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Solar
Flares Set The Sun Quaking
Friday, April 18, 2008
Best in Full Screen
This mega-flare was seen
being spewed out by the Sun starting at 20:29 CET on 4
November 2003. This video sequence was captured by SOHO's
Extreme ultraviolet Imaging Telescope.
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Credits: SOHO / EIT
/ ESA / NASA
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.
Hi-Res
Version
Solar flares set the Sun
quaking
A frequency-time diagram of
global oscillations in the Sun as measured by SOHO. The
color-coding represents the strength of the oscillations.
The bright horizontal lines are strongly correlated with
solar flares, represented by the solar X-ray flux in the
right panel. These lines are most prominent in the
high-frequency region of the spectrum. The vertical line at
5.55 MHz in the left panel is an instrumental artifact.
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Credits: SOHO /
VIRGO / ESA / NASA
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.
Source:
ESA
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