To astronomers,
one of the sun's most puzzling mysteries has centered on why the
star, fiercely hot at its core, cools with distance, only to have
temperatures in its extended outer atmosphere soar again.
"It's
as though you walk away from a campfire, get cooler and cooler,
then you start getting hotter again," says John Leibacher,
an astronomer at the National Solar Observatory in Tucson, Ariz.
In research
to be published next month, astronomers at Lockheed Martin's Solar
and Astrophysics Laboratory in Palo Alto, Calif., have taken a
crucial step toward solving the mystery. In the process, some
scientists say, the team's work may shed light on enormous outbursts
of charged particles that can wreak havoc on satellites, trigger
power blackouts on Earth, and send ribbons of blue, green, and
red aurora weaving through the night skies.
While they
don't yet know the reason for the strange temperature pattern,
their research has given scientists a new understanding of where
the sun's corona is heated – toppling long-held theories
and getting astronomers closer to answers.
"This
overturns a picture basic to the field for about 30 years,"
says Craig DeForest, an astrophysicist at the Southwest Research
Institute in Boulder, Colo. "This is important to all aspects
of the field."
Until now,
conventional wisdom had held that the corona was like a thin gaseous
atmosphere, heated uniformly.
This led to
at least 10 competing explanations for the high temperatures in
the sun's corona. (The sun's core burns at about 27 million degrees
F., its surface falls to about 10,000 degrees, then the corona
– stretching millions of miles into space – rockets
back up to between 1.8 million and 9 million degrees.)
But the Lockheed
Martin team, using data from NASA's TRACE satellite, found that
the corona is heated unevenly from below, as if by enormous numbers
of gas jets that emerge from beneath the corona.
"To solve
the [heating] puzzle," says Lockheed astronomer Markus Aschwanden,
"you first have to know where the corona is heated, then
you can sort out the physical mechanisms. We have identified where
the heating occurs."
Observations
from TRACE, launched into Earth orbit in 1998, have shown that
the sun's corona is like a loose-weave carpet. Magnetic fields
rise and fall in massive loops made up of many "threads"
of magnetic fields, some of which are perhaps as narrow as 60
miles.
These threads
act as conduits along which hot, electrically charged gas travels.
The team studied
41 loops that extend to distances ranging from about 2,500 miles
to more than 180,000 miles. The researchers found that threads
in shorter loops appear to heat uniformly along their length,
but the longer threads tend to cool noticeably with height.
Calculations
involving the loops' energy levels suggested that they are heated
from below. The team estimates that heating typically occurs in
the first 6,000 miles of a loop's length.
Moreover,
light from the loops brightens and fades, suggesting that energy
and mass are being injected into the loops in pulses, Dr. Aschwanden
says.
He adds that
the pulses may result as the threadlike magnetic fields break
and reconnect – a process driven by stresses they undergo
as they try adjust to the random motion of the magnetic field
on the solar surface.
Understanding
how these coronal loops work can shed light on their much-larger
but rarer cousins, coronal-mass ejections, according Richard Fisher,
who heads the Laboratory for Astronomy and Solar Physics at NASA's
Goddard Space Flight Center in Greenbelt, Md.
When these
outbursts are directed toward Earth, the charged particles they
hurl outward can disrupt or destroy satellite components, and
prompt surges in electrical transmission lines that can cause
blackouts and touch off brilliant auroral displays.
Understanding
the mechanics of these loops could improve efforts to forecast
such solar storms, he says. "The cause of coronal-mass ejections
has never really been settled," Dr. Fisher notes. "One
of the big questions has been: What does a coronal-mass ejection
look like close to the sun? If it looks like this, then what would
make it happen" as suddenly as it does?
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