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the mix of active region emission that generates the solar Ca II
irradiance was somewhat different than is currently seen in the
Sun, the radiative output of the Sun can be estimated for the
Maunder Minimum (Figure 2.3).
Current ability to predict solar activity is at best primitive.
Statistical methods predict sunspot numbers and the 10.7 cm radio
flux 12 months in the future with moderate success. There are also
precursor methods that predict the strength of the next solar cycle
from the behavior of polar structure on the Sun and geomagnetic
activity in the declining phase of the current cycle (e.g.,
Schatten and Pesnell, 1993; Thompson, 1993). But there is limited
physical understanding of why these precursor methods should be
appropriate except that the magnetic fields and corona near the
solar poles change near solar maximum and hence may herald the
onset of the new cycle before the next generation of sunspots
On century time scales, the periodicities of 11 and 88 years
identified in the sunspot record, together with the 208 year
periodicity found in the 14C
record, provide limited guidance to future solar behavior, such as
the occurrence of the next Maunder Minimum. The time span of solar
measurement is simply too short for reliable prediction of solar
extrema occurring sporadically every 200 years or so. Nevertheless,
it has been speculated that the concatenations of the 208 and 88
year periods may have contributed to generally increasing solar
activity levels during the twentieth century, with maximum activity
predicted to occur during the first half of the twenty-first
century (Damon and Sonnet, 1991).
Predictive capability will be substantially improved when a
complete understanding is obtained of the mechanisms within the
solar atmosphere that produce the emitted radiation, and form
sunspots and plages. Predicting the Sun-as-a-star energy quantities
needed for global change studies will ultimately require
development of a theory for the solar dynamo that can accommodate
known solar behavior. Yet, the very nature of solar variability,
whether driven by an internal chronometer (Dicke, 1978) or by
stochastic or chaotic processes (Mundt et al., 1991; Morfill et
al., 1991; Kremliovsky, 1994), remains elusive. Solar activity
levels may well defy reliable prediction in the near future.