The variation in sunspot numbers from 1700-1993, courtesy of NOAA/NGDC.
Click on image for full size
The number of sunspots
on the Sun is not constant. In addition to the obvious variation
caused by the Sun's rotation (sunspots disappear from view and then
re-appear), over time new sunspot groups form and old ones decay and
When viewed over short periods of time (a few weeks or months), this
variation in the number of sunspots might seem to be random.
However, observations over many years reveal a remarkable feature
of the Sun: the number of sunspots varies in a periodic manner, usually
described as the 11 year cycle (in actuality, the period varies, and
has been closer to 10.5 years this century). The 11 year sunspot
cycle is related to a 22 year cycle for the reversal of the
Sun's magnetic field. In 1848 Johann Rudolf Wolf
devised a method of counting sunspots on the solar disk called
the Wolf number. Today the Wolf number (averaged from many
observing sites) is used to keep track of the solar cycle.
While the cycle has been relatively uniform this century,
there have been large variations in the past. From about
1645 to 1715, a period
known as the Maunder minimum, apparently few sunspots were present
on the Sun.
During the solar cycle,the migration of sunspots in latitude has a
Although the number of sunspots is the most easily observed feature,
essentially all aspects of the Sun and
solar activity are influenced
by the solar cycle. Because solar activity
(such as coronal mass ejections) is more frequent at solar maximum and
less frequent at solar minimum, geomagnetic activity also follows
the solar cycle. Why is there a solar cycle? No one knows
the answer to this question.
A detailed explanation of the solar cycle is a fundamental
physics problem still waiting to be solved.
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