New Explanation Of Solar Eruptions Ignites Debate

College Park, MD (November 15, 1999)--Increased danger to satellites, the threat of power outages, and a greater risk of cell phone disruptions are being forecast for early next year. That's when the Sun will experience the most active part of its 11-year cycle with a heightened burst of energy. At a meeting in Seattle this week, researchers from the Naval Research Laboratory in Washington, D.C. will present a controversial new explanation for the origin of the violent solar eruptions which can trigger these disturbances. While some scientists say their theory is premature and hard to test, others applaud it as an intriguing new idea.

"The degree to which the data agrees with the new theory is unprecedented," says Jonathan Krall of the Naval Research Laboratory. Team leader James Chen of the Naval Research Lab will present the theory at the American Physical Society Division of Plasma Physics Meeting in Seattle on November 16.

Everyone agrees that this is an important question to pursue.

"There is a compelling national interest in predicting and understanding these eruptions, because they can affect electronic communications and electric power transmission here on Earth," according to Joe Gurman of the NASA Goddard Space Flight Center in Greenbelt, Maryland. "The question of where these eruptions really originate is a very important one," he says. Answering this question, scientists believe, will ultimately improve efforts to forecast and prepare for electrical and communications disruptions on our planet.

Earth is affected by the Sun in more ways than one. In addition to shining light on our planet, the Sun erupts regularly, sending forth streams of charged particles that can pass by the Earth. These particles can disrupt the magnetic field of the Earth, creating disturbances to cell-phone communications and even knocking out power grids in extreme cases.

The prevailing theory says that the energy responsible for these eruptions, called "coronal mass ejections," comes from the corona, the Sun's outermost atmosphere. "But these theories have not been shown to reproduce observations," says Krall.

In contrast, Krall and Chen argue that the energy responsible for these eruptions is stored below the photosphere, the visible solar surface underneath the corona.

To bolster their theory, the scientists have examined a wealth of data recently collected from the LASCO (Large Angle and Spectrometric Coronagraph Experiment) instrument on the SOHO spacecraft launched in 1995. "With observations extending out to an unprecedented 30 solar radii [where one solar radius is the distance between the Sun's center and its visible surface], we were able to compare theory to observation in great detail," Krall says.

Their explanation involves the concept of "solar flux ropes," giant magnetic field loops rooted below the photosphere. According to the theory, electrical current increases along a flux rope prior to solar eruption. As the current flows, the rope's magnetic field is increased in what's called a "flux injection."

Flux injection causes the rope to expand, taking electrically charged particles (mostly protons and electrons) with it and ejecting them into interplanetary space. Each such eruption can eject up to 100 trillion grams of matter at speeds of up to 1000 kilometers per second. According to Krall, a significant fraction of all solar eruptions--more than 30%--are caused by flux ropes.

"We have evaluated several different mechanisms of eruption. The purpose of doing so was to compare our model with traditional ones," Krall says. We have found that our proposed scenario provides the best description of observed solar eruptions, as well as the properties of observed ‘magnetic clouds,'" says Krall.

"Magnetic clouds" are collections of charged particles in interplanetary space. They originate from the solar wind, streams of charged particles emitted regularly by the Sun. Believed to be associated with the solar eruptions, these clouds have distinctive magnetic fields that have been observed by spacecraft.

However, solar astrophysicists have mixed reactions to this new proposal. Here are a few:

"I believe the new model raises more problems than it solves," says Leon Golub of the Harvard-Smithsonian Center for Astrophysics. "There have been some problems with the ‘old paradigm,' but they are rapidly disappearing as the result of more recent work," he said.

"It is a well accepted observation that the photosphere remains very passive during an eruption," says Terry Forbes of the University of New Hampshire. "Although the researchers claim that flux can be injected without disturbing the surface enough to be noticeable, they have yet to provide a convincing demonstration of why this is so."

"This theory reflects the authors' support of what is an emerging paradigm in solar physics," says Richard Canfield of Montana State University. "There is certainly evidence that it may be correct, though there is much work to be done before the case is proven."

And according to Joe Gurman of NASA-Goddard, "We are at least 5 or 10 years away from the time in which we can definitively test this model and the others, using local helioseismology," in which scientists measure vibrations on the surface of the Sun. "In fact, it's only been very recently that researchers have obtained good magnetic field measurements in the corona," he says. "We need to rule out the possibility that the eruptions originate in the corona, and this will be hard to do."

"At first sight, the Chen and Krall model is as credible as any of the other explanations that are on the table, and it is certainly a new and interesting idea," says Craig DeForest of NASA-Goddard. "I'm certainly interested to see how it develops and whether it agrees with observations."