As of 24 May 2021 this site has switched from using a decades-old quadratic relationship between the 10.7cm solar radio flux (F10.7) and the solar sunspot number (SSN). This relationship was based on data collected prior to 1972. We have been using it in order to retain some continuity over the many solar cycles despite a known deficiency around solar minimum (a single linear relationship starts to break down below a sunspot number of 25-35).
Over the past several years the international solar research community has rescaled the sunspot number counts, which results in this algorithm no longer being valid. There are several relationships between F10.7 and the new sunspot number series, but rather than use one of these I have decided to develop my own tailored to this relationship over the past two cycles. In the Tapping and Morgan (2017) paper listed below, evidence is presented that the relationship between these two indexes has been changing over the past two cycles. Also in that paper, they present a relationship derived from all cycles for which there are F10.7 observations available (from mid-Cycle 18 through Cycle 24) which includes a "knee" at around SSN=20 which they speculate as being due to a change of the energetics involved at low activity.
Based on this, I have derived a simple relationship using two linear fits to data from the
past two cycles (23 and 24) with the two linear segments matched near SSN=20:
Tapping, K., and C. Morgan (2017), Changing relationships between sunspot number, total sunspot area and F10.7 in Cycles 23 and 24, Solar Phys., 292, DOI 10.1007/s11207-017-111-6.
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