The Sciences

Forecasting the Sun's Weather

If we want to be a spacefaring nation, we'll have to get better at predicting the weather the nearest star throws at us.

By Dana MackenzieMay 29, 2004 5:00 AM


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The perfect storm came with plenty of advance warning. For two weeks last fall, a cluster of sunspots—group number 10486—churned from west to east across the face of the sun like a hurricane that wouldn’t quit. The activity twisted and contorted lines in the sun’s magnetic field until they snapped and reconnected, triggering giga-lightning bolts that showered Earth’s satellites 93 million miles away with X-rays and gamma rays. Giant eruptions of hot plasma and high-energy particles spewed forth, a Mount Everest’s weight of gas in a single belch.

Solar flares and coronal mass ejections are commonplace on our sun, where one moderate eruption a day is normal. But 10486 was extraordinary. On October 28 it fired one of the strongest flares ever recorded—a flare pointed directly at Earth. And that blast was a pip-squeak compared with what came next. On November 4 the most powerful solar storm ever recorded shot off a flare so bright that it swamped the X-ray detectors of the Geostationary Operational Environmental Satellite measuring it. The National Oceanic and Atmospheric Administration’s Space Environment Center in Boulder, Colorado, ranked it as a 28 on a scale for high-intensity flares that normally runs from 1 to 20. But without an accurate detector, astronomers could only approximate its value. Some scored it as high as 40.

“I was at a conference on magnetic reconnection at the time,” says Robert Lin, a solar physicist at the University of California at Berkeley. Although he was attending a lecture, his eyes were riveted on his laptop computer, which was connected to the Space Environment Center’s Web site. “I noticed the X-ray count going straight up,” Lin says. “Then it got to X18, and it went straight horizontal, which meant the detector had been saturated. After the talk ended, I announced that we were watching one of the biggest solar flares of the last 100 years.”

Fortunately, the consequences of the greatest solar storm in at least a decade were mild. When the October 28 coronal mass ejection got to Earth, its magnetic field was lined up in the same direction as Earth’s, a configuration that tends to repel incoming radiation. As a result, the only major damage at Earth’s surface was a power outage in Sweden. As for the November 4 event, it was never a threat, because the accompanying coronal mass ejection was not pointed toward Earth.

In the atmosphere and farther out in space, the situation was dicier. Pilots flew planes at lower altitudes to avoid exposing passengers to the increased incoming radiation. Airlines had to reroute their transoceanic flights farther south because static was disrupting radio communications near the poles. And astronauts had to take temporary shelter in a radiation-protected section of the International Space Station. At least 30 NASA satellites had problems over a two-week period, and one Japanese satellite was lost. Even as far away as Mars, one of the instruments aboard the Odyssey spacecraft—an instrument designed to assess radiation risk to humans—was damaged.

The storms of October and November brought to public attention what solar physicists have known for a long time: The sun is not the peaceable neighbor it may seem to be. And as astronauts begin to spend much more time in space, their survival may depend on how well we can predict the sun’s violent episodes.

Until the 1950s nobody even knew what powered the sun. Then physicists figured out that the sun’s energy comes from hydrogen atoms fusing in the interior to form helium. The sun’s core is literally a nonstop hydrogen-bomb explosion that keeps the solar furnace revved up to tens of millions of degrees.

Outside the core, the sun has a relatively quiet radiative zone. About 120,000 miles beneath the surface, the radiative zone gives way to a much more turbulent convective zone that is constantly churning like a pot of boiling water. This region is made up of hydrogen plasma, a gas of atoms whose electrons have been stripped away by the ferocious temperature, leaving just protons behind. The sun’s magnetic field stretches the plasma into ropes that break through the surface and become loops or prominences. Nearly all the activity scientists see on the sun’s surface—sunspots, flares, coronal mass ejections—is governed by mysterious twists and turns in the field.

At the solar surface, the temperature drops to about 11,000 degrees Fahrenheit, making it appear yellow. If the surface were hotter, the sun would look bluish; if it were cooler, the sun would look orange or red. Its intermediate temperature and size make it a garden-variety star, one of billions of class G stars in our galaxy.

Just above the surface, something strange happens in the tenuous outer layer known as the corona. There the sun’s temperature rises to a million degrees or more. This searing heat also strips electrons away from atoms, creating exotic ions like iron-XII (iron with 11 electrons removed). Because physicists know the precise amount of energy needed to create an ion like iron-XII, they can tell how hot different parts of the corona are by looking at the particular wavelengths of light the ions emit. In effect, astronomers put on different kinds of ultraviolet and X-ray filters to see heat in the corona. When they detect iron-XII, they know they are witnessing the sun’s most energetic event—a solar flare.



Harm to astronauts

Instrument damage

Atmospheric drag

on spacecraft


Airline-passenger radiation


Electronic-grid disruption



Radio-wave disturbance

Over the past 10 years, 10 major satellites have scrutinized the sun in every conceivable way, short of diving into it. Some analyze the sun’s light, X-rays, and gamma rays. Others listen to the rumbling of its convection cells, sniff the solar wind as it blusters by, or look from above at the magnificent auroras produced on Earth by incoming solar particles. With this bonanza of new data, physicists are finally starting to understand the sun’s less obvious effects on Earth. Perhaps most amazingly, they have begun to look through and inside the sun.

They aren’t doing it just out of scientific curiosity. Radiation and electromagnetic disturbances from the sun impose a real cost: disrupted radio signals, blown transformers, crippled satellites, and perhaps—if we are not careful—irradiated astronauts. Space weather is every bit as real as Earth weather, but we cannot forecast it reliably. “We’re just about where weather forecasters were in the 1950s,” says Chris St. Cyr, a senior project scientist for NASA. We’re just getting data-driven models, the first-generation models of how Earth’s magnetic field reacts to the sun. We’re able to show what happens—after it happens.”

One of the sun’s newest spectators is a satellite called Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Besides a tongue-twisting name, it has the distinction of being the first space mission in NASA’s Explorer program to be operated solely by a university. Lin, the ship’s designer, commands it from UC Berkeley.



When magnetic field lines interact, they can release high-energy ions and electrons, which produce X-rays and gamma rays. The blue spots indicate X-rays. The red indicates gamma rays. This is the first image of gamma rays detected in a solar flare.

As a college student in the late 1950s, Lin seemed an unlikely candidate to become one of the world’s leading experts on the sun. He liked to sleep late, and in those days solar astronomers had to get up before sunrise. The advent of satellite astronomy in the 1960s, however, revolutionized the way astronomers look at the sun. Some of NASA’s earliest satellites surprised scientists by revealing that Earth is bombarded by high-energy radiation, such as X-rays and gamma rays. A whole new branch of astronomy opened up, called X-ray astronomy.

In the 1990s and early 2000s, the Japanese satellite Yohkoh gave scientists their first systematic look at the sun’s X-ray emissions. In Yohkoh’s spectacular, almost sinister-looking portraits, bright pinpricks of light and fantastic curlicues of flame play across the face of the sun. The bright spots show where sunspots congregate and solar flares explode. The curlicues trace the paths of highly charged particles racing along magnetic fields in the sun’s atmosphere. The rest of the surface appears dark because most of the sun’s surface doesn’t emit any type of X-ray.

But Yohkoh could not detect the most energetic—and short-lived—forms of radiation: high-energy X-rays and gamma rays. These particles are the by-products of solar flares, the most powerful explosions anywhere in the solar system. In effect, Yohkoh could see the cloud of smoke after the bomb blast but not the spark that triggered it. RHESSI’s ultimate goal was actually to see the fuse being lit and the moment of detonation.

Launched in February 2002, RHESSI was almost too late to catch what it was looking for. Like hurricanes in the Atlantic, solar flares have a season: the two-to-three-year period of the solar cycle when the sun is most active. This solar maximum comes around once every 11 years—and like so much about the sun, the reasons are unknown. But Lin got lucky. The current cycle is the 23rd since records have been kept, and odd-numbered solar cycles are thought to have a late surge of big solar flares. Although the largest number of flares occurred in 2000, the sun saved some of its most spectacular fireworks for 2002 and 2003.

On July 23, 2002, Lin bagged his quarry. Just after midnight Greenwich mean time, a magnitude X4.8 flare erupted from a spot just south of the solar equator. The eruption wasn’t as enormous as last fall’s flares, but it was big enough to emit gamma rays. RHESSI caught a few thousand of them, enough to provide the anatomy of an explosion in unprecedented detail.

Courtesy of Metatech Corp./“Space Weather” (CAGU)

This series of images shows a sunspot cluster the size of Jupiter moving across the face of the sun between October 24 and November 4, 2003. The accompanying bars graph the magnitude of huge explosions, known as solar flares, that occurred on each day. Sunspots are areas of intense magnetism, and solar flares erupt when lines of magnetic energy intersect, break, and reconnect, releasing billions of tons of electrically charged particles. During its two-week transit, the cluster produced more than a dozen flares that were of medium intensity (M) or higher (X) and two flares that were extremely intense (X+). The chance alignment of the sun’s and Earth’s magnetic fields shielded the planet from the intense October 28 eruption. The first record of a solar flare and a magnetic storm was noted by astronomer Richard Carrington in 1859.

Some of Lin’s observations agreed with existing theories, and some made no sense at all. According to the leading theory, a flare is set off when nearby magnetic fields of opposite polarity break and reconnect, creating a closed loop with two footprints on the solar surface. High-energy protons and electrons come screaming out of the reconnection site, flow along the loop, and crash into the denser plasma at the sun’s surface. The electrons release a salvo of X-rays when they collide with the atoms. The protons are powerful enough to smash an atom’s nucleus and create all sorts of exotic by-products, including antimatter. The antimatter doesn’t last long. Positrons (antielectrons) meet up with electrons and annihilate each other, releasing ultrahigh-energy gamma rays. Lin estimates that the July 23 flare manufactured about a pound of antimatter, enough to power the entire United States for two days.

As expected, Lin’s images showed big X-ray sources at the two footprints and a 40-million-degree afterglow that he believes marks the reconnection site. But there was also a mystery: The protons didn’t stay on the same loop as the electrons, as theory would predict. They crashed to the surface about 10,000 miles away from the electron footprints. Craig de Forest, a solar physicist at Southwest Research Institute in Boulder, compares the phenomenon to dynamiting a hillside only to discover that all the dirt has gone in one direction and all the gold in another. “This upsets the canonical picture,” Lin says. “People are beginning to think about what it means, but there’s not a satisfactory idea yet.”

Three flares from last autumn’s storms produced measurable gamma rays, and one of the flares produced at least seven times as many as the July 2002 flare. Analyzing these recent events may allow physicists to determine the significance of the unexpected distribution observed in the 2002 flare. “With the new flares, we want to do an even better job than we did last time,” says Lin.

All the new data may provide a window into the life cycles of stars and open up the sun’s past. Many other stars are known to have flares, many more violent than the sun’s. But even ordinary G-class stars like our own have been known to double in brightness for a period of a few minutes to a few days. These rare events, called superflares by astronomer Bradley Schaefer of Louisiana State University in Baton Rouge, are 100 to 10 million times more powerful than anything our sun has dished out in human history.

If a large superflare happened on our sun, says Schaefer, it would cause an ecological holocaust on Earth. “Even a medium-size superflare would cause mass extinctions,” he says. “The reason is ozone depletion.” Radiation bombarding the upper atmosphere would obliterate the protective ozone layer, rendering us defenseless against ultraviolet light. Some astronomers speculate that Earth could have experienced a superflare in the distant past. Superflares would be more common in younger stars, they reason, because they spin faster and have stronger magnetic fields.

Even now, the ozone layer is not completely safe from solar attack. One of the unexpected consequences of the October 28 flare was a fivefold increase in ozone-destroying nitric oxide at 70 miles above Earth’s surface. The increase lasted less than two days, and it doesn’t seem to have bothered the ozone layer, which lies 40 miles below. Still, the solar weather watchers at NASA are puzzled. “We don’t really know what this means,” says Chris St. Cyr. “I wouldn’t say it’s disturbing, but it is curious.”

Across San Francisco Bay from Lin, a group of scientists at Stanford University is looking at the sun’s interior through a remarkable technique called helioseismology. The method uses optical sensing devices to detect movement of solar gases, much as seismologists can detect tremors in the earth. “There are sound waves going every which way in the sun,” says group member Philip Scherrer, one of the 12 principal investigators for the Solar and Heliospheric Observatory (SOHO), a joint project of NASA and the European Space Agency. The sound waves come from the churning of gases in the sun’s convective layer. In SOHO videos, the sun appears to have hundreds of thousands of flickering lights on its surface. Each flicker, called a granule, is a column of gas about 600 miles wide, which vibrates up and down about once every five minutes. Think of each one as a 600-mile-wide drumhead and you can understand why the sun is a noisy place.

Monitoring a Tempestuous and Unpredictable Powerhouse

The sun converts 4 million tons of matter into energy every second. The reactions occur in its core, where hydrogen is compressed to such intense heat that helium forms and energy is released. Today the sun consists of mostly hydrogen (78 percent) and helium (20 percent); the rest are heavy elements. Solar physicists calculate that the sun is about halfway through its 12-billion-year life span. It is currently a yellow dwarf, a star that is average in size and temperature. In 7 billion years, it will balloon to 250 times its current size and become a red giant. About half a billion years later, it will contract to a tiny white dwarf one-hundredth the size it is now. Ultimately, it will become a black dwarf—a hunk of matter that no longer emits light. Although physicists can project the sun’s life span, they have many questions about its inner life. The foremost question is, what causes the magnetic fields that drive solar phenomena?

Graphic by Don Foley


Ionized gases extend millions of miles away from the solar surface. During an eclipse the corona is visible as a bright halo around the sun. The corona is extremely hot—1,800,000°F. The cause of this intense heat is unknown.

Earth’s protective magnetic field

Magnetic forces that form around Earth protect the planet and humankind from most of the intense solar activity. Interactions between the magnetic fields of Earth and the sun produce the beautiful polar phenomena known as auroras.

Scherrer and his colleagues at Stanford watch for echoes of these drumbeats. Every drumbeat will echo numerous times, as the sound waves go into the sun and come back out and bounce off the surface and go back in, over and over. Eventually, after several bounces, they may come out close to their starting point again. A roomful of computers watch for these reverberations 24 hours a day. Amid the cacophony of a million drums, they try to pick out the sound of a single drum and its echo from roughly a million miles away on the opposite side of the sun.

When they do pick up an echo, physicists can learn a great deal about the sun’s interior. Sound waves do not travel equally fast at all temperatures. The hotter the gases, the faster they go. So a sound wave that passes deep through the interior of the sun to the opposite side and bounces back will return sooner than one that makes lots of little bounces close to the surface. The difference in transit time allows physicists to take the temperature of the regions the sound waves pass through. The technique is even sensitive enough to measure the temperature under a single sunspot. Scherrer and his colleagues have showed that although a sunspot is cooler on the sun’s surface, it traps a layer of hotter gas beneath it.

The most spectacular consequence of this discovery is the ability to detect sunspots on the unseen side of the star—a possibility first broached by Douglas Braun and Charles Lindsey, solar researchers at NorthWest Research Associates in Boulder, more than 10 years ago. If there is an active sunspot region on the other side of the sun, Braun and Lindsey argued, the sound speed should increase underneath it because of the hotter gas under the spots. That speeds up the round trip of a drumbeat by a few seconds.

At first “we heard a lot of skepticism about the idea,” Braun says. Lindsey is blunter: “Some people thought it was pie-in-the-sky craziness.” And it didn’t work when Lindsey and Braun looked at the sun through Earth-based telescopes. “All we got was noise,” Braun says. “We really needed 24 hours of observation time to reduce the noise.”

When SOHO was launched in 1995, it provided the perfect round-the-clock viewing platform. So in 1999, Braun and Lindsey decided to try their idea again. This time it was a stunning success: Their computer program produced fuzzy yet unmistakable images of sunspots on the far side. Five years later, this amazing feat has become routine. (The far side of the sun can be observed every day at

Far-side imaging even provided plenty of advance warning of the gamma-ray flare in 2002 that pleased Lin. The active region it came from, 10039, had grown larger throughout its transit across the far side. Similarly, region 10486, the main source of last fall’s storms, drew researchers’ attention long before the sun rotated and the region came around to the front. Besides showing up like a big red bruise on the helioseismological maps of the far side, it had released coronal mass ejections that could not be traced to any region on the near side. The eruption had also emitted ultraviolet light that reflected off interplanetary hydrogen gas and back into SOHO’s cameras. “When you see all three of these things together, it gives you the picture that something spectacular is happening on the far side,” says St. Cyr.

While solar physicists refine their forecasting techniques, some people are just getting acclimated to the concept of space weather. “Blizzards, a miserably cold and wet spring, a weirdly dreary June, a nasty hurricane, and now the weather has turned terrible in space,” wrote columnist Joel Achenbach in The Washington Post after hearing of the October 28 flare.

But the people who run power grids and satellites have wanted this information for a long time. The eye-opener was a blackout in Quebec on March 13, 1989, that plunged some 6 million people into darkness for more than nine hours. The cause was surges of electricity induced by a geomagnetic storm that burned out a few key transformers. Last fall’s storm didn’t cause any blackouts in the United States, but it did create power surges in long-range transmission lines. If the flares had happened at a time when there was more demand on the power grid, the consequences could have been more severe.

For satellites, the danger comes from energetic particles, which can confuse computers by changing bits of data in memory chips. Also, solar particles can short-circuit electronic instruments, and drag induced by the particles heating the atmosphere around a spacecraft can alter its orbit. In one of the great ironies of the space age, Skylab—the first manned U.S. spacecraft to observe the sun—was dragged to Earth prematurely by this effect.

Although SOHO, RHESSI, and other satellites have improved our ability to prepare for solar hazards, they are only a start. SOHO can see an eruption lift off from the sun, but it can’t tell when it will get to Earth or whether it has the proper magnetic orientation to pose a threat. By the time particles actually hit SOHO, they are only an hour away from Earth. “All it can say is, ‘Look, here comes a geomagnetic st—!’ ” says Scherrer.

Several upcoming space missions may fill in the blanks. The mission of the Solar Terrestrial Relations Observatory, scheduled for launch in 2005, is to send two identical satellites into solar orbit, one ahead of Earth and one behind. That will give astronomers their first three-dimensional view of coronal mass ejections. Now they have to make do with views that cannot easily distinguish a coronal mass ejection headed toward Earth from one headed in the opposite direction. The Solar Dynamics Observatory, planned for 2008, will allow more complete study of the sun’s interior and perhaps enable the prediction of sunspots before they appear.

Still, the list of what scientists don’t understand about the sun is daunting. For example: What drives the 11-year sunspot cycle, what makes the corona so hot, and how big do solar flares get? After the unexpected intensity of the November 4 flare, some ideas about the scale of solar fury are likely to change. St. Cyr notes that we do know what a 100-year hurricane is like, “but on the sun, we don’t know what the 100-year events are. We don’t really have a clue.”

Late 2006 will mark the close of a solar cycle that began in 1996. The graph below tracks the frequency of sunspots during the current cycle, the 23rd since astronomers have been monitoring solar activity. The large orange solar image—created from helium emissions—was taken in 2000 as the cycle neared its peak. The green images—created from iron emissions—show the trend toward increased solar activity. Bright areas indicate intense magnetic activity. During active periods, up to 250 sunspots may form, but only a handful occur during the quiet periods. German astronomer Samuel Schwabe recorded sunspots over 17 years and reported in 1843 that sunspot activity is cyclic. A typical solar cycle is about 11 years, although some have been as long as 17 years or as short as 8 years. Solar physicists believe that waxing and waning magnetic activity within the sun drives the solar cycle, but they do not know why the cycle occurs. Some climatologists suspect that changes in solar activity can provoke climate change on Earth. For example, Europe’s “little ice age” coincided with a period of minimal solar activity between 1640 and 1710. But other researchers believe changes in current solar activity will have a minimal effect, if any, on Earth’s climate.

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