Chapter 15--Our Sun - Geological Sciences
Chapter 15--Our Sun - Geological Sciences
Chapter 15--Our Sun - Geological Sciences
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Figure <strong>15</strong>.20 An X-ray image of the<br />
<strong>Sun</strong> reveals the million-degree gas of the<br />
corona. Brighter regions of this image<br />
(yellow) correspond to regions of stronger<br />
X-ray emission. The darker regions are<br />
the coronal holes from which the solar<br />
wind escapes. (From the Yohkoh space<br />
observatory.)<br />
X-ray<br />
<strong>15</strong>.5 Solar Weather<br />
and Climate<br />
Individual sunspots, prominences, and flares are short-lived<br />
phenomena, somewhat like storms on Earth. They constitute<br />
what we call solar weather or solar activity.You know<br />
from personal experience that the Earth’s weather is notoriously<br />
unpredictable. The same is true for the <strong>Sun</strong>: We<br />
cannot predict precisely when or where a particular sunspot<br />
or flare will appear. Earth’s climate, on the other hand, is<br />
quite regular from season to season. So it is with the <strong>Sun</strong>,<br />
where despite day-to-day variations the general nature and<br />
intensity of solar activity follow a predictable cycle.<br />
The <strong>Sun</strong>spot Cycle<br />
Long before we realized that sunspots were magnetic disturbances,<br />
astronomers had recognized patterns in sunspot<br />
activity. The most notable pattern is the number of sunspots<br />
visible on the <strong>Sun</strong> at any particular time. Thanks to<br />
telescopic observations of the <strong>Sun</strong> recorded by astronomers<br />
since the 1600s, we know that the number of sunspots<br />
gradually rises and falls in a sunspot cycle with an average<br />
period of about 11 years (Figure <strong>15</strong>.21a). At the time of<br />
solar maximum,when sunspots are most numerous, we<br />
may see dozens of sunspots on the <strong>Sun</strong> at one time. In contrast,<br />
we see few if any sunspots at the time of solar minimum.The<br />
frequency of prominences and flares also follows<br />
the sunspot cycle, with these events being most common<br />
at solar maximum and least common at solar minimum.<br />
Although we’ll call it an “11-year” cycle, the interval<br />
between solar maxima is sometimes as long as <strong>15</strong> years<br />
or as short as 7 years. The number of sunspots also varies<br />
dramatically (Figure <strong>15</strong>.21a). In fact, sunspot activity virtually<br />
ceased between the years 1645 and 17<strong>15</strong>, a period<br />
sometimes called the Maunder minimum (after E. W.<br />
Maunder, who identified it in historical sunspot records).<br />
Another feature of the sunspot cycle is a gradual<br />
change in the solar latitudes at which individual sunspots<br />
form and dissolve (Figure <strong>15</strong>.21b). As a cycle begins at solar<br />
minimum, sunspots form primarily at mid-latitudes (30°<br />
to 40°) on the <strong>Sun</strong>. The sunspots tend to form at lower latitudes<br />
as the cycle progresses, appearing very close to the<br />
solar equator as the next solar minimum approaches.<br />
A less obvious feature of the sunspot cycle is that<br />
something peculiar happens to the <strong>Sun</strong>’s magnetic field<br />
at each solar minimum. The field lines connecting all pairs<br />
of sunspots (see Figure <strong>15</strong>.17) tend to point in the same<br />
direction throughout an 11-year solar cycle (within each<br />
hemisphere). For example, all compass needles might point<br />
from the easternmost sunspot to the westernmost sunspot<br />
in a pair. However, as the cycle ends at solar minimum, the<br />
magnetic field reverses: In the subsequent solar cycle, the<br />
field lines connecting pairs of sunspots point in the opposite<br />
direction. Apparently, the entire magnetic field of the <strong>Sun</strong><br />
flip-flops every 11 years.<br />
The magnetic reversals hint that the sunspot cycle<br />
is related to the generation of magnetic fields on the <strong>Sun</strong>.<br />
They also tell us that the complete magnetic cycle of the<br />
<strong>Sun</strong>, called the solar cycle, really averages 22 years, since it<br />
takes two 11-year cycles before the magnetic field is back<br />
the way it started.<br />
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