The core of
the Sun holds secrets into how it and the planets formed billions of years ago,
but the bright solar surface obscures the view of its heart.
Now after a
30-year search, astrophysicists may have detected hints of ripples on the
surface, just a few yards high, that could finally help shed light on the mysterious
core.
The results
suggest the
Sun's core rotates more slowly than theorists have predicted.
Long
search
Astronomers
first detected waves on the solar surface 30 years ago. Soaring miles high, those
"p-mode" waves are generated by sound running through the star. They hinted
at the existence of far more subtle ripples driven by gravity. These
"g-mode" waves are believed to occur when
gas churning below the surface plunges even deeper and collides with denser
material, sending ripples propagating through the interior and up to the
surface--the equivalent of dropping a stone in a pond.
Scientists
wanted to detect g-mode waves since they pass through the Sun's mysterious
heart and carry vital information concerning internal activity. For instance,
the speed at which the solar core rotates is uncertain. Knowing this detail
could shed light on the birth of the entire solar system, because it represents
the hub of rotation for the dusty cloud of matter that eventually formed the
sun and planets.
Unfortunately,
g-modes are badly degraded during their passage to
the solar surface, and by the time they reach the exterior, they are little
more than ripples a few yards high.
To
make matters harder, the g-modes take between two and seven hours to rise and
fall just once, which means astronomers are faced with having to detect a swell
that moves just a few millimeters per second at most.
Finally, success
Now,
however, an international team of astronomers employing the ESA-NASA Solar and
Heliospheric Observatory (SOHO) may have finally caught glimpses of these
long-sought waves, findings detailed online May 3 in the journal Science.
The
key was the telescope's Global Oscillation at Low Frequency (GOLF) instrument.
This device looks for bright, distinctive signals given off by sodium vapor,
allowing researchers to measure ripples on the Sun with great sensitivity. The
g-modes are very distinct from the p-modes in terms of both size and the speed
at which they rise and fall.
The
instrument cannot distinguish lone g-modes. Instead, astrophysicist Rafael
Garcia at DSM/DAPNIA/Service d'Astrophysique in France and his colleagues looked
for the signature of a large number of these oscillations from 10 years of GOLF
data.
"By
analogy, imagine that the Sun was an
enormous piano playing all the notes simultaneously. Instead of looking for a
particular note--middle C, for instance--it would be easier to search for all the
C's, from all the octaves together," Garcia said. "So that's what we
looked for, the cumulative effect of several g-modes."
What was learned
The
way the g-modes got distorted as they passed through the Sun have given the
first hints of the core's rotation speed. The surface of the Sun rotates at
different rates depending on location, with the equator spinning faster (about 25
days) than the poles (roughly 36 days).
"The
core of the Sun seems to rotate about three to five times faster on
average," Garcia told LiveScience.
Current
theories of solar formation suggest the original cloud of matter that gave rise
to the solar system had a high rate of rotation, a remnant of which "could
exist in the deepest regions of the Sun," Garcia said. "It seems that
the solar core rotation is slower than expected by those theories," he
added.
Garcia
noted a magnetic field leftover from the solar system's formation could have
contributed to slowing down the solar core. A new generation of instruments now
being built that can measure individual g-modes--for instance, "the GOLF
New-Generation prototype that could fly in the DynaMICCS spacecraft presented
inside the Cosmic-Vision 2015-2025 ESA program"--could help provide the
insight needed to explain the mysteries of the solar core, he said.
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