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April
14, 2009: This just in: The Sun is blasting the solar
system with croissants.
Researchers
studying data from NASA's twin STEREO probes have found that
ferocious solar storms called CMEs (coronal mass ejections)
are shaped like a French pastry. The elegance and simplicity
of the new "croissant model" is expected to dramatically
improve forecasts of severe space weather.
"We
believe we can now predict when a CME will hit Earth with
only 3-hours of uncertainty," says Angelos Vourlidas
of the Naval Research Lab, who helped develop the model. "That's
a four-fold improvement over older methods."
Above:
An artist's concept of a croissant-shaped CME. Click on the
image to launch a
movie of the explosion. Credit: NASA
Coronal
mass ejections are billion-ton clouds of hot magnetized gas
that explode away from the sun at speeds topping a million
mph. Sometimes the clouds make a beeline for Earth and when
they hit they can cause geomagnetic storms, satellite outages,
auroras, and power blackouts. The ability to predict the speed
and trajectory of a CME is key to space weather forecasting.
"This
is an important advance," says Lika Guhathakurta, STEREO
program scientist at NASA headquarters in Washington DC. "From
a distance, CMEs appear to be a complicated and varied population.
What we have discovered is that they are not so varied after
all. Almost all of the 40-plus CMEs we have studied so far
with STEREO have a common shape--akin to a croissant."
Thousands
of CMEs have been observed by NASA and European Space Agency
spacecraft, but until now their common shape was unknown.
That's because in the past observations were made from only
a single point of view. The STEREO mission has the advantage
of numbers. It consists of two probes that flank the sun and
photograph explosions from opposite sides. STEREO's sensitive
wide-field cameras can track CMEs over a wider area of sky
than any other spacecraft, following the progress of the storm
all the way from the sun to the orbit of Earth.
"STEREO
has done what no previous mission could," notes Guhathakurta.
Vourlidas
says he is not surprised that CMEs resemble French pastries.
"I have suspected this all along. The croissant shape
is a natural result of twisted magnetic fields on the sun
and is predicted by a majority of theoretical models."
He
offers the following analogy: Take a length of rope and hold
one end in each hand. Start twisting the ends in opposite
directions. Twist, twist and continue twisting until the middle
of the rope is a fat knotted mess.
 "That's
how CMEs get started—as twisted ropes of solar magnetism.
When the energy in the twist reaches some threshold, there
is an explosion which expels the CME away from the sun. It
looks like a croissant because the twisted ropes are fat in
the middle and thin on the ends."
Right:
A computer model of a croissant-shaped CME. Models like this
can be rapidly fit to real CMEs as soon as they are observed,
allowing forecasters to accurately estimate the speed and
trajectory of the storms: movie.
Credit: NASA.
The
shape alone, however, does not tell the full story of a CME.
The contents of the CME must be considered, too. How much
plasma does it contain? What is the orientation and strength
of its internal magnetic field? When a CME strikes, the havoc
it causes will depend on the answers—answers the croissant
model does not yet provide.
"There
is more work to do. We must learn to look at a CME and not
only trace its shape, but also inventory in contents,"
says Guhathakurta. "We are halfway there."
Eventually,
the quest to learn what lies inside the croissant will be
taken up by other spacecraft such as the Solar Dynamics Observatory,
slated to launch in August 2009, and Solar Probe+, a daring
mission (still on the drawing board) to fly close to the sun
and actually enter these storms near their source.
STEREO
isn't finished, though. The two probes are continuing their
journeys to opposite sides of the sun for a 24/7, 360-degree
view of the star. Along the way, they'll actually run into
a few CMEs and have the chance to sample the 'croissants'
in situ.
Stay
tuned for updates.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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