Like hair energized by static electricity, thousands upon thousands of glowing
tendrils reach out from the Sun. Since these "spicules" of hot gas were discovered
in 1877, scientists have been scratching their heads over what causes them.
New observations by three observatories coupled with powerful computer modeling
appear to have solved the mystery.
The spikes of superheated gas, called plasma, are small compared to many of
the Sun's prominent features, such as giant loops of magnetic energy that are
flung many thousands of miles into the solar atmosphere. The spicules are typically
300 miles (480 km) in diameter and shoot a relatively modest 3,000 miles (4,830
km) above the Sun's surface. They scream upward at 50,000 mph (22 kps) and then
vanish within five minutes, making them hard to study.
More than 100,000 spicules tickle the solar atmosphere, called the corona,
at any time.
Coincident cycles
The key to their origin is in the regular cycle with which spicules regenerate.
Every five minutes or so, one leaps forth from the same spot, according to the
new study. It so happens that the Sun has a well-known inner ring, much like
that of a bell, that works on the same cycle.
"We developed a computer model of the Sun's atmosphere to show that the
periodicity of the spicules is caused by sound waves at the solar surface that
have the same five-minute period," said Robertus Erdelyi von Fáy-Siebenbürgen,
a professor of applied mathematics at the University of Sheffield, UK.
The ringing of the Sun involves seismic waves similar to those in an earthquake.
They are usually damped down before they can leave the roiling surface and so
continue bouncing around inside the Sun.
"They're basically sound waves that are trapped," said Bart De Pontieu, a solar
physicist at the Lockheed Martin Solar and Astrophysics Lab. "For the most part
they can't escape to the outside."
But the study found that some of the sound waves leak out, developing shock
waves that propel matter upward to generate spicules and push them into the
corona, De Pontieu explained in a telephone interview.
Feeding the solar wind?
The research team now wonders if spicules might contribute to the solar
wind, a constant stream of charge particles emanating from the Sun and coursing
through the solar system.
Scientists don't know exactly how the mass of the solar wind gets into the
corona nor what accelerates the particles into space. The spicules are a good
candidate, as they carry into the atmosphere more than 100 times the mass needed
to feed the solar wind, the new study concludes.
"Most of the mass of the spicules comes back down," De Pontieu said. But only
a fraction would have to remain high up to account for the solar wind. However,
he said, it's not clear how much if any of the mass from the spicules remains
in the corona, or if it is heated to the extent that appears to be required
to create the super energetic particles of the solar wind.
Oddly, the corona is 1.8 million degrees Fahrenheit
(1 million Celsius), while the Sun's surface is typically no more than 11,000
degrees Fahrenheit (6,000 Celsius). The reason remains one of the great mysteries
of the Sun.
Group activity
Previous studies had found hints of the 5-minute cycle of spicules. But scientists
had only studied them on the limb of the Sun, the periphery of the hot disk
as seen from our vantagepoint. The new observations allowed the researchers
to examine spicules across the face of the Sun.
The investigation showed that spicules occur in groups.
Spicules are usually associated with concentrated magnetic tubes called flux
tubes, giant loops that link one region of magnetic polarity to another by arcing
out from the surface and back to another spot thousands of miles away. The tubes
are mostly vertical, but some are inclined away from the vertical, and that's
where the sound waves tend to leak out, De Pontieu said.
The findings are detailed in the July 29 issue of
the journal Nature. The work was based on observations by the Swedish
Solar Telescope on the Spanish island of La Palma, as well as the orbiting SOHO
and TRACE observatories.
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