NASA-funded scientists have created the first three-dimensional (3-D) view of massive solar eruptions called Coronal Mass Ejections (CMEs). The result is critical for a complete understanding of CMEs, which, when directed at Earth, may disrupt radio communications, satellites and power systems.
The researchers analyzed ordinary two-dimensional images from the joint NASA/European Space Agency Solar and Heliospheric Observatory (SOHO) spacecraft in a new way to yield the 3-D images.
"We need to see the structure of CMEs in three dimensions to fully understand their origin and the process that launches them from the sun," said Dr. Thomas Moran of the Catholic University of America, Washington. "Views in three dimensions will help to better predict CME arrival times and impact angles at the Earth," he said.
Moran developed the analysis technique. He is lead author of a paper on this research published today in Science. Dr. Joseph Davila of NASA's Goddard Space Flight Center, Greenbelt, Md., is co-author of the paper.
CMEs are among the most powerful eruptions in the solar system, with billions of tons of electrified gas being blasted from the sun's atmosphere into space at millions of miles (kilometers) per hour.
Researchers believe CMEs are launched when solar magnetic fields become strained and suddenly "snap" to a new configuration, like a rubber band that has been twisted to the breaking point. Complex and distorted magnetic fields travel with the CME cloud and sometimes interact with the Earth's own magnetic field to pour tremendous amounts of energy into the space near the planet.
The magnetic fields are invisible, but because the CME gas is electrified (a plasma), it spirals around the magnetic fields, tracing out their shapes. A view of the CME gas in 3-D therefore gives scientists valuable information on the structure and behavior of the magnetic fields powering it.
The new analysis technique for SOHO data determines the three-dimensional structure of a CME. A sequence of three SOHO Large Angle and Spectrometric Coronagraph (LASCO) images is taken through polarizers at separate angles. The ratio of polarized-to-unpolarized brightness at each pixel is then computed. Based on the way light scatters off electrically charged particles (electrons) in CME plasma, light from the structures at angles closer to the plane-of-the-sun will be more polarized than light from those at angles farther from the plane.
The distance from the plane is computed from the measurements, giving the three-dimensional coordinates of the mean scattering position to construct a view in 3-D. (Light which has an electric field oriented randomly in all directions is unpolarized, while light with an electric field oriented in just one direction is polarized.)
With the technique, the team has confirmed that the structure of Earth-directed (halo) CMEs is an expanding arcade of loops, rather than a bubble or rope-like structure. Although the CME eventually disconnects from the sun, the team also discovered the loops remained connected to the source region for an unexpectedly long time, for at least as long as the CME was visible to the SOHO instrument.
The team learned the technique was previously independently developed and used to study relatively static structures in the solar atmosphere during total solar eclipses. The team believes its method will complement the upcoming Solar Terrestrial Relations Observatory (STEREO) mission. The mission, scheduled for launch in February 2006, will use two widely separated spacecraft to construct 3-D views of CMEs by combining images from the two different vantage points of the twin spacecraft.
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