Probes capture 3-D images of solar explosions

By: JEFFREY SIEGEL

Posted: 4/30/09

Researchers using data from NASA probes have discovered that a certain type of solar storm looks like a croissant! NASA's Solar Terrestrial Relations Observatory (STEREO) probes, a pair of spacecraft tracking solar weather, are allowing scientists to track the speed, trajectory and 3-D shape of solar explosions, known as coronal mass ejections, as they leave the sun, improving our ability to forecast solar weather.

"Coronal mass ejections (CMEs for short) are solar 'hurricanes' that erupt from the sun's surface and spew billions of tons of plasma into space at speeds of thousands of miles per hour," said Kristi Marren, public affairs representative for the Hopkins Applied Physics Laboratory (APL).

"Aside from creating auroral lights, when these massive storms hit our atmosphere they can disrupt satellite communications, GPS and cell phone signals and can cause power outages or blackouts," Marren said.

As a result, tracking solar weather is crucial; If we know when these disruptions are likely to occur, we can manage them more effectively and be more prepared for their arrival.

Angelos Vourlidas, a researcher at the Naval Research Lab who developed the croissant model for CMEs, said, "Before the STEREO probes, measurements and the subsequent data of a CME observed near the sun had to wait until the ejections arrived at Earth three to seven days later. Now we can see a CME from the time it leaves the solar surface until it reaches Earth, and we can reconstruct the event in 3-D directly from the images."

From this reconstruction, he realized that CMEs are shaped like a series of twisted magnetic fields, fat in the middle and thin on the edges - in short, a croissant.

According to Vourlidas, the twisted shape of CMEs makes sense. "CMEs get started as twisted ropes of solar magnetism. When the energy in the twist reaches some threshold, there is an explosion which expels the CME away from the sun," he said.

This model, along with the vantage point offered by the STEREO satellites, allows improved forecasting of solar weather. "We believe we can now predict when a CME will hit Earth with only three hours of uncertainty, a four-fold improvement over older methods," Vourlidas said.

The STEREO probes are a pair of spacecraft placed at different vantage points, one ahead of the Earth in its orbit around the sun and the other lagging behind. These two different vantage points are what allow for the 3-D images to be generated, much like how having our eyes in slightly different positions on our faces gives us the ability to perceive depth.

However, the probes have to be aligned just right to make sure the data is consistent over time. "APL spacecraft experts are helping STEREO scientists reconstruct 3-D imagery, collected by the spacecraft, by very accurately pointing the spacecraft and reducing its jitter or movement," Marren said. From the reconstructed images, scientists can track a CME's shape, velocity, mass and trajectory from when it leaves the surface of the sun to when it reaches Earth.

Knowing these factors improves our ability to forecast when a CME arrives, but not what it will do. For that, the contents of the CME need to be known. The effects of solar storms depend on how much plasma they contain and on the strength and orientation of their magnetic fields, not only on their shape. For that, new satellites will be needed, ranging from new space-based observatories to probes that would approach the sun's surface and actually enter emerging CMEs.

Nevertheless, STEREO still has a role to play. Its probes will continue to track the sun on opposite sides, and may even directly encounter CMEs, allowing scientists to observe a solar storm as it travels through space.

"We plan to analyze a number of CMEs that are directed to Earth or one of the STEREO spacecraft and quantify the improvement in predicting when the CME will impact [Earth or STEREO] by using our 3-D information," Vourlidas said.