Strange 'spin cycle'
inside the sun may explain sunspots, solar flares and
winds
BY MARK SHWARTZ
Few things in the universe
seem as constant as the sun.
But now scientists have
discovered that two parallel layers of gas deep beneath
the solar surface are actually speeding up and slowing
down in a strange, synchronous pattern.
It turns out that, as the
sun rotates on its axis, one gas layer gradually spins
faster while the other reduces speed.
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Scientists are at a loss
to explain the phenomenon, which occurs in regular
12-to-16-month cycles.
"It's not what we
expected at all," says Stanford research physicist
Jesper Schou. "It comes totally out of the
blue."
Schou is part of an
international team of researchers using satellite and
ground-based observatories to monitor the sun.
Writing in the March 31
issue of the journal Science, Schou and
postdoctoral fellow Rasmus Larsen point out that these
unusual but predictable changes in rotational speed only
occur above and below a section of the sun known as the
interface layer or tachocline.
Located about 135,000
miles below the solar surface, the tachocline separates
the sun's two major regions of gas: the radiative zone,
which includes the energy-generating core, and the
convective zone near the surface (see illustration
below).
Interior of
the sun
Courtesy
NASA
Solar experts believe that
the tachocline may be the source of powerful magnetic
fields that produce strong solar flares and solar winds,
and create sunspots that mysteriously appear and
disappear during an 11-year cycle.
No one knows how the sun's
enormous magnetic fields are generated, or why they
reverse polarity from positive to negative every 11
years.
But the discovery that the
area surrounding the tachocline varies its rotation in a
regular pattern could be a clue to solving the mystery.
The research team used
independent data from two instruments to detect changes
in the solar spin-rate between May 1995 and Nov. 1999.
Stanford team members
Schou and Larsen used data from the Michelson Doppler
Imager (MDI) on board the Solar and Heliospheric
Observatory (SOHO).
Launched in 1995, SOHO is
positioned about a million miles from Earth.
The MDI instrument on
board SOHO creates a picture of the solar interior by
measuring millions of sound waves that constantly
ricochet inside the sun.
Independent observations
of the sun were made by the Global Oscillation Network
Group (GONG), a team of scientists that collects data
from six Earth-based solar observatories.
Funded by the National
Science Foundation, GONG is managed by researchers from
the Tuscon-based National Solar Observatory, including
Rachel Howe, lead author of the March 31 Science
article.
After analyzing
approximately four years of data, the MDI and GONG teams
came up with remarkably similar findings.
Their results showed that
the convective zone just above the tachocline gradually
increased its rotational speed by about 60 feet per
second between July 1996 and Feb. 1997, then slowly
returned to its original velocity some eight months
later.
Meanwhile, the radiative
zone just below the tachocline demonstrated the exact
opposite behavior, slowing down between July and
February, then gradually accelerating eight months later.
This cycle of acceleration
and deceleration repeated itself roughly every 16 months,
or 1.3 years, at the equator, but only recurred every 12
months at the 60-degree latitude.
The discovery that the
inner sun spins at different rates at different latitudes
is consistent with earlier studies showing that the
surface of the sun also rotates at different speeds.
For example, at the
equator, it takes about 25 days for the surface of the
sun to rotate on its axis, but at the poles, surface
rotation requires roughly 33 days.
That's because the sun is
made of gas, so different parts of its surface spin
independently unlike the surface of Earth, Mars and
other solid planets.
But why are the gas layers
above and below the tachocline speeding up and slowing
down at opposite rates?
Perhaps this puzzling
behavior is somehow related to the mysterious forces that
generate the sun's magnetic field and the 11-year sunspot
cycle.
"For the interior to
change speed every 11 years would make sense," notes
Schou. "But a 1.3-year period was unexpected. We
don't know what it means, but isn't it interesting!"
And practical, too,
because if researchers can determine what drives the
sun's magnetic field, they also may be able to forecast
solar flares and winds that can knock out satellites,
increase the risk of radiation to airline passengers and
even cause power outages on Earth.
The ability to predict
solar storms could help people avoid such incidents and
may even provide researchers insight into the sun's
long-term impact on the Earth's climate.
The SOHO spacecraft, which
is jointly operated by NASA and the European Space
Agency, made headlines in 1998 when it temporarily
stopped functioning an event that caused MDI
scientists to lose five months of solar data.
The SOHO mission is
scheduled to end in 2003, but Schou and his colleagues
would like to continue their observations to see if the
12-to-16-month rotation cycle varies year after year.
NASA has announced plans
to replace SOHO with the Solar Dynamics Observatory
(SDO).
If launched, SDO would
remain in geosynchronous orbit about 22,000 miles above
the Stanford campus, allowing researchers to start
downloading new acoustical data from the sun beginning in
2006.
Stanford Professor Philip
Scherrer, principal investigator of MDI, hopes that NASA
will obtain funding for SDO, allowing space-based solar
research to continue.
"It's pretty neat to
look at that kind of detail inside a star," says
Scherrer. "It's really fun!"
The March 31 Science
article is co-authored by Rachel Howe, Frank Hill and
Rudi Komm of the National Solar Observatory (NSO);
Jørgen Christensen-Dalsgaard of Aarhus University in
Denmark; Michael Thompson of Queen Mary & Westfield
College in London; Juri Toomre of the University of
Colorado, Boulder; and Rasmus Munk Larsen and Jesper
Schou of Stanford. SR
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