The sun's surface seethes with flares, sunspots, and curious jets of gas that spout upward like geysers. Now, after more than a century of mystery, researchers think they have figured out what causes the jets: constant pulsations inside the sun that drive gas up through narrow, magnetically confined tubes. The scenario, described in the 29 July issue of Nature, intrigues solar physicists but hasn't yet convinced them.
Solar scientists first saw the spiky jets, called "spicules," in 1877. Since then, they've learned that about 100,000 spicules constantly dot the sun, rifling more than 5000 kilometers high within a few minutes. Most of the gas cascades back onto the sun, but a small fraction may escape into the outer atmosphere to feed the solar wind--a steady stream of particles that blows far into space. Physicists found it hard to determine how spicules arise, because they are just 500 kilometers wide--close to the limit of what most solar telescopes can see clearly.
The new observations expose the dynamics of spicules in striking
detail. A team led by solar physicist Bart De Pontieu of the
Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto,
California, used a Swedish telescope equipped with a rapidly
flexing mirror, which cancels the blur caused by Earth's air. The
crisp images of the solar surface, combined with satellite photos
of the sun's lower atmosphere, revealed when and where individual
spicules appear.The team discovered that the jets ebb and flow
roughly every 5 minutes, often from the same spot. That matches the
main rhythm of acoustic waves that echo through the sun's interior,
forcing the gaseous surface to throb up and down. A model developed
by co-author Robertus Erdélyi of the University of
Sheffield, U.K., shows that certain alignments of the sun's
magnetic fields can channel the vibrations through narrow tunnels
in the fields, driving spicules upward like regular thrusts from a
piston."It's an impressive and exciting paper, but their case is
not yet complete," says solar physicist Alphonse Sterling of NASA's
Marshall Space Flight Center in Huntsville, Alabama. Both Sterling
and solar physicist Jacques Beckers of the University of Chicago in
Illinois note that the team based their model on spicules observed
indirectly, as cooler ribbons of gas that appear dark in the sun's
hot atmosphere. "I'm not convinced those features are the same as
the spicules we see elsewhere on the sun," Beckers says.Related
sites
More
spicule images and movies
Primer on
solar physics
Swedish
Solar Telescope, used in the study TRACE satellite, also used in the
study
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