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- As dawn breaks on our
understanding of the sun's eruptions
a controversial idea has
re-emerged that attempts to explain the source
of coronal mass
ejections -- those periodic enormous releases of energy
that threaten
satellites, cell phones and even power grids.
-
- The theory, being presented
this week by scientists from
the Naval Research Laboratory at a plasma
physicists meeting in Seattle,
posits that the triggering mechanism for
these ejections comes from deeper
within the star than current theories
hold.
-
- Proponents say the theory provides the best description
of the
events, but other scientists are highly skeptical. Real answers
may
still be years away.
-
- But the stakes are high and the need for greater
knowledge
is somewhat urgent. Space weather is nearing an 11-year high
as the sun
ratchets ever closer to what researchers call a solar
maximum -- expected
early next year. Being able to understand space
weather and its origins,
and to predict its effects on Earth, would
allow satellite operators and
utility companies to prepare for events
that might cripple services.
-
- During a maximum, heightened sunspot activity
corresponds
with more intense and frequent coronal mass ejections. The
last time around,
more than a decade ago, the eruptions caused power
outages and crippled
satellites -- all in an era when the world was far
less reliant on space-based
communications.
-
- Eruptive
origins
-
- Standard models maintain that these explosions originate
in the corona -- a luminous atmosphere of energy around the sun. But the
new theory claims that the eruptions start far deeper, below the
photosphere,
which scientists describe as the visible layer of the sun
just beneath
the corona.
-
- James Chen and Jonathan Krall, of the Naval Research
Laboratory, explain a process involving giant loops of magnetic energy,
called "solar flux ropes" (see the illustration at the top of
this page). As the electrical current in the ropes increases, they expand
upward and outward until they fling their energy -- mostly electrons and
protons -- into space at speeds up to 2.2 million miles per hour (1000
kilometers per second).
-
- The researchers say this process accounts for about 30
percent of all solar eruptions.
-
- "We have found that our proposed scenario provides
the best description of observed solar eruptions," said Krall.
"The
degree to which the data agrees with the new theory is
unprecedented."
-
- The researchers based the current version of their model
on data stretching back to 1995 from SOHO, a solar-imaging spacecraft
hovering
midway between Earth and the sun. They studied observations
that extended
outward 15 times the diameter of the sun. Krall said the
new theory also
accounts for "magnetic clouds" -- pockets of
charged particles
in space -- that researchers believe are associated
with solar eruptions.
-
- Like all theories, it's only a
theory
-
- Several astrophysicists and other experts expressed
doubt
about the Chen and Krall model, an earlier version of which was
proposed
in 1989.
-
- "The view put forward by Dr. Chen that the energy
for powering coronal mass ejections comes from below the photosphere is
not generally accepted by the solar community," said Terry Forbes,
a researcher in astrophysics at the University of New Hampshire.
-
- Prevailing models
hold that the sun's energy migrates
to the surface more slowly, Forbes
said. Pockets of concentrated energy
are seen on the solar surface as
sunspots. Energy is also stored in the
sun's atmosphere. Some
as-yet-unknown triggering mechanism forces the coronal
mass ejection,
which radiates outward and also pushes its way back toward
the sun's
surface. Evidence of this process is seen in ripples that occur
on the
sun's surface roughly two minutes after the event has been observed
above the solar surface.
-
- "I don't see how they can transport energy from
below, and have a big display in the corona, and yet at the surface you
see nothing," Forbes said in an interview.
-
- Chen and Krall do not, in fact,
describe the energy source
that might trigger a release. Instead, they
start with the assumption that
a deep-seated source could trigger the
solar flux ropes, and then they
set out to describe the
process.
-
- "This theory has been tested against the observational
data more carefully than any other, and with excellent results,"
Krall
said. "We are challenging others to test their ideas in the
same way."
-
- Other responses to the Chen and Krall model showed a
mix of
criticism and skepticism, and also a suggestion that more data is
needed to prove anything.
-
- "This theory reflects the authors' support of what
is an emerging paradigm in solar physics," said Richard Canfield,
a Montana State University researcher who was involved in research
published
earlier this year describing an above-the-surface triggering
mechanism.
"There is certainly evidence that it may be correct,
though there
is much work to be done before the case is
proven."
-
- Joe Gurman, an observer at NASA's Goddard Space Flight
Center,
said the tools needed for evaluating what's going on below the
sun's
surface are just now being developed. New data, not yet published,
reveals pockets of energy migrating to the surface -- a precursor to
sunspots
that scientists have never before glimpsed, Gurman told
space.com.
-
- "There are some coronal mass ejections observed
by the
LASCO chronograph on SOHO that suggest that this model could be
correct," Gurman said. But, he added that it could be five years or
more before scientists have the ability to gather the data needed to test
the model.
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