Spotting Old Sol's Hot Flashes
The sun is showing its mean streak these days. It's at the peak of its 11-year cycle of violent events, known as the solar maximum. These temper tantrums are marked by giant sunspots, huge flares and bursts of fast-moving ionized gas. When the charged particles slam into the earth's magnetic field they create a spectacular light show of northern and southern lights.
But the charged particles can also disrupt communications, knock out satellites, damage the electric power grid, and expose astronauts and airline passengers alike to extra-high levels of solar radiation. The last time the sun acted so violently, in 1989, radiation blacked out the Hydro-Quebec electric power system, leaving 6 million people in Canada without power for more than nine hours.
That's why satellite observatories and ground-based monitoring stations around the world went on top alert in mid-March when a sunspot suddenly began growing larger. Sunspots are caused by powerful distortions in the sun's magnetic field which slows the flow of heat from the Sun's interior. Because they are slightly cooler than their surroundings, sunspots appear dark to the eye.
Sunspots usually presage stormy solar weather. By Mar. 28, the new sunspot, designated AR 9393 by astronomers and the biggest since 1991, was like a giant thunder cloud covering an area equivalent to the surface of 14 earth's. Over the next two days, four huge flares erupted near the sunspot. These can release as much energy as a billion megatons of TNT. Two of these explosions, known as coronal mass ejections (CMEs), hurled billions of tons of electrified, magnetic gas in the direction of Earth.
These clouds, travelling at speeds of approaching 5 million miles her hour, take about two days to collide with the magnetic fields surrounding Earth. The first reached Earth early on the morning of Mar. 31, triggering a geomagnetic storm that produced spectacular northern lights visible as far south as Texas and Northern Mexico. But the anticipated double whammy -- a second burst arriving nearly 24 hours later -- fizzled. Instead of intensifying the storm, the interacting magnetic fields seemed to have neutralized each other.
Although no damage was reported, astronomers believe that the show of this solar max is far from over. At the low ebb of the solar cycle, there may be one CME every few days. But at its peak, Old Sol can belch out three or more a day. Indeed, on Apr. 2, AR 9393 unleashed two more enormous solar flares that were apparently not headed toward Earth.
Here's an eerie sidelight: Astronomers have recently discovered that solar eruptions have an unsavory social habit -- cannibalism. Pictures captured by the Solar & Heliospheric Observatory (SOHO) show fast-moving solar eruptions overtaking and devouring their slower-moving kin. "Coronal mass ejection cannibalism is the most violent form of interaction between CMEs," Natchimuthuk Gopalswamy of the NASA Goddard Space Flight Center reported at a late March meeting of the European Geophysical Society.
According to Gopalswamy, collisions between CMEs change the speed at which the gas clouds travel, making it more difficult for astronomers to predict the estimated time of arrival at Earth. They may also alter the way those eruptions distort the atmosphere, leading to longer and more intense storms when they reach Earth.
So far 21, cannibalistic ejections have been identified since they were discovered by telltale radio signals in April 1997. And the NASA researchers think there may be even more events that aren't detected. "Collisions between CMEs may be more common than previously thought and may play a key role in determining the interplanetary traffic of CMEs," adds Gopalswamy.
Scientists concede that there is no way to change the sun's behavior, but many hope that they can learn to better predict such tantrums, so that valuable earthbound systems can be protected. "We are now at the stage where global weather forecasting was 30 years ago, when satellites first became available," says Bernard V. Jackson, a solar physicist at University of California, San Diego. "We discovered then that we could see hurricanes really well from a satellite, tell what direction they were going in, and predict where they were going to make landfall."
Jackson and his colleague, Masayoshi Kojima, of Nagoya University's Solar-Terrestrial Environment Laboratory in Japan, found a way to predict which CME's might hit the Earth's atmosphere. They combined four radio telescopes so they function like a CT scanner to produce three-dimensional images of fast-moving gas clouds traveling between the sun and Earth. By focusing the telescopes on powerful sources of natural radio emissions, the physicists deduce the direction of motion of these storms by the "twinkle," or variation, they produce in the radio signals.
The sun watchers also believe that the accuracy of solar weather forecasts will be improved dramatically with the scheduled launch in December, 2001, of a U.S. Air Force satellite equipped with an instrument capable of making direct pictures of solar mass ejections. "We'll get a thousand times more data from the Solar Mass Ejection Imager, and we'll be able to resolve these things by an order of magnitude better," says Jackson.
With the solar max beginning to wane, the researchers will have another 11 years to perfect their techniques. But they're likely to be better prepared when the next season of the sun rolls around.
For additional information and frequently updated solar weather reports: www.spaceweather.com
By Alan Hall in New York
Edited by Douglas Harbrecht