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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



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