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12.5 The Solar Cycle
The number of sunspots changes from year to year in what is called the solar cycle. This variability can
be seen in figure 12.23, which shows the number of sunspots detected over the last 140 years or so. The
number of sunspots rises and falls approximately every 11 years. For example, the cycle had peaks in
1958, 1969, 1980, 1990, and 2001. This cyclical change in the number of sunspots was discovered by
Samuel Heinrich Schwabe, a pharmacist and amateur astronomer. He studied the Sun almost every day,
hunting for a hypothesized planet orbiting closer to the Sun than Mercury, but after 17 years his detailed
notes revealed this entirely unexpected phenomenon.
Flares and prominences also follow the solar cycle, and climate patterns on Earth may, too. For this
reason, astronomers have sought to understand not only the cause of the solar cycle but also how it
influences terrestrial climate.
Cause of the Solar Cycle
As the Sun rotates, gas near its equator circles the Sun faster than gas near its poles; that is, it spins
differentially, a property common in gaseous bodies (recall from chapter 10 that Jupiter and the other
giant planets rotate differentially). The Sun's differential rotation is such that its equator rotates in about
25 days and its poles in about 35 days. Thus, a set of points arranged from pole to pole in a straight line
would move over the course of time into a curve, as shown in figure 12.24.
Figure 12.23
Plot of sunspot numbers showing solar cycle.
Figure 12.24
Sketch showing solar differential rotation. Points near the Sun's equator rotate faster than points near the
poles.
Differential rotation should similarly distort the Sun's magnetic field, “winding up” the field below the
Sun's surface.* Astronomers think such winding of the Sun's magnetic field may cause the solar cycle,
though the exact mechanism is still not well understood. According to one hypothesis, the Sun's rotation
wraps the solar magnetic field into coils below the surface, making the field stronger and increasing solar
activity: spots, prominences, and flares. The wrapping occurs because the Sun's magnetic field is
“frozen” into the gas, as discussed in section 12.4. Thus, if the gas moves, so does the field, and vice
versa.
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Because the field and gas are tightly connected, differential rotation causes gas at the equator, which is
moving faster than the gas at the poles, to drag the magnetic field with it, so that a field, initially straight
north to south, is wound into two subsurface loops, as shown in figure 12.25. As the loops are wound
tighter, they develop kinks, as when you twist a rubber band too tight. The cycle ends when the field
twists too “tightly” and collapses, and the process repeats.
Figure 12.25
Sketch showing how differential rotation can wind up the Sun's subsurface magnetic field. The field
becomes twisted into coils that can break through the surface, making sunspots.
Sunspots form when kinks in the magnetic field rise to the Sun's surface and break through the
photosphere (last panel of fig. 12.25). Here, the field slows the outward flow of heat, making the surface
cooler and darker than in surrounding areas and thereby creating sunspots, as we discussed in section
12.4. Each kink breaks the photosphere in two places—one where it leaves and one where it enters. We
therefore expect that spots will occur in pairs or paired groupings.
In such pairs, one grouping has a north polarity and the other a south polarity.* That is, in one the field
emerges from the surface, while in the other it descends so that the field direction is reversed. The false­
color map of the Sun's field in Figure 12.19B shows this effect clearly. Yellow areas have a north
polarity and dark blue areas a south polarity. Notice that the pattern of the polarity differs between the
two hemispheres of the Sun. In one, the yellow areas are on the right, while in the other they are on the
left. This reversal arises because the subsurface field is coiled in opposite directions in the two
hemispheres, additional evidence that sunspots and the solar cycle are caused by winding of the Sun's
magnetic field.
Changes in the Solar Cycle
The solar cycle is not always 11 years: it may be as short as 7 or as long as 16 years. Moreover, if we
consider the polarity of the spot groups, the cycle averages 22 years, rather than 11, because the polarity
of the Sun's field reverses at the end of each 11­year cycle. It therefore takes two 11­year cycles for the
field to return to its original configuration.
Figure 12.26 illustrates this effect. In the first frame, we see spot pairs as the cycle begins. A right­hand
spot in the top hemisphere (technically, the “leading spot” because it leads in the direction the Sun
rotates) has a south polarity, while a left­hand spot (technically called a “trailing spot”) has a north
polarity.
Figure 12.26
Diagram illustrating the approximate 22­year periodicity of solar magnetic activity.
You can also see an additional feature of the cycle: all leading spots in one hemisphere have the same
polarity. In the other hemisphere, the leading spots have the opposite polarity. Eleven years later, the
polarity of the pairs will be reversed, as you can see by looking at Cycle 2 in figure 12.26. Only in the
next cycle, another 11 years later, will the spot fields have returned to their original directions. Thus, the
full cycle of magnetic activity takes 22 years on the average.
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Links Between the Solar Cycle and Terrestrial Climate
Climatologists find this 22­year period interesting because of a cycle in which droughts occur
approximately every 22 years in the midwestern United States and Canada. Does the Sun's cycle affect
the Earth's climate cycle? If so, how? One possibility is as follows.
The Sun's magnetic field heats the corona. The corona drives the solar wind. The solar wind alters the
Earth's upper atmosphere; in particular, it changes the way the temperature varies with altitude. This in
turn alters the atmosphere's circulation and may shift the jet stream to a new location. The jet stream
steers storms and hence rainfall.
ANIMATION
Wrapping of the Sun's subsurface magnetic field
Although this hypothesis has not been verified yet, many scientists think that solar activity affects our
climate. The evidence to support this hypothesis is based in part on the work of E. W. Maunder, a British
astronomer who studied sunspots. Maunder noted in 1893 that, according to historical records, very few
sunspots were seen between 1645 and 1715 (fig. 12.27). He concluded that the solar cycle turned off
during that period. The period is now called the Maunder minimum in honor of his discovery.
Wrapping of Sun's Subsurface Magnetic Field
Figure 12.27
Plot illustrating that the number of sunspots changes with time, showing the Maunder minimum and the
solar cycle.
The Maunder minimum coincides with an approximately 70­year spell of abnormally cold winters in
Europe and North America. Glaciers in the Alps advanced; rivers froze early and remained frozen late;
the North Sea froze. The cold was so abnormal that meteorologists call the epoch part of the “little ice
age.” If only one such episode were known, we might dismiss the sunspot–climate connection as a
coincidence, but three other cold periods have also occurred during times of low solar activity. This
strengthens our conclusion that somehow the Sun's magnetic activity affects our climate.
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Although scientists have concluded that a link exists between solar activity and Earth's climate, there are
clearly other factors affecting our climate as well. Figure 12.28 shows how the ocean temperature
(expressed as deviations from the normal average) changed from 1860 to 2000. The figure also shows
how the number of sunspots changed over the same time span. Notice that from 1860 to 1980, when the
number of spots is high, the ocean is warmer than average, and when the number of spots is low, the
ocean is colder than average. But from 1990 on, the curves no longer are in step, which most scientists
interpret as strong indication that increasing levels of greenhouse gases in Earth's atmosphere are to
blame, not the Sun.
Figure 12.28
Curves showing the change in ocean temperatures on Earth and the change in sunspot numbers over
more than a century. Notice that the curves were approximately in step until the 1980s. Over the last two
decades the sea surface temperature has continued to rise while sunspot counts have declined.
Astronomers deduce that solar activity affects our climate, but other factors must explain the recent
temperature rise.