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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 1)

“The space around us isn’t as benign, friendly and accommodating to our technology as we had assumed,” Schrijver says.

By documenting the origins of these storms in unprecedented detail, SDO gives researchers their best chance yet to understand the Sun’s destructive capabilities. The goal is to forecast space weather—to read the Sun’s moods far enough in advance that we can take precautions against them. Success will rely upon gazing through the Sun’s surface to see magnetic outbursts as they develop, in much the same way that meteorologists use cloud-penetrating radar to see signs of a tornado before it roars to the ground.

But for now, the Sun’s activity is so complex that its convulsions baffle the field’s top minds. When asked to explain the physics that drives the Sun’s violence, SDO scientist Philip Scherrer of Stanford University minces no words: “We fundamentally don’t know.”

Our parent star is just eight minutes away, as the light flies. The Sun gets more telescope time than any other object in space, and the research is a global enterprise. The most successful satellite prior to SDO, a joint NASA-European Space Agency mission called the Solar and Helio­spheric Observatory (SOHO), still sends back images of the Sun 15 years after its launch. A smaller explorer now in space, called Hinode, is a Japan-NASA collaboration that studies how the Sun’s magnetic fields store and release energy. And NASA’s Solar Terrestrial Relations Observatory (STEREO) mission consists of two nearly identical satellites traveling in Earth’s orbit, one in front of our planet and one behind. The satellites allow scientists to create 3-D images of solar ejections. Now on opposite sides of the Sun, this past February they took the first photo of the Sun’s entire surface. On the ground, telescopes in the Canary Islands, California and elsewhere examine the Sun with techniques that eliminate the blurring effects of Earth’s atmosphere.

The Sun is a spinning ball of gas large enough to contain 1.3 million Earths. Its core is a furnace of nuclear fusion, converting 655 million tons of hydrogen into helium every second at a temperature of 28 million degrees Fahrenheit. This fusion creates energy that ultimately reaches us as sunlight. But the core and inner layers of the Sun are so dense that it may take a million years for a photon of the energy to fight just two-thirds of the way out. There it reaches what solar physicists call the “convective zone.” Above that is a thin layer we perceive as the Sun’s surface. Solar gases continue far into space beyond this visible edge in a blazing hot atmosphere called the corona. A tenuous solar wind blows through the entire solar system.

Things get especially interesting in the convective zone. Giant gyres of charged gas rise and fall, as in a pot of boiling water, only more turbulent. The Sun rotates at different speeds—about once every 24 days at its equator and more slowly, about every 30 days, at its poles. This difference in velocity shears the gas and tangles its electrical currents, fueling the Sun’s magnetic fields. The overall magnetic field has a direction, just as Earth’s north and south poles attract our compasses. However, the Sun’s field is full of curves and kinks, and every 11 years, it flips: the north pole becomes the south, then back to north again 11 years later. It’s a dynamic cycle that scientists don’t fully grasp, and it’s at the heart of most efforts to understand how the Sun behaves.

During those flips, the Sun’s deep magnetic field gets really gnarled. It rises up and pokes through the visible surface to create sunspots. These dark patches of gas are cooler than the rest of the Sun’s surface because the knotted magnetic fields act as barriers, preventing some of the Sun’s energy from escaping into space. The fields in sunspots have the potential to erupt. Above sunspots, the Sun’s magnetic field loops and swirls through the corona. These writhings ignite the explosions on Lockheed’s video screens in Palo Alto.

Schrijver and his boss, Alan Title, have worked together for 16 years, long enough to complete each other’s sentences. Their group’s latest creation, the Atmospheric Imaging Assembly—a set of four telescopes that take pictures of million-degree gases in the corona—is one of three instruments deployed on SDO. NASA compares it to an IMAX camera for the Sun.

“This bubble of gas blowing off is 30 times Earth’s diameter, moving at a million miles an hour,” Title says, pointing on the screen to an expanding red vortex caught by SDO soon after the satellite’s launch. And, he notes almost casually, this was a fairly minor eruption.

Magnetic fields keep the Sun’s gases in line as they arch into space, Title says, much as a bar magnet puts iron filings into neat patterns. The more tangled the fields become, the less stable they are. Solar outbursts happen when the magnetic fields snap into a new pattern—an event that physicists call “reconnection.”

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