This plot illustrates the difference between the Estimated International Sunspot Number (EISN), which is calculated from optical observations of the sun and a sunspot number derived from the 10.7cm solar radio flux (SSNf). These indices are used as inputs to models of the ionosphere for use in communications-performance predictions.

The algorithm used to calculate the F10.7-derived SSN (SSNf) parameter is described here.

Note: The foF2-based effective sunspot number (SSNe) which had been included in this plot is no longer in production.