"S" marks the spot for scientists trying to forecast solar eruptions that can damage satellites, disrupt communications networks and cause power outages.
Using the Japanese Yohkoh spacecraft, NASA-sponsored scientists have discovered that an S-shaped structure often appears on the Sun in advance of a violent eruption, called a coronal mass ejection, that is as powerful as billions of nuclear explosions.
"Early warnings of approaching solar storms could prove useful to power companies, the communications industry and organizations that operate spacecraft, including NASA," said Dr. George Withbroe, science director for Sun-Earth Connection research at NASA Headquarters. "This is a major step forward in understanding these tremendous storms."
"S marks the spot," said Dr. Alphonse Sterling of Computational Physics, Inc., Fairfax, VA, detailed to the Institute of Space and Astronautical Science (ISAS), Japan. "We have found a strong correlation between an S-shaped pattern on the Sun, called a sigmoid, and the likelihood that an ejection will occur from that region within days. Each sigmoid is like a loaded gun that we now know has a high probability of going off."
"The S-shaped regions are the dangerous ones," said Dr. Richard Canfield, a research professor of physics at Montana State University-Bozeman, and lead author on a paper to be published in the March 15 issue of Geophysical Research Letters. "As soon as we can recognize an S-shaped region, we know that it is more likely to erupt. Other common structures look like a butterfly, quite symmetric, and these rarely erupt."
The sigmoid structures are likely the result of twisted solar magnetic fields, said Dr. Sarah Gibson of the University of Cambridge, UK. "The key to the coronal mass ejection is its magnetic field, which can structure and propel the mass outward," said Gibson.
Coronal mass ejections are violent discharges of electrically charged gas from the Sun's corona, or outer atmosphere. The largest explosions in the solar system, they hurl up to 10 billion tons of gas into space at speeds of one to two million miles an hour. The outbursts occur several times a day, but not all are hurled toward Earth.
Images from various spacecraft have provided often spectacular images and information after a coronal mass ejection had already erupted, but scientists have been trying for some time to identify a precursor for these events. Sterling and Dr. Hugh Hudson of the Solar Physics Research Corporation, Tucson, AZ, working at ISAS, first observed a relationship between a sigmoid shape before a coronal mass ejection, and an arch-shape afterwards. Later, Hudson and others found the same pattern in several other ejections.
That finding prompted Canfield, Hudson and Dr. David McKenzie, a research scientist at Montana State University, to look for a statistical correlation between the sigmoid shape and subsequent eruptions. They viewed a total of two years of daily X-ray images from the Japanese/US/UK Soft X-ray Telescope on Yohkoh. The composite pictures -- 50 images each day -- were made into movies for analysis.
"We need to get past simple classifications such as, 'Is it sigmoidal or not, is the sunspot big or small,' and get to quantitative measurements that answer, 'how twisted are the magnetic fields, how big is the spot'," Canfield said. "As well, we want to know in which direction the ejection is going to go and how many regions are likely to erupt."
Ultimately, Canfield continued, the National Oceanic and Atmospheric Administration (NOAA) may be able to include warnings of coronal mass ejections in its space weather forecasts. NOAA is building a Solar X-ray Imager similar to that on Yohkoh, scheduled for launch next year, he said.
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