RHESSI Discovers the Sun Is Not A Perfect Sphere
- From: baalke@xxxxxxxxxxxxx
- Date: Fri, 3 Oct 2008 15:13:55 -0700 (PDT)
http://science.nasa.gov/headlines/y2008/02oct_oblatesun.htm
How Round is the Sun?
NASA Science News
10.02.2008
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."
"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.
"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 physicsâthe 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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