The Sun’s Halo in 3-D

New perspectives on solar explosions

For 4 minutes and 7 seconds early on the afternoon of March 29, thousands of people who had trekked deep into the southern Sahara Desert saw blazing day turn into night. Wearing turbans to keep the sand out of their hair, the sky watchers in Libya were treated to a picture-perfect view of the sun being blocked by the shadow of the moon. It was also the longest such eclipse ever seen.

A new simulation of the sun’s outer atmosphere, or corona, is the first to accurately describe its global activity in three dimensions. The model reveals, for example, the tangle of magnetic field lines along which streams of ionized gas head from the sun into space. SAIC, NASA, NSF

HEAVENLY MATCH. A model (left) of the solar eclipse that took place on March 29 agrees well with a composite image (right) of the real thing. The eclipse image, recorded in Kastellorizo, Greece, is superimposed with observations of the solar disk and streams of ionized gas, both depicted in orange, taken by the Solar and Heliospheric Observatory. SAIC, NASA, NSF; Williams College Eclipse Expedition, NSF, National Geographic, NASA, ESA

SOLAR STORM. Theorists relied on this solar eruption, which took place on March 12, 1997, to hone their computer model of the corona. NASA, ESA

As the sun disappeared, a white, lacy halo popped into view. It was the sun’s wispy outer atmosphere, or corona, which is rarely seen because it’s normally washed out by the sun’s glare.

Whether looking through telescopes or special protective glasses, the observers in Libya were jubilant. So were some researchers half a globe away. Their computer model had accurately predicted the appearance and behavior of the corona. In developing the first accurate model of the corona, Zoran Mikic, Jon Linker, and their colleagues at Science Application International Corp. in San Diego have produced the equivalent of a weather map for the sun.

“No other simulation has had the high resolution, physical accuracy, and global coverage” of this model, says solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo.

Scientists plan to use the map ultimately to predict the appearance and location of solar flares and coronal-mass ejections. The ejections are billion-ton clouds of hot, electrified gas that are hurled from the corona and can damage orbiting satellites and communications and power systems on Earth. With advance warning, people on Earth might minimize such damage, and astronauts might reduce their exposure to harmful radiation from these turbulent events.

Following on the success of the new map, a pair of NASA spacecraft set for launch on the same rocket on Aug. 31 is expected to greatly advance scientists’ knowledge of the corona. As the two nearly identical craft, known as Solar Terrestrial Relations Observatory (STEREO), slowly separate during their mission, they will observe the corona from different perspectives and provide the first three-dimensional views of the sun’s outer atmosphere.

The model developed by the San Diego scientists will play a crucial role in interpreting images taken by STEREO, says mission scientist Russ Howard of the Naval Research Laboratory in Washington, D.C.

At the same time, says Linker, he and his collaborators plan to use the STEREO images to hone the model they have developed.

Anatomy of an eruption

During a coronal mass ejection, a magnetized cloud of material that has lifted off the sun travels at 1.5 million kilometers per hour. On its 2-to-3-day journey to Earth, the cloud rams into the slower-moving solar wind, the stream of particles continually blown out by the sun. The collision creates a shock wave that in turn sweeps up other charged particles in space, strengthening the moving cloud.

When a coronal mass ejection nears Earth, it can wreak havoc. It can compress the magnetosphere, the magnetic shield that surrounds our planet. Satellites that had been orbiting just inside the magnetosphere may now lie just outside it, where they are no longer protected from an onslaught of energetic charged particles that can harm their sensitive electronics.

Like a bar magnet, Earth’s magnetic field has two poles. If the magnetic field of a coronal mass ejection happens to point opposite that of Earth’s, the eruption can do further damage. It can connect directly with Earth’s field in a catastrophic magnetic handshake that releases vast amounts of energy. Researchers suspect that just such an interaction knocked out power grids, causing the vast power outage that afflicted Quebec in March 1989.

Coronal mass ejections strong enough to cause such damage happen only about twice a year, DeForest says. Depending on solar activity, the sun can launch a coronal mass ejection once every few hours to every few days. About 10 percent of these ejections head toward Earth. With advance warning, engineers can power down satellites and turn them away from the sun, delay launches of craft and make sure astronauts are not out spacewalking, and mitigate widespread damage to electronic systems on Earth.

Sunny simulation

The new model of the corona, like most simulations of the sun, relies on the assumption that magnetic activity drives solar explosions. The sun’s corona is threaded with long, looping magnetic fields that are created deep within the sun. The loops are anchored on the sun’s visible surface, thousands of kilometers below the corona.

As the sun rotates, its polar regions make a complete circle in about 34 days, compared with the 25 days required by its equator. As a result, the magnetic fields generated at the sun’s core become twisted and tangled. Every so often, according to the prevailing theory, the entangled fields snap like rubber bands, releasing a torrent of energy. That energy somehow gets channeled into outbursts such as flares and coronal mass ejections.

Researchers had previously modeled magnetic activity in the corona, but the complexity of the physics and the amount of computer time required had limited the simulations to two-dimensional slices.

“The equations themselves and the effects they describe were known, but … the simulation was simply too hard to do in three dimensions,” says Mikic. The latest advance came about, he says, because he and other theorists finally managed to solve the equations that describe the transport of heat in the corona in three dimensions. Heat flows through the sun’s outer atmosphere via three processes. Light can radiate heat away, the solar wind can carry it outward, or the magnetic fields can transport it along their arches.

Some magnetic fields form loops with both their north and south poles anchored on the solar surface. Such fields form closed circuits that extend as far as 300,000 kilometers from the sun and trap the solar wind. Other fields are called open—they extend into the vastness of interplanetary space, beyond Pluto, before looping back and reconnecting to the sun’s surface. These dark-appearing, open regions, known as coronal holes, permit solar wind to escape into space.

Because the team has determined the geometry of the coronal holes, researchers can predict the speed of the solar wind. “Knowledge of the solar wind tells us how quickly coronal mass ejections reach us, [which in turn] helps us to better forecast geomagnetic storms,” Mikic notes.

The team has also confirmed that the solar corona “is denser in regions of closed magnetic field lines that trap the flow of the solar wind [than in open regions],” says Mikic. “This is why some of the corona is brighter … than in other places.”

With a better understanding of the corona, Mikic and his colleagues completed their three-dimensional model. Making a prediction required 700 computers to run for 4 days.

Because scientists have made relatively few direct measurements of the coronal magnetic field, the theorists had to extrapolate its strength from magnetic measurements made at the sun’s visible surface.

Relying on magnetic field data from the orbiting Solar and Heliospheric Observatory (SOHO), the National Solar Observatory at Kitt Peak in Arizona, and the Wilcox Solar Observatory in Stanford, Calif., Mikic and his colleagues posted a simulation of the March 29 eclipse online 2 weeks before the event. They updated it on March 24. The model predicted what the corona would look like, including its density.

When the team observed the actual eclipse, Mikic says that he knew right away that the prediction was spot on.

Dynamic duo

Three months from now, a pair of spacecraft is scheduled to begin simultaneously broadcasting to Earth visible-light and ultraviolet images of the sun. Like SOHO, each of the craft has an occulting mask, or coronagraph, that blocks the glare of the sun. This creates an artificial eclipse that permits the observatories to directly view the corona.

One of the STEREO craft will initially lead Earth in orbit around the sun, while the other will trail behind. In a manner similar to the way in which the brain integrates information coming from two eyes, scientists will take advantage of the offset between the two observatories to construct three-dimensional views of the sun and its corona, and to trace the volume and flow of matter and energy from the sun to Earth. This information will enable researchers to more accurately assess the direction and speed of a coronal mass ejection. That capability could provide earlier warning of an eruption headed toward Earth and more precise knowledge of when it will arrive.

Besides taking pictures, each STEREO craft will record bursts of radio waves emitted by coronal mass ejections and solar flares. This will provide an independent three-dimensional view of a solar eruption.

Furthermore, as high-speed particles from coronal mass ejections pass the STEREO craft, detectors on each will record particle density and energy. That will enable scientists to compare images and particle measurements made by the same craft, notes Howard.

During the mission, the two craft will slowly drift farther apart. Powered by solar batteries, they will also move out of the Earth-sun line. By the end of their second year in space, the two craft will be 1.4 times as far apart from each other as Earth is from the sun. The two observatories will then be examining the sun from perpendicular positions.

“We’ve never had that [perspective] before,” says Howard.

During part of the mission, the two craft will face the far side of the sun, recording disturbances that ground-based observatories and Earth-orbiting satellites can’t see. It takes 2 weeks for the back of the sun to rotate to the front.

These perspectives, says Howard, should enable STEREO not only to record coronal mass ejections as they lift off the sun and travel toward Earth but also to determine the instabilities that cause these solar temper tantrums in the first place.

Another spacecraft, set for launch this fall, will dramatically improve magnetic field measurements of the sun and increase the accuracy of the new model, notes Linker. A team led by the Japanese Aerospace Exploration Agency will use Solar-B’s combination of visible-light, extreme-ultraviolet, and X-ray detectors to study the interaction between the sun’s magnetic fields and the corona.

Solar-B doesn’t have a coronagraph to artificially mimic solar eclipses, leaving that to SOHO and STEREO.

The chance to view a natural total eclipse anyplace on Earth comes, on average, less than once a year. Mikic observed the real thing—the March 29 eclipse—on a beach near the town of Side in Turkey. Fortunately, there was a bar right next to the observatory.

“We celebrated the way all good scientific results are celebrated—with a good glass of beer,” he says.

A Corona, of course.

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