Dark Edge of Sunspots Reveal Magnetic Melee

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 last 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.