The Sun is the prime source of energy in our solar system and it is the prime source of space weather. This chapter provides an overview on the main forms of solar energy output ± fast and slow solar wind streams, co-rotating interaction regions,¯ares, coronal mass ejections and their interplanetary counterparts, solar energetic particle events ± that determine space weather conditions in the interplanetary medium and in geospace and their variation with the solar activity cycle. The chapter also addresses the processes through which the energy transfer is modulated by solar, interplanetary and terrestrial conditions. The outlook of the chapter aims at de®ning the required observations that are crucial to help establishing real-time space weather forecasts.On Earth ± only at the visible wavelengths and part of the radio wavelength regime ± the atmosphere is fully transparent to the Sun's radiation (see Figure 3.3) so that solar observations at X-ray and EUV wavelengths have to be achieved through space missions. The Yohkoh mission, launched on August 31, 1991 can be regarded as a milestone in terms of continuous high spatial resolution, full disk solar remote-sensing observations at X-ray wavelengths. Until the end of its mission life-time ± on December 14, 2001 ± it provided stunning new views of the Sun's X-ray corona (Figure 3.4, color section).Although the Sun's total irradiance ± the`Solar Constant', being roughly 1367 W/m 2 as measured at the distance of the Earth ± varies only at the order of 0.1% in the course of the solar cycle (e.g., Froehlich, 2003), the variation at speci®c wavelength intervals can be much larger (see Chapter 8). Since intensity variations at UV-and EUV-wavelengths may have important eects on the Earth's atmosphere, this subject is one of the hot current research topics. Figure 3.5 (color section) shows the variation of the longitudinal component of the photospheric magnetic ®eld from solar activity maximum around 1992 until the next one around 2000, as measured by the US Kitt Peak National Solar Observatory (KPNSO), Tucson, Arizona, together with the measurements of the Sun's coronal soft X-ray emission, as measured by the Soft X-ray Telescope (SXT) onboard the Japanese/US Yohkoh satellite. The total variation of irradiance ± in the range 2±30 A Ê ± was roughly at the order of 10 2 between 1992 and 1996, but can in principle be much larger in case the solar photospheric magnetic ®elds are more frequent and intense, as can be expected, for example, from the highly varying number of sunspots in the dierent c. 11-yr long solar activity cycles (Figure 3.6). Benevolenskaya et al. (2002) found that the soft X-ray intensity shows Total power input Solar radiation
