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.

EFFECTS OF MAGNETIC STORMS

Space

Harm to astronauts


Instrument damage

Atmospheric drag 

on spacecraft

Atmosphere

Airline-passenger radiation

Ground

Electronic-grid disruption

Telecommunication

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.

SOLAR FLARE

Courtesy NASA/LMSAL

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.