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SEASON OF SOLAR STORMS / The volatile sun is in the most active phase of its 11-year cycle

Published 4:00 am, Monday, May 14, 2001


  • Peak in the Solar Cycle. Chronicle Graphic
    Peak in the Solar Cycle. Chronicle Graphic


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A giant sunspot's thrashing magnetic fields blotched the brilliant face of the sun with darkness last month and unleashed a solar flare that exploded with the energy of a million hydrogen bombs.

It jetted out from the sun's surface at precisely 2:51 p.m. Pacific Daylight Time on April 2. Within eight minutes, bursts of X-rays and gamma rays began penetrating the Earth's protective magnetic field at the speed of light.

Half an hour later, an eruption of the sun's corona - its outer atmosphere -

blew a huge cloud of charged nuclear particles, mostly protons and electrons from hydrogen and helium atoms, through the solar system at almost 5 million mph.

It was one of the most violent outbursts of solar activity on record - a particularly dramatic example of the sun's electromagnetic forces at the peak of its roughly 11-year cycle of activity.

Known as the "solar maximum," the peak period began early last year and is expected to continue into 2002.

"Everything that the sun can do it's doing," said physicist Joseph Kunches, acting chief of the government's Space Forecast Center in Boulder, Colo.

The bursts of activity can disrupt electric power grids and communication links and damage orbiting satellites, as many such flares have done in the past. Last month's event, however, originated near the edge of the rotating sun, so most of its energy was directed away from the Earth. It did briefly black out many radio transmissions on the Earth's sunlit side, but otherwise it caused little discernible damage to electric sites.

The monstrous eruption that followed the flare - known as a coronal mass ejection - sent ionized gases speeding at almost 5 million mph through interplanetary space and enveloped the Earth inside a cloud of the gas particles 50 million miles across.

Its most visible effects on Earth were colorful auroral displays - the "Northern Lights," which are rarely seen south of Canada, but that shimmered in reds and greens for several nights in the skies as far south as the Bay Area to the west and North Carolina to the east.

What Earthlings were witnessing was a giant geomagnetic storm triggered first by the sunspot that astronomers call "Active Region 9393," and then by the twisting magnetic fields of the solar flare and the coronal mass ejection that followed.

Joseph B. Gurman of the National Aeronautics and Space Administration's Goddard Space Flight Center called it "the King Kong of all sunspots."

It was actually a cluster of sunspots 86,000 miles wide across a small fraction of the sun's face, but 10 times the diameter of the Earth. From the region of its magnetic fields came the flares and coronal mass ejections that followed for several days afterward, Kunches said.

Smaller flares and eruptions of electrically charged gas from the sun's corona continue even now. "A fresh sunspot has just come into view as the sun rotates," Kunches said as he tracked the data recently. "It's keeping us hectically busy trying to forecast the impact on space weather."

Kunches and his team of solar forecasters are part of the National Oceanic and Atmospheric Administration's Space Environment Center. Just as the National Weather Service uses satellites to warn of storms over land and sea, so the space weather forecasters watch the sun to warn satellite operators, electric companies and radio and television systems that geomagnetic storms are on the way.

Although not nearly as large or violent as last month's, more sunspots and flares will continue showing up as the sun rotates on its axis roughly every 27 days.

In another five or six years, the sun will quiet down during "solar minimum, " when sunspots will be fewer and geomagnetic storms on Earth much less violent.

"There's not the smallest orb which thou behold'st

But in his motion like an angel sings."

Lorenzo, contemplating the stars to his Jessica in "The Merchant of Venice, " could not realize that the sun, too, is a star, and, like the stars, the sun sings with the thrumming resonance of bass guitars and the harmonics of kettledrums.

These vibrations and pressure pulses deep inside the atmospheres of our sun,

and in more distant stars, let scientists analyze the interiors through their low-frequency sound waves, much the way scientists measure seismic waves from earthquakes vibrating through the Earth's crust or the deep tremors of lava thrusting upward from magma chambers inside volcanoes.

Scientists who measure the sun's pressure pulses call their trade "helioseismology." The 5-year-old Solar and Heliospheric Observatory, called SOHO, now stationed a million miles out from Earth, carries an instrument that monitors the solar sounds 24 hours a day.

SOHO's Michelson Doppler Imager has revealed surprising details of the sun's layered interior and has even probed all the way through it to detect reflections from the back side of the sun as it rotates.

"The new part is that we can now do it in near real time," said solar physicist Jasper Schou, a member of the imager team at Stanford University.

Schou and Philip H. Scherrer at Stanford re-examined their imager data from mid-March recently and realized that they had actually mapped the same huge sunspot that caused all the flares and mass ejections of April 2 a full two weeks earlier - while the spot itself was still on the sun's back side.

"Too bad we didn't make some noise about it," Schou lamented, for it would have been the first prediction of a major solar disturbance well ahead of time.

"Some other time, maybe . . . " he said.

A five-nation network of mountaintop telescopes known collectively as the Global Oscillation Network Group, or GONG, is also taking the sun's heartbeat. And one day, the scientists speculate, it should be possible to forecast dangerous sunspot activity weeks ahead of time.

The danger is not only real, but it has struck sharply in the recent past. A huge geomagnetic storm in March 1989 during the last solar maximum, for example, short-circuited the entire electric grid of Hydro Quebec, the big Canadian power company, and plunged all of its service area's 8 million people into darkness.

Geomagnetic storms have often crippled commercial communications satellites,

and insurance companies have paid out more than $3.4 billion for the damages, according to Sten Odenwald, an astronomer who works with NASA's IMAGE satellite team and has recently written a book on solar flares, coronal mass ejections and their effects called "The 23rd Cycle" (Columbia University Press). Astronauts aboard the International Space Station can be endangered from storms of solar protons if radiation levels are high, and when Kunches and his forecast center colleagues warn of hazards, space station crews are trained to retreat into shielded areas.

"Solar maximum" this year marks the 23rd time astronomers and physicists have possessed the sophisticated space instruments to measure and analyze each full cycle of the sun's activity. The scientists concede they will need much more understanding of what Odenwald calls its "messy physics" before they can predict events with full confidence - or perhaps even learn the full details of the source of Earth's warmth and light for the past 4.5 billion years.


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