Physicists Gain in Effort to Predict Disruptive Solar Eruptions

March 10, 1999

Physicists Gain in Effort to Predict Disruptive Solar Eruptions

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By JOHN NOBLE WILFORD

Analyzing X-ray images of transient S-shaped patterns on the sun, scientists think they have found a reliable way to forecast powerful solar eruptions that can cause disruptive magnetic storms in Earth's atmosphere, knock out electrical systems and communications, and disable orbiting spacecraft.
The scientists reported Tuesday that two years of observations by the Yohkoh Soft X-Ray Telescope, a Japanese-American-British satellite, showed a strong correlation between the appearance of S or inverse S shapes, called sigmoids, and the likelihood of a violent eruption in that region a few days later.
These eruptions, called coronal mass ejections, are discharges of hot, electrically charged gas from the sun's outer atmosphere, presumably caused by interactions of twisted solar magnetic field lines. They are as powerful as billions of nuclear explosions and send out blasts of electrified gases at least once a day that, if pointed in the right direction, can reach Earth in four days. Such a solar storm two years ago silenced a communications satellite.
In a news conference at the Washington offices of the National Aeronautics and Space Administration, solar physicists showed time-lapse movies from the Yohkoh spacecraft that illustrated the relationship between the emergence of sigmoids, particularly the large ones, and subsequent coronal mass ejections. Each sigmoid, they said, is like a loaded gun with a high probability of going off.
"We have here a tool which we expect will be very useful in forecasting space weather," said Dr. Richard C. Canfield, a physicist at Montana State University in Bozeman, who was a leader in interpreting the new findings. He is the principal author of a report on the research being published next Monday in the journal Geophysical Research Letters.
In the report, Canfield's group noted that sigmoids in all sizes showed eruptive signatures but that the noneruptive ones were almost always small. Yet in even the smallest-size category, the authors said, "at least twice as many show eruptive signatures as don't."
Another speaker at the news conference, Dr. Sarah Gibson, a solar physicist at Cambridge University in England, described the sigmoid phenomenon as "observable tracers of twisted magnetic fields, quite possibly some of the field lines lighting up with a lot of energy."
Dr. David M. Rust, a physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., noted one mystifying and possibly significant aspect of the phenomenon: the regular S pattern is the rule in the sun's southern hemisphere, and the inverse S is the most frequent occurrence in the north.
"What we are really seeing is telling you something about the fundamental physics of the sun," Rust said, but what that is, he was not ready to say.

The scientists noted another peculiarity that they did not understand. If the research showed that S marks the spot of a likely coronal eruption, they said, other common magnetically disturbed regions of the sun that have a symmetrical butterfly shape only rarely lead to eruptions.
The apparent relationship between the sigmoid patterns and the coronal ejections was first observed by Dr. Hugh S. Hudson and Dr. Alphonse Sterling, American physicists working at the Institute of Space and Astronautical Science in Japan, where the X-ray images transmitted by the Earth-orbiting Yohkoh spacecraft were processed.
As a result of that early finding, Hudson, Canfield and Dr. David E. McKenzie, also of Montana State, embarked on a detailed examination of Yohkoh's daily X-ray images -- 50 a day -- over the course of two years, 1993 and 1997. This established "the tight statistical relationship between sigmoids and eruptions," Canfield said Tuesday.
The next step, he said, is to transform the correlations into a more refined predictive tool leading to early warnings of impending magnetic storms caused by the coronal mass ejections.
"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 coronal mass eruption is going to go, and how many regions are likely to erupt."
Rust said it would take several years of observations by other spacecraft and further study before the new knowledge could be translated into a reliable forecasting system. The National Oceanic and Atmospheric Administration is building a solar X-ray imaging satellite, similar to Yohkoh, to be launched next year.
And NASA is planning a spacecraft mission, Solar Terrestrial Relations Observatory, to make three-dimensional observations of the dynamics of coronal mass eruptions and their surrounding regions.
The ultimate goal, scientists said, is to be able to include early warnings of stormy solar weather in space in the same way meteorologists make long-range forecasts of weather on Earth.

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