Chapter 15--Our Sun - Geological Sciences

Figure 15.20 An X-ray image of the
Sun reveals the million-degree gas of the
corona. Brighter regions of this image
(yellow) correspond to regions of stronger
X-ray emission. The darker regions are
the coronal holes from which the solar
wind escapes. (From the Yohkoh space
observatory.)
X-ray
15.5 Solar Weather
and Climate
Individual sunspots, prominences, and flares are short-lived
phenomena, somewhat like storms on Earth. They constitute
what we call solar weather or solar activity.You know
from personal experience that the Earth’s weather is notoriously
unpredictable. The same is true for the Sun: We
cannot predict precisely when or where a particular sunspot
or flare will appear. Earth’s climate, on the other hand, is
quite regular from season to season. So it is with the Sun,
where despite day-to-day variations the general nature and
intensity of solar activity follow a predictable cycle.
The Sunspot Cycle
Long before we realized that sunspots were magnetic disturbances,
astronomers had recognized patterns in sunspot
activity. The most notable pattern is the number of sunspots
visible on the Sun at any particular time. Thanks to
telescopic observations of the Sun recorded by astronomers
since the 1600s, we know that the number of sunspots
gradually rises and falls in a sunspot cycle with an average
period of about 11 years (Figure 15.21a). At the time of
solar maximum,when sunspots are most numerous, we
may see dozens of sunspots on the Sun at one time. In contrast,
we see few if any sunspots at the time of solar minimum.The
frequency of prominences and flares also follows
the sunspot cycle, with these events being most common
at solar maximum and least common at solar minimum.
Although we’ll call it an “11-year” cycle, the interval
between solar maxima is sometimes as long as 15 years
or as short as 7 years. The number of sunspots also varies
dramatically (Figure 15.21a). In fact, sunspot activity virtually
ceased between the years 1645 and 1715, a period
sometimes called the Maunder minimum (after E. W.
Maunder, who identified it in historical sunspot records).
Another feature of the sunspot cycle is a gradual
change in the solar latitudes at which individual sunspots
form and dissolve (Figure 15.21b). As a cycle begins at solar
minimum, sunspots form primarily at mid-latitudes (30°
to 40°) on the Sun. The sunspots tend to form at lower latitudes
as the cycle progresses, appearing very close to the
solar equator as the next solar minimum approaches.
A less obvious feature of the sunspot cycle is that
something peculiar happens to the Sun’s magnetic field
at each solar minimum. The field lines connecting all pairs
of sunspots (see Figure 15.17) tend to point in the same
direction throughout an 11-year solar cycle (within each
hemisphere). For example, all compass needles might point
from the easternmost sunspot to the westernmost sunspot
in a pair. However, as the cycle ends at solar minimum, the
magnetic field reverses: In the subsequent solar cycle, the
field lines connecting pairs of sunspots point in the opposite
direction. Apparently, the entire magnetic field of the Sun
flip-flops every 11 years.
The magnetic reversals hint that the sunspot cycle
is related to the generation of magnetic fields on the Sun.
They also tell us that the complete magnetic cycle of the
Sun, called the solar cycle, really averages 22 years, since it
takes two 11-year cycles before the magnetic field is back
the way it started.
chapter 15 • Our Star 513