Scientists begin to unravel a stubborn solar mystery

To astronomers, one of the sun's most puzzling mysteries has centered on why the star, fiercely hot at its core, cools with distance, only to have temperatures in its extended outer atmosphere soar again.

"It's as though you walk away from a campfire, get cooler and cooler, then you start getting hotter again," says John Leibacher, an astronomer at the National Solar Observatory in Tucson, Ariz.

In research to be published next month, astronomers at Lockheed Martin's Solar and Astrophysics Laboratory in Palo Alto, Calif., have taken a crucial step toward solving the mystery. In the process, some scientists say, the team's work may shed light on enormous outbursts of charged particles that can wreak havoc on satellites, trigger power blackouts on Earth, and send ribbons of blue, green, and red aurora weaving through the night skies.

While they don't yet know the reason for the strange temperature pattern, their research has given scientists a new understanding of where the sun's corona is heated - toppling long-held theories and getting astronomers closer to answers.

"This overturns a picture basic to the field for about 30 years," says Craig DeForest, an astrophysicist at the Southwest Research Institute in Boulder, Colo. "This is important to all aspects of the field."

Until now, conventional wisdom had held that the corona was like a thin gaseous atmosphere, heated uniformly.

This led to at least 10 competing explanations for the high temperatures in the sun's corona. (The sun's core burns at about 27 million degrees F., its surface falls to about 10,000 degrees, then the corona - stretching millions of miles into space - rockets back up to between 1.8 million and 9 million degrees.)

But the Lockheed Martin team, using data from NASA's TRACE satellite, found that the corona is heated unevenly from below, as if by enormous numbers of gas jets that emerge from beneath the corona.

"To solve the [heating] puzzle," says Lockheed astronomer Markus Aschwanden, "you first have to know where the corona is heated, then you can sort out the physical mechanisms. We have identified where the heating occurs."

Observations from TRACE, launched into Earth orbit in 1998, have shown that the sun's corona is like a loose-weave carpet. Magnetic fields rise and fall in massive loops made up of many "threads" of magnetic fields, some of which are perhaps as narrow as 60 miles.

These threads act as conduits along which hot, electrically charged gas travels.

The team studied 41 loops that extend to distances ranging from about 2,500 miles to more than 180,000 miles. The researchers found that threads in shorter loops appear to heat uniformly along their length, but the longer threads tend to cool noticeably with height.

Calculations involving the loops' energy levels suggested that they are heated from below. The team estimates that heating typically occurs in the first 6,000 miles of a loop's length.

Moreover, light from the loops brightens and fades, suggesting that energy and mass are being injected into the loops in pulses, Dr. Aschwanden says.

He adds that the pulses may result as the threadlike magnetic fields break and reconnect - a process driven by stresses they undergo as they try adjust to the random motion of the magnetic field on the solar surface.

Understanding how these coronal loops work can shed light on their much-larger but rarer cousins, coronal-mass ejections, according Richard Fisher, who heads the Laboratory for Astronomy and Solar Physics at NASA's Goddard Space Flight Center in Greenbelt, Md.

When these outbursts are directed toward Earth, the charged particles they hurl outward can disrupt or destroy satellite components, and prompt surges in electrical transmission lines that can cause blackouts and touch off brilliant auroral displays.

Understanding the mechanics of these loops could improve efforts to forecast such solar storms, he says. "The cause of coronal-mass ejections has never really been settled," Dr. Fisher notes. "One of the big questions has been: What does a coronal-mass ejection look like close to the sun? If it looks like this, then what would make it happen" as suddenly as it does?

(c) Copyright 2000. The Christian Science Publishing Society