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 Sunspot Cycle
Some sources for sunspot numbers are listed below. 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 11 years. This 11 year periodicity is
called the sunspot cycle.
The last solar maximum (period of maximal sunspot activity was in the year
1990-1991. Thus, we may expect the next solar maximum early in the next
The Active and Quiet Sun
Sunspot maxima correspond generally to periods of high solar activity. This
activity includes increased solar wind and phenomena like aurorae and magnetic
storms that are correlated with the solar wind, increased flares and
prominences, and increased non-thermal radio and X-ray emission.
Conversely, near sunspot minima the Sun is much quieter with respect to these
phenomena. In addition, as we have seen there are significant differences in
the nature of the corona during periods of active and quiet Suns.
Sources for Sunspot Data: