NASA Spacecraft Make First 3-D Images of Sun

NASA's twin Solar Terrestrial Relations Observatory (STEREO) spacecraft have made the first three-dimensional images of the sun. The new view will greatly aidscientists' ability to understand solar physics and thereby improve space weather forecasting.

"The improvement with STEREO's 3D view is like going from a regular X-ray to a 3D CAT scan in the medical field," says Michael Kaiser, STEREO project scientist at NASA's Goddard Space Flight Center, Greenbelt, Md.

The STEREO spacecraft were launched October 25, 2006. On January 21 they completed a series of complex maneuvers, including flying by the moon, to position the spacecraft in their mission orbits. The two observatories are now orbiting the sun, one slightly ahead of Earth and one slightlybehind, separating from each other by approximately 45 degrees per year. Just as the slight offset between a person's eyes provides depth perception, the separation of spacecraft allow 3-D images of the sun.

Violent solar weather originates in the sun's atmosphere, or corona, and can disrupt satellites, radio communication, and power grids on Earth. The corona resembles wispy smoke plumes, which flow outward along the sun'stangled magnetic fields. It's difficult for scientists to tell which structures are in front and which are behind.

"In the solar atmosphere, there are no clues to help us judge distance. Everything appears flat in the 2D plane of the sky. Having a stereoperspective just makes it so much easier," says Russell Howard of the Naval Research Laboratory, Washington, the principal investigator for the SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) suite of telescopes on the spacecraft.

"With STEREO's 3D imagery, we'll be able to discern where matter andenergy flows in the solar atmosphere much more precisely than with the 2D views available before. This will really help us understand the complex physics going on," says Howard.

STEREO's depth perception also will help improve space weather forecasts. Of particular concern is a destructive type of solar eruptioncalled a Coronal Mass Ejection (CME). CMEs are eruptions of electrically charged gas, called plasma, from the sun's atmosphere. A CME cloud can contain billions of tons of plasma and move at a million miles per hour.

The CME cloud is laced with magnetic fields, and CMEs directed toward Earth smash into the planet's magnetic field. If the CME magnetic fieldshave the proper orientation, they dump energy and particles into Earth'smagnetic field, causing magnetic storms that can overload power lineequipment and radiation storms that disrupt satellites.

Satellite and utility operators can take precautions to minimize CME damage, but they need an accurate forecast of when the CME will arrive. To do this, forecasters need to know the location of the front of the CME cloud. STEREO will allow scientists to accurately locate the CME cloud front. "Knowing where the front of the CME cloud is will improve estimates of the arrival time from within a day or so to just a few hours," says Howard. "STEREO also will help forecasters estimate how severe the resulting magnetic storm will be."

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