ANTIMATTER FACTORY ON SUN YIELDS CLUES TO
SOLAR EXPLOSIONS
The best look yet at how a solar explosion becomes an
antimatter factory gave unexpected insights into how the
tremendous explosions work. The observation may upset theories
about how the explosions, called solar flares, create and
destroy antimatter. It also gave surprising details about how
they blast subatomic particles to almost the speed of
light.
Solar flares are among the most powerful explosions in the
solar system; the largest can release as much energy as a
billion one-megaton nuclear bombs. A team of researchers used
NASA's Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) spacecraft to take pictures of a solar flare on July
23, 2002, using the flare's high-energy X-rays and gamma
radiation. (The high-energy radiation from the flare is
depicted by red and blue contour lines in Item 3. Click on
Item 4 to see a movie of the flare with the RHESSI
observations (purple, blue, and red areas) superimposed on
movies of the flare made by other observatories. For another
view of the flare, click on Items 5 and 6 to see a movie made
using the SOHO spacecraft. Item 7 is a multi-spacecraft view
of a different flare illustrating how observations can be
combined to see more than what is observed with one spacecraft
alone. Refer to captions in the right column for more detailed
descriptions of the images and movies.)
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Item 3 |
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"We are taking pictures of flares in an entirely new color,
one invisible to the human eye, so we expect surprises, and
RHESSI gave us a couple already," said Dr. Robert Lin of the
University of California, Berkeley, who is the Principal
Investigator for RHESSI.
Gamma-rays and X-rays are the most energetic forms of
light, with a particle of gamma ray light at the top of the
scale carrying millions to billions of times more energy than
a particle of visible light. The results are part of a series
of papers about the RHESSI observation to be published in
Astrophysical Journal Letters October 1.
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Item 4 |
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Antimatter annihilates normal matter in a burst of energy,
inspiring science fiction writers to use it as a supremely
powerful source to propel starships. Current technology only
creates minute quantities, usually in miles-long machines
employed to smash atoms together, but scientists discovered
the July 2002 flare created a half-kilo (about one pound) of
antimatter, enough to power the entire United States for two
days. According to the RHESSI images and data, this antimatter
was not destroyed where expected.
Antimatter is often called the "mirror image" of ordinary
matter, because for every type of ordinary matter particle, an
antimatter particle can be created that is identical except
for an opposite electric charge or other fundamental
properties.
Antimatter is rare in the present-day universe. However, it
can be created in high-speed collisions between particles of
ordinary matter, when some of the energy from the collision
goes into the production of antimatter. Antimatter is created
in flares when the fast-moving particles accelerated during
the flare collide with slower particles in the Sun's
atmosphere. (Click on Item 1 for a computer animation
illustrating how flares accelerate particles to high speeds.
Click on Item 2 for a close-up of the collision region.)
According to flare theory, these collisions happen in
relatively dense regions of the solar atmosphere, because many
collisions are required to produce significant amounts of
antimatter. Scientists expected that the antimatter would be
annihilated near the same places, since there are so many
particles of ordinary matter to run into. "Antimatter
shouldn't get far," said Dr. Gerald Share of the Naval
Research Laboratory, Washington, D.C., lead author of a paper
on RHESSI's observations of the antimatter destruction in the
July 23 flare.
However, in a cosmic version of the shell game, it appears
that this flare might have shuffled antimatter around,
producing it in one location and destroying it in another.
RHESSI allowed the most detailed analysis to date of the gamma
rays emitted when antimatter annihilates ordinary matter in
the solar atmosphere. The analysis indicates that the flare's
antimatter might have been destroyed in regions where high
temperatures made the particle density 1,000 times lower than
where the antimatter should have been created.
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Item 7 |
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Alternatively, perhaps there is no "shell game" at all, and
flares are able to create significant amounts of antimatter in
less dense regions, or flares somehow may be able to maintain
dense regions despite high temperatures, or the antimatter was
created "on the run" at high speeds, and the high-speed
creation gave the appearance of a high-temperature region,
according to the team.
Solar flares are also capable of blasting electrically
charged particles in the Sun's atmosphere (electrons and ions)
to almost the speed of light (about 186,000 miles per second
or 300,000 km/sec.). The new RHESSI observation revealed that
solar flares somehow sort particles, either by their masses or
their electric charge, as they propel them to ultra-high
speeds.
The solar atmosphere is a gas of electrically charged
particles (electrons and ions). Since these particles feel
magnetic forces, they are constrained to flow along magnetic
fields that permeate the Sun's atmosphere. It is believed that
solar flares happen when magnetic fields in the Sun's
atmosphere become twisted and suddenly snap to a new
configuration, like a rubber band breaking when overstretched.
This is called magnetic reconnection (represented by the
orange lines that come together and break in Item 1).
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Item 8 |
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Previously, scientists believed that the particles in the
solar atmosphere were accelerated when they were dragged along
with the magnetic field as it snapped to a new shape, like a
stone in a slingshot. However, if it were this simple, all the
particles would be shot in the same direction. The new
observations from RHESSI show that this is not so; heavier
particles (ions) end up in a different location than lighter
particles (electrons).
"The result is as surprising as gold miners blasting a
cliff face and discovering that the explosion threw all the
dirt in one direction and all the gold in another direction,"
said Dr. Craig DeForest, a solar researcher at the South West
Research Inst. Boulder, Colo.
The means by which flares sort particles by mass is
unknown; there are many possible mechanisms, according to the
team. Alternatively, the particles could be sorted by their
electric charge, since ions are positively charged and
electrons negatively charged. If this is so, an electric field
would have to be generated in the flare, since particles move
in different directions in an electric field according to
their charge. In either case, magnetic reconnection still
provides the energy, but the acceleration process is more
complex.
The clue that tipped scientists off to this surprising
behavior was the RHESSI observation that gamma rays from the
July 23 flare were not emitted from the same locations that
emitted the X-rays, as theory predicts. According to solar
flare theories, electrons and ions are accelerated to
high-speeds during the flare and race down arch-shaped
magnetic structures. The electrons slam into the denser solar
atmosphere near the two footpoints of the arches, emitting
X-rays when they encounter electrically charged protons there
that deflect them. (Refer to Item 2 for a close-up view of the
collision region.) Gamma rays should be emitted from the same
locations when the high-speed ions also crash into these
regions.
While RHESSI observed two X-ray emitting regions at the
footpoints, as expected, it only detected a diffuse gamma-ray
glow centered at a different location some 15,000 kilometers
(approximately 9,300 miles) south of the X-ray sites.
"Each new discovery shows we are only just beginning to
understand what happens in these gigantic explosions," said
Dr. Brian Dennis of NASA's Goddard Space Flight Center,
Greenbelt, Md., who is the Mission Scientist for RHESSI.
RHESSI was launched February 5, 2002, with the University of
California, Berkeley, responsible for most aspects of the
mission, and NASA Goddard responsible for program management
and technical oversight. For a list of the scientific papers
from this observation and the researchers involved, refer to:
http://sprg.ssl.berkeley.edu/~hhudson/rhessi/
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