Sunspots are indicators of other phenomena that can be important to space weather, and ultimately to ionospheric effects on telecommunication systems. Actually, sunspots are probably overrated as indicators of space weather phenomena. Nevertheless, sunspots have been monitored for centuries and have proved to be a useful if imprecise index. Indeed, because of its historical record and availability, most predictive models are at least partly based upon some measure of the sunspot number.
The most common index of solar activity is based upon a count of the number of sunspots on the solar disk. The fundamental index is the relative sunspot (or Wolf) number that is reckoned daily. It is given by the following relationship developed by Rudolf Wolf who was the first director of the Swiss Federal Observatory in Zurich:
In Equation 2.4, A: is a correction factor dependent upon the observatory, g represents the number of sunspot groups, and s is the number of individual spots. For many years the Wolf number was compiled from measurements compiled at Zurich. Until 1981, when it was discontinued, it formed the basis for many solar and ionospheric studies. After 1981, the Zurich number (termed Rz) was replaced by the International sunspot number, Ri. About 25 stations are involved in the construction of Rj. Throughout the chapter we will use the term SSN as a generic for various forms of sunspot number, including Rz and Rj.
Records of daily and averaged sunspot numbers are archived by the World Data Center A for Solar-Terrestrial Physics through the National Geophysical Data Center located in Boulder, Colorado. The Solar Influences Data Center (SIDC) in Brussels, also a Regional Warning Center, compiles the International Sunspot number and various other products (See Chapter 5).
Another index of solar activity used by many because of its ease of determination and its power as a representative index of solar activity is the noontime value of the 10.7 cm (i.e., 2800 MHz) solar flux, </>, from either the Penticton Radio Observatory or from Ottawa. This index is expressed as a monthly mean value in units of 10"22 Watts m"2 Hz"1. Stewart and Leftin  have compared the Ottawa flux index with the sunspot number and have derived the following relationship:
(f>i2 = 63.7 + 0.728 RI2 + 8.9 (10"4)/?,/ (2.5)
where <J)12 and Rn are the 12-month running mean values of <f> and R respectively. Note that at solar minimum, the flux level is not zero. For a sunspot number of 100, the solar flux would be ~ 145 according to the Equation 2.5 approximation.
Figure 2-7 gives the range of daily variability in the sunspot number for a period of 170 years, and Figure 2-8 exhibits the 10.7 cm (2800 MHz) solar flux from 1947-2003. Both day-to-day and month-to-month variability in both <f> and R may be significant; but is it important from a practical standpoint? We will explore this matter in Section 2.2.6. But first we will mention a few things about prediction of the sunspot cycle.
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