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a variety of space missions. Taken together, these observations
have revealed new insights into how solar magnetic activity
modulates terrestrial solar energy inputs and how magnetized plasma
from the Sun evolves as its flows to the Earth. These observations
have established beyond doubt that the Sun's energy output varies
continuously on all observed time scales.
Predicting, understanding, and monitoring global change are the
ultimate objectives of the USGCRP (Chapter 1). Yet contemporary
measurements of solar energy inputs alone reveal little about
future solar variability nor of past solar variations that might
have influenced the paleoclimate record, which is the focus of the
Earth System History science element of the USGCRP. To begin to
understand how the Sun varied in the past and how it might vary in
the future, we must first understand why the Sun varies at all.
The fundamental physical processes that generate the variations
observed in solar energy production are associated with the 22-year
magnetic cycle of the Sun. The sunspot number time series remains
the principal historical indicator of this cycle, and it is shown
in Figure 6.1. This is the record of solar activity that was
compared with the 14C and
temperature time series in Figure 1.3 and with surface temperature
anomalies in Figure 2.4. Recent monitoring from space indicates
that both the total solar irradiance (Figure 2.1) and the UV
irradiances (Figure 3.2) increase near the peak of the sunspot
cycle and decrease during times of few sunspots. Likewise, the flow
of energy, plasma, and magnetic fields from the Sun into the
Earth's environment depends on the magnetic cycle. Fundamental to
understanding the Sun's behavior as a variable star is
understanding how variations in its emitted energy are generated
from the magnetic activity cycle.
Origins of Solar Variability
The 22-year magnetic cycle of the Sun manifests itself as the
familiar 11-year sunspot cycle, the 22-year cycle being simply two
11-year cycles having reversed magnetic field polarities.
Physically, the sunspot cycle is a roughly periodic emergence,
approximately every 11.1 years, of strong magnetic flux tubes at
the solar surface in the form of sunspots. More