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X-ray
images of the Sun taken by the Solar and Hemispheric Observatory
(SOHO).
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Our
Sun, the star closest to Earth, offers us the opportunity to make
detailed observations of the exterior and interior of a typical
star in its mid-life phase. In addition, the Sun offers us a natural
laboratory for the study of plasmas (high-temperature gases in which
atoms have been stripped of their electrons) that are influenced
by magnetic fields. From the details of the Sun's behavior, we can
address questions about key processes within stars. How do they
generate energy through nuclear fusion at their centers? How does
that energy pass through hundreds of thousands of kilometers of
surrounding layers of gas? How does that energy, released in the
form of highly energetic particles and ultraviolet and infrared
light waves, affect Earth and the Sun's other planets?
Careful
study of the Sun's output of energy as different types of radiation
will improve our knowledge of these processes. Better ground-based
solar telescopes can reveal conditions on the Sun's surface at scales
of distance as small as 70 kilometers - an important scale because
the Sun's atmospheric pressure and temperature change noticeably
over distances this small. Other solar phenomena, including the
temporary cool and dark regions called sunspots, and the violent
outbursts visible at and above the Sun's surface, have their origin
below the visible layers of the Sun. With specialized instruments,
we can deduce the conditions that exist below the surface layers
and determine how these transient events arise and develop.
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Three
x-ray images of the Sun taken almost 3 years apart illustrate
how the level of solar activity increases significantly as it
approaches solar maximum once every 11 years. The last solar
maximum occurred in 2000.
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Our
Sun continues to pose some difficult questions. How can the solar
surface, with temperatures of only about 10,000 degrees Fahrenheit,
produce the much hotter chromosphere above it and, above the chromosphere,
the still hotter solar corona, where the temperature rises to a
million degrees or more? How do the boiling motions in the outer
layers of the Sun concentrate magnetic energy? What governs the
sudden release of this energy as huge coronal mass ejections and
solar flares? These outbursts eject huge numbers of electrons, protons,
and other charged atomic nuclei, which disrupt the patterns of Earth's
magnetic field when they reach us a few days later. Understanding
the details of the Sun's behavior has important applications for
both short-and long-term weather forecasting. In addition, solar
outbursts disrupt radio, television, radar, and power transmission
and create a danger for astronauts; changes in the Sun's energy
output, even at a modest level, produce long-term, still poorly
understood climate changes on Earth. To the extent that we can better
understand how the Sun and its outbursts work, we can take steps
to better protect ourselves and our planetary environment.
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