Chapter 15--Our Sun - Geological Sciences
Chapter 15--Our Sun - Geological Sciences
Chapter 15--Our Sun - Geological Sciences
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e quator<br />
N N N<br />
magnetic<br />
field line<br />
differing<br />
rotation<br />
rates<br />
time<br />
time<br />
S S S<br />
Figure <strong>15</strong>.22 The <strong>Sun</strong> rotates more quickly at its equator than it does near its poles. Because gas circles<br />
the <strong>Sun</strong> faster at the equator, it drags the <strong>Sun</strong>’s north-south magnetic field lines into a more twisted configuration.<br />
The magnetic field lines linking pairs of sunspots, depicted here as green and black blobs, trace out<br />
the directions of these stretched and distorted field lines.<br />
The magnetic field associated with the solar wind constantly<br />
interacts with Earth’s magnetic field. Occasionally<br />
these fields interconnect. When that happens, large amounts<br />
of energy are released from the magnetic field into the<br />
charged particles near the interconnection zone. Many<br />
of these energized particles then flow down Earth’s magnetic<br />
field lines toward the poles (Figure <strong>15</strong>.23a). Collisions<br />
between the charged particles and atoms in Earth’s upper<br />
atmosphere cause electrons in the atoms to jump to higher<br />
energy levels [Section 4.4].These excited atoms subsequently<br />
emit visible-light photons as they drop to lower energy levels,<br />
creating the shimmering light of auroras (Figure <strong>15</strong>.23b).<br />
Because coronal mass ejections are particularly energetic, the<br />
auroras they stimulate can be especially spectacular.<br />
Particles streaming from the <strong>Sun</strong> after the occurrence<br />
of solar flares, coronal mass ejections, or other major solar<br />
storms can also have practical impacts on society. For example,<br />
these particles can hamper radio communications,<br />
disrupt electrical power delivery, and damage the electronic<br />
components in orbiting satellites. During a particularly<br />
powerful magnetic storm on the <strong>Sun</strong> in March 1989, the<br />
U.S. Air Force temporarily lost track of over 2,000 satellites,<br />
SPECIAL TOPIC<br />
Long-Term Change in Solar Activity<br />
Figure <strong>15</strong>.21 shows that the sunspot cycle varies in length and<br />
intensity, and it sometimes seems to disappear altogether. With<br />
these facts as background, many scientists are searching for longerterm<br />
patterns in solar activity. Unfortunately, the search for longerterm<br />
variations is difficult because telescopic observations of<br />
sunspots cover a period of only about 400 years. Some naked-eye<br />
observations of sunspots recorded by Chinese astronomers go<br />
back almost 2,000 years, but these records are sparse, and nakedeye<br />
observations may not be very reliable. We can also guess at<br />
past solar activity from descriptions of solar eclipses recorded<br />
around the world: When the <strong>Sun</strong> is more active, the corona tends<br />
to have longer and brighter “streamers” visible to the naked eye.<br />
Another way to gauge past solar activity is to study the amount<br />
of carbon-14 in tree rings. High-energy cosmic rays [Section 19.2]<br />
coming from beyond our own solar system produce radioactive<br />
carbon-14 in Earth’s atmosphere. During periods of high solar<br />
activity, the solar wind tends to grow stronger, shielding Earth<br />
from some of these cosmic rays. Thus, production of carbon-14<br />
drops when the <strong>Sun</strong> is more active. All the while, trees steadily<br />
breathe in atmospheric carbon, in the form of carbon dioxide,<br />
and incorporate it year by year into each ring. We can therefore<br />
estimate the level of solar activity in any given year by measuring<br />
the level of carbon-14 in the corresponding ring. No clear evidence<br />
has yet been found of longer-term cycles of solar activity,<br />
but the search goes on.<br />
Theoretical models predict a very long term trend of lessening<br />
solar activity. According to our theory of solar system formation,<br />
the <strong>Sun</strong> must have rotated much faster when it was young<br />
[Section 9.3].Because a combination of convection and rotation<br />
generates solar activity, a faster rotation rate should have meant<br />
much more activity. Observations of other stars that are similar<br />
to the <strong>Sun</strong> but rotate faster confirm that these stars are much<br />
more active. We find evidence for many more “starspots” on these<br />
stars than on the <strong>Sun</strong>, and their relatively bright ultraviolet and<br />
X-ray emissions suggest that they have brighter chromospheres<br />
and coronas—just as we would expect if they are more active<br />
than the <strong>Sun</strong>.<br />
chapter <strong>15</strong> • <strong>Our</strong> Star 5<strong>15</strong>