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SUN
CRANKS OUT RARE COOKIE CUTTER FLARES
A
series of rare, nearly identical solar flares with associated
coronal mass ejections blasted from the Sun in November 2000.
Like snowflakes, each event is believed to be unique, because
flares get their energy from the destruction of magnetic fields
in the Sun's atmosphere, so they can't repeat in exactly the
same way. Finding a series of such similar flares is therefore
highly surprising. The observation may help reveal the link
between flares and certain types of coronal mass ejection,
the billion-ton eruptions of electrified gas that affect Earth's
space environment.
"A
sequence of flare-associated coronal mass ejections (CMEs)
that look and behave almost the same is interesting,"
said Dr. Nariaki Nitta of the Lockheed Martin Solar and Astrophysics
Laboratory, Palo Alto, California, who is the lead author
of a paper on this research published in Geophysical Research
Letters. "It may be due to a huge, organized magnetic
structure rising from within the Sun to trigger these similar
great explosions, one after the other."
According
to the researchers, the rising magnetic field could repeatedly
rebuild a similar magnetic structure in the solar atmosphere,
setting it up for carbon-copy annihilation. "It was surprising
that all six short-duration flares in this sequence were associated
with a CME in a similar pattern, all within 60 hours; that
is, before the magnetic field could possibly recover from
the previous flare/CME event in the sequence. We believe a
vigorous magnetic field emergence in the active region is
the key to understand this," said Nitta.
The
series of solar explosions, each releasing approximately 100,000
times humanity's total annual energy consumption, was observed
with the Yohkoh and the Solar and Heliospheric Observatory
(SOHO) spacecraft. They occurred near the center of the solar
disk, in the vicinity of a sunspot group. (Sunspots are relatively
cool, dark areas on the surface of the Sun). Six flares were
observed over a 60-hour period beginning at 04:50 UT on November
24. The first three in the series were similar to each other,
as were the final two. The other flare, however, was less
powerful than the rest, and lasted longer. The flares were
similar in several ways; their shape, the way they developed
over time, their ejection of high-energy particles, and their
generation of high-energy light.
Each
of the flares in this series was also associated with a coronal
mass ejection (CME). During the period, a total of 14 CMEs
were emitted from the flare region, six of which were associated
with the homologous flares. Scientists are hopeful that this
discovery will help them shed light on the complex relationship
between flares and CMEs, both of which occasionally disrupt
high-technology systems at Earth.
"Most
CMEs are associated with long-duration flares, those lasting
more than two hours; but all the flares in this series were
short-duration, and a bunch of CMEs happened anyway,"
said Nitta. "This is exciting because short-duration
flares may sometimes participate in the processes that launch
CMEs, but we don't know how. We have much more to discover
about the connection between flares and CMEs."
According
to the researchers, it is difficult to determine if flares
cause CMEs or vice versa, because they might be just different
aspects of one phenomenon. "Early ideas about causality,
one way or the other, seem to have been based upon incomplete
observations. Right now we are pretty sure that flares and
CMEs are inseparable physically, with some events showing
more or less of the eruptive and flare-like behavior. These
particular events were very impulsive and therefore help us
understand these time relationships better," said co-author
Dr. Hugh Hudson of the Solar Physics Research Corporation
and the University of California, Berkeley.
Yohkoh
is an international collaboration led by Japan, and SOHO is
a mission of cooperation between the European Space Agency
and NASA. Yohkoh is managed by NASA's Marshall Space Flight
Center in Huntsville, Ala.
Since
December 15, 2001, Yohkoh scientific operations have been
suspended due to a power loss associated with an unexpected
rotation in the satellite as it crossed the path of an annular
eclipse over the Pacific Ocean the same day.
Yohkoh
experienced an unexpected satellite-body rotation when the
satellite attitude (orientation) control switched from the
normal Sun-acquisition mode to the stand-by mode as the orbit
crossed the annular-eclipse belt. This prevented Yohkoh's
solar panels from receiving a sufficient, stable supply of
sunlight.
Satellite
operators have been successful at minimizing Yohkoh's power
consumption by shutting down the power lines of the other
instruments and heaters, and no more decrease of the battery
voltage is expected at the moment. However, the satellite
is still spinning, and measures to regain the normal control
of Yohkoh are continuing. Many steps will be required for
recharging the battery, re-establishing attitude control,
and so forth. Therefore, it may take some time before scientific
observations can resume.
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