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When a coronal mass ejection reaches Earth, solar particles stream along magnetic field lines, energize gases in the atmosphere and shine as norther lights. (Federico Buchbinder)

Something New Under the Sun

Scientists are probing deep beneath the surface of our nearest star to calculate its profound effect on Earth

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(Continued from page 2)

A typical solar outburst expelled toward Earth, called a coronal mass ejection, might contain ten billion tons of charged gas racing across space. “You have to imagine a set of forces sufficient to launch all of the water in the Mississippi River to a velocity 3,000 times faster than a jet plane flies, in 15 to 30 seconds,” he says, pausing a moment to let that sink in. “There is no counterpart to this on Earth. We have trouble explaining these processes.”

Previous solar missions took fuzzy snapshots of large coronal mass ejections. Other telescopes zoomed in for fine details but could focus on only a tiny portion of the Sun. SDO’s high resolution of an entire hemisphere of the Sun and its rapid-fire recordings reveal how the surface and atmosphere change minute to minute. Some features are so unexpected that the scientists haven’t yet named them, such as a corkscrew-like pattern of gas that Schrij-ver traces on the screen with his finger. He thinks it’s a spiraling magnetic field seen along its edge, lacing through gas as it ascends into space. “It’s like [the gas] is being lifted in slings,” he says.

Before the mission was a year old, the scientists had analyzed hundreds of events, covering many thousands of hours. (The August 1 eruptions, they found, were linked by magnetic “fault zones” spanning hundreds of thousands of miles.) The team is working under pressure, from NASA and elsewhere, for better forecasts of space weather.

“Good Lord, this is complicated,” says Schrijver, playing a movie of the Sun’s mood on another day. “There is no quiet day on the Sun.”

A few miles away, on the campus of Stanford, solar physicist Philip Scherrer is wrestling with the same question that animates the Lockheed Martin group: Will we be able to predict when the Sun will cataclysmically hurl charged gas toward Earth? “We’d like to give a good estimate whether a given active region will produce flares or mass ejections, or if it will just go away,” he says.

Scherrer, who uses a satellite downlink for television reception, explains the impact of space weather by recalling an event in 1997. “One Saturday, we woke up and all we saw was fuzz,” he says. A coronal mass ejection had swept past Earth the night before. The magnetic cloud apparently took out the Telstar 401 satellite used by UPN and other networks.

“I took that personally, because it was ‘Star Trek’ [I was unable to watch],” Scherrer says with a wry smile. “If it had happened on the morning of the Super Bowl, everyone would have known about it.”

Scherrer’s team and Lockheed Martin engineers developed SDO’s Helioseismic and Magnetic Imager, an instrument that probes into the Sun’s churning interior and monitors the direction and strength of the magnetic field, creating black-and-white maps called magnetograms. When sunspots come along, the maps show magnetic turmoil at the bases of arching structures in the Sun’s atmosphere.

The instrument also measures vibrations on the Sun’s surface. On Earth, seismologists measure surface vibrations to reveal earthquake faults and geologic structures far underground. On the Sun, vibrations come not from sunquakes but from pulsations caused by gases heaving up and down on the surface at speeds of some 700 miles per hour. As each blob of gas crashes down, it propels sound waves into the Sun, and they jiggle the entire star. Scherrer’s device gauges those vibrations across the Sun’s face.

The key, says Scherrer, a leading expert in helioseismology, as this science is known, is that the sound waves move faster through hotter gas, such as turbulent knots far below the surface that often presage sunspots. The sound waves also accelerate when they move through gases flowing in the same direction. Although these measurements create mathematical nightmares, computers can create pictures of what’s happening under the Sun’s surface.

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