SOHO craft gets the lowdown on
Sunspots have fascinated scientists ever since Galileo sketched these
dark, Earth-size blemishes in 1611 and shattered the notion that the sun
is a divine sphere devoid of flaws. Now known as sites of intense
magnetic activity that can hurl flares and belch clouds of ionized gas
toward Earth, sunspots have puzzled astrophysicists for decades.
It's easy to see why they've been perplexed. Because the bundles of
magnetic fields that cluster in a sunspot all point in the same
direction, they should repel each other-just as bar magnets do when
their like poles are brought together. Yet somehow, instead of flying
apart, the fields of a sunspot stay bundled, enabling sunspots to last
for days to weeks.
By using sound waves to obtain the first clear picture of the structure
beneath the surface of a
sunspot, scientists now have a solution to the puzzle, they say Beneath
the surface lies a planet-size hurricane that pulls in ionized gas. The
inflow of gas acts like a collar,
keeping the magnetic fields together and the sunspot intact.
Dynamics of a sunspot: A planet-size hurricane below
the surface of a sunspot (expanded area) funnels
ionized gas, stabilizing the sunspotís magnetic
structure. Arrows show flow of gas.
Alexander G. Kosovichev of Stanford University in Palo Alto, Calif.,
reported the findings last week at a press conference in Washington,
D.C. He and his colleagues Junwel Zhao of Stanford and Thomas L. Duvall
Jr. of NASA's Goddard Space Flight Center in Greenbelt, Md., also had
detailed some of their work in the Aug. 10 ASTROPHYSICAL JOURNAL.
To probe the inside of a sunspot, the team used an instrument aboard the
SOHO (Solar and Heliospheric Observatory) spacecraft that detects
ripples on the sun's surface. The ripples are caused by low-frequency
sound waves generated by roiling gases inside the solar cauldron (SN:
3/18/00, p. 183). The sound waves reveal interior structures because the
waves travel faster through regions with higher temperatures and
stronger magnetic fields.
Kosovichev and his collaborators used the SOHO detector to study a
sunspot in June 1998. Measuring the speed of sound
waves in the solar orb, they produced a map of the region extending from
the sunspot's surface to some 16,000 kilometers below it.
The surface of a sunspot is cooler and darker than adjacent regions
because its bundled magnetic fields act like a plug, capping the flow of
heat from the solar interior. The team's analysis reveals that sound
waves near the surface of the June sunspot move 10 percent slower than
their average speed for the sun's surface. This indicates a lower
temperature than the surrounding surface.
Sunspots keep their cool, but it's only skin deep. The study shows that
at a shallow depth-less than 1 percent of the distance to the sun's
core-the sound waves speed up significantly. This implies that beneath
their surface, sunspots are in fact hotter than their surroundings.
The data also indicate that above the shallow magnetic plug, gas cools
and grows denser, and it sinks rapidly. This leaves a void, which is
filled by hotter gas and associated magnetic fields sucked toward the
sunspot from nearby areas. This vortex increases pressure on the
sunspot, preventing its magnetic field from dispersing. As a result, the
sunspots stay intact, notes Kosovichev.
Moreover, the additional magnetic fields from the inflowing gas
strengthen the plug. This prompts further cooling and sinking of gas
from the surface and perpetuates the process that maintains sunspots,
the team says.
"If confirmed, the reported results represent a major step forward
in our understanding of sunspots," says Robert Rosner of the
University of Chicago.
Science News, VOL. 160 November 17, 2001