very far into the atmosphere.

The flare was "like the flashbulb on a camera," dumping much energy in a short time, says Woods. That energy stripped electrons from atoms of high-altitude gases, heating the atmosphere, causing it to expand farther into space, and adding to the drag on low-orbiting satellites. On Earth's sunlit side, the density of free electrons in the ionosphere--the atmospheric layer that lies at altitudes above 75 kilometers or so--abruptly jumped about 25 percent, thereby disrupting high-frequency radio communications whose signals travel over the horizon by bouncing off this layer.