Researchers Plan Mission
To Reveal Secrets
Of The Solar Wind
By James Cornell
Smithsonian News Service
Slewing out of the sun at a
million miles an hour, the great
gashing geyser of broiling gas
and supercharged particles
1.1 .:-wn as the solar wind blast
f 'jiigh space, enveloping al
the planets, moons and small
bodies of the solar system in a
hot, seething soup that affects
everything in this comer of the
universe, including human life
itself.
As this cosmic jet stream
rams Into the Earth’s magnetic
field, most of it is thankfully
deflected into interstellar space.
But billions of wind-driven
particles still manage to break
through the planet’s warped and
twisted protective shield. They
spiral down into the atmosphere
over the poles to produce
aurorae, geomagnetic storms,
disruptions in radio
communications and power
surges along transmission lines.
The existence of this
powerful solar wind has been
known since the early days of
space exploration, but its origin
inside the sun-like much else
about our nearest star-remains
a mystery.
Sometime in the next
decade, however, a satellite
experiment called the
Ultraviolet Coronagraph
Spectrometer, working ii
concert with several other space
instruments, may at last reveal
the secrets of the solar wind.
The project is part of a massive
international campaign to study
the complex relationship among
the sun. Earth and human
activities.
”An ultraviolet coronagraph-
spectrometer is our name for an
artificial eclipse machine," says
Dr. John Kohl of the
Smithsonian Astrophysical
Observatory in Cambridge,
Mass., and principal
investigator for the project now
being developed with
researchers in Italy and
Switzerland as well as at
several U.S. universities.
The Smithsonian’s solar wind
experiment is one of several
aboard a satellite called the
Solar and Heliospheric
Observatory (SOHO) to be built
by the European Space Agency
in cooperation with the National
Aeronautics and Space
Administration.
"The solar wind seems to
originate in the sun’s corona, or
outer atmosphere," Kohl
explains. "This pearly white
ring of hot gas is visible from
Earth only during a total solar
eclipse. To see what’s
happening in this region on a
regular basis, we have to block
out the sun’s bright disk, which
otherwise simply overwhelms
the corona."
In fact, most aspects of the
hot gas and charged particles in
the wind-detectable as high-
energy ultraviolet light-are
invisible to ground-based
optical telescopes. But
observations of this light to
determine the solar wind’s
velocity, temperature and
composition can be made from
space.
In this artist's conception, NASA’s Solar and Heliospheric Observatory--known as SOHO—
watches the sun while a small armada of satellites called "Cluster” circles the Earth to observe
interactions between the solar wind and the Earth’s magnetic field.
"Our coronagraph-
spectrometer is a device that
creates a permanent eclipse,"
says Kohl. "Thus, we can
observe the corona-and the
outpouring of the wind-
continuously.
Continuous observation is
absolutely necessary because
the flow of the solar wind is not
constant. It changes
dramatically and frequently,
often over periods of a few
hours. Since the sun makes one
full rotation every 27 days,
streams of hot, high-speed gas
spiral out like the spray of water
from a spinning lawn sprinkler.
The wind also displays other
long-term variations linked,
perhaps, with the same 11-year
cycle that brings sunspots and
fiares to the sun’s surface.
The solar wind was first
The halo” orbit planned for NASA’s Solar and Heliospheric Observatory
will allow the satellite to monitor the sun continuously. The orbit is
located at the "Lagrangian point,” an area between the sun and the Earth
where the gravitational pull of the two bodies on the satellite is equal. Here,
the satellite can maintain a near-perfect circular orbit. (A kilometer is
about five-eighths of a mile.)
detected by equipment aboard
the Mariner 2 spacecraft in
1961. Since then, numerous
spacecraft in orbit around both
the Earth and the sun have
measured the wind at distances
as close as 30 million miles
from the solar surface.
In the mid-1970s, for
example. X-ray observations
from the manned Skylab space
station revealed that the sun’s
outer atmosphere was laced by
great dark regions and that
some of the powerful solar
wind was pouring out of these
"coronal holes."
"Except for the coronal
holes," says Dr. George
Withbroe, an associate director
at the Harvard-Smithsonian
Center for Astrophysics, also in
Cambridge, "we have so far
been unable to determine where
the wind originates-or what
mechanism drives it away from
the sun. Some of the energy
required to accelerate the wind
and, allow it to escape the sun’s
great gravitational pull must be
associated with the same
unknown mechanism that heats
the coronal gas to 1 million
degrees Celsius."
The coronal holes, some
large enough to cover 20
percent of the sun’s visible
surface change shape, size and
location with time. At periods
of maximum solar flare and
sunspot activity, they may
disappear completely and, with
them, the high-speed solar wind
streams.
The variability of the solar
wind, combined with the
dynamic and complicated
system of magnetic fields and
electrical currents in the vast
domain ot geospacc
surrounding Earth, add up to a
complex array of physical
effects.
The largest region of
geospace, for example, is the
"magnetosphere,” where all
phenomena are dominated by
the Earth’s own magnetism.
The gusty solar wind
compresses the magnetosphere
into a giant, bowlike shockwave
on the day side of Earth and
pulls it out like a string of taffy
on the opposite, or night, side.
The second region is the
"ionosphere” -tJ e broad band of
atmosphere some 40 to 300
miles above the Earth. Here,
solar interactions, including the
charged particles driven inward
by the wind, give rise to the
great magnetic storms that
produce spectacular Northern
and Southern lights displays as
well as disrupt communications.
In the Earth’s lower
atmosphere, chemical reactions
and wind patterns, triggered by
solar wind interactions
hundreds of miles above, may
affect local weather conditions
and, perhaps, even global
climatic patterns. - The
mechanism linking these
various phenomena, however, is
still poorly understood.
More alarming, increasing
evidence suggests that many
human activities-fossil fuel
burning and unrestrained
release of chlorofluorocarbons
in aerosols among them-may be
affecting not only the lower
atmosphere, but the entire
geospace. The outstanding
example of this is the "ozone
hole" detected in recent years
over Antarctica.
Understanding the solar wind
is essential to unraveling this
intertwining of many related
phenomena, and a diverse
battery of experiments planned
in the next decade under the
broad nmErella the
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