THE
SUN'S DARK SECRET: HOW SUNSPOTS PULL THEMSELVES TOGETHER
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What
Lies Beneath a Sunspot Image 1 |
| Scientists
now have the first clear picture of what lies beneath sunspots, enigmatic planet-sized
dark areas on the Sun's surface, and have peered inside the Sun to see swirling
flows of electrified gas or plasma that create a self-reinforcing cycle, which
holds a sunspot together. The
new research, gathered from the Michelson Doppler Imager (MDI) onboard the Solar
and Heliospheric Observatory (SOHO) spacecraft, will deepen understanding of the
stormy areas on the Sun in which sunspots appear. Vast explosions associated with
these active magnetic regions occasionally affect high-technology systems. Sunspots
have fascinated people since Galileo's observations of them contradicted the common
belief that heavenly objects were flawless. Sunspots remain mysterious because
at first glance, it seems they should rapidly disappear. Instead, they persist
for weeks or more. "They obey what is a fundamental finding of observational
science: Anything that does happen, can happen," said Philip Scherrer of
Stanford University, Palo Alto, Calif., Principal Investigator for SOHO's MDI.
"We now have a hint at 'how.'"
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Under
the Sunspot movie Image 2 | |
Astronomers
know sunspots are regions where magnetic fields become concentrated. Yet, anyone
who played with magnets as a child has felt how magnetic fields of like polarities
repel each other. The strong solar magnetic fields should naturally repel each
other also, causing the sunspot to dissipate. In fact, observations show that
surface material clearly flows out of the spots. Alexander
Kosovichev and Junwei Zhao of Stanford University and Thomas Duvall of NASA's
Goddard Space Flight Center, Greenbelt, Md., used MDI's unique ability to look
just below the sunspot's surface and clearly observed inward-flowing material
for the first time. The Astrophysical Journal published their research August
10.
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Internal
Ebb and Flow movie Image 3 | |
"We
discovered that the outflowing material was just a surface feature," said
Zhao. "If you can look a bit deeper, you find material rushing inward, like
a planet-sized whirlpool or hurricane. This inflow pulls the magnetic fields together." Solar
astronomers have long known that the intense magnetic field below a sunspot strangles
the normal up-flow of energy from the hot solar interior, leaving the spot cooler
and therefore darker than its surroundings. The suppression of the bubbling convective
motions forms a kind of plug that prevents some of the energy in the interior
from reaching the surface.
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What
is a sunspot? Image 4 | |
The
material above the plug cools and becomes denser, causing it to plunge downward
at up to 3,000 miles per hour, according to the new observations. That draws the
surrounding plasma and magnetic field inward toward the sunspot's center. The
concentrated field promotes further cooling, and as that cooling plasma sinks
it draws in still more plasma, thereby setting up a self-perpetuating cycle. As
long as the magnetic field remains strong, the cooling effect will maintain an
inflow that makes the structure stable. The superficial outflows seen right at
the surface are confined to a very narrow layer. Since
the magnetic plug prevents heat from reaching the solar surface, the regions beneath
the plug should become hotter. A June 1998 observation provided evidence for this
also. "We were surprised at how shallow sunspots are," said Kosovichev.
Below 3,000 miles the observed sound speed was higher, suggesting that the roots
of the sunspots were hotter than their surroundings, just the opposite of the
conditions at the surface. "The cool part of a sunspot has the shape of a
stack of two or three nickels," he added.
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Galileo's
Sunspots Image 5 | |
"The
cool downward flows dissipate at the same depth where the hot upward flows diverge,"
said Duvall. "With these data one cannot get a sharp enough picture to really
explain the details. Until now we've looked down at the top of sunspots like we
might look down at the leaves in treetops. For the first time we're able to observe
the branches and trunk of the tree that give it structure. The roots of the tree
are still a mystery." MDI
explores beneath the surface of the Sun by analyzing sound-generated ripples at
its surface using a technique called acoustic tomography -- a novel method similar
to ultrasound diagnostics in medicine that use sound waves to image structures
inside the human body. SOHO continues to mark an era of successful partnership
between the European Space Agency and NASA within the Solar Terrestrial Science
program.
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TRACE
Coronal Loops Image 6 | |
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TRACE
satellite image Image 7 | |
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TRACE
satellite image Image 8 | |
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