Fury on The Sun

By LEON JAROFF

Strolling outside Arizona's Kitt Peak National Observatory during a work break, staff observer Paul Avellar at first thought the angry red glow in the night sky was caused by forest fires. Then, seeing a greenish fringe and vertical streamers stretching like ribbons above the horizon, he realized what was happening. He raced to a telephone and called his wife and friends, awakening them and insisting they share the view. "A chance like this doesn't come along very often," says Avellar. "To see the northern lights is very humbling and awe-inspiring. You realize the sun is just going about its business and making our nighttime sky glow without any trouble at all. It makes you wonder what would happen if the sun ever really got mad."

Some 93 million miles away, the sun was, at the very least, agitated. In early March, an area of sunspots large enough to contain 70 earth-size planets had come into view around the eastern rim* of the glowing orb. Created by intense magnetic fields and cooler than the surrounding gases, the sunspots were visible as dark blemishes on the fiery surface. Just as astronomers were turning their attention to the mottled region, a bright spot suddenly appeared in its midst. It spread like a prairie wildfire, glowing white hot on the sun's yellow face and quickly expanding to cover hundreds of thousands of square miles. The monster blotch was an unusually large solar flare, a stupendous explosion that belched radiation and billions of tons of matter far into space.

The great flare, and its coterie of sunspots, was an unmistakable signal. It heralded the imminent arrival of the solar maximum: the period every eleven years or so when the sun reaches its peak levels of activity and pointedly reminds earth dwellers of its awesome power. At maximum, the sun

bombards the planet with radiation and particles, causing unusually brilliant auroras, communications blackouts and power failures. But it also gives scientists a fresh opportunity to solve some of the mysteries surrounding the star that provides the earth with energy, drives the weather and sustains life itself.

During a maximum, marked by a jump in the number of sunspots and flares, giant loops of incandescent gases, called prominences, proliferate, shooting tens of thousands of miles above the solar surface, sometimes hanging suspended for months. The solar corona, the halo around the sun visible during total eclipses, becomes fuller and brighter; great blobs of the corona, containing billions of tons of hot gas, occasionally burst free, shooting into space at speeds as high as 2 million m.p.h. And the earth's upper atmosphere, pummeled by solar particles, is laced by electrical currents of as much as a million amperes. These in turn create powerful magnetic fields that raise havoc below.

Because the previous maximum occurred in late 1979, astronomers had targeted 1991 as the year when solar frenzy would again peak. But the sun is notably capricious. While the intervals between maximums average eleven years, some have been as short as seven, others as long as 17. Ever since the sun began revving up three years ago toward the next maximum, its activity has mounted with unprecedented speed.

"It is the fastest riser on record," says Ron Moore, an astronomer at NASA's Marshall Space Flight Center in Huntsville, Ala. So fast, in fact, that astronomers are betting on 1990 or perhaps even later this year, instead of 1991, as the beginning of the maximum. And what a maximum it could be. Despite the ferocity of the March flares, Moore warns, "this cycle is still in its early phase. It's got quite a way to go." Solar buffs are speculating it might approach the violence reached by the 1957-58 maximum, which touched off five disruptive geomagnetic superstorms and vivid auroral displays. Says astronomer Donald Neidig at the National Solar Observatory outpost on Sacramento Peak, near Sunspot, N. Mex.: "We can't rule out a record breaker."

In anticipation of the fireworks, astronomers scheduled a two-week, worldwide solar-observation period during the second half of June. The project was timed to benefit from the observations of the Solar Maximum Mission satellite (nicknamed Solar Max) before it plunges to its death. Lofted into earth orbit in 1980 to monitor the sun's activity, the satellite is gradually descending and will probably re-enter the earth's atmosphere in November and be incinerated. Solar Max's readings of the sun's activity were coordinated with observations made all over the world by ground-based telescopes and instruments mounted on high-flying rockets. A hundred solar centers around the globe were linked by an electronic-mail network designed to provide the latest data on the sun's behavior.

A major goal of the project was to catch a flare in the act, mapping all the solar high jinks associated with it from beginning to end. The sun's timing could not have been better. During the first week of observations, it set off several large flares and ejected billions of tons of matter in a prominence that extended more than 200,000 miles into space.

The intense solar observations should provide clues to many of the still unanswered or only partly resolved questions about the sun: Does the solar cycle affect terrestrial weather? What internal mechanisms control the cycle? Is the sun growing cooler? Hotter? Is there a basic flaw in the current theory about the fusion process that powers the solar furnace?

While the recent flares did not measure up to the March conflagration, astronomers were jubilant. "We have been exceptionally lucky," says Alan Kiplinger, a solar physicist at the University of Colorado. "It's unusual to have the sun cooperate."

Fortunately for earth dwellers, the March flare occurred on the easternmost edge of the sun and thus aimed its full force away from the earth. But on March 10, when the sun's stately rotation brought the turbulent group of sunspots to a position more directly facing the earth, a second, only slightly less powerful flare erupted in the region. Eight minutes later, traveling at the speed of light, a blast of X ray and ultraviolet radiation seared the earth's upper atmosphere. Within an hour, high-energy protons began to arrive, followed in three days by a massive bombardment of lower-energy protons and electrons.

Among the first to feel the effects of the flare's fury was the orbiting Solar Max. As the radiation saturated Solar Max's instruments, a NASA spokesman reported, "the satellite was stunned for a minute and then recovered." Heated by the incoming blast of radiation, the upper fringe of the atmosphere expanded farther into space. Low-orbiting satellites, encountering that fringe and running into increasing drag, slowed and dropped into still lower orbits. A secret Defense Department satellite began a premature and fatal tumble, and the tracking system that keeps exact tabs on some 19,000 objects in earth orbit briefly lost track of 11,000 of them. Solar Max descended by as much as half a mile in a single day, almost certainly hastening its demise.

On the earth, the flare's effects were equally disruptive. Shortwave transmissions were interrupted, some for as long as 24 hours, and satellite communication and a Coast Guard loran navigation system were temporarily overwhelmed. Powerful transient magnetic fields, generated in the upper atmosphere by the flare, induced electrical currents in transmission lines and wiring, and mystified homeowners reported automatic garage doors opening and closing on their own. A surge of flare-induced current was blamed by Hydro- Quebec officials for shutting down the power company's system and blacking out parts of Montreal and the province of Quebec for as long as nine hours. These startling phenomena were shrugged off by Sacramento Peak's Neidig. "A really big flare," he says, "can produce enough energy to supply a major city with electricity for 200 million years."

By far the most dramatic manifestation of the solar flare was the two-night, spectacular display of the aurora borealis, or northern lights, that awed Paul Avellar and millions of others. Arriving high-energy electrons, deflected by the earth's magnetic field, spilled into the upper atmosphere near the north and south polar regions, which are unprotected by magnetic-field lines. Acting much as does the electrical current in a neon sign, the electrons banged into oxygen atoms, causing them to emit red and green light.

Ordinarily far less intense and visible only in arctic climes, the glowing, flickering aurora was seen as far south as Brownsville, Texas, and Key West, Fla. Alarmed Floridians, unfamiliar with the lights and fearing that a catastrophe had occurred somewhere in the north, flooded police switchboards with calls.

The two great flares of March were not isolated events. Nine other major outbursts and hundreds of smaller ones were recorded during the two weeks it took for the sunspot region to rotate out of view. In the months since, as the sun moves erratically toward its maximum, several flares have been observed every day.

The sun has long been pre-eminent in human thoughts and actions. Almost from the beginning, people worshiped the sun as the beneficent provider of light and life, and as a god, called Ra by the Egyptians, Helios by the Greeks and Sol by the Romans. To the Aztecs, the sun god was Huitzilopochtli, whom they nourished with human sacrifices. Egypt's great pyramids at Giza were built with their sides aligned with the rising sun at the vernal equinox, and the temple complex at Karnak was dedicated to Ra. The ancient circle at Stonehenge, in England, was apparently constructed so that the sun would rise over one of the great stones at the time of the summer solstice.

From the beginnings of history and literature, human beings have also invoked the sun. In rejecting peace offers from Darius before the battle of Gaugamela, Alexander the Great explained, "Heaven cannot brook two suns, nor earth two masters." And in 1911, Kaiser Wilhelm II of Germany, speaking of his nation, declared, "No one can dispute with us the place in the sun that is our due."

Through the centuries, few natural phenomena have inspired as much fear and awe as solar eclipses. The ancient Chinese used firecrackers and gongs to drive away the spirit they thought was devouring the sun. Mark Twain's Connecticut Yankee, aware that a most timely total eclipse was going to occur, escaped being burned at the stake by King Arthur's knights when he predicted that the sun would disappear. A benign form of sun worship continues to this day, not only among beachgoers but also by a group of intrepid American astronomy buffs who have traveled around the world by plane, ship and jeep, from Java to Siberia to Africa, to view each of the past dozen total eclipses.

Even in ancient times, however, an occasional hardy soul refused to deify the sun. The Greek philosopher Anaxagoras brazenly claimed that it was merely a ball of fiery stone, and was arrested and banished from Athens for his blasphemy. But his radical concept caught on and was later refined by Aristotle, who proclaimed the sun an unchanging sphere of pure fire, devoid of any imperfections.

Aristotle's view prevailed through the Middle Ages, was embraced by Christianity and went largely unquestioned until Galileo and other early 17th century sky watchers pointed the newly invented telescope at the sun and saw black spots on its surface. So much for solar purity. Despite clerical disapproval, the reality of sunspots was quickly accepted. Still, more than two centuries passed before Samuel Heinrich Schwabe, a German apothecary and amateur astronomer, discovered the strange, cyclic behavior of the solar blemishes.

Schwabe had been searching for the hypothetical planet Vulcan, supposedly the closest one to the sun, hoping to spot it in silhouette as it moved across the solar disk. In the process, he observed and kept meticulous records of sunspots over a 17-year period. Finally, in 1843, he recognized and announced the eleven-year cyclic nature of the spots and wrote, "I may compare myself to Saul, who went to seek his father's ass and found a Kingdom."

In the years since, by tabulating sunspot records going back to the early 18th century and using improved telescopes, satellites, advanced instruments and modern theory, scientists have become ever more familiar with the bizarre dance of the sunspots. Each cycle begins when spots show up in both the northern and southern hemispheres about 35 degrees away from the solar equator. As the cycle matures and the older sunspots fade away (some last only a few hours, others for weeks and even months), new and more numerous spots appear at lower latitudes. Toward the end of the cycle, diminished in number, they appear at latitudes some 5 degrees from the equator.

Sunspots tend to travel in pairs or groups of opposite polarity, like the ends of horseshoe magnets poking through the solar surface. During one eleven- year cycle, as the blemishes traverse the face of the sun in an east-west direction, the leading spots of each group in the northern hemisphere will generally have positive polarity, the trailing spots negative. In the southern hemisphere, the leading spots will be negative. During the next cycle, the hemisphere polarities will reverse. On average, then, 22 years will pass between solar maximums of the same sunspot polarity. This suggests to many astronomers that the fundamental solar cycle is 22 years rather than eleven.

Since the sun in myriad ways governs the very existence of all terrestrial life, the cyclic changes in the sunspot population have, ever since Schwabe, inspired speculation about their effect on solar radiation and, consequently, on the earth. Though the sun is a rather ordinary star, its vital statistics are breathtaking by earthly standards. Some 865,000 miles in diameter, it consists largely of hydrogen (72%) and helium (27%) and is 333,000 times as massive as the earth. Solar temperatures range from about 27 million degrees F* in the core, where 600 million tons of hydrogen are fused into helium every second, to 10,000 degrees F on the photosphere, or surface.

Like a giant nuclear-fusion furnace in the sky, the sun radiates stupendous amounts of energy. Some of it departs in the form of speeding particles, mostly electrons and protons, that form a solar wind blowing from the sun in all directions. It is this continuously flowing wind that feeds particles into the earth's Van Allen radiation belts and distorts the terrestrial magnetic field into a teardrop shape. It also sets off the frequent minor auroral displays visible at higher latitudes.

Also radiating from the solar surface is energy in the form of visible light, ultraviolet and X rays. Enough of this energy penetrates the atmosphere to deliver some 100 trillion kW of power to the earth. Reduced to more comprehensible terms, solar radiation amounts to 1.35 kW falling on every square meter of earth, a number that scientists call the solar constant.

It is this vital sunlight that provides the energy for photosynthesis, the process plants use to produce carbohydrates for sustaining their growth. And it is solar energy -- stored in ancient plants that have become today's fossil fuels -- that powers factories and runs automobiles. Sunlight also drives the earth's weather system, supplying the heat that causes atmospheric circulation and evaporates seawater to form clouds and rain. It bombards oxygen in the atmosphere, converting it into the ozone that, paradoxically, screens out much of the sun's lethal ultraviolet radiation. The ultraviolet that does reach the earth gives sun worshipers their tans and, taken in excess, their skin cancers.

Sunlight, so far, has been most reliable. For more than 3.5 billion years it has kept global temperatures within the narrow range necessary to sustain life. Even small changes in solar-energy output could have a profound influence on the planet. A long-term variation in global temperatures of only a few degrees could melt the ice caps, inundating coastal cities -- or bringing on a new ice age.

But is the solar constant a misnomer? Is the sun's energy output really constant, or does it vary with the ebb and flow of sunspots? And can such short-term changes have any significant effect on terrestrial existence? "This has always been an area where cranks and charlatans thrive," says Harry van Loon, at the National Center for Atmospheric Research (NCAR) in Boulder. "Ever since the sunspot cycle has been known, it's been correlated with every darn thing: the length of women's dresses, the number of polar bears, plague in India, even, for a time, the number of Republicans in the U.S. Senate."

Still, some of the coincidences cannot be easily dismissed. One of the most striking involves the Maunder minimum, named for the British astronomer who, after a search of old records, concluded that only a handful of sunspots had been observed during the 70-year period between 1645 and 1715. That long interval of apparently minimum solar activity happened to coincide with one of the coldest stretches of the "little ice age," the period from the mid-15th to the mid-19th centuries. During that chilly interval, many glaciers advanced farther south than they had in 10,000 years, conquistadores walked their horses over the frozen Rio Grande, and the English Channel became the site of winter skating festivals.

In a 20th century follow-up on Maunder's work, John Eddy, at the National Center for Atmospheric Research, also scoured historical records and discovered two other signs of a slackening of solar activity during that 70- year period: auroras were extremely rare, and the corona, the radiant white halo that ordinarily becomes visible during total solar eclipses, could be seen only as a dull, reddish ring of light.

To confirm evidence of the sun's strange hiatus, Eddy turned to one of nature's most reliable forms of record keeping -- tree rings, each of which represents a year of growth. In the rings is a record of cosmic rays, high- speed particles from outer space that constantly collide with molecules in the atmosphere, creating a radioactive isotope called carbon 14. When trees assimilate carbon dioxide through photosynthesis, some carbon 14 is also incorporated.

Analyzing tree-ring data from 5,000-year-old living bristlecone pines and even older dead ones, Eddy reported in 1976 that their carbon-14 content seemed to vary in rhythm with sunspot numbers. When sunspots were rare, as they were during the Maunder minimum, the amount of carbon 14 in the tree rings increased markedly; when they were numerous, the amount decreased. The explanation: during the sun's more active periods, its magnetic field, which ordinarily deflects some cosmic rays away from the earth, expands and becomes an even greater barrier to the rays. As a result, less carbon 14 is created in the atmosphere and less finds its way into trees.

Eddy's tree-ring data revealed other 50-to-100-year intervals in the past when carbon-14 production was high and the sun apparently quiescent. But did this mean that all of these periods were times of extreme cold? Many scientists doubted it, suggesting that the correlation between the Maunder minimum and the little ice age might be nothing more than sheer coincidence. Changes in solar cyclic activity, the doubters argued, were not necessarily accompanied by variations in the sun's output of heat and light and probably did not affect terrestrial weather and climate.

Solar Max has undermined those arguments. A sensitive radiometer aboard the satellite has confirmed that between 1980 and 1986 average solar output declined one-tenth of 1%, then leveled off, and now has begun to climb. The finding strongly suggests that solar radiation varies with the sunspot cycle and that the solar constant is not that constant after all.

But is so small a cyclic change able to have a noticeable effect on weather? Two scientists suspect it may. Karin Labitzke of Berlin's Free University and NCAR's Van Loon have discovered a relationship between the solar cycle and the stratospheric winds over the tropics. During a 28-month period, these winds reverse direction, blowing half the time from the east, the other half from the west, a phenomenon meteorologists call the QBO, or quasi-biennial oscillation. Depending on the direction of the QBO flow, Labitzke and Van Loon found, solar maximums and minimums seem linked to changes in air pressure, temperatures, the number of storms and perhaps even the size of the notorious hole in the Antarctic ozone layer.

"It's pure statistics," Van Loon concedes. "We have no physical explanation for what we've found." That explanation may be hard to come by. Experts have calculated that the tiny change in the solar constant detected by Solar Max can supply less than a millionth of the energy needed to produce the observed changes in weather. "If there really is an effect," says Van Loon, "there must be an enhancing mechanism, and we don't have the foggiest idea of what that enhancing mechanism might be." Yet the statistical evidence is so compelling that many scientists are taking it seriously. The QBO data have persuaded meteorologist Anthony Barnston, of the National Climate Analysis Center, to incorporate the solar cycle into the computer algorithms for his monthly and 90-day seasonal forecasts.

While astronomers who study the sun get more attention during periods of solar maximums, they generally feel somewhat neglected, underfunded and unappreciated, poor cousins to those who observe distant stars and galaxies in the night skies and who consider the sun boring. Then why do solar astronomers persist? "We are driven to an understanding of the sun," says Robert Howard, an astronomer at the National Solar Observatory in Tucson. "It is an enormous lab. It is a Rosetta stone for the study of the stars. With other stars, all you have is a pinpoint of light. By understanding more about the sun, we can learn more about the distant stars."

While all that may be true, says Caltech physicist Robert Leighton, "if the sun didn't have a magnetic field, it would be as dull as most nighttime astronomers think it is." What a difference a field makes. Twisted and stretched by both the sun's rotation and its roiling interior, the magnetic lines of force orchestrate the intriguing solar cycle.

Most explanations of that phenomenon liken the sun to a dynamo. Mighty currents of electricity flowing in the solar interior generate magnetic-field lines that, like the earth's, tend to be oriented in a north-south direction. But because the sun, unlike the earth, is gaseous, it does not rotate uniformly: bands of gases around the equator circle the solar axis once every 27 days, compared with a 34-day rotation rate near the poles.

At the same time, hot gases, being lighter, rise from the interior to the surface, while cooler, heavier gases descend -- a process called convection (similar to what occurs in a hot oven). As a result of these massive convection currents and the differing rates of solar rotation, the magnetic lines of force begin wrapping around the sun like ropes. The wrapping action stretches the ropes and creates magnetic fields so strong that they repel the surrounding solar gases. In effect, this makes the magnetic regions lighter than the gases, and they begin to rise. Some reach the surface and become sunspots, dark because they are cooler than surrounding incandescent gases.

The darker central portions of sunspots, or umbras, have the strongest magnetic fields; the lighter exteriors, or penumbras, the weaker fields. Occasionally, the penumbras of two sunspots of opposite polarity merge as they move past each other, putting the oppositely charged umbras in contact. The results are spectacular. "Because the umbras have opposite polarities, they attract each other," says the Marshall Center's Moore. "The closer they are together, the stronger the pull. Then, as they push past each other, it's like an earthquake fault slipping. In this case the stored energy is released in a flare." In the sunspot group that produced the flares of March, he notes, spots of opposite polarity were close together.

The wrapping and stretching of magnetic-field lines is also believed to be responsible for the spots' appearing progressively closer to the solar equator and the switch of magnetic polarities after each cycle. But ingenious as it seems, the dynamo model of the sun may need some serious revision. Astronomer Richard Altrock, at Sacramento Peak, has observed a brightening of the sun's corona that begins near the poles -- just when the first sunspots of a cycle break out around 35 degrees latitude -- then slowly progresses toward the equator. The brightening, he suspects, marks the beginning of still another cycle, long before the current one has expired. With that much overlap, he says, "our feeling is that the ((dynamo)) model cannot be correct."

To refine or revise this model, scientists must learn more about the interior structure and behavior of the sun. A new tool has evolved that should help them in their quest -- helioseismology, which, simply stated, involves "listening" to the interior of the sun as it bubbles, gurgles and swirls. The entire outer third of the sun is a seething ocean of gas, constantly churned by thermal convection. And convection, says astronomer John Harvey of the National Solar Observatory at Kitt Peak, "is a very noisy process. So the sun makes noise, just as a pot of water does as it boils."

These sound waves, or seismic waves, cannot travel in space because there is no air or other medium to carry them. So when the waves reach the surface of the sun from below, they bounce back into the interior, where the greater heat bends them toward the surface again. The result, says astronomer Robert Noyes of the Harvard-Smithsonian Center for Astrophysics, is a "sun ringing like a bell, but not one that is being struck by a clapper. Rather, it is vibrating somewhat like a bell suspended in a sandstorm, continuously struck by tiny grains of sand."

While scientists cannot monitor these waves directly, they can see the effects on the solar surface. "On reaching the surface," explains Juri Toomre, an astrophysicist at the University of Colorado, "the waves cause the gases there to move up and down" -- oscillations that astronomers can measure. To date, they have discovered millions of different oscillations, up- and-down motions with cycles ranging from 2 1/2 to 13 minutes. Some are caused by seismic waves confined to a zigzag path near the surface, others by waves that plunge as far as four-fifths the distance to the solar center before being deflected back.

Just as seismologists describe the structure and nature of the earth's interior by studying seismic waves caused by earthquakes or explosions, astronomers are using helioseismology to learn more about the structure of the sun. Although still in its infancy, the new science has already led to several discoveries. Says NSO's Harvey: "It looks as if the frequencies of the oscillating waves vary with the solar cycle: they decreased a bit as the sun went toward solar minimum. Now we expect them to increase again."

The telltale seismic-wave patterns also suggest that while the sun's outer layer rotates faster at the equator than at the poles, the inner region rotates uniformly. This creates a shear force, like the blades of scissors sliding past each other, that Harvey suspects distorts the magnetic field, giving rise to the solar cycle.

Not everyone agrees with Harvey's view, but further advances in helioseismology could resolve the issue. For example, the technique might help confirm or undermine a new idea that sunspots and flares are produced by giant cylindrical-shaped flows of hot material kept in motion by convection. Adjacent cylinders rotate in opposite directions in the 250,000-mile-deep solar convection zone and gradually migrate toward the equator. Like rollers in an old-fashioned washing machine, one proposal suggests, these cylinders squeeze magnetic fields and in effect wring out sunspots in the process.

For the continuous pursuit of helioseismology, uninterrupted when the sun dips below the horizon, the National Science Foundation is planning to commit $18 million toward a Global Oscillation Network Group, which sponsors hope will begin full operation in 1993. The program will link six helioseismological stations around the world so that the sun will, literally, never set on GONG. Oscillations will also be measured by a satellite that NASA and the European Space Agency plan to launch in the early 1990s.

One possible benefit that scientists could gain from these observations would be the ability to predict powerful flares, an art that will become increasingly important in the space age. While the only probable casualties of the approaching solar maximum will be satellites in low orbit, blown electrical transformers and some interrupted radio and television transmissions, future maximums could endanger humans venturing beyond the earth's protective magnetic field. Space travelers in high orbit, in interplanetary space or on the surface of the moon or another planet during maximums would be vulnerable to lethal radiation from high-energy particles hurled into space by solar flares. That threat has influenced plans for manned missions to Mars, which call for spacecraft with lead-lined "storm cellars," into which astronauts can duck if a dangerous solar flare appears or is anticipated.

Does the approaching solar maximum, or future ones, pose any threat to earthbound humans? Astronomer Eddy does not think so. "For the most part," he says, "the sun is a very benign, well-behaved, middle-age* star." Still, he muses, "it's not entirely regular. We have looked at the sun for such a small fraction of its life, we should not be at all surprised if it does something outside our experience."

Of one thing astronomers are sure: solar behavior will eventually change -- and drastically. When the sun runs low on hydrogen, it will swell into a red giant, ballooning out to engulf Mercury and perhaps Venus. Even if it does not expand far enough to swallow the earth, its radiant heat will boil away the oceans, leaving behind a dead, incinerated planet. But that cosmic calamity will not occur for about 5 billion years. That gives astronomers a fair amount of time to comprehend the still mysterious workings of the star closest to the earth.

FOOTNOTE: *To astronomers, the eastern edge of the sun is to the left, as viewed from earth.

FOOTNOTE: *If current theory is correct. But only about a third the number of neutrinos (particles with little or no mass that travel at the speed of light) that the sun should be producing at this temperature have been detected, leading some scientists to speculate that the core temperature is lower.

FOOTNOTE: *The sun flared into life around 4.5 billion years ago.

With reporting by J. Madeleine Nash/Sunspot