Solar flares set the Sun quaking (Forwarded)

ESA News
http://www.esa.int

18 April 2008

Solar flares set the Sun quaking

Data from the ESA/NASA spacecraft SOHO shows clearly that powerful
starquakes ripple around the Sun in the wake of mighty solar flares that
explode above its surface. The observations give solar physicists new
insight into a long-running solar mystery and may even provide a way of
studying other stars.

The outermost quarter of the Sun's interior is a constantly churning
maelstrom of hot gas. Turbulence in this region causes ripples that
criss-cross the solar surface, making it heave up and down in a patchwork
pattern of peaks and troughs.

The joint ESA-NASA Solar and Heliospheric Observatory (SOHO) has proved to
be an exceptional spacecraft for studying this phenomenon. Discovering how
the ripples move around the Sun has provided valuable information about the
Sun's interior conditions. A class of oscillations called the 5-minute
oscillations with a frequency of around 3 millihertz have proven
particularly useful.

According to conventional thinking, the 5-minute oscillations can be thought
of as the sound you would get from a bell sitting in the middle of the
desert and constantly being touched by random sand grains, blown on the
wind. But what Christoffer Karoff and Hans Kjeldsen, both at the University
of Aarhus, Denmark, saw in the data, was very different.

"The signal we saw was like someone occasionally walking up to the bell and
striking it, which told us that there was something missing from our
understanding of how the Sun works," Karoff says.

So they began looking for the culprit and discovered an unexpected
correlation with solar flares. It seemed that when the number of solar
flares went up, so did the strength of the 5-minute oscillations.

"The strength of the correlation was so strong that there can be no doubt
about it," says Karoff.

A similar phenomenon is known on Earth in the aftermath of large
earthquakes. For example, after the 2004 Sumatra-Andaman Earthquake, the
whole Earth rang with seismic waves like a vibrating bell for several weeks.

The correlation is not the end of the story. Now the researchers have to
work to understand the mechanism by which the flares cause the oscillations.
"We are not completely sure how the solar flares excite the global
oscillations," says Karoff.

In a broader context, the correlation suggests that, by looking for similar
oscillations within other stars, astronomers can monitor them for flares.
Already, Karoff has used high-technology instruments at major ground-based
telescopes to look at other Sun-like stars. In several cases, he detected
the tell-tale signs of oscillations that might originate from flares.

"Now we need to monitor these stars for hundreds of days," he says. That
will require dedicated spacecraft, such as the CNES mission with ESA
participation, COROT. The hard work, it seems, is just starting.

Notes for editors:

'Evidence that solar flares drive global oscillations in the Sun' by C.
Karoff & H. Kjeldsen will be published in The Astrophysical Journal letters
on 1 May 2008.

For more information:

Christoffer Karoff, University of Aarhus, Denmark
Email: Karoff @ phys.au.dk

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

[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaCP/SEM4SB4XQEF_index_1.html ]