This plot illustrates the differences between the "real" sunspot number (SSN), which is calculated from optical observations of the sun, a sunspot number derived from the 10.7cm solar radio flux (SSNf), and a sunspot number derived from fitting an ionospheric model to ionospheric measurements. All of these indices are used as inputs to models of the ionosphere for use in communications-performance predictions - this plot shows that they don't always agree as to what the SSN should be in that particular context.

Note: The F10.7-derived SSN (SSNf) is calculated from the 10.7cm solar radio flux (the Penticton Radio Observatory noon value) using the following relationship: