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June
17, 2009: The
sun is in the pits of a century-class solar minimum, and sunspots
have been puzzlingly scarce for more than two years. Now,
for the first time, solar physicists might understand why.
At
an American Astronomical Society press conference today in
Boulder, Colorado, researchers announced that a jet stream
deep inside the sun is migrating slower than usual through
the star's interior, giving rise to the current lack of sunspots.
Rachel
Howe and Frank Hill of the National Solar Observatory (NSO)
in Tucson, Arizona, used a technique called helioseismology
to detect and track the jet stream down to depths of 7,000
km below the surface of the sun. The sun generates new jet
streams near its poles every 11 years, they explained to a
room full of reporters and fellow scientists. The streams
migrate slowly from the poles to the equator and when a jet
stream reaches the critical latitude of 22 degrees, new-cycle
sunspots begin to appear.
Above:
A helioseismic map of the solar interior. Tilted red-yellow
bands trace solar jet streams. Black contours denote sunspot
activity. When the jet streams reach a critical latitude around
22 degrees, sunspot activity intensifies. [larger
image] [more
graphics]
Howe
and Hill found that the stream associated with the next solar
cycle has moved sluggishly, taking three years to cover a
10 degree range in latitude compared to only two years for
the previous solar cycle.
The
jet stream is now, finally, reaching the critical latitude,
heralding a return of solar activity in the months and years
ahead.
"It
is exciting to see", says Hill, "that just as this
sluggish stream reaches the usual active latitude of 22 degrees,
a year late, we finally begin to see new groups of sunspots
emerging."
The
current solar minimum has been so long and deep, it prompted
some scientists to speculate that the sun might enter a long
period with no sunspot activity at all, akin to the Maunder
Minimum of the 17th century. This new result dispells those
concerns. The sun's internal magnetic dynamo is still operating,
and the sunspot cycle is not "broken."
Because
it flows beneath the surface of the sun, the jet stream is
not directly visible. Hill and Howe tracked its hidden motions
via helioseismology. Shifting masses inside the sun send pressure
waves rippling through the stellar interior. So-called "p
modes" (p for pressure) bounce around the interior and
cause the sun to ring like an enormous bell. By studying the
vibrations of the sun's surface, it is possible to figure
out what is happening inside. Similar techniques are used
by geologists to map the interior of our planet.
In
this case, researchers combined data from GONG and SOHO. GONG,
short for "Global Oscillation Network Group," is
an NSO-led network of telescopes that measures solar vibrations
from various locations around Earth. SOHO, the Solar and Heliospheric
Observatory, makes similar measurements from space.
"This is an important discovery," says Dean Pesnell
of NASA's Goddard Space Flight Center. "It shows how
flows inside the sun are tied to the creation of sunspots
and how jet streams can affect the timing of the solar cycle."
 There
is, however, much more to learn.
"We
still don't understand exactly how jet streams trigger sunspot
production," says Pesnell. "Nor do we fully understand
how the jet streams themselves are generated."
To
solve these mysteries, and others, NASA plans to launch the
Solar Dynamics Observatory (SDO) later this year. SDO is equipped
with sophisticated helioseismology sensors that will allow
it to probe the solar interior better than ever before.
Right:
An artist's concept of the Solar Dynamics Observatory. [more]
"The
Helioseismic and Magnetic Imager (HMI) on SDO will improve
our understanding of these jet streams and other internal
flows by providing full disk images at ever-increasing depths
in the sun," says Pesnell.
Continued
tracking and study of solar jet streams could help researchers
do something unprecedented--accurately predict the unfolding
of future solar cycles. Stay tuned for that!
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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