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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 3)

In this way, Scherrer’s team can detect sunspots on the far side of the Sun days before they rotate into view and before they are in position to spew harmful particles and gas toward Earth. The scientists also hope to spot active regions bubbling up from within the Sun a day or more before they are visible as sunspots.

These techniques provide previews of coming attractions. The challenge, Scherrer says, is finding the right signs of magnetic entanglement that—like the radar images of a newly forming tornado—give reliable warnings. Some researchers have keyed in on the shapes of magnetic fields, noting that a particular S-shaped curvature often heralds an outburst. Others look at whether magnetic strength across the center of a sunspot changes quickly—an indication that it might be ready to snap.

Scherrer calls up some pictures on his screen, apologizing that they don’t rival the Lockheed movies. The helioseismic images remind me of the knobby surface of an orange, with nodules of gas surging upward across the Sun’s entire sphere. The magnetic graphics cast the Sun in mottled gray tones, but when Scherrer zooms in, black and white flecks grow into irregular patches. These are the ribbons of magnetic force, poking into or out of the Sun’s constantly moving surface.

When magnetic field lines reconnect high in the Sun’s atmosphere, Scherrer says, “it’s very much like a short circuit when you touch two wires with a current. The energy flowing in the current turns into heat or light.” The sudden sparks shoot down along the magnetic field and slam into the Sun’s surface, setting off a powerful flare.

The strongest of the Sun’s arching magnetic fields can trap billions of tons of gas beneath them, setting the stage for coronal mass ejections. When a magnetic reconnection suddenly releases all that tension, the gas lifts off into space with the solar wind. “It’s like cutting the string on a helium balloon,” Scherrer says.

By studying many such events, Scherrer thinks he and his colleagues can devise a system that ranks the odds of the Sun aiming an eruption at Earth—a scale that might run from “all clear” to “take precautions.” Such guidelines would not be predictions, he admits, and he acknowledges, too, that solar forecasting may never rival earthly weather reports. Solar predicting requires the team to compare recent activity on the Sun with computer models. But the models are so involved that by the time the computer spits out an answer, the Sun may already have popped off or stayed quiet.

One of the biggest solar surprises in the past 50 years wasn’t something the Sun did but something it didn’t do: it didn’t produce many sunspots for most of 2008 and 2009. “We’d go 60, 70, 80, 90 days without a single sunspot,” says NASA science editor Tony Phillips, who independently publishes SpaceWeather.com. “In the lifetime of solar physicists, no one had seen this. It surprised the entire community.”

No one knows what caused the eerie quiet. The deep magnetic field apparently did not twist up in its usual way, perhaps because electrical currents inside the Sun grew weaker. Some scientists speculated that the Sun was powering down, at least temporarily. A panel of solar physicists studied these changes and projected that the Sun’s activity might reach just half of its recent levels in its next 11-year sunspot cycle. This could have minor implications for climate change. For the past century, human activity far outweighed the Sun’s modulations in affecting Earth’s climate. If the pattern of reduced solar activity continues through another of the Sun’s cycles and beyond, the subtle decrease in energy from the Sun could slightly offset global warming.

The Sun is projected to reach the peak of its current sunspot cycle in late 2013 or early 2014. But there’s no reason to think a more sedate Sun will stay that way. “The biggest particle event and geomagnetic storm in recorded history”—the 1859 event observed by Carrington—“occurred during a solar cycle of about the same size as the one we’re projecting in the next couple of years,” says Phillips. Moreover, a recent study by Suli Ma and colleagues at the Harvard-Smithsonian Center for Astrophysics showed that one-third of the solar storms striking Earth arise without solar flares or other warning signs. These sneak attacks suggest that the Sun can be hazardous even when it appears quiet.

There’s no way to shield the Earth from the Sun’s eruptions; powerful storms will always disrupt our planet’s magnetic field. But advance warning can limit their impact. Precautions include reducing power loads to prevent surges on electrical lines, putting satellites into an electronic safe mode, and—in NASA’s case—telling astronauts to take shelter within the most fortified parts of their spacecraft.

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