How sunspots are counted
Sunspots are dark, cooler patches on the Sun’s surface where strong magnetic fields hold back the flow of heat. The sunspot number R is computed from the number of individual spots s and the number of groups g: R = k(10g + s), where k is a correction factor for each observer or instrument. Different organisations publish slightly different values (for example the international SILSO number and the NOAA SWPC number), so compare like with like.
The 11-year cycle
Sunspot numbers rise to a maximum and fall to a minimum about every 11 years, with strong cycle-to-cycle variation. Around the maximum, the ionosphere is highly ionised and the upper HF bands (15, 12, 10 and even 6 metres) open on many days. Around the minimum, the upper bands are mostly shut and 40 to 160 metres become the workhorses. Smoothed (13-month average) numbers are used to describe where a cycle stands, while the daily number jumps around considerably.
Sunspots versus solar flux
Sunspot number and the solar flux index are closely related. An empirical relationship (Holland and Vaughan) is F10.7 = 63.7 + 0.728 R + 0.00089 R². The SFI to sunspot number converter applies it in both directions. For propagation, the flux is more useful because it is a smoother, objective measurement.
