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SPACE PHYSICISTS FIND THE ENERGY THAT POWERS EXPLOSIVE CORONAL
MASS EJECTIONS AND DISCOVER SIGNATURES OF THEIR ORIGIN AND
Using spacecraft and supercomputers, scientists from the
International Solar Terrestrial
Physics (ISTP) program have developed a new theory for the
explosive, high velocity
coronal mass ejections (CMEs) that will erupt from the Sun with
during the maximum of the new solar cycle. CMEs are eruptions of
gas from the Sun that can trigger magnetic storms around Earth.
occasionally disturb spacecraft, navigation and communications
systems, and electric
Recent experimental and theoretical observations from ISTP indicate
that the interaction
of magnetic fields high above the Sun’s surface—not low in the
corona, as was
previously thought—allows tremendous energy to build up and to
release CMEs at speeds
approaching 2000 kilometers per second. Scientists also have
determined that “halo”
CMEs almost always lead to geomagnetic storms, an observation of
for the prediction of space weather. Finally, ISTP researchers
have been able to create
realistic visual simulations of the effects that CMEs can have on
Earth's magnetic field.
Coronal mass ejections are the largest structures that erupt from
the Sun and they one of
the principal ways that the Sun ejects material and magnetic energy
into the solar system.
Most CMEs travel at 400 km per second, but “fast CMEs can reach
speeds double or
triple that. The first fast CME of the new solar cycle was
observed on November 6, 1997.
Slow or fast, the eruptions produce magnetic clouds that can cause
and increase the intensity of the aurora (Northern and Southern
lights). Fast CMEs also
can accelerate protons in interplanetary space to the point where
they can harm spacecraft
or astronauts in space.
Inspired by images of CMEs collected by the Large Angle
Spectrometric Coronograph on
the SOHO spacecraft, Dr. Spiro Antiochos of the U.S. Naval Research
Washington, DC, has proposed a new answer to the long-standing
question of how the
Sun can build up the energy to produce the violent explosions of
fast CMEs. Using
supercomputers from NASA and the Department of Defense, Antiochos
has created a
model that simulates the complex, interwoven magnetic structures of
the Sun. After
observing how magnetic fields abut and interact, Antiochos
theorizes that the Sun’s
magnetic fields tend to restrain each other and force the buildup
of tremendous energy.
When a “stressed” magnetic field low in the Sun’s atmosphere pushes
higher into the
corona, it gets held back by the surrounding and overlying fields.
As the stressed field
builds up more potential energy, it pushes harder against these
magnetic ropes and moves
higher into the corona. Eventually, through a process known as
where opposing magnetic lines of force merge and cancel--the field
is released from its
bonds and escapes the Sun as high speed.
Antiochos compares the process to that of filling a helium balloon.
“If you fill it without
anchoring it, the balloon will slowly drift upward,” he said. “But
if you hold the balloon
down as you fill it, you can generate a lot of upward force, which
makes the balloon take
off at higher speed once you release it. Fast CMEs are released in
the same way.”
Antiochos added that such high-speed bursts will be more frequent
as the Sun approaches
its maximum period of activity in 2000-2001.
In other research, ISTP teams headed by David Webb of Boston
College and the Research
Laboratory at Hanscom Air Force Base and by Dr. Guenter Brueckner
of the Naval
Research Laboratory used ISTP’s SOHO and Wind spacecraft to study
nine “halo” type
CMEs that occurred from December 1996 to May 1997. They found that
almost always result in magnetic activity at Earth. Halo CMEs are
so named because
they appear as expanding halos around the Sun when seen from the
perspective of Earth.
According to Dr. Nancy Crooker, a space physicist at Boston
University, detection of halo
events “will vastly improve” methods of predicting geomagnetic
storms. “If you see a
halo CME, you are pretty sure that you are going to see a storm at
Earth,” Crooker said.
“In the past, scientists could only look at solar flares to know if
a storm was headed
toward Earth. And while some flares have accompanying CMEs, some
don’t. But now
we have instruments sensitive enough to see the CMEs themselves.”
ISTP does not predict space weather. Forecasting is the domain of
Environment Center of the National Oceanic and Atmospheric
Crooker and Webb also have identified the signature of halo CMEs on
the Sun's surface.
These remnants of CMEs show up in X rays and ultraviolet light as
brightening of magnetic arches, with dimming areas on either side.
These dimming areas
seem to mark the roots of CMEs already launched into space.
Downwind from those eruptions, the invisible magnetic shell (or
shields Earth from the Sun’s particles and radiation is regularly
shaped and shorn by
CMEs. To better understand this interaction, Dr. Charles Goodrich,
of the University of
Maryland, has developed a series of animations that depict how the
responds to the shock of a CME.
Using a Cray C-90 and other powerful computers, Goodrich and
colleagues have for the
first time depicted the evolution of the magnetosphere as it is
bombarded by a real CME.
The simulation reconstructs 42 hours of time centered around the
arrival of a CME on
January 10,, 1997. The simulation reveals the shape and
orientation of Earth’s magnetic
field, as well as the density of plasma in Earth’s space
In fact, the real January CME produced magnetic storms and
spectacular auroral displays,
and poured as much as 1400 Gigawatts of electricity into the
atmosphere (almost double
the power generating capacity of the United States). The magnetic
cloud from the CME
smacked the magnetosphere with a burst of plasma 30 times denser
than the normal solar
wind. The shock compressed the leading edge of the magnetosphere
geosynchronous orbit, where many satellites are positioned.
“We have created a global picture of what is going on in the
Goodrich. “Since we only have a few spacecraft, and they can only
measurements, this is the only way to look at the whole system.”
Such simulations will
provide researchers with insights about the conditions that lead
to--and perhaps trigger--
geomagnetic storms, Goodrich added.
In the past year, the primary spacecraft of ISTP--SOHO, Wind,
Polar, and Geotail--and
the program’s network of cooperating satellites, ground sensors,
and theory centers have
monitored approaching interplanetary storms for the first time,
from their genesis to their
impact on Earth. ISTP was designed as a joint, comprehensive
effort to observe and
understand our star, the Sun, and its effects on Earth’s
environment in space.
Participating institutions include NASA, the European Space Agency
Japanese Institute of Space and Astronautical Sciences (ISAS), the
Research Institute (IKI), as well as the Max Planck Institute, the
U.S. National Oceanic
and Atmospheric Administration, the Los Alamos National Laboratory,
the U.S. Air
Force, the Canadian Space Agency, the British Antarctic Survey, the
Science Foundation, and the Johns Hopkins Applied Physics
For more information about ISTP and the physics of the Sun and
Earth, go to
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Author: Mike Carlowicz
Official NASA Contact: ISTP-Project
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