The sunspot cycle
The number of sunspots visible on the solar disk at any given time varies continuously, as sunspots are carried on and off the visible disk by solar rotation. At first glance, observations carried out over time periods of weeks suggest that the appearance of sunspots are stochastic, but observations over time periods of decades reveal an intriguing cyclic pattern of gradual increase and decrease in the average number of sunspots visible on the solar disk. This was first noted in 1843 by H. Schwabe, an amateur solar astronomer, and provided the first hint of the existence of the sunspot cycle, whose period Schwabe estimated to be about 10 years. Further work revealed that the length of successive sunspot cycles is not strictly constant but varies from  to 11.5 years, with an average cycle period of about 10.8 years.

The Corona through the solar cycle
Not surprisingly, the changes in the surface magnetic field distribution through the solar cycle, as evidenced by the evolving numbers and spatial distributions of sunspots, prominences and filaments, are also reflected in the corona.

The top row of images are X-ray images from the Yohkoh satellite, spaced approximately 10 months apart during the descending part of cycle 22. The overall decrease in X-ray luminosity for the solar disk as a whole is in many ways as spectacular as the decrease in the number of active regions seen at a given time on the disk. The bottom row shows a few eclipse photographs spanning the time period 1966---1988, together with a coronal image constructed from Solar Maximum Mission data for 1985, essentially at solar minimum. Note how the corona is reduced to a belt of streamers symmetrically straddling the solar equator.

The Solar Corona over 6 yrs