This plot illustrates the differences between the Estimated International Sunspot Number (EISN), 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.
The algorithm used to calculate the F10.7-derived SSN (SSNf) parameter is described here.
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