|Sunspot number is the most important index for tracking the level of solar activity. It is calculated as
where n is the number of individual spots, g is the number of sunspot groups and k is a constant that is different for each station. Records of sunspot number go back to the mid-17th century. This site lists values of the daily mean sunspot number from 1818 to the present, monthly sunspot counts from 1749 to the present, and yearly sunspot counts from 1700 to the present. The most obvious period in these records is an 11.4 year period called the solar cycle.
Another measure of the solar activity is provided by the 10.7 centimeter radio flux. Since intense emission at radio wavelengths are produced in magnetically active regions, a proxy can be created for sunspot number using measurements of the radio flux. Statistical correlations between the sunspot number and f10.7 flux have been made using 40 years of data and is given by
R = (1.14).S - 73.21
where S is the solar flux (density) value in solar flux units. Tabulated values of the f10.7 fluxfor use in this calculation are available from DRAO, National Research Council of Canada