One measure of the structure of the ionosphere is the free electron density, which is an indicator of the degree of ionization. Here are electron density contour maps of the ionosphere for months in 1957 to the present. Compare these simulations of the variation by month of the ionosphere for the year 1990 (a period of high solar activity with many sunspots) and 1996 (a period of low solar activity with few sunspots):

The adjacent images are based on these electron density contour maps of the ionosphere for months in the year 1957 to the present. Additional animations may be found in this NOAA directory.

Notice the substantial differences in these two animations, corresponding to the strong influence of solar activity on the structure of the Earth's ionosphere.