Instead of the sunspot number, the total area of all spots observed on the solar disk might seem to be a less arbitrary measure of solar activity. However, these data have been available since 1874 only, covering a much shorter period of time than the sunspot number data. In addition, the determination of sunspot areas, especially farther from disk center, is not as trivial as it may seem, resulting in significant random and systematic errors in the area determinations. Area measurements performed in two different observatories often show discrepancies reaching 30% for smaller spots (cf. the figure and discussion in Appendix A of Petrovay et al., 1999).
A number of other direct indicators of solar activity have become available from the 20th century. These include, e.g., various plage indices or the 10.7 cm solar radio flux – the latter is considered a particularly good and simple to measure indicator of global activity (see Figure 2). As, however, these data sets only cover a few solar cycles, their impact on solar cycle prediction has been minimal.
Of more importance are proxy indicators such as geomagnetic indices (the most widely used of which is the aa index), the occurrence frequency of aurorae or the abundances of cosmogenic radionuclides such as 14C and 10Be. For solar cycle prediction uses such data sets need to have a sufficiently high temporal resolution to reflect individual 11-year cycles. For the geomagnetic indices such data have been available since 1868, while an annual 10Be series covering 600 years has been published very recently by Berggren et al. (2009). Attempts have been made to reconstruct the epochs and even amplitudes of solar maxima during the past two millennia from oriental naked eye sunspot records and from auroral observations (Stephenson and Wolfendale, 1988; Nagovitsyn, 1997), but these reconstructions are currently subject to too many uncertainties to serve as a basis for predictions. Isotopic data with lower temporal resolution are now available for up to 50 000 years in the past; while such data do not show individual Schwabe cycles, they are still useful for the study of long term variations in cycle amplitude. Inferring solar activity parameters from such proxy data is generally not straightforward.
Living Rev. Solar Phys. 7, (2010), 6
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