In what may be one of the most important steps in understanding sunspots since they were discovered by Chinese sky watchers more than two millennia ago, researchers have discovered that the lines of magnetic force that surge out of sunspots appear to peel apart like husk off an ear of corn as some of the lines are dragged back beneath the surface by a sort of solar quicksand. This "quicksand" and the magnetic fields it bends create the penumbrae around some sunspots, the strange rings of mid-darkness that have eluded explanation by astronomers since Galileo first sketched them. With the help of sophisticated computer models and data from solar telescopes that give spectacular views of the sun, researchers at the University of Rochester, University of Colorado, University of Cambridge, and University of Leeds have reported an answer to several mysteries of sunspots in the current issue of Nature.
"We believe we have found the key to understanding the structure of sunspots," says John H. Thomas, professor of mechanical and aerospace sciences and of astronomy at the University of Rochester. "It's the missing link of sunspot evolution-explaining why the main magnetic tube gets torn apart like a peeled banana, why some lines of force dive back below the surface of the sun, and why sunspots grow a penumbra in the first place."
Sunspots are created when giant bundles of magnetic lines, or flux tubes, wider than the Earth, rise up from deep within the sun and expand out into space. The force exerted by these tubes tends to inhibit the motion of the normally roiling gas just below the surface, slowing down the transfer of heat from deep in the sun to the surface. This is why sunspots are dark, because they are relatively cool-a chilly 6,000 degrees Fahrenheit instead of the regular 10,000. But dark sunspots are ringed by a region brighter than the spot's center, yet still darker than the rest of the sun's surface. These penumbrae first puzzled Galileo four centuries ago, and when modern researchers found that they consist of magnetic tubes bent in ways that made little sense, the mystery only deepened.
The team (Thomas, Nigel O. Weiss, professor of mathematical astrophysics at the University of Cambridge, Nicholas H. Brummel, professor of astrophysical and planetary sciences at the University of Colorado, and Steven M. Tobias, professor of applied mathematics at the University of Leeds) tackled the problem from a theoretical perspective using sophisticated computer simulations of the surface layers of the sun, and the results show a sort of magnetic quicksand lurking along the surface of the sun.
The giant magnetic flux tube that forms a sunspot expands outward as it emerges through the solar surface, and some of the outer parts of the tube are pulled downward while the rest continues out into space, much like the husk being peeled from an ear of corn. Thomas and Weiss realized that the convection currents outside the sunspot were pumping those peels back down into the sun and keeping them there. Together with Tobias and Brummell, they modeled these convection currents in which hotter gas rises and lifts the magnetic flux and cooler gas sinks and depress the flux, and found that the currents do not lift and sink in the same way. This difference is the key to why some of the sunspot's magnetic field is pulled back down.
The hot gas lifts like a geyser in a wide column, but as this gas gives up its heat to space and becomes cooler and denser it begins to sink between other upcoming geysers, forming a thin, sheet-like flow. Since this flow is thinner than the rising column, it must flow faster-the way a river runs faster in its narrowest sections. Magnetic fields respond more to the faster-moving fluids than the slower ones, so the descending sheets of gas dominate, dragging sections of the sunspot's magnetic tube deep into the sun as they fall.|
The bent-over lines of magnetic force create the penumbra, and likely also cause other observed phenomena in and around sunspots, such as the long channels of gas that stream out of sunspots. These flows follow the magnetic lines and are pushed along by pressure differences created by the downward-pumping effect.
This research was funded by NASA, the United Kingdom Particle Physics and Astrophysics Research Council, and the Nuffield Foundation.