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Figure 6.1 Solar activity variations during the past four centuries, as indicated by monthly means since 1750 of the sunspot number (solid line) with yearly means from 1610 to 1750 (from the National Geophysical Data Center, dashed line, and Eddy, 1976, crosses). Prominent in the record is a cycle of about 11 years. According to Eddy, the 11-year sunspot cycle was severely depressed between 1645 and 1715 (see also Ribes et al., 1989). Thus, during the contemporary epoch when the Sun has been observed most intensely, its activity has been at relatively high levels compared with the past 300 years. This is confirmed by cosmogenic isotope records (McHargue and Damon, 1991). The Sun is likewise at high activity levels compared with other Sun-like stars (Figure 2.3). Recent analysis of sunspot observations in the 18th century indicate that the sunspot number may overestimate solar activity levels in that period (Hoyt et al., 1994). From J. Lean, Reviews of Geophysics, 29, 506, 1991, copyright by the American Geophysical Union.

generally, the solar activity cycle pertains to the periodic emergence ofmagnetic flux that generates not just sunspots, which are dark, but avariety of phenomena, especially bright regions known as plages andfaculae that radiate strongly at UV and EUV wavelengths. The darksunspots and bright plages and faculae modify the radiation from the solardisk, thereby generating the variations observed by spaceborne solarradiometers. Also, changes in the Sun's magnetic field topology, due to

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