Magnetic twists and turns
Will we ever be able to truly predict CMEs? Today, forecasts issued shortly after a CME emerges from the Sun provide warnings from hours to several days in advance of a potential geomagnetic storm. To give even more lead time, scientists will have to learn the precursors of CMEs. They'll need to illuminate the plasma contortions below the solar surface that give birth to a CME and the coronal magnetic fields that shape its evolution. Spotting a newborn CME is now routine, but viewing the magnetism that lies at its heart--and throughout the surrounding corona--isn't so easy.
Because there's so little mass in the corona, it's a major technical challenge to detect the presence of coronal magnetic fields in the weak light it emits. "The only things we have to work with are extrapolations of coronal magnetic field from the more readily measured field in the photosphere," says Gibson. But magnetic activity in the denser photosophere, the lowest layer of the Sun's atmosphere, doesn't provide enough information on its own to specify the magnetic field in the much thinner corona higher up.
As a CME emerges (above), it typically contains a lower-density cavity (lighter area at center) behind its crest (dark band at right), and it often has a bright prominence core embedded within it (the line labeled at center left). The magnetic field of the corona is responsible for creating the three-part structure of front, cavity and core seen in CME photos such as the ones on page 6. (Illustration adapted from Low and Hundhausen, Astrophysical Journal 443 (1995), pg. 818.)
Since 1998, NASA's Transition Region and Coronal Explorer satellite (TRACE) has parted the curtains somewhat. With a tight focus on small regions, it measures how the magnetic field shapes coronal plasma from the photosphere up through the corona at a horizontal resolution as small as one arc second.
While intrigued by the TRACE images, coronal experts have been tantalized by what's still unseen. The arching structures uncovered by TRACE denote only a few of the corona's intricately nested magnetic field lines--arches within arches, as it were. To help see these multilayered structures, many coronal specialists have turned to animation. Technology is on their side: desktop computers and software packages are now powerful enough to produce useful animations in short order.
The Transition Region and Coronal Explorer (TRACE) satellite captured this image of a magnetically active region at the edge of the Sun. The cylindrical loops connect one magnetic polarity to the other. (Image courtesy NASA.)
Gibson is using visualization routines based on modeling by Yuhong Fan to see how an idealized twisted tube of magnetic flux--a CME in the making--might appear in observations. She concentrates on a sigmoidal (S-shaped) portion of the twisted field (the purple lines in the illustration below). This zone is the interface between field lines that are firmly tethered to the dense solar surface and those field lines that have a portion suspended in the atmo-sphere and can move more freely. "This is the region where heating is likely to happen during an eruption, or to a lesser extent as the flux rope generally jiggles about," says Gibson.
Recent X-ray data show that hot coronal gas can take on a sigmoidal structure a few days to weeks before a CME emerges in the same area. While the relationship isn't fail-safe, and it currently has limited use as a forecasting tool, "the link is definitely intriguing from a scientific point of view," says Gibson. "If we can figure out the science behind the eruptions, we'll be in a much better position for making future forecasts."
This computer graphic by Sarah Gibson, based on modeling from Yuhong Fan, shows areas likely to be heated during coronal eruptions (group of purple field lines), overlaid on other sample field lines from an emerging magnetic flux rope. Color contours at lower boundary represent the normal magnetic field at the Sun's surface.
NCAR's Mk4 coronagraph at Mauna Loa is giving David Foster yet another angle on the CME mystery. A graduate student at the University of Colorado, Foster is examining cavities, the low-density regions behind the crest of a CME. Some cavities appear in the corona well before a CME occurs, he says, hinting that the flux-tube structure believed to frame a cavity may be in place as well. The analysis isn't easy, however, says Foster: "You have to be looking down these tunnel-like cavities in order to see them."
Only about 30% of CMEs have been linked to sigmoidal structures, so there is still plenty of room for solar specialists to debate what other processes might be at work. Some of the debates may not last long, though. A new array of space-based instruments is on NASA's drawingboard, including a pair of satellites that would bracket the Sun and give the first coordinated, multidimensional view of CMEs. Meanwhile, NCAR's coronal multichannel polarimeter (see sidebar) may soon provide the first-ever Sun-wide readings of the coronal magnetic field.
"It's something we've really wanted to do well for years,” says Holzer.
by Bob Henson
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