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Oct.
2, 2008: Scientists using NASA's RHESSI spacecraft
have measured the roundness of the sun with unprecedented
precision, and they find that it is not a perfect sphere.
During years of high solar activity the sun develops a thin
"cantaloupe skin" that significantly increases its
apparent oblateness. Their results appear the Oct. 2nd edition
of Science Express.
"The
sun is the biggest and smoothest natural object in the solar
system, perfect at the 0.001% level because of its extremely
strong gravity," says study co-author Hugh Hudson of
UC Berkeley. "Measuring its exact shape is no easy task."
The
team did it by analyzing data from the Reuven Ramaty High-Energy
Solar Spectroscopic Imager, RHESSI for short, an x-ray/gamma-ray
space telescope launched in 2002 on a mission to study solar
flares. Although RHESSI was never intended to measure the
roundness of the sun, it has turned out ideal for the purpose.
RHESSI observes the solar disk through a narrow slit and spins
at 15 rpm. The spacecraft's rapid rotation and high data sampling
rate (necessary to catch fast solar flares) make it possible
for investigators to trace the shape of the sun with systematic
errors much less than any previous study. Their technique
is particularly sensitive to small differences in polar vs.
equatorial diameter or "oblateness."
Above:
"Cantaloupe ridges" on the sun. The glowing white
magnetic network is what gives the sun its extra oblateness
during times of high solar activity. Los Angeles astronomer
Gary Palmer took the
picture in July 29, 2005, using a violet calcium-K solar filter.
[larger image]
"We
have found that the surface of the sun has rough structure:
bright ridges arranged in a network pattern, as on the surface
of a cantaloupe but much more subtle," describes Hudson.
During active phases of the solar cycle, these ridges emerge
around the sun's equator, brightening and fattening the "stellar
waist." At the time of RHESSI's measurements in 2004,
ridges increased the sun's apparent equatorial radius by an
angle of 10.77 +- 0.44 milli-arcseconds, or about the same
as the width of a human hair viewed one mile away.
"That
may sound like a very small angle, but it is in fact significant,"
says Alexei Pevtsov, RHESSI Program Scientist at NASA Headquarters.
Tiny departures from perfect roundness can, for example, affect
the sun's gravitational pull on Mercury and skew tests of Einstein's
theory of relativity that depend on careful measurements of
the inner planet's orbit. Small bulges are also telltale signs
of hidden motions inside the sun. For instance, if the sun had
a rapidly rotating core left over from early stages of star
formation, and if that core were tilted with respect to its
outer layers, the result would be surface bulging. "RHESSI's
precision measurements place severe constraints on any such
models."
The
"cantaloupe ridges" are magnetic in nature. They
outline giant, bubbling convection cells on the surface of
the sun called "supergranules." Supergranules are
like bubbles in a pot of boiling water amplified to the scale
of a star; on the sun they measure some 30,000 km across (twice
as wide as Earth) and are made of seething hot magnetized
plasma. Magnetic fields at the center of these bubbles are
swept out to the edge where they form ridges of magnetism.
The ridges are most prominent during years around Solar Max
when the sun's inner dynamo "revs up" to produce
the strongest magnetic fields. Solar physicists have known
about supergranules and the magnetic network they produce
for many years, but only now has RHESSI revealed their unexpected
connection to the sun's oblateness.
Right:
In this diagram, the sun's oblateness has been magnified 10,000
times for easy visibility. The blue curve traces the sun's
shape averaged over a three month period. The black asterisked
curve traces a shorter 10-day average. The wiggles in the
10-day curve are real, caused by strong magnetic ridges in
the vicinity of sunspots. [larger
image]
"When
we subtract the effect of the magnetic network, we get a 'true'
measure of the sun's shape resulting from gravitational forces
and motions alone," says Hudson. "The corrected
oblateness of the non-magnetic sun is 8.01 +- 0.14 milli-arcseconds,
near the value expected from simple rotation."
"These
results have far ranging implications for solar physics and
theories of gravity," comments solar physicist David
Hathaway of the NASA Marshall Space Flight Center. "They
indicate that the core of the sun cannot be rotating much
more rapidly than the surface, and that the sun's oblateness
is too small to change the orbit of Mercury outside the bounds
of Einstein's General Theory of Relativity."
Further
analysis of RHESSI oblateness data could also help researchers
detect a long-sought type of seismic wave echoing through
the interior of the sun: gravitational oscillations or "g-modes."
The ability to monitor g-modes would open a new frontier in
solar physics—the study of the sun's internal core.
"All
of this," marvels Hathaway, "comes from clever use
of data from a satellite designed for something entirely different.
Congratulations to the RHESSI team!"
The
paper reporting these results, "A large excess in apparent
solar oblateness due to surface magnetism," was authored
by Martin Fivian, Hugh Hudson, Robert Lin and Jabran Zahid,
and appears in the Oct. 2nd issue of Science Express.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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