Has SOHO ended a 30-year quest for solar ripples? (Forwarded)

ESA Science News
http://sci.esa.int

3 May 2007

Has SOHO ended a 30-year quest for solar ripples?

The ESA-NASA Solar and Heliospheric Observatory (SOHO) may have glimpsed
long-sought oscillations on the Sun's surface. The data will reveal
details about the core of our star and it contains clues on how the Sun
formed, 4.6 thousand million years ago.

The subtle variations reveal themselves as a miniscule ripple in the
overall movement of the solar surface. Astronomers have been searching for
ripples of this kind since the 1970s, when they first detected that the
solar surface was oscillating in and out.

The so-called 'g-modes' are driven by gravity and provide information
about the deep interior of the Sun. They are thought to occur when gas
churning below the solar surface plunges even deeper into our star and
collides with denser material, sending ripples propagating through the
Sun's interior and up to the surface. It is the equivalent of dropping a
stone in a pond.

Unfortunately for observers, these waves are badly degraded during their
passage to the solar surface. By the time g-modes reach the exterior, they
are little more than ripples a few metres high.

To make matters more difficult, the g-modes take between two and seven
hours to oscillate just once. So, astronomers are faced with having to
detect a swell on the surface that rises a metre or two over several
hours.

Now, however, astronomers using the Global Oscillation at Low Frequency
(GOLF) instrument on SOHO think they may have caught glimpses of this
behaviour. Instead of looking for an individual oscillation, they looked
for the signature of the cumulative effect of a large number of these
oscillations.

As an 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.

In the piano their frequencies are related to each other just as on the
Sun, one class of g modes are separated by about 24 minutes.

"So that's what we looked for, the cumulative effect of several g modes,"
says Rafael A. García, DSM/DAPNIA/Service d'Astrophysique, France. They
combined ten years of data from GOLF and then searched for any hint of the
signal at 24 minutes. They found it.

"We must be cautious but if this detection is confirmed, it will open a
brand new way to study the Sun's core," says García.

Until now, the rotation rate of the solar core was uncertain. If the GOLF
detection is confirmed, it will show that the solar core is definitely
rotating faster than the surface.

The rotation speed of the solar core is an important constraint for
investigating how the entire Solar System formed, because it represents
the hub of rotation for the interstellar cloud that eventually formed the
Sun and all the bodies around it.

The next step for the team is to refine the data to increase their
confidence in the detection. To do this, they plan to incorporate data
from other instruments, both on SOHO and at ground-based observatories.

"By combining data from space (VIRGO and MDI, on SOHO) and ground (GONG
and BiSON) instruments, we hope to improve this detection and open up a
new branch of solar science," says García.

Notes for editors

The findings are published in the 4 May 2007 edition of the journal
Science, in the article: "Tracking solar gravity modes: the dynamics of
the solar core", by R. García, S. Turck-Chièze, S. Jiménez-Reyes, J.
Ballot, P. Pallé, A. Eff-Darwich, S. Mathur, J. Provost.

For more information:

Rafael A. García
DSM/DAPNIA/Service d'Astrophysique, France
Email: Rafael.Garcia @ cea.fr

Alan Gabriel, SOHO GOLF Principal Investigator
Institut d'Astrophysique Spatiale, Universite Paris XI, France
Email: Gabriel @ ias.u-psud.fr

Bernard Fleck, ESA SOHO Project Scientist
Email: Bfleck @ esa.nascom.nasa.gov

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