Controversy Erupts
Over Theory On
Solar Coronal Mass
Ejections (CMEs)
By Robert Roy Britt
Senior Science Writer
As dawn breaks on our understanding of the sun's eruptions a controversial idea has re-emerged that attempts to explain the source of coronal mass ejections -- those periodic enormous releases of energy that threaten satellites, cell phones and even power grids.
The theory, being presented this week by scientists from the Naval Research Laboratory at a plasma physicists meeting in Seattle, posits that the triggering mechanism for these ejections comes from deeper within the star than current theories hold.
Proponents say the theory provides the best description of the events, but other scientists are highly skeptical. Real answers may still be years away.
But the stakes are high and the need for greater knowledge is somewhat urgent. Space weather is nearing an 11-year high as the sun ratchets ever closer to what researchers call a solar maximum -- expected early next year. Being able to understand space weather and its origins, and to predict its effects on Earth, would allow satellite operators and utility companies to prepare for events that might cripple services.
During a maximum, heightened sunspot activity corresponds with more intense and frequent coronal mass ejections. The last time around, more than a decade ago, the eruptions caused power outages and crippled satellites -- all in an era when the world was far less reliant on space-based communications.
Eruptive origins
Standard models maintain that these explosions originate in the corona -- a luminous atmosphere of energy around the sun. But the new theory claims that the eruptions start far deeper, below the photosphere, which scientists describe as the visible layer of the sun just beneath the corona.
James Chen and Jonathan Krall, of the Naval Research Laboratory, explain a process involving giant loops of magnetic energy, called "solar flux ropes" (see the illustration at the top of this page). As the electrical current in the ropes increases, they expand upward and outward until they fling their energy -- mostly electrons and protons -- into space at speeds up to 2.2 million miles per hour (1000 kilometers per second).
The researchers say this process accounts for about 30 percent of all solar eruptions.
"We have found that our proposed scenario provides the best description of observed solar eruptions," said Krall. "The degree to which the data agrees with the new theory is unprecedented."
The researchers based the current version of their model on data stretching back to 1995 from SOHO, a solar-imaging spacecraft hovering midway between Earth and the sun. They studied observations that extended outward 15 times the diameter of the sun. Krall said the new theory also accounts for "magnetic clouds" -- pockets of charged particles in space -- that researchers believe are associated with solar eruptions.
Like all theories, it's only a theory
Several astrophysicists and other experts expressed doubt about the Chen and Krall model, an earlier version of which was proposed in 1989.
"The view put forward by Dr. Chen that the energy for powering coronal mass ejections comes from below the photosphere is not generally accepted by the solar community," said Terry Forbes, a researcher in astrophysics at the University of New Hampshire.
Prevailing models hold that the sun's energy migrates to the surface more slowly, Forbes said. Pockets of concentrated energy are seen on the solar surface as sunspots. Energy is also stored in the sun's atmosphere. Some as-yet-unknown triggering mechanism forces the coronal mass ejection, which radiates outward and also pushes its way back toward the sun's surface. Evidence of this process is seen in ripples that occur on the sun's surface roughly two minutes after the event has been observed above the solar surface.
"I don't see how they can transport energy from below, and have a big display in the corona, and yet at the surface you see nothing," Forbes said in an interview.
Chen and Krall do not, in fact, describe the energy source that might trigger a release. Instead, they start with the assumption that a deep-seated source could trigger the solar flux ropes, and then they set out to describe the process.
"This theory has been tested against the observational data more carefully than any other, and with excellent results," Krall said. "We are challenging others to test their ideas in the same way."
Other responses to the Chen and Krall model showed a mix of criticism and skepticism, and also a suggestion that more data is needed to prove anything.
"This theory reflects the authors' support of what is an emerging paradigm in solar physics," said Richard Canfield, a Montana State University researcher who was involved in research published earlier this year describing an above-the-surface triggering mechanism. "There is certainly evidence that it may be correct, though there is much work to be done before the case is proven."
Joe Gurman, an observer at NASA's Goddard Space Flight Center, said the tools needed for evaluating what's going on below the sun's surface are just now being developed. New data, not yet published, reveals pockets of energy migrating to the surface -- a precursor to sunspots that scientists have never before glimpsed, Gurman told
"There are some coronal mass ejections observed by the LASCO chronograph on SOHO that suggest that this model could be correct," Gurman said. But, he added that it could be five years or more before scientists have the ability to gather the data needed to test the model.