The Sunspot Cycle
Sunspots have been monitored since the time of Galileo. One striking
feature that emerges from the long-term data
is that the number of sunspots observed in a given year
varies in a dramatic and highly predictable way.
The 11-Year Cycle
Some sources for sunspot numbers are listed in the right panel. If one plots the total
number of sunspots observed in a year as a function of the year the plot shown
to the right is obtained. There is a striking variation in the number
of sunspots that is cyclic, with a
period of approximately eleven years. This eleven-year periodicity is
called the sunspot cycle.
The last two solar maxima (period of maximal sunspot activity) were in the years
1990-1991 and 2000-2001. Thus, we may expect the next solar maximum
around the year 2012.
Double Peaks in the Sunspot Maximum
There is some evidence in each of the last three sunspot maxima
that the number of sunspots plotted versus time
formed a weakly-doubled peak near maximum. For example,
in mid-2000 the number of sunspots
reached a peak and began to decline, but a few months later
it increased
again and peaked a second time near mid-2001. Thus, the
sunspot cycle is perhaps a little more complicated than the simple
picture of a single 11-year period would suggest. However, these
double-peaked effects are rather small and the simple 11-year
period is still a very good description of the frequency of sunspots.