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Figure 1.1 (a) The Sun's spectral irradiance (solid line, typical of solar minimum conditions) is compared with the spectrum of a black body radiator at 5770 K (dashed line). The broad spectral bands identified along the top of this figure are the ultraviolet (UV), visible (VIS), and infrared (IR). Not shown, at wavelengths longer than the IR, is the microwave or radio portion of the solar spectrum. (b) Approximate amplitude of the Sun's spectral irradiance variation from the maximum to minimum of the 11-year activity cycle. The solar cycle variation in the spectrally integrated, or total, solar irradiance is indicated by the dot-dash line. From J. Lean, Reviews of Geophysics, 29, 506, 1991, copyright by the American Geophysical Union.