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HYPERVELOCITY
WINDS RAGE IN THE SUN'S ATMOSPHERE Winds
of electrified gas rip through the solar atmosphere at nearly the speed of sound
there, according to new observations from NASA's Transition Region and Coronal
Explorer (TRACE) spacecraft and the European Space Agency/NASA Solar and Heliospheric
Observatory (SOHO) spacecraft. The
new result, from a team of astronomers at the Harvard-Smithsonian Center for Astrophysics
(CfA) in Cambridge, Mass., shows that the winds and storms of the solar atmosphere
-- at speeds up to 200,000 miles per hour (about 320,000 kilometers/hour) -- are
so intense that they are more important than gravity in determining the density
of the atmosphere. The Sun's gravity at its visible surface is about 28 times
stronger than that at the Earth's surface; a 150-pound (68-kilogram) person would
face an epic struggle to support 4,200 pounds (1,900 kilograms) if he or she could
somehow stand on the solar surface. For the Earth's atmosphere to behave similarly,
winds over 3,000 mph (4,800 kph) would be common on the surface. "This
discovery completely changes our understanding of coronal loops, immense, arch-shaped
structures of electrified gas that comprise the Sun's outer atmosphere (corona),"
said Amy Winebarger, lead author of a paper on this research published in March
in the Astrophysical Journal. "We are excited about this because it increases
our understanding of the corona, which is the location of explosive solar activity
that occasionally disrupts high-technology systems at Earth." Winebarger,
formerly at CfA, is a solar physicist for Computational Physics Incorporated,
Springfield, Va., and is now with the Naval Research Laboratory in Washington. The
solar atmosphere is permeated with magnetic fields, generated by electrified gas,
or plasma, churning violently beneath the visible surface. Solar astronomers have
long observed loops of plasma, called coronal loops, which appear to trace out
the corona's complex magnetic-field structure, much as iron filings reveal the
invisible magnetic field surrounding a magnet. Coronal loops come in various sizes,
but most are enormous, capable of spanning several Earths. Solar
astronomers know the particles comprising plasma are electrically charged and
feel magnetic forces. Thus, scientists thought coronal loops were tubes of plasma
trapped by and enclosed in the arch-shaped magnetic fields of the corona. The
coronal loops have puzzling features, however. The strong pull of solar gravity
led astronomers to believe that the plasma should be dense at the bases of the
loop and thin at the top, just as the Earth's gravity pulls our atmosphere close
to the surface, causing it to thin with increasing altitude. In fact, coronal
loops seem to be about the same density throughout their height, even though some
of them extend several hundred thousand miles (over a million kilometers) above
the solar surface. In
their new observation, the astronomers saw bright blobs of plasma racing up and
down the coronal loops in movies made from TRACE images. SOHO data confirmed that
these plasma blobs were moving at tremendous speeds, leading the researchers to
their new view that coronal loops are not static plasma-filled structures, but
rather hypervelocity currents of plasma blasted from the solar surface and squirted
between the magnetic structures in the corona. "Rather
than being tubes of plasma enclosed within a magnetic container, they are jets
of hot plasma flowing along in the alleys between the strong coronal magnetic
fields," said Leon Golub of CfA, a co-author of the paper. If coronal loops
are indeed currents of plasma being propelled against solar gravity, they would
have about the same density along their entire height, just like the arc of water
from a water fountain. The
researchers observe plasma flows in approximately half of all coronal loops visible
by TRACE. Flows may be present in the remainder, but they might be too faint for
TRACE to detect, according to the team. New instruments will be required to determine
if all coronal loops have high-speed flows, because some loops are too small,
or are of a different temperature than what TRACE can see. According
to the researchers, the plasma current that forms a coronal loop is probably caused
by uneven heating at the bases of the loop, with plasma racing from the hotter
end to the cooler end. "The bases of a coronal loop are separated by many
thousands of miles, and there is no reason to assume that the environment at one
end will be exactly the same, and input exactly the same amount of heat, as the
environment at the other end," said Golub. The
researchers aren't sure what causes coronal-loop heating in the first place, but
their work may help discover the mechanism, shedding light on the long-standing
mystery of why the corona is hundreds of times hotter than the solar surface.
"There are many theories to explain coronal heating, but only a few of them
can produce the hypervelocity plasma flows we see in coronal loops, so this discovery
narrows the possibilities greatly," said Winebarger. Back
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