Images on this page courtesy of SOHO/NASA/ESA

October 28, 2003, at 6:12 a.m. EST, the sun flung the fourth most powerful solar flare ever recorded directly at Earth. The flare’s emissions appear as white light in the green (false color) ultraviolet image.

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.”

Eighteen minutes later the explosion’s first particles rise into space.

The explosion originated in the southernmost group of sunspots, shown in visible light.

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.